Patent Publication Number: US-8993297-B2

Title: Preparation and uses of gene sequences encoding chimerical glycosyltransferases with optimized glycosylation activity

Description:
The present invention relates to the production of gene sequences encoding chimerical glycosyltransferases presenting optimized glycosylation activity, and to their uses in the frame of the preparation of recombinant proteins of interest by cells transformed with said sequences and sequences encoding said recombinant proteins. 
     Glycosylation of Proteins and Lipids 
     Proteins and lipids are major components of cell membranes. Membrane associated carbohydrates are exclusively in the form of oligosaccharides covalently attached to proteins forming glycoproteins, and to lipids forming glycolipids. Glycoconjugates (including glycolipids and glycoproteins) are most often key cell surface molecules which are considered to be involved in cell-cell interactions and cell adhesion (Feizi, 1993; Crocker &amp; Feizi, 1996). 
     The predominant monosaccharides found in eukaryotic glycoproteins are glucose (Glc), galactose (Gal), mannose (Man), fucose (Fuc), N-Acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc) and sialic acid (Sia) most often as neuraminic acid (NeuAc) which may be N-acetylated or N-glycolylated in mammals but only N-acetylated in humans. Carbohydrate chains also designated as glycans are linked to the polypeptide backbone through either O-glycosidic or N-glycosidic bonds. The N-glycosidic linkage is through the amide group of asparagine. The O-glycosidic linkage is to the hydroxyl of serine, threonine or hydroxylysine. In ser- or thr-type O-linked glycoproteins, the monosaccharide directly attached to the protein is frequently GalNAc while in N-linked glycoproteins, only GlcNAc is found. 
     The N-glycosidic linkage is conserved throughout the eukaryotic kingdom including yeast; plants, insects, mammals and humans. The site of N-linked glycosylation occurs within a consensus sequence of amino acids, Asn-X-Ser/Thr (N-X-S(T)), where X is any amino acid except proline. When a protein analysis in the public databases is carried out, it can be shown that approximately 65% of all the proteins contain at least one such consensus sequence. N-linked glycoproteins all contain a common (invariant) pentasaccharide attached to the polypeptide. This core consists of three Man and two GlcNAc residues. Antennae of variable sequence are completing the glycan and allowing the classification into three major N-linked subclasses:
     1. Mannose-rich glycans contain only mannose as terminal sugars.   2. Hybrid glycans contains at least a GlcNAc-Gal antenna.   3. Complex glycans contain from 2 to 5 of those antennae terminated in GlcNAc, Gal or sialic acid.   

     In all eukaryotic cells, N-linked glycoproteins are synthesized co-transitionally from polyribosomes bound to the endoplasmic reticulum (ER). Processing of N-linked glycans occurs early in the lumen of the ER and continues in the golgi apparatus and transgolgi network where glycoproteins achieved their final and functional polymorphism. Attachment of O-linked glycans occurs post-translationally in the golgi apparatus where glycosylation of lipids also occurs. Sugars used for both N- and O-glycosylation are activated by coupling to specific nucleotides in the cytoplasm and are imported within the lumen of organelles through specific transporters. 
     Glycosylation is the most sophisticated set of post-translational modifications which may occur in proteins. They aimed to: i) control the biological activity of proteins, ii) signal proteins for binding lectin-like receptors and/or degradation systems, iii) address proteins to the cell surface (secretion), iv) target proteins to cellular compartments, v) define an immunological identity (blood groups). As a result, glycosylated proteins exist as a mixture of glycoforms whose physico-chemical and biological properties differ from the product directly coded by the relevant gene. It is widely admitted that post-translational modifications of proteins allow a better adaptation the protein to its biological function (Helenius &amp; Aebi, 2001). 
     Many human glycoproteins are of high clinical relevance. For example, on cell surfaces, they are important for communication between cells, for maintaining cell structure and for self-recognition by the immune system. 
     Glycoproteins are most abundant in soluble forms in biological fluids such as milk and blood. In this case, glycans protect the proteins against proteases, increase their solubility govern their plasmatic clearance and address them to target organs. 
     Glycosylation Enzymes 
     Glycosylation reactions are of major biological importance to both prokaryotes and eukaryotes and require the coordinated action of a large number of enzymes designated as Glycosyltransferases (GTs) (Breton &amp; Imberty, 1999). 
     Glycosyltransferases (GTs) constitute a large family of enzymes involved in the biosynthesis of polysaccharides and glycoconjugates in the prokaryotic and eukaryotic kingdoms respectively. Sequences of the prokaryotic enzymes are not homologous to mammalian glycosyltransferases while enzymes from human, mammals and drosophila often share significant homology. So far, more than a thousand of GTs are known to mediate a wide array of biological functions. Developments in the molecular biology of these enzymes have revealed an unexpected diversity suggesting that glycosylation process probably require the involvement of about 250 genes in a single human living cell (Breton et al., 2001). About 500 glycosyltransferases including 160 human enzymes have been cloned to date. The number of cloned enzymes is increasing in human and may reach 200 shortly (Narimatsu, 2004). GTs are classified according to the stereochemistry of the reaction substrates and products as either retaining or inverting enzymes (Sinnott, 1990). A classification of glycosyltransferases using nucleotide diphospho-sugar, nucleotide monophospho-sugars and sugar phosphates and related proteins into distinct sequence-based families has been established. It shows that human GTs distribute so far into 42 structural families. 
     Glycosyltransferases catalyze the transfer of sugar residues from a nucleotide-sugar (activated donor substrate) to an acceptor (lipid, protein or growing carbohydrate chain). In addition to the official classification of enzymes, they can be grouped into functional families based on their sequence similarities, which may reflect enzymatic characteristics such as donor specificity, acceptor specificity, and linkage specificity between donor and acceptor. Based on the sugar they transfer, GTs are named according to the sugar they transfer such as N-Acetylglucosaminyltransferase (GlcNAcT), galactosyltransferases (GalT), and, fucosyltransferases (FucT) or sialyltransferases (SiaT or STs). Owing to the accumulation of glycosyltransferase gene data, it is thought that glycosyltransferases have high specificity for the type of linkage in either the donor and acceptor substrates as well as for the nature of the glycoconjugate acceptor (N-/O-linked glycoproteins or lipids). 
     Structure of Glycosyltransferases: 
     All GTs cloned, so far in vertebrates display the same topology. They are type II membrane proteins (N-terminal cytoplasmic domain) composed of four main domains: a short cytoplamic tail (CT) at the N-terminal end, a membrane anchor region (TMD) of 10 to 20 amino acids, a stem region (SR) and a large C-terminal catalytic domain (CD) ( FIG. 1 ) (Paulson et al., 1987). The anchor region acts as a non cleavable signal peptide and also as a transmembrane domain (Wickner &amp; Lodisch, 1985), orientating the catalytic domain of GTs in the lumenal part of the golgi apparatus. The SR is widely considered as a flexible region allowing also the orientation of the CD in the lumenal part of the golgi apparatus. On a general way, the CD is reasonably conserved within GTs of various species whereas the SR constitutes an hypervariable portion of the transferase. Some of these enzymes may be cleaved at their SR by an endogenous protease, or proteases, and secreted out of the cell (Paulson &amp; Colley, 1989) to produce milk or blood enzymes. It has been well documented that the proteolytic cleavage and secretion of glycosyltransferases are affected by various pathological conditions such as malignant transformation and inflammation but the molecular mechanisms underlying the cleavage and secretion have not yet been clarified. 
     GTs and neoglycosylated proteins/lipids have been localized inside the ER and the subcompartments of Golgi using both subcellular fractionation of cellular membranes and immunoelectron microscopy (Roth, 1987). Early studies suggest that the compartmentalization of GTs may reflect the sequence of the oligosaccharide chain modification (Kornfeld &amp; Kornfeld, 1985). It was thought that this strict localization ensure an optimal biosynthesis of the glycan chains by providing an efficient vicinity between enzyme, substrate and sugar nucleotide donor. Further studies showed that many of glycosylation enzymes overlap in localization and demonstrated cell-type specific golgi subcompartmentation (Colley, 1997). GTs are spread out in the golgi stacks and this can differ between cell types for a given protein (Roth, 1991). 
     Generally, it has been demonstrated that the transmembrane domain (TMD) of proteins is a determinant key to confer golgi localization essential for at least GalNAcTs, GlcNAcTs, GalT, FucT and SiaT to find their acceptors. It was demonstrated that the flanking region of TMD and/or the lumenal portion contribute to localize as well (Munro, 1998; Yang et al., 1996). Moreover, the CT and the SR of GTs specify their in vivo functional sublocalisation and stability in the golgi apparatus. Substituting either of the three domains (CT, TMD and SR) does not change the catalytic activity of the enzyme but contribute to alter its distribution in the golgi compartments (Grabenhorst &amp; Conradt, 1999). No clear specific targeting sequences have been found over the last decade and only critical regions of the GTs have been identified for their compartmentalization (Opat et al., 2001). More recently, the inventors have found that the soluble catalytic domain of ST6Gal followed a different subcellular route than the full-length enzyme (Ronin, Biochimie 2003). 
     Among the large number of GTs, the families which are of greatest interest are those which are in charge of terminal glycosylation because these sugars play an important role in phenomenons of recognition and signalization in humans. Those essentially include sialyltransferases (SiaTs), fucosyltransferases (FucTs) and galactosyltransferases (GalTs). Galactose and fucose are involved in recognition of blood group (ABH/Lewis) antigens. Sialylated oligosaccharides of glycoproteins and glycolipids are implicated in many biological processes such as cell adhesion and receptor recognition in inflammation and cancer (sialyLewis antigen) as well as neuronal outgrowth (Polysialic antigen) (Paulson, 1989). The structural diversity and regulated expression of sialylglycoconjugates appear to be well correlated with their functions (Sasaki, 1996). 
     Sialyltransferases: 
     The sialic acid family is composed of more than a hundred of derivatives among which neuraminic acid is the most frequent in mammals and humans. Sialyltransferases (STs) catalyze the transfer of a sialic acid residue from its activated form, cytidyl-monophospho-N-acetyl-neuraminic acid (CMP-Neu5Ac), to a non-reducing terminal position on a glycan acceptor in glycoproteins or glycolipids. The catalyzed reaction is as follows:
 
CMP-β-Neu5Ac+HO-acceptor→CMP-H+Neu4Ac-α-O-acceptor
 
     Each ST is classified according to the type of linkage established between the sialic acid residue and the acceptor substrate specificity (which can be either a protein or a lipid). Thus, three groups are distinguished: α2,3-sialyltransferases (α2,3-STs), α2,6-sialyltransferases (α2,6-STs), and α2,8-sialyltransferases (α2,8-STs). α2,6-STs transfer a sialic acid residue at an alpha2,6 position (ST6Gal) to a galactose residue, or to a N-Acetyl-D-galactosamine (ST6GalNAc), or to a N-acetyl-D-glucosamine (ST6GlcNAc). However, the enzyme involved in the formation of this last type of linkage is still unknown. The α2,3-STs transfer a sialic acid to the carbon 3 of galactose (ST3Gal), and the α2,8-STs to the carbon 8 of an other acid sialic residue (ST8Sia). 
     STs have also been identified in all animals (from birds to humans), in bacterial cells and in viruses (Sujino et al., 2000). The family of human STs is composed of 20 different genes cloned as shown in table 1 (Sasaki, 1996, Ronin 2003). Their substrates specificities are as follows. 
     1) α2,6-STs 
     ST6Gal: 
     ST6Gal I transfers a sialic acid residue on the disaccharide Galβ1-4GlcNAc of N-glycans almost (van den Eijnden et al., 1980; Weinstein et al., 1982). It is expressed in an ubiquitous manner in human tissues except in testis and brain where it is expressed a lower levels (Table 2) (Kitagawa &amp; Paulson, 1994a). 
     ST6Gal II, identified recently (Krzewinski-Recchi et al., 2003; Takashima et al., 2002), recognizes the disaccharide Galβ1-4GlcNAc as acceptor substrate particularly when it is on a free oligosaccharide (unknown protein or lipid). The expression pattern of ST6Gal II is restricted to the brain and shows a low expression level in testis, thyroid, lymphatic ganglia and some fetal tissues (Table 2). 
     ST6GalNAc: 
     The second subfamily of α2,6-STs is represented by the group of ST6GalNAc, which transfer a sialic acid residue on an N-acetyl galactosamine residue. Six members have been identified in human (Table 2). 
     ST6GalNAc I and ST6GalNAc II possess the broadest substrate specificity as they are able to transfer a sialic acid on the following O-glycan structures: Galβ1-3GalNAc, GalNAcα-O-Ser/Thr, Siaα2-3 Galβ1-3GalNAcα-O-Ser/Thr (Ikehara et al., 1999; Kono et al., 2000; Kurosawa et al., 1994; Kurosawa et al., 1996; Harduin-Lepers et al., 2001). Their expression pattern is different, ST6GalNAc I is expressed in submaxillary and mammary glands, spleen and colon whereas ST6GalNAc II is expressed in many tissues such as testis and lactating mammary glands (Kurosawa et al., 1996; Kurosawa et al., 2000). 
     ST6GalNAc III, IV, V and VI possess a reduced substrate specificity: they recognize only the trisaccharide Siaα2,3Galβ1-3GalNAc. However, it is now established that ST6GalNAc III, V and VI catalyze preferentially the formation of the GM1b glycolipid (Sjoberg et al., 1996; Lee et al., 1999) whereas ST6GalNAc IV catalyzes the transfer of sialic acid on O-glycans (Harduin-Lepers et al., 2000, Ikehara et al., 1999; Lee et al., 1999, Okajima et al., 2000). ST6GalNAc IV shows restricted substrate specificity using only the trisaccharide sequence Siaα2,3Galβ1-3GalNAc and does not discriminate between α- and β-linked GalNAc. Two different isoforms have been found. Northern-blot analysis detected a 2.2 kb transcript in various adult tissues and lower levels of expression of an additional transcript in brain, heart and skeletal muscle (Harduin-Lepers et al., 2000). 
     2) α2,3-STs 
     They all are ST3Gal transferring sialic acid of a galactose residue in protein or lipid acceptor. 
     ST3Gal I was cloned from submaxillary glands cDNA library and its ubiquitous expression (Table 2) was confirmed by Northern-blot (Chang et al., 1995). It synthesizes Siaα2,3Galβ1-3-GalNAc, a structure common to many O-linked oligosaccharides. It differs from other STs in its ability to use glycolipid acceptor substrates in vitro (Kitagawa &amp; Paulson, 1994b). 
     ST3Gal II was cloned by Kim et al. (1996) from a liver cDNA library and the Northern-blot analysis revealed high levels expression in heart, liver and skeletal muscle, intermediate levels in thymus, lymph node, appendix and spleen, and lower levels in lung peripheral blood lymphocytes (Table 2). ST3Gal II can transfer sialic acid residues on Galβ1-3-GalNAc or on the gangliosides (lipid) GM1 or asialo-GM1 as acceptor substrates (Giordanengo et al., 1997). 
     ST3Gal III has been cloned by screening a human placental cDNA library with a probe based on sialylmotif, the cDNA encoding ST3Gal III was isolated (Kitagawa &amp; Paulson, 1993). Transcript is abundantly expressed in skeletal muscle and fetal tissue and lower expressed in placenta. This enzyme catalyzes the transfer of sialic acid to galactose-containing substrates (Table 2). 
     ST3Gal IV is involved in sialylation of O-linked Galβ1-3GalNAc (Table 2) (Tetteroo et al., 1987). There are 5 different mRNAs expressed, and they encode for identical protein sequences except at the 5′-ends. These transcripts are produced by a combination of alternative splicing. Northern-blot analysis showed that one of them is specifically expressed in placenta, testis and ovary, indicating that its expression is independently regulated from the others (Kitagawa et al., 1996). 
     ST3Gal V has been isolated from a human cell library of cDNA and called GM3 synthase. A major 2.4 kb transcript is expressed in many tissues particularly in brain, skeletal muscle, placenta and testis. It is widely distributed in human brain with slightly elevated expression in the cerebral cortex, temporal lobe and putamen. The substrate specificity of this enzyme is highly restricted to lacosylceramide as the acceptor (Table 2) (Ishii et al., 1998). 
     ST3Gal VI was cloned from a human melanoma cDNA library. This ST exhibits restricted substrate specificity: it is involved in the synthesis of sialyl-paragloboside, a precusor of the sialyl-Lewis X determinant (Okajima et al., 1999). There are 2 forms of ST3Gal VI mRNA (called type 1 and 2), differing only in the 5′-untranslated region. This enzyme is expressed at similar levels in most tissues (Table 2) (Taniguchi et al., 2001). 
     3) α2,8-STs 
     ST8Sia I is also called ganglioside GD3 synthase and catalyzes the transfer of a sialic acid molecule to the terminal sialic acid of GM3 via an α2,8 linkage (Sasaki et al., 1994). It has been shown that this enzyme can also use GM1b, GD1a and GT1b as acceptor substrate (Table 2) (Nakayama et al., 1996; Nara et al., 1996; Watanabe et al., 1996). 
     ST8Sia II is also called STX, Scheidegger et al. (1995) used sequences of rodent STX to clone the human cDNA from a fetal heart library. STX is primarily expressed in embryonic tissues and modestly in adult heart, brain and thymus (Angata et al., 1997). This enzyme regulates the linkage between neural cell adhesion molecule (NCAM) and polysialic acid (PSA) and modulates by this way the adhesive properties of NCAM. STX catalyzes the transfer of the first sialic acid via an α2,8 linkage on an other sialic acid linked in α2,3 or α2,6 (Table 2) (Angata et al., 1997) on a N-Glycan, then it is involved in the elongation process making successive α2,8-linkage over the previous residues. 
     ST8Sia III transfers a sialic acid on Siaα2-3Galβ1-4GlcNAc structures inside N-glycans and glycolipids such as GM3 (Table 2) (Lee et al., 1998; Yoshida et al., 1995a; Yoshida et al., 1995b). 
     ST8Sia IV, also called PST (Polysialyltransferase), was cloned by Nakayama et al. (1995). Northern-blot analysis revealed that PST is expressed in many fetal tissues and in adult heart, spleen, thymus and at lower levels in other organs and tissues (Table 2). This enzyme also regulates the linkage between neural cell adhesion molecule (NCAM) and polysialic acid (PSA). In addition, it is shown that it catalyzes the same reaction as the STX (Angata et al., 2000; Nakayam et al., 1995). 
     ST8Sia V presents a transfer activity towards the gangliosides GM1b, GD1a, GT1b and GD3 (Table 2) (Kono et al., 1996). 
     ST8Sia VI has been cloned quite recently in human and little is known about it. The mouse ST8Sia VI possesses a transfer activity of sialic acid on the NeuAcα2,3(6)Galβ structure found at the non reduced end of O-glycans, N-glycans and free oligosaccharides such as the sialyllactose (Table 2) (Takashima et al., 2002). 
     4) Structural Organization Needed for Promoting Sialyltransferase Activity 
     All members of the STs family possess three conserved region in their CD named Sialylmotifs: motif L for Large, S for Small and VS for very small (Datta &amp; Paulson, 1995; Geremia et al., 1997) based on the comparison of their primary sequences A survey of the animal genomes has been published lately (Harduin-Lepers, 2005) to find out unknown sequences possessing the sialylmotifs and thus potentially new STs. 
     The sialylmotif L consisted of 44 or 45 amino acids and contained between 5 invariant residues among all the human enzymes as shown in  FIG. 2 . This region is in the center of the CD. The second sialylmotif S, in the COOH-terminal portion, consisted of 23 amino acids residues, two of which residues being identical among all the STs (Drickamer, 1993). The third sialylmotif VS, at the terminal part of the enzyme, consisted of 13 amino acids with two conserved residues (histidine and glutamate). 
     It has been demonstrated that the sialylmotif L is involved in the binding of the donor substrate CMP-sialic acid (Datta &amp; paulson, 1995). Some amino acids of this motif can also participate in the catalytic activity of enzymes (Sasaki, 1996). It has been proposed that the sialylmotif S participate to both donor and acceptor binding (Datta et al., 1998). The precise role of VS is still unclear but recent studies on STX and PST examined the functional rule of the conserved His in this motif (Kitazume-Kawaguchi et al., 2001). The change of the Histidine by a Lysine reside affect their catalytic activity showing that the motif VS is involved in catalysis. The sialylmotif VS is necessary for optimal catalytic efficiency and it is part of the active site, mainly on the acceptor site or at the vicinity of both donor and acceptor sugar substrates (Jeanneau et al., 2004). From previous work, it is now clear that the C-terminal part of the CD (part of sialylmotif S, motif 3 and sialylmotif VS) is primarily dedicated to the recognition of acceptor substrates whereas the N-terminal part (sialylmotifs L and part of S) is mostly involved in nucleotide sugar binding (Datta &amp; Paulson, 1995; Datta et al., 1998; Laroy et al., 2001; Kitazume-Kawaguchi et al., 2001; Jeanneau et al., 2004). However, it cannot been excluded that the Sialylmotif L could also be involved in acceptor recognition (Legaigneur et al., 2001). 
     As mentioned above, the amino acid sequences deduced from the cloned human sialyltransferase cDNA show the same organization in four domains shared by many GTs: i) a short N-terminal cytoplasmic tail (CT; around 10 amino-acids), ii) a transmembrane domain (TMD; around 20 amino acids), iii) a stem region (SR), highly variable in length and iv) a catalytic domain (CD; around 300 amino acids. 
     The SR is defined as the peptide region between the TMD and the CD which can be removed without altering the enzymatic activity (Paulson 1989, Ronin, 2001, Vallejo-Ruiz et al., 2001; Jeanneau et al., 2004). However, the inventors have shown that this hypervariable region may be essential to define a fine recognition of the glycan acceptor (tri&gt;bi&gt;tetraantennary) and proposed that it may be involved in a conformational change to open the catalytic site (Ronin, 2001). As a result, the catalytic efficiency is increased by a factor 35 and the recognition of glycan acceptor is broadened. 
     The CD is crucial for STs to display enzymatic activity. It contains at least three highly conserved sequences maintaining identical amino acids positions found in all mammalian STs cloned. Those domains are L-, S- and VS-sialylmotifs (Livingston &amp; Paulson, 1993; Geremia et al., 1997). An additional domain (motif 3) has also been recently isolated between the sialylmotifs S and VS, it contains four highly conserved residues, with the following consensus sequence: (H/y)Y(Y/F/W/h)(E/D/q/g). (Capital letters and lowercase letters indicate a strong or a low occurrence of the amino acid, respectively.) (Jeanneau et al., 2004). Many studies described in the literature aim to get insight into structure/function relationships in the large STs family and thus particularly to define a minimal catalytic domain inside. The minimal CD has been so defined experimentally by site-directed mutagenesis or alternatively by sequence alignment and comparison for a few STs (Vallejo-Ruiz et al., 2001; Chen &amp; Colley, 2000, Ronin 2003). 
     Despite the broad definition of the CD and the catalytic activity, delineating a minimal CD of STs is still uneasy. ST6Gal I is the most studied enzyme among STs and most of the research has been realised on it to define both minimal catalytic domain and catalytic activity. This is due to the absence of ST6Gal in all cells used to produce human recombinant proteins. This lack is a technological bottleneck for heterologous systems using non animal cells (yeast, insects and plants) since they do not contain any sialic acid. In CHO cells alternatively, only a ST3 activity is expressed. 
     The delineation between the SR and the CD has never been well defined except experimentally. Based on bioinformatics, the inventors have designed a strategy of identifying the end of the SR for the 3 ST families (Ronin., 2003). 
     The CD generally coincides with the end of the hypervariable region in the N-terminal half of the enzymes (Ronin., 2003). The catalytic domain is therefore assumed to start around 70-90 residues upstream from the sialylmotif L and around 40-45 residues for ST6GalNAc III to VI. The average size of the CD is estimated around 300 (±20) amino acids, including the sialylmotifs (Jeanneau et al., 2004) (Table 3). The minimal CD has been defined for a few STs—either by truncation experiments or sequence comparison—(Legaigneur et al., 2001; Vallejo-Ruiz et al., 2001; Chen &amp; Colley, 2000), such as for hST3Gal I (minimal CD consisting in amino acids 57-340; Vallejo-Ruiz et al., 2001) and hST8Sia IV (minimal CD consisting in amino acids 62-359; Angata et al., 2004). 
     When transfected in CHO cells, the soluble CD of hST6Gal I (amino acids 90-406) was found to display an enlarged specificity towards endogenous acceptors as it follows the intracellular secretory pathway within the Golgi apparatus. It has thus been possible to delineate a minimal CD for hST6Gal I containing a critical sequence capable of displacing the acceptor recognition from intracellular resident acceptors to cell surface glycoconjugates (Donadio et al., 2003). In addition, the soluble secreted CD of hST6Gal I showed increased transfer efficiency, irrespectively of the branching pattern of the glycan acceptor (Legaigneur et al., 2001). 
     Glycosylation of Protein (Drugs) Produced in Heterologous Expression Systems 
     Proteins of therapeutic interest were first extracted from natural sources such as blood, placenta, human or animal tissues. However, this approach is limited by the source, amount and availability of human tissues and may contain life threatening contaminants (prions, oncogenes, viruses . . . ) as well as potential allergens generated by proteins from animals. With the rise drug-approved proteins and clinical needs, new approaches have been developed to produce proteins using different expression systems. 
     In most instances, intensive work is currently aiming at humanizing the glycosylation pattern of the recombinant proteins to approach the pattern found in the natural glycoproteins as closely as possible to improve pharmacokinetics and lower immunogenicity of the product. 
     The machinery required for the synthesis, the activation and the introduction of sialyl residues is poorly expressed in the various existing recombinant proteins expression systems. The recombinant human proteins produced are therefore often under or even not-sialylated compared to their native counterparts. 
     One of the most used systems is the bacteria  Escherichia coli  ( E. coli ) (Swartz, 2001; Baneyx, 1999), but its main inconvenient is that the human post-traductionnal modifications, particularly protein glycosylations are not carried out by this prokaryote because no such glycosyltranferases are expressed in  E. coli . This can lead to the reject of the therapeutic proteins of interest by the immune system, the reduction of their circulatory half-life and/or of their biological activity. The protein produced can eventually be misfolded and aggregate as inclusion bodies. 
     Yeasts and filamentous fungi are also well-known eukaryote expression systems and they possess cellular machinery similar to those of human cells. The yeast produces complex proteins and is able to carry out several post-traductional modifications such as simple glycosylations. However, although the N-glycosylation process performed by yeast and fungi is the same as the mammalian process for the initial steps in the endoplasmic reticulum, no complex oligosaccharide containing sialic acid, galactose, fucose and N-acetylgalactosamine have been found inside the glycoproteins produced by these organisms (Blanchard, 2004); both yeast and fungi typically produce mannose-rich glycans by adding up to 100 mannose residues (in the case of yeast) on the pentasaccharidic core (Tanner &amp; Lehele, 1987; Herscovics &amp; Orlean, 1993) in the golgi apparatus. Those hypermannosylation foster an immune response in human. 
     Engineering glycosylation in yeast (Hamilton et al., 2003; Roy et al., 2000) has first allowed the reduction of the mannoses residues number added. Then, enzymes which are necessary to peripheral N-acetylglucosaminylation and galactosylation and have been added in these systems (Maras et al., 1999; Bretthauer, 2003; Vervecken et al., 2004). However, the addition of the terminal sialic acid is still difficult to achieve, due to the large number of enzymes involved in this terminal step. 
     Proteins expressed in insect cells are properly folded, may undergo post-traductional modifications and be secreted. Early N-linked glycosylation carried out by these cells are similar to those performed by mammalian cells. However, the glycan structures obtained in this case are of the paucimannosidic type i.e. truncated-due to the presence of an undesirable N-acetylhexosaminidase activity which degrades the neoglycoproteins expressed during Baculovirus expression (Blanchard, 2004). In addition, in some insect cells lines, α1,3/fucose residues may be found and these residues generally trigger an undesirable immune response in human. Thus, the use of this system is restricted to the production vaccinal antigens. 
     In insect cells, the GTs catalyzing the transfer of immunogenic sugars have been deleted and sialylation could be achieved by adding 3 genes encoding for the N-acetylglucosamine 2-epimerase, N-acetylneuraminyl lyase and CMP-Neu5Ac synthase (Jarvis et al., 1998; Aumiller et al., 2003). A new insect cell line (SfSWT-3) designed to synthesize its own CMP-sialic acid has been created The resulting cells express all the 7 mammalian genes, can produce CMP-sialic acid and sialylate an heterologous protein when cultured in a serum-free growth medium (Aumiller et al., 2003). 
     As eukaryotic cells, plants exhibit a complex and sophisticated cellular machinery which may be used to produce therapeutic proteins. Recombinant proteins possess a very good pharmacological quality because plants have many of the enzymes required for the maturation of the proteins. However, the glycosylation pathway needs major adjustments not to produce allergenic proteins. Indeed, the N-glycosylations in plants (Lerouge et al., 2000) are similar to those performed in humans as far as core glycosylation is concerned. However, glycans are still lacking sialylated antennae and contain inner β1,2-xylose and α1,3-fucose. Both residues are highly immunogenic for human and currently compromise the approval of transgenic plants as expression systems for therapeutics. 
     In plants, the first strategy used aimed at preventing the addition of allergenic sugars by preventing proteins to exit the endoplasmic reticulum. As a result, N-linked glycans can not mature to complex type. Another strategy was based on the inhibition of several GTs inside the golgi apparatus. This inhibition can not be complete and/or can enter in competition with the endogenous machinery for maturation. 
     Mammalian cell expression system, namely CHO cells, is currently the only drug-approved system to produce recombinant therapeutics. These cells show a major advantage because they are able to synthesize complex N-linked glycoproteins of high molecular weight and/or multimeric. Mammalian cells naturally express, not only the enzymes involved in the synthesis and the transport of the nucleotide-sugars, but also the glycosyltransferases required to achieve complex glycosylation of the recombinant proteins with a satisfactory content in 3-linked sialic acid. However, there is a lack of some enzymes such as the α1,3/4-fucosyltransferases and α2,6-sialytransferases, which realize terminal O-linked and N-linked glycosylation. Moreover, in mammalian cells, the sialylation occurs through N-glycolyneuraminic acid (NeuGc) which significantly differs also from the N-Acetylated derivative (NeuAc) found in human cells 
     In the case of mammalian cells, work has been performed through the over-expression of an α2,3-ST and a β1,4GalT (Weikert et al., 1999); both enzymes are present in the genome but their activities are highly variable upon cell culture conditions. This led to a wide variability concerning the presence of the terminal Gal and sialic acid and to an extensive microheterogeneity in glycosylated proteins. Work has also been oriented towards the optimization of the galactosylation and sialylation (Granbenhorst et al., 1999) by introducing an α2,6-ST (Bragonzi et al., 2000) in mammalian cells. A CHO cell line stably expressing native rat α2,6-ST has been established. The glycoproteins produced by these CHO cells display both α2,6 and α2,3-linked terminal sialic acid residues, similar to human; the ratio observed between α2,6 and α2,3-linked terminal sialic acid residues carried was of 40.4% of α2,6- and 59.6% α2,3-sialic acid residues, which improved pharmacokinetics in clearance studies (Bragonzi et al., 2000). Despite these improvements in humanization of cells, the ratio between α2,6 and α2,3-linked terminal sialic acid residues cannot be controlled and has never been found even favorable to the 6-activity. It thus appears that first, sialylation is a critical step to control glycan structures and secondly, the difficulty resides also in expressing the heterologous ST in a specific compartment of the cell (addressing), in optimizing its activity provided that the donor substrate for this enzyme is present (carrier). 
     It is therefore worth noting that terminal sialylation of approved glycoprotein drugs is the most difficult step to obtain in all expression systems available so far. 
     GOAL OF THE INVENTION 
     The goal of the present invention is to provide a process for generating a panel of new gene sequences encoding chimerical membrane enzymes with glycosyltransferase activity, and in particular for designing gene sequences encoding innovative chimerical sialyltransferases to equip cells with a needed sialylating activity and improve the quality and yield of recombinant glycosylated proteins. 
     The invention relates to a process for producing gene sequences encoding chimerical membrane glycosyltransferases presenting an optimized glycosylation activity in cells transformed with said sequences, when compared with the glycosylation activity of the corresponding native glycosyltransferases, i.e. a glycosylation less selective and/or more efficient (up to at least 30-fold higher than the initial activity of the native full length glycosyltransferases) towards the acceptor substrate, said process comprising the fusion: 
     of a first nucleic acid sequence coding for a C-terminal minimal fragment of the catalytic domain (CD) of the native full length glycosyltransferase, said C-terminal minimal CD fragment displaying a transferase activity (which can be enhanced up to at least 30-fold higher than the initial activity of to the native CD), said first nucleic sequence being obtained by removing nucleotides coding for one or several contiguous amino acids extending from the first amino acid of the N-terminal end of said CD, and selection of the nucleic acid encoding said C-terminal minimal CD fragment which is such that if n represents the number of contiguous amino acids as defined above which has been deleted, then the fragment obtained when deleting at least n+1 contiguous amino acids as defined above, has no substantial transferase activity, 
     to a second nucleic acid of variable sequence coding for a transmembrane peptide chain specifying the anchorage of the glycosyltransferases in intracellular compartments, and comprising (or consisting of) in its N-terminal region a cytoplasmic tail (CT) region located upstream from a transmembrane domain (TMD), itself located upstream of a stem region (SR) or of a fragment of at least 3 contiguous amino acids of the SR, said SR or part thereof being linked to said catalytic domain, optionally via a linker or connection peptide of at least 2 amino acids encoded by a restriction site which does not exist in the nucleotide sequence coding for the native CD mentioned above, 
     provided that at least one of these CT, TMD, SR peptides being different from the primary structure of the native corresponding peptides in the native glycosyltransferase from which is derived the CD fragment with optimal glycosyltransferase activity as defined above, 
     this fusion being carried out in such way that the first nucleic acid is located downstream from the second nucleic acid and provides a protein product in which the CD is in the C-terminal half. 
     By chimerical membrane glycosyltransferases presenting an optimized glycosylation activity in cells transformed with said sequences, when compared with the glycosylation activity of the corresponding native glycosyltransferases, one should understand that said chimerical membrane glycosyltransferases: 
     have a sugar transfer activity less selective towards acceptor substrates than the corresponding native glycosyltransferases when tested in vitro with exogeneous/commercially available acceptor glycoproteins having known bi-, tri- or tetraantennary glycans, i.e. have a in vivo glycosylation activity in cells towards all cell glycoprotein acceptors, said glycosylation activity being measurable preferably in intracellular and cell surface compartments according to the following general procedure using lectin SNA binding, 
     and/or have a more efficient sugar transfer activity towards their acceptor substrates than the corresponding native glycosyltransferases, i.e. show a glycosylation activity up to at least 30-fold higher than the initial activity of the native full length glycosyltransferases, said glycosylation activity being measurable according to the general in vitro procedure with exogeneous/commercially available acceptor glycoproteins having known bi-, tri- or tetraantennary glycans. 
     The expression “chimerical glycosyltransferases” used above corresponds to a glycosyltransferase whose full length sequence is not the direct product of a native gene or a transcript but has been designed using sequences from other glycosyltransferases exclusively. 
     The expression “native glycosyltransferases” used above corresponds to the sequence of the full length enzyme as represented by a naturally occurring coding sequence. 
     The expression “glycosylation activity” used above corresponds to the catalytic reaction of transferring a sugar from a nucleotide donor to an acceptor substrate. 
     The expression “minimal catalytic domain (CD)” used above corresponds to the C-terminal peptide domain of a glycosyltransferase sequence which cannot be deleted further without loss of transfer activity. 
     The expression “transmembrane domain (TMD)” used above corresponds to a peptide portion composed of a stretch of 17-24 essentially hydrophobic amino acids 
     The expression “cytoplasmic tail (CT)” used above corresponds to the N-terminal peptide of a glycosyltransferase which may encompass at least more than 3 amino acids upstream from the TMD. 
     The expression “stem region (SR)” used above corresponds to a stretch of at most 246 amino acids downstream the TMD/upstream from the CD. 
     The invention relates more particularly to a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively, in glycosyltransferases from eukaryotic origin, preferably mammals and humans, said glycosyltransferases being involved in:
         O-glycosylation of the proteins in cells, such as N-acetylgalactosaminyl-, N-acetylglucosaminyl-, glucosyl-, galactosyl-, or sialyltransferases,   N-glycosylation of the proteins in cells such as N-acetylglucosaminyl-, galactosyl-, fucosyl-, or sialyltransferases,   Glycosylation of lipids such as the N-acetylgalactosaminyl-, N-acetylglucosaminyl-, fucosyl-, galactosyl-, or sialyltransferases.       

     The expression “O-glycosylation” used above corresponds to the biosynthetic pathway elaborating monosaccharides or oligosaccharides covalently attached to amino acids which are not asparagine residues but can be preferably hydroxy amino acids such as serine, threonine or hydroxylysine residues. 
     The expression “N-glycosylation” used above corresponds to the biosynthetic pathway elaborating oligosaccharides attached to asparagine residues within the consensus tripeptide Asn-X-Ser/Thr (X being not Proline). 
     The expression “galactosyltransferases” used above corresponds to a glycosyltransferase which transfers a galactose residue from UDP-Gal to an N-/O-linked protein or glycolipid acceptor. 
     The expression “sialyltransferases” used above corresponds to a glycosyltransferase which transfers a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to a N-/O-linked protein or glycolipid acceptor. 
     The invention concerns more particularly a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively in N-acetylgalactosaminyltransferases, N-acetylglucosaminyltransferases, glucosyltransferases, galactosyltransferases, fucosyltransferases, or sialyltransferases. 
     The invention concerns more particularly a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively in alpha 6 fucosyltransferases (core glycosylation), beta 2/4/6 N-acetylglucosaminyltransferases (branching), beta 4 galactosyltransferases, and alpha 3/6/8 sialyltransferases (terminal glycosylation). 
     Advantageously, the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, of enzymes implicated in the N-glycosylation pathway. 
     The invention relates more particularly to a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively, in sialyltransferases. 
     The invention concerns more particularly a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively, in α2,6-sialyltransferases, α2,3-sialyltransferases, or α2,8-sialyltransferases. 
     Advantageously, nucleotide sequences encoding CT, TMD and/or SR or SR fragment mentioned above are preferably from synthetic origin. 
     The expression “α2,6-sialyltransferases” used above corresponds to a glycosyltransferase able to transfer a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 6-position of a carbohydrate acceptor from the N-/O-linked protein or glycolipid type. 
     The expression “α2,3-sialyltransferases” used above corresponds to a glycosyltransferase able to transfer a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 3-position of an carbohydrate acceptor from the N-/O-linked protein or glycolipid type. 
     The expression “α2,8-sialyltransferases” used above corresponds to a glycosyltransferase able to transfer a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 8-position of a sialylated acceptor from the N-/O-linked protein or glycolipid type. 
     The invention relates more particularly to a process as defined above, characterized in that the first and second nucleic acids are derived from nucleotide sequences encoding CD domains, or CT, TMD, and SR regions, respectively, in: 
     α2,6-sialyltransferases chosen among:
         the human β1,4-galactoside α2,6-sialyltransferases I and II (hST6Gal I, and hST6Gal II) represented by SEQ ID NO: 2, and SEQ ID NO: 4, respectively, encoded by the nucleotide sequences SEQ ID NO: 1, and SEQ ID NO: 3, respectively, or   the human N-acetylgalactosaminide-α2,6-sialyltransferases I to VI (hST6GalNAc I to VI) represented by SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 14, and SEQ ID NO: 16, respectively, encoded by the nucleotide sequences SEQ ID NO: 5, SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11, SEQ ID NO: 13, and SEQ ID NO: 15, respectively,       

     α2,3-sialyltransferases chosen among the human galactoside-α2,3-sialyltransferases I to VI (hST3Gal I to VI) represented by SEQ ID NO: 18, SEQ ID NO: 20, SEQ ID NO: 22, SEQ ID NO: 24, SEQ ID NO: 26, and SEQ ID NO: 28, respectively, encoded by the nucleotide sequences SEQ ID NO: 17, SEQ ID NO: 19, SEQ ID NO: 21, SEQ ID NO: 23, SEQ ID NO: 25, and SEQ ID NO: 27, respectively, or the rat galactoside-α2,3-sialyltransferases I to VI (rST3Gal I to VI), such as the rST3Gal III represented by SEQ ID NO: 30 encoded by the nucleotide sequence SEQ ID NO: 29, or any ST from other animal origin provided that it shares at least 85% homology with the human enzyme, 
     α2,8-sialyltransferases chosen among the human sialic acid-α2,8-sialyltransferases I to VI (hST8Sia I to VI) represented by SEQ ID NO: 32, SEQ ID NO: 34, SEQ ID NO: 36, SEQ ID NO: 38, SEQ ID NO: 40, and SEQ ID NO: 42, respectively, encoded by the nucleotide sequences SEQ ID NO: 31, SEQ ID NO: 33, SEQ ID NO: 35, SEQ ID NO: 37, SEQ ID NO: 39, and SEQ ID NO: 41, respectively. 
     The expression “galactoside-α2,6-sialyltransferases” used above corresponds to a glycosyltransferase which transfers a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 6-position of a galactosylated acceptor from the N-/O-linked protein or lipid type. 
     The expression “N-acetylgalactosaminide α2,6-sialyltransferases” used above corresponds to a glycosyltransferase which transfers a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 6-position of a N-acetylgalactosaminyl residue of a N-/O-linked protein or glycolipid acceptor. 
     The expression “galactoside α2,3-sialyltransferases” used above corresponds to a glycosyltransferase which transfers a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to the 3-position of a galactosylated N-/O-linked protein or lipid acceptor. 
     The expression “sialic acid α2,8-sialyltransferases” used above corresponds to a glycosyltransferase which transfers a sialic acid residue, preferably a derivative of neuraminic acid from CMP-NeuAc to a sialylated N-/O-linked protein or glycolipid acceptor. 
     The invention concerns more particularly a process as defined above, characterized in that the nucleotide sequence encoding the CD domain comprised in the first nucleic acid is chosen among the sequences constituted of, or comprising: 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 268 to 330 and in its 3′ end by the nucleotide located in position 1218 of SEQ ID NO: 1, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6Gal I delimited in its N-terminal end by the amino acid located in position 90 to 110 and in its C-terminal end by the amino acid located in position 406 of SEQ ID NO: 2, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 307 to 369 and in its 3′ end by the nucleotide located in position 1587 of SEQ ID NO: 3, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6Gal II delimited in its N-terminal end by the amino acid located in position 103 to 123 and in its C-terminal end by the amino acid located in position 529 of SEQ ID NO: 4, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 814 to 876 and in its 3′ end by the nucleotide located in position 1800 of SEQ ID NO: 5, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac I delimited in its N-terminal end by the amino acid located in position 272 to 292 and in its C-terminal end by the amino acid located in position 600 of SEQ ID NO: 6, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 172 to 234 and in its 3′ end by the nucleotide located in position 1122 of SEQ ID NO: 7, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac II delimited in its N-terminal end by the amino acid located in position 58 to 78 and in its C-terminal end by the amino acid located in position 374 of SEQ ID NO: 8, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 73 to 135 and in its 3′ end by the nucleotide located in position 915 of SEQ ID NO: 9, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac III delimited in its N-terminal end by the amino acid located in position 25 to 45 and in its C-terminal end by the amino acid located in position 305 of SEQ ID NO: 10, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 61 to 123 and in its 3′ end by the nucleotide located in position 906 of SEQ ID NO: 11, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac IV delimited in its N-terminal end by the amino acid located in position 21 to 41 and in its C-terminal end by the amino acid located in position 302 of SEQ ID NO: 12, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 121 to 183 and in its 3′ end by the nucleotide located in position 1008 of SEQ ID NO: 13, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac V delimited in its N-terminal end by the amino acid located in position 41 to 61 and in its C-terminal end by the amino acid located in position 336 of SEQ ID NO: 14, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 154 to 216 and in its 3′ end by the nucleotide located in position 999 of SEQ ID NO: 15, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST6GalNac VI delimited in its N-terminal end by the amino acid located in position 52 to 72 and in its C-terminal end by the amino acid located in position 333 of SEQ ID NO: 16, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 145 to 207 and in its 3′ end by the nucleotide located in position 1020 of SEQ ID NO: 17, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal I delimited in its N-terminal end by the amino acid located in position 49 to 69 and in its C-terminal end by the amino acid located in position 340 of SEQ ID NO: 18, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 175 to 237 and in its 3′ end by the nucleotide located in position 1050 of SEQ ID NO: 19, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal II delimited in its N-terminal end by the amino acid located in position 59 to 79 and in its C-terminal end by the amino acid located in position 350 of SEQ ID NO: 20, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 199 to 261 and in its 3′ end by the nucleotide located in position 1332 of SEQ ID NO: 21, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal III delimited in its N-terminal end by the amino acid located in position 67 to 87 and in its C-terminal end by the amino acid located in position 444 of SEQ ID NO: 22, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 79 to 141 and in its 3′ end by the nucleotide located in position 987 of SEQ ID NO: 23, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal IV delimited in its N-terminal end by the amino acid located in position 27 to 47 and in its C-terminal end by the amino acid located in position 329 of SEQ ID NO: 24, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 136 to 198 and in its 3′ end by the nucleotide located in position 1086 of SEQ ID NO: 25, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal V delimited in its N-terminal end by the amino acid located in position 46 to 66 and in its C-terminal end by the amino acid located in position 362 of SEQ ID NO: 26, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 73 to 135 and in its 3′ end by the nucleotide located in position 993 of SEQ ID NO: 27, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST3Gal VI delimited in its N-terminal end by the amino acid located in position 25 to 45 and in its C-terminal end by the amino acid located in position 331 of SEQ ID NO: 28, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 103 to 165 and in its 3′ end by the nucleotide located in position 1122 of SEQ ID NO: 29, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of rat ST3Gal III delimited in its N-terminal end by the amino acid located in position 35 to 55 and in its C-terminal end by the amino acid located in position 374 of SEQ ID NO: 30, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 133 to 195 and in its 3′ end by the nucleotide located in position 1068 of SEQ ID NO: 31, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia I delimited in its N-terminal end by the amino acid located in position 45 to 65 and in its C-terminal end by the amino acid located in position 356 of SEQ ID NO: 32, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 190 to 252 and in its 3′ end by the nucleotide located in position 1125 of SEQ ID NO: 33, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia II delimited in its N-terminal end by the amino acid located in position 64 to 84 and in its C-terminal end by the amino acid located in position 375 of SEQ ID NO: 34, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 205 to 267 and in its 3′ end by the nucleotide located in position 1140 of SEQ ID NO: 35, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia III delimited in its N-terminal end by the amino acid located in position 69 to 89 and in its C-terminal end by the amino acid located in position 380 of SEQ ID NO: 36, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 145 to 207 and in its 3′ end by the nucleotide located in position 1077 of SEQ ID NO: 37, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia IV delimited in its N-terminal end by the amino acid located in position 49 to 69 and in its C-terminal end by the amino acid located in position 359 of SEQ ID NO: 38, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 211 to 273 and in its 3′ end by the nucleotide located in position 1128 of SEQ ID NO: 39, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia V delimited in its N-terminal end by the amino acid located in position 71 to 91 and in its C-terminal end by the amino acid located in position 376 of SEQ ID NO: 40, 
     the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 277 to 339 and in its 3′ end by the nucleotide located in position 1194 of SEQ ID NO: 41, said nucleotide sequence encoding the polypeptide sequence corresponding to the CD domain of hST8Sia VI delimited in its N-terminal end by the amino acid located in position 93 to 113 and in its C-terminal end by the amino acid located in position 398 of SEQ ID NO: 42. 
     The invention relates more particularly to a process as defined above, characterized in that the first nucleic acid is the nucleotide sequence SEQ ID NO: 43 corresponding to the sequence delimited by the nucleotides located in position 268 and 1218 of SEQ ID NO: 1, said nucleotide sequence SEQ ID NO: 43 encoding the polypeptide sequence SEQ ID NO: 44 corresponding to the C-terminal minimal fragment of the CD domain of hST6Gal I delimited by the amino acids located in positions 90 to 406 of SEQ ID NO: 2. 
     The invention concerns more particularly a process as defined above, characterized in that: 
     the nucleotide sequence encoding the CT region comprised in the second nucleic acid is chosen among:
         the nucleotide sequence SEQ ID NO: 45 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 27 of SEQ ID NO: 1, said nucleotide sequence SEQ ID NO: 45 encoding the polypeptide sequence SEQ ID NO: 46 corresponding to the CT region of hST6Gal I delimited by the amino acids located in positions 1 to 9 of SEQ ID NO: 2,   the nucleotide sequence SEQ ID NO: 47 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 30 of SEQ ID NO: 3, said nucleotide sequence SEQ ID NO: 47 encoding the polypeptide sequence SEQ ID NO: 48 corresponding to the CT region of hST6Gal II delimited by the amino acids located in positions 1 to 10 of SEQ ID NO: 4,   the nucleotide sequence SEQ ID NO: 49 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 42 of SEQ ID NO: 5, said nucleotide sequence SEQ ID NO: 49 encoding the polypeptide sequence SEQ ID NO: 50 corresponding to the CT region of hST6GalNAc I delimited by the amino acids located in positions 1 to 14 of SEQ ID NO: 6,   the nucleotide sequence SEQ ID NO: 51 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 21 of SEQ ID NO: 7, said nucleotide sequence SEQ ID NO: 51 encoding the polypeptide sequence SEQ ID NO: 52 corresponding to the CT region of hST6GalNac II delimited by the amino acids located in positions 1 to 7 of SEQ ID NO: 8,   the nucleotide sequence SEQ ID NO: 53 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 9, said nucleotide sequence SEQ ID NO: 53 encoding the polypeptide sequence SEQ ID NO: 54 corresponding to the CT region of hST6GalNAc III delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 10,   the nucleotide sequence SEQ ID NO: 55 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 18 of SEQ ID NO: 11, said nucleotide sequence SEQ ID NO: 55 encoding the polypeptide sequence SEQ ID NO: 56 corresponding to the CT region of hST6GalNAc IV delimited by the amino acids located in positions 1 to 6 of SEQ ID NO: 12,   the nucleotide sequence SEQ ID NO: 57 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 13, said nucleotide sequence SEQ ID NO: 57 encoding the polypeptide sequence SEQ ID NO: 58 corresponding to the CT region of hST6GalNAc V delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 14,   the nucleotide sequence SEQ ID NO: 59 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 129 of SEQ ID NO: 15, said nucleotide sequence SEQ ID NO: 59 encoding the polypeptide sequence SEQ ID NO: 60 corresponding to the CT region of hST6GalNAc VI delimited by the amino acids located in positions 1 to 43 of SEQ ID NO: 16,   the nucleotide sequence SEQ ID NO: 61 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 39 of SEQ ID NO: 17, said nucleotide sequence SEQ ID NO: 61 encoding the polypeptide sequence SEQ ID NO: 62 corresponding to the CT region of hST3Gal I delimited by the amino acids located in positions 1 to 13 of SEQ ID NO: 18,   the nucleotide sequence SEQ ID NO: 63 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 18 of SEQ ID NO: 19, said nucleotide sequence SEQ ID NO: 63 encoding the polypeptide sequence SEQ ID NO: 64 corresponding to the CT region of hST3Gal H delimited by the amino acids located in positions 1 to 6 of SEQ ID NO: 20,   the nucleotide sequence SEQ ID NO: 65 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 21, said nucleotide sequence SEQ ID NO: 65 encoding the polypeptide sequence SEQ ID NO: 66 corresponding to the CT region of hST3Gal III delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 22,   the nucleotide sequence SEQ ID NO: 67 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 23, said nucleotide sequence SEQ ID NO: 67 encoding the polypeptide sequence SEQ ID NO: 68 corresponding to the CT region of hST3Gal IV delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 24,   the nucleotide sequence SEQ ID NO: 69 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 15 of SEQ ID NO: 25, said nucleotide sequence SEQ ID NO: 69 encoding the polypeptide sequence SEQ ID NO: 70 corresponding to the CT region of hST3Gal V delimited by the amino acids located in positions 1 to 5 of SEQ ID NO: 26,   the nucleotide sequence SEQ ID NO: 71 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 12 of SEQ ID NO: 27, said nucleotide sequence SEQ ID NO: 71 encoding the polypeptide sequence SEQ ID NO: 72 corresponding to the CT region of hST3Gal VI delimited by the amino acids located in positions 1 to 4 of SEQ ID NO: 28,   the nucleotide sequence SEQ ID NO: 73 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 29, said nucleotide sequence SEQ ID NO: 73 encoding the polypeptide sequence SEQ ID NO: 74 corresponding to the CT region of ratST3Gal III delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 30,   the nucleotide sequence SEQ ID NO: 75 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 87 of SEQ ID NO: 31, said nucleotide sequence SEQ ID NO: 75 encoding the polypeptide sequence SEQ ID NO: 76 corresponding to the CT region of hST8Sia I delimited by the amino acids located in positions 1 to 29 of SEQ ID NO: 32,   the nucleotide sequence SEQ ID NO: 77 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 18 of SEQ ID NO: 33, said nucleotide sequence SEQ ID NO: 77 encoding the polypeptide sequence SEQ ID NO: 78 corresponding to the CT region of hST8Sia II delimited by the amino acids located in positions 1 to 6 of SEQ ID NO: 34,   the nucleotide sequence SEQ ID NO: 79 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 27 of SEQ ID NO: 35, said nucleotide sequence SEQ ID NO: 79 encoding the polypeptide sequence SEQ ID NO: 80 corresponding to the CT region of hST8Sia III delimited by the amino acids located in positions 1 to 9 of SEQ ID NO: 36,   the nucleotide sequence SEQ ID NO: 81 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 21 of SEQ ID NO: 37, said nucleotide sequence SEQ ID NO: 81 encoding the polypeptide sequence SEQ ID NO: 82 corresponding to the CT region of hST8Sia IV delimited by the amino acids located in positions 1 to 7 of SEQ ID NO: 38,   the nucleotide sequence SEQ ID NO: 83 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 51 of SEQ ID NO: 39, said nucleotide sequence SEQ ID NO: 83 encoding the polypeptide sequence SEQ ID NO: 84 corresponding to the CT region of hST8Sia V delimited by the amino acids located in positions 1 to 17 of SEQ ID NO: 40,   the nucleotide sequence SEQ ID NO: 85 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 9 of SEQ ID NO: 41, said nucleotide sequence SEQ ID NO: 85 encoding the polypeptide sequence SEQ ID NO: 86 corresponding to the CT region of hST8Sia VI delimited by the amino acids located in positions 1 to 3 of SEQ ID NO: 42,       

     the nucleotide sequence encoding the TMD region comprised in the second nucleic acid is chosen among:
         the nucleotide sequence SEQ ID NO: 87 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 28 and 78 of SEQ ID NO: 1, said nucleotide sequence SEQ ID NO: 87 encoding the polypeptide sequence SEQ ID NO: 88 corresponding to the TMD region of hST6Gal I delimited by the amino acids located in positions 10 to 26 of SEQ ID NO: 2,   the nucleotide sequence SEQ ID NO: 89 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 31 and 90 of SEQ ID NO: 3, said nucleotide sequence SEQ ID NO: 89 encoding the polypeptide sequence SEQ ID NO: 90 corresponding to the TMD region of hST6Gal II delimited by the amino acids located in positions 11 to 30 of SEQ ID NO: 4,   the nucleotide sequence SEQ ID NO: 91 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 43 and 105 of SEQ ID NO: 5, said nucleotide sequence SEQ ID NO: 91 encoding the polypeptide sequence SEQ ID NO: 92 corresponding to the TMD region of hST6GalNAc I delimited by the amino acids located in positions 15 to 35 of SEQ ID NO: 6,   the nucleotide sequence SEQ ID NO: 93 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 22 and 84 of SEQ ID NO: 7, said nucleotide sequence SEQ ID NO: 93 encoding the polypeptide sequence SEQ ID NO: 94 corresponding to the TMD region of hST6GalNAc II delimited by the amino acids located in positions 8 to 28 of SEQ ID NO: 8,   the nucleotide sequence SEQ ID NO: 95 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 84 of SEQ ID NO: 9, said nucleotide sequence SEQ ID NO: 95 encoding the polypeptide sequence SEQ ID NO: 96 corresponding to the TMD region of hST6GalNAc III delimited by the amino acids located in positions 9 to 28 of SEQ ID NO: 10,   the nucleotide sequence SEQ ID NO: 97 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 19 and 81 of SEQ ID NO: 11, said nucleotide sequence SEQ ID NO: 97 encoding the polypeptide sequence SEQ ID NO: 98 corresponding to the TMD region of hST6GalNAc IV delimited by the amino acids located in positions 7 to 27 of SEQ ID NO: 12,   the nucleotide sequence SEQ ID NO: 99 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 87 of SEQ ID NO: 13, said nucleotide sequence SEQ ID NO: 99 encoding the polypeptide sequence SEQ ID NO: 100 corresponding to the TMD region of hST6GalNAc V delimited by the amino acids located in positions 9 to 29 of SEQ ID NO: 14,   the nucleotide sequence SEQ ID NO: 101 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 130 and 177 of SEQ ID NO: 15, said nucleotide sequence SEQ ID NO: 101 encoding the polypeptide sequence SEQ ID NO: 102 corresponding to the TMD region of hST6GalNAc VI delimited by the amino acids located in positions 44 to 59 of SEQ ID NO: 16,   the nucleotide sequence SEQ ID NO: 103 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 40 and 102 of SEQ ID NO: 17, said nucleotide sequence SEQ ID NO: 103 encoding the polypeptide sequence SEQ ID NO: 104 corresponding to the TMD region of hST3Gal I delimited by the amino acids located in positions 14 to 34 of SEQ ID NO: 18,   the nucleotide sequence SEQ ID NO: 105 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 19 and 81 of SEQ ID NO: 19, said nucleotide sequence SEQ ID NO: 105 encoding the polypeptide sequence SEQ ID NO: 106 corresponding to the TMD region of hST3Gal II delimited by the amino acids located in positions 7 to 27 of SEQ ID NO: 20,   the nucleotide sequence SEQ ID NO: 107 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 84 of SEQ ID NO: 21, said nucleotide sequence SEQ ID NO: 107 encoding the polypeptide sequence SEQ ID NO: 108 corresponding to the TMD region of hST3Gal III delimited by the amino acids located in positions 9 to 28 of SEQ ID NO: 22,   the nucleotide sequence SEQ ID NO: 109 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 78 of SEQ ID NO: 23, said nucleotide sequence SEQ ID NO: 109 encoding the polypeptide sequence SEQ ID NO: 110 corresponding to the TMD region of hST3Gal IV delimited by the amino acids located in positions 9 to 26 of SEQ ID NO: 24,   the nucleotide sequence SEQ ID NO: 111 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 16 and 78 of SEQ ID NO: 25, said nucleotide sequence SEQ ID NO: 111 encoding the polypeptide sequence SEQ ID NO: 112 corresponding to the TMD region of hST3Gal V delimited by the amino acids located in positions 6 to 26 of SEQ ID NO: 26,   the nucleotide sequence SEQ ID NO: 113 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 13 and 75 of SEQ ID NO: 27, said nucleotide sequence SEQ ID NO: 113 encoding the polypeptide sequence SEQ ID NO: 114 corresponding to the TMD region of hST3Gal VI delimited by the amino acids located in positions 5 to 25 of SEQ ID NO: 28,   the nucleotide sequence SEQ ID NO: 115 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 84 of SEQ ID NO: 29, said nucleotide sequence SEQ ID NO: 115 encoding the polypeptide sequence SEQ ID NO: 116 corresponding to the TMD region of rat ST3Gal III delimited by the amino acids located in positions 9 to 28 of SEQ ID NO: 30,   the nucleotide sequence SEQ ID NO: 117 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 88 and 144 of SEQ ID NO: 31, said nucleotide sequence SEQ ID NO: 117 encoding the polypeptide sequence SEQ ID NO: 118 corresponding to the TMD region of hST8Sia I delimited by the amino acids located in positions 30 to 48 of SEQ ID NO: 32,   the nucleotide sequence SEQ ID NO: 119 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 19 and 69 of SEQ ID NO: 33, said nucleotide sequence SEQ ID NO: 119 encoding the polypeptide sequence SEQ ID NO: 120 corresponding to the TMD region of hST8Sia II delimited by the amino acids located in positions 7 to 23 of SEQ ID NO: 34,   the nucleotide sequence SEQ ID NO: 121 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 28 and 99 of SEQ ID NO: 35, said nucleotide sequence SEQ ID NO: 121 encoding the polypeptide sequence SEQ ID NO: 122 corresponding to the TMD region of hST8Sia III delimited by the amino acids located in positions 10 to 33 of SEQ ID NO: 36,   the nucleotide sequence SEQ ID NO: 123 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 22 and 60 of SEQ ID NO: 37, said nucleotide sequence SEQ ID NO: 123 encoding the polypeptide sequence SEQ ID NO: 124 corresponding to the TMD region of hST8Sia IV delimited by the amino acids located in positions 8 to 20 of SEQ ID NO: 38,   the nucleotide sequence SEQ ID NO: 125 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 52 and 114 of SEQ ID NO: 39, said nucleotide sequence SEQ ID NO: 125 encoding the polypeptide sequence SEQ ID NO: 126 corresponding to the TMD region of hST8Sia V delimited by the amino acids located in positions 18 to 38 of SEQ ID NO: 40,   the nucleotide sequence SEQ ID NO: 127 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 10 and 72 of SEQ ID NO: 41, said nucleotide sequence SEQ ID NO: 127 encoding the polypeptide sequence SEQ ID NO: 128 corresponding to the TMD region of hST8Sia VI delimited by the amino acids located in positions 4 to 24 of SEQ ID NO: 42,       

     the nucleotide sequence encoding the SR region comprised in the second nucleic acid, or encoding a fragment of at least 2 amino acids thereof, is chosen among sequences constituted of, or comprising:
         the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 79 and in its 3′ end by the nucleotide located in position 267 to 327 of SEQ ID NO: 1, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6Gal I delimited in its N-terminal end by the amino acid located in position 27 and in its C-terminal end by the amino acid located in position 89 to 109 of SEQ ID NO: 2,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 91 and in its 3′ end by the nucleotide located in position 306 to 336 of SEQ ID NO: 3, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6Gal II delimited in its N-terminal end by the amino acid located in position 31 and in its C-terminal end by the amino acid located in position 102 to 112 of SEQ ID NO: 4,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 106 and in its 3′ end by the nucleotide located in position 813 to 873 of SEQ ID NO: 5, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc I delimited in its N-terminal end by the amino acid located in position 36 and in its C-terminal end by the amino acid located in position 271 to 291 of SEQ ID NO: 6,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 85 and in its 3′ end by the nucleotide located in position 171 to 231 of SEQ ID NO: 7, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc II delimited in its N-terminal end by the amino acid located in position 29 and in its C-terminal end by the amino acid located in position 57 to 77 of SEQ ID NO: 8,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 85 and in its 3′ end by the nucleotide located in position 102 to 132 of SEQ ID NO: 9, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc III delimited in its N-terminal end by the amino acid located in position 29 and in its C-terminal end by the amino acid located in position 34 to 44 of SEQ ID NO: 10,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 82 and in its 3′ end by the nucleotide located in position 90 to 120 of SEQ ID NO: 11, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc IV delimited in its N-terminal end by the amino acid located in position 28 and in its C-terminal end by the amino acid located in position 30 to 40 of SEQ ID NO: 12,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 88 and in its 3′ end by the nucleotide located in position 120 to 180 of SEQ ID NO: 13, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc V delimited in its N-terminal end by the amino acid located in position 30 and in its C-terminal end by the amino acid located in position 40 to 60 of SEQ ID NO: 14,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 178 and in its 3′ end by the nucleotide located in position 183 of SEQ ID NO: 15, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST6GalNAc VI delimited in its N-terminal end by the amino acid located in position 60 and in its C-terminal end by the amino acid located in position 61 of SEQ ID NO: 16,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 103 and in its 3′ end by the nucleotide located in position 144 to 204 of SEQ ID NO: 17, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal I delimited in its N-terminal end by the amino acid located in position 35 and in its C-terminal end by the amino acid located in position 48 to 68 of SEQ ID NO: 18,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 82 and in its 3′ end by the nucleotide located in position 174 to 234 of SEQ ID NO: 19, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal II delimited in its N-terminal end by the amino acid located in position 28 and in its C-terminal end by the amino acid located in position 58 to 78 of SEQ ID NO: 20,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 85 and in its 3′ end by the nucleotide located in position 198 to 258 of SEQ ID NO: 21, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal III delimited in its N-terminal end by the amino acid located in position 29 and in its C-terminal end by the amino acid located in position 66 to 86 of SEQ ID NO: 22,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 79 and in its 3′ end by the nucleotide located in position 108 to 138 of SEQ ID NO: 23, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal IV delimited in its N-terminal end by the amino acid located in position 27 and in its C-terminal end by the amino acid located in position 36 to 46 of SEQ ID NO: 24,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 79 and in its 3′ end by the nucleotide located in position 135 to 195 of SEQ ID NO: 25, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal V delimited in its N-terminal end by the amino acid located in position 27 and in its C-terminal end by the amino acid located in position 45 to 65 of SEQ ID NO: 26,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 76 and in its 3′ end by the nucleotide located in position 102 to 132 of SEQ ID NO: 27, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST3Gal VI delimited in its N-terminal end by the amino acid located in position 26 and in its C-terminal end by the amino acid located in position 34 to 44 of SEQ ID NO: 28,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 85 and in its 3′ end by the nucleotide located in position 105 to 165 of SEQ ID NO: 29, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of rST3Gal III delimited in its N-terminal end by the amino acid located in position 29 and in its C-terminal end by the amino acid located in position 35 to 55 of SEQ ID NO: 30,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 145 and in its 3′ end by the nucleotide located in position 162 to 192 of SEQ ID NO: 31, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia I delimited in its N-terminal end by the amino acid located in position 49 and in its C-terminal end by the amino acid located in position 54 to 64 of SEQ ID NO: 32,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 70 and in its 3′ end by the nucleotide located in position 189 to 249 of SEQ ID NO: 33, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia II delimited in its N-terminal end by the amino acid located in position 24 and in its C-terminal end by the amino acid located in position 63 to 83 of SEQ ID NO: 34,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 100 and in its 3′ end by the nucleotide located in position 204 to 264 of SEQ ID NO: 35, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia III delimited in its N-terminal end by the amino acid located in position 34 and in its C-terminal end by the amino acid located in position 68 to 88 of SEQ ID NO: 36,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 61 and in its 3′ end by the nucleotide located in position 144 to 204 of SEQ ID NO: 37, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia IV delimited in its N-terminal end by the amino acid located in position 21 and in its C-terminal end by the amino acid located in position 48 to 68 of SEQ ID NO: 38,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 115 and in its 3′ end by the nucleotide located in position 210 to 270 of SEQ ID NO: 39, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia V delimited in its N-terminal end by the amino acid located in position 39 and in its C-terminal end by the amino acid located in position 70 to 90 of SEQ ID NO: 40,   the nucleotide sequence delimited in its 5′ end by the nucleotide located in position 73 and in its 3′ end by the nucleotide located in position 276 to 336 of SEQ ID NO: 41, said nucleotide sequence encoding the polypeptide sequence corresponding to the SR region of hST8Sia VI delimited in its N-terminal end by the amino acid located in position 25 and in its C-terminal end by the amino acid located in position 92 to 112 of SEQ ID NO: 42,   or any fragment of at least 6 nucleotides of the nucleotide sequences encoding polypeptides sequence corresponding to SR regions mentioned above, and encoding at least 2 contiguous amino acids of said SR regions, such as:
           the fragment SEQ ID NO: 129 delimited by the nucleotides located in positions 106 to 222 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 130 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 74 of SEQ ID NO: 6,   the fragment SEQ ID NO: 131 delimited by the nucleotides located in positions 109 to 222 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 132 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 37 to 74 of SEQ ID NO: 6,   the fragment SEQ ID NO: 133 delimited by the nucleotides located in positions 106 to 420 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 134 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 140 of SEQ ID NO: 6,   the fragment SEQ ID NO: 135 delimited by the nucleotides located in positions 106 to 774 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 136 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 258 of SEQ ID NO: 6,   the fragment SEQ ID NO: 137 delimited by the nucleotides located in positions 85 to 138 of SEQ ID NO: 21, encoding the polypeptide sequence SEQ ID NO: 138 corresponding to the fragment of the SR region of hST3Gal III delimited by the amino acids located in positions 29 to 46 of SEQ ID NO: 22,   the fragment SEQ ID NO: 139 delimited by the nucleotides located in positions 85 to 138 of SEQ ID NO: 29, encoding the polypeptide sequence SEQ ID NO: 140 corresponding to the fragment of the SR region of ratST3Gal III delimited by the amino acids located in positions 29 to 46 of SEQ ID NO: 30,   the fragment SEQ ID NO: 141 delimited by the nucleotides located in positions 70 to 237 of SEQ ID NO: 33, encoding the polypeptide sequence SEQ ID NO: 142 corresponding to the fragment of the SR region of hST8Sia II delimited by the amino acids located in positions 24 to 79 of SEQ ID NO: 34,   the fragment SEQ ID NO: 143 delimited by the nucleotides located in positions 61 to 201 of SEQ ID NO: 37, encoding the polypeptide sequence SEQ ID NO: 144 corresponding to the fragment of the SR region of hST8Sia IV delimited by the amino acids located in positions 21 to 67 of SEQ ID NO: 38.   
               

     The invention relates more particularly to a process as defined above, characterized in that: 
     the nucleotide sequence encoding the CT region comprised in the second nucleic acid is chosen among:
         the nucleotide sequence SEQ ID NO: 49 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 42 of SEQ ID NO: 5, said nucleotide sequence SEQ ID NO: 49 encoding the polypeptide sequence SEQ ID NO: 50 corresponding to the CT region of hST6GalNAc I delimited by the amino acids located in positions 1 to 14 of SEQ ID NO: 6,   the nucleotide sequence SEQ ID NO: 65 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 21, said nucleotide sequence SEQ ID NO: 65 encoding the polypeptide sequence SEQ ID NO: 66 corresponding to the CT region of hST3Gal III delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 22,   the nucleotide sequence SEQ ID NO: 73 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 24 of SEQ ID NO: 29, said nucleotide sequence SEQ ID NO: 73 encoding the polypeptide sequence SEQ ID NO: 74 corresponding to the CT region of ratST3Gal III delimited by the amino acids located in positions 1 to 8 of SEQ ID NO: 30,   the nucleotide sequence SEQ ID NO: 77 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 18 of SEQ ID NO: 33, said nucleotide sequence SEQ ID NO: 77 encoding the polypeptide sequence SEQ ID NO: 78 corresponding to the CT region of hST8Sia II delimited by the amino acids located in positions 1 to 6 of SEQ ID NO: 34,   the nucleotide sequence SEQ ID NO: 81 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 1 and 21 of SEQ ID NO: 37, said nucleotide sequence SEQ ID NO: 81 encoding the polypeptide sequence SEQ ID NO: 82 corresponding to the CT region of hST8Sia IV delimited by the amino acids located in positions 1 to 7 of SEQ ID NO: 38,       

     the nucleotide sequence encoding the TMD region comprised in the second nucleic acid is chosen among:
         the nucleotide sequence SEQ ID NO: 91 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 43 and 105 of SEQ ID NO: 5, said nucleotide sequence SEQ ID NO: 91 encoding the polypeptide sequence SEQ ID NO: 92 corresponding to the TMD region of hST6GalNAc I delimited by the amino acids located in positions 15 to 35 of SEQ ID NO: 6,   the nucleotide sequence SEQ ID NO: 107 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 84 of SEQ ID NO: 21, said nucleotide sequence SEQ ID NO: 107 encoding the polypeptide sequence SEQ ID NO: 108 corresponding to the TMD region of hST3Gal III delimited by the amino acids located in positions 9 to 28 of SEQ ID NO: 22,   the nucleotide sequence SEQ ID NO: 115 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 25 and 84 of SEQ ID NO: 29, said nucleotide sequence SEQ ID NO: 115 encoding the polypeptide sequence SEQ ID NO: 116 corresponding to the TMD region of rST3Gal III delimited by the amino acids located in positions 9 to 28 of SEQ ID NO: 30,   the nucleotide sequence SEQ ID NO: 119 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 19 and 69 of SEQ ID NO: 33, said nucleotide sequence SEQ ID NO: 119 encoding the polypeptide sequence SEQ ID NO: 120 corresponding to the TMD region of hST8Sia II delimited by the amino acids located in positions 7 to 23 of SEQ ID NO: 34,   the nucleotide sequence SEQ ID NO: 123 corresponding to the nucleotide sequence delimited by the nucleotides located in positions 22 and 60 of SEQ ID NO: 37, said nucleotide sequence SEQ ID NO: 123 encoding the polypeptide sequence SEQ ID NO: 124 corresponding to the TMD region of hST8Sia IV delimited by the amino acids located in positions 8 to 20 of SEQ ID NO: 38,       

     the nucleotide sequence encoding the SR region or fragment thereof comprised in the second nucleic acid, is chosen among:
         the sequence SEQ ID NO: 129 delimited by the nucleotides located in positions 106 to 222 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 130 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 74 of SEQ ID NO: 6,   the sequence SEQ ID NO: 131 delimited by the nucleotides located in positions 109 to 222 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 132 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 37 to 74 of SEQ ID NO: 6,   the sequence SEQ ID NO: 133 delimited by the nucleotides located in positions 106 to 420 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 134 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 140 of SEQ ID NO: 6,   the sequence SEQ ID NO: 135 delimited by the nucleotides located in positions 106 to 774 of SEQ ID NO: 5, encoding the polypeptide sequence SEQ ID NO: 136 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 258 of SEQ ID NO: 6,   the sequence SEQ ID NO: 137 delimited by the nucleotides located in positions 85 to 138 of SEQ ID NO: 21, encoding the polypeptide sequence SEQ ID NO: 138 corresponding to the fragment of the SR region of hST3Gal III delimited by the amino acids located in positions 29 to 46 of SEQ ID NO: 22,   the sequence SEQ ID NO: 139 delimited by the nucleotides located in positions 85 to 138 of SEQ ID NO: 29, encoding the polypeptide sequence SEQ ID NO: 140 corresponding to the fragment of the SR region of ratST3Gal III delimited by the amino acids located in positions 29 to 46 of SEQ ID NO: 30,   the sequence SEQ ID NO: 141 delimited by the nucleotides located in positions 70 to 237 of SEQ ID NO: 33, encoding the polypeptide sequence SEQ ID NO: 142 corresponding to the fragment of the SR region of hST8Sia II delimited by the amino acids located in positions 24 to 79 of SEQ ID NO: 34,   the sequence SEQ ID NO: 143 delimited by the nucleotides located in positions 61 to 201 of SEQ ID NO: 37, encoding the polypeptide sequence SEQ ID NO: 144 corresponding to the fragment of the SR region of hST8Sia IV delimited by the amino acids located in positions 21 to 67 of SEQ ID NO: 38.       

     The invention concerns more particularly a process as defined above, characterized in that the CT, TMD, SR, or SR fragment peptides comprised in the second nucleic acid, are homologous sequences deriving from the same native glycosyltransferase, this latter being different from peptides in the native glycosyltransferase from which is derived the CD fragment with optimal glycosyltransferase activity as defined above. 
     The invention relates more particularly to a process as defined above, characterized in that the second nucleic acid is chosen among the following sequences: 
     the sequence SEQ ID NO: 145 delimited by the nucleotides located in positions 1 to 222 of SEQ ID NO: 5, containing SEQ ID NO: 49, 91, and 129, and encoding the polypeptide sequence SEQ ID NO: 146 corresponding to the fragment of hST6GalNAc I delimited by the amino acids located in positions 1 to 74 of SEQ ID NO: 6, and containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 130, respectively, 
     the sequence SEQ ID NO: 147 delimited by the nucleotides located in positions 1 to 420 of SEQ ID NO: 5, containing SEQ ID NO: 49, 91, and 133, and encoding the polypeptide sequence SEQ ID NO: 148 corresponding to the fragment of the SR region of hST6GalNac I delimited by the amino acids located in positions 1 to 140 of SEQ ID NO: 6, and containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 134, respectively, 
     the sequence SEQ ID NO: 149 delimited by the nucleotides located in positions 1 to 774 of SEQ ID NO: 5, containing SEQ ID NO: 49, 91, and 135, and encoding the polypeptide sequence SEQ ID NO: 150 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 1 to 258 of SEQ ID NO: 6, and containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 136, respectively, 
     the sequence SEQ ID NO: 151 delimited by the nucleotides located in positions 1 to 138 of SEQ ID NO: 21, containing SEQ ID NO: 65, 107, and 137, and encoding the polypeptide sequence SEQ ID NO: 152 corresponding to the fragment of the SR region of hST3Gal III delimited by the amino acids located in positions 1 to 46 of SEQ ID NO: 22, and containing the CT, TMD and SR fragment regions of hST3Gal III corresponding to SEQ ID NO: 66, 108, and 138, respectively, 
     the sequence SEQ ID NO: 153 delimited by the nucleotides located in positions 1 to 138 of SEQ ID NO: 29, containing SEQ ID NO: 73, 115, and 139, and encoding the polypeptide sequence SEQ ID NO: 154 corresponding to the fragment of the SR region of rST3Gal III delimited by the amino acids located in positions 1 to 46 of SEQ ID NO: 30, and containing the CT, TMD and SR fragment regions of hST3Gal III corresponding to SEQ ID NO: 74, 116, and 140, respectively, 
     the sequence SEQ ID NO: 155 delimited by the nucleotides located in positions 1 to 237 of SEQ ID NO: 33, containing SEQ ID NO: 77, 119, and 141, and encoding the polypeptide sequence SEQ ID NO: 156 corresponding to the fragment of the SR region of hST8Sia II delimited by the amino acids located in positions 1 to 79 of SEQ ID NO: 34, and containing the CT, TMD and SR regions of hST8Sia II corresponding to SEQ ID NO: 78, 120, and 142, respectively, 
     the sequence SEQ ID NO: 157 delimited by the nucleotides located in positions 1 to 201 of SEQ ID NO: 37, containing SEQ ID NO: 81, 123, and 143, and encoding the polypeptide sequence SEQ ID NO: 158 corresponding to the fragment of the SR region of hST8Sia IV delimited by the amino acids located in positions 1 to 67 of SEQ ID NO: 38, and containing the CT, TMD and SR regions of hST8Sia IV corresponding to SEQ ID NO: 82, 124, and 144, respectively. 
     The invention concerns more particularly a process as defined above, characterized in that the CT, TMD, SR, or SR fragment peptides comprised in the second nucleic acid, are heterologous sequences deriving from different natural glycosyltransferase gene or transcript. 
     The invention relates more particularly to a process as defined above, characterized in that the second nucleic acid is the sequence SEQ ID NO: 159 corresponding the fusion of the nucleotide sequence SEQ ID NO: 177 containing SEQ ID NO: 65 and 107 encoding the CT and TMD regions of hST3Gal III corresponding to SEQ ID NO: 66 and 108 respectively, with the nucleotide sequence SEQ ID NO: 131 encoding the polypeptide sequence SEQ ID NO: 132 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 37 to 74 of SEQ ID NO: 6, said sequence SEQ ID NO: 159 encoding the fusion polypeptide SEQ ID NO: 160 between the CT and TMD regions of hST3Gal III, on the one hand, and the 37-74 fragment of the SR region of hST6GalNAc I, on the other hand. 
     The invention relates more particularly to a process as defined above, characterized in that the second nucleic acid is the sequence SEQ ID NO: 179 corresponding the fusion of the nucleotide sequence SEQ ID NO: 65 encoding the CT of hST3Gal III corresponding to SEQ ID NO: 66, with the nucleotide sequence SEQ ID NO: 119 encoding the TMD region of hST8Sia II corresponding to SEQ ID. NO: 120, and with the nucleotide sequence SEQ ID NO: 129 encoding the polypeptide sequence SEQ ID NO: 130 corresponding to the fragment of the SR region of hST6GalNAc I delimited by the amino acids located in positions 36 to 74 of SEQ ID NO: 6, said sequence SEQ ID NO: 179 encoding the fusion polypeptide SEQ ID NO: 180 between the CT region of hST3Gal III, the TMD region of hST8Sia II, and the 36-74 fragment of the SR region of hST6GalNAc I. 
     The invention concerns more particularly a process as defined above, characterized in that it comprises the fusion of the sequence SEQ ID NO: 43 as the first nucleic acid, with a second acid nucleic chosen among: 
     the sequence SEQ ID NO: 145, leading to the nucleotide sequence SEQ ID NO: 161 encoding the fusion protein SEQ ID NO: 162 containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 130, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 147, leading to the nucleotide sequence SEQ ID NO: 163 encoding the fusion protein SEQ ID NO: 164 containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 134, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 149, leading to the nucleotide sequence SEQ ID NO: 165 encoding the fusion protein SEQ ID NO: 166 containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 136, respectively, linked via a SR linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 151, leading to the nucleotide sequence SEQ ID NO: 167 encoding the fusion protein SEQ ID NO: 168 containing the CT, TMD and SR fragment regions of hST3Gal III corresponding to SEQ ID NO: 66, 108, and 138, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 153, leading to the nucleotide sequence SEQ ID NO: 169 encoding the fusion protein SEQ ID NO: 170 containing the CT, TMD and SR fragment regions of ratST3Gal III corresponding to SEQ ID NO: 74, 116, and 140, respectively, linked via a SR linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 155, leading to the nucleotide sequence SEQ ID NO: 171 encoding the fusion protein SEQ ID NO: 172 containing the CT, TMD and SR regions of hST8Sia II corresponding to SEQ ID NO: 78, 120, and 142, respectively, linked via a KL linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 157, leading to the nucleotide sequence SEQ ID NO: 173 encoding the fusion protein SEQ ID NO: 174 containing the CT, TMD and SR regions of hST8Sia IV corresponding to SEQ ID NO: 82, 124, and 144, respectively, linked via a KL linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 159, leading to the nucleotide sequence SEQ ID NO: 175 encoding the fusion protein SEQ ID NO: 176 containing the CT and TMD regions of hST3Gal III corresponding to SEQ ID NO: 66 and 108 respectively, and the 37-74 fragment of the SR region of hST6GalNac I corresponding to SEQ ID NO: 132, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, 
     the sequence SEQ ID NO: 179, leading to the nucleotide sequence SEQ ID NO: 181 encoding the fusion protein SEQ ID NO: 182 containing the CT region of hST3Gal III, the TMD region of hST8Sia II, and the 36-74 fragment of the SR region of hST6GalNAc I, corresponding to SEQ ID NO: 66, 120, and 130 respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I. 
     The invention also relates to gene sequences encoding chimerical glycosyltransferases such as obtained according to the process as defined above. 
     The invention concerns more particularly gene sequences as defined above, chosen among: 
     the sequence SEQ ID NO: 161 encoding the fusion protein SEQ ID NO: 162 containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 130, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 161 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 145, 
     the sequence SEQ ID NO: 163 encoding the fusion protein SEQ ID NO: 164 containing the CT, TMD and SR fragment regions of hST6GalNAc I corresponding to SEQ ID NO: 50, 92, and 134, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 163 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 147, 
     the sequence SEQ ID NO: 165 encoding the fusion protein SEQ ID NO: 166 containing the CT, TMD and SR fragment regions of hST6GalNac I corresponding to SEQ ID NO: 50, 92, and 136, respectively, linked via a SR linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 165 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 149, 
     the sequence SEQ ID NO: 167 encoding the fusion protein SEQ ID NO: 168 containing the CT, TMD and SR fragment regions of hST3Gal III corresponding to SEQ ID NO: 66, 108, and 138, respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 167 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 151, 
     the sequence SEQ ID NO: 169 encoding the fusion protein SEQ ID NO: 170 containing the CT, TMD and SR fragment regions of ratST3Gal III corresponding to SEQ ID NO: 74, 116, and 140, respectively, linked via a SR linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 169 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 153, 
     the sequence SEQ ID NO: 171 encoding the fusion protein SEQ ID NO: 172 containing the CT, TMD and SR regions of hST8Sia II corresponding to SEQ ID NO: 78, 120, and 142, respectively, linked via a KL linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 171 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 155, 
     the sequence SEQ ID NO: 173 encoding the fusion protein SEQ ID NO: 174 containing the CT, TMD and SR regions of hST8Sia IV corresponding to SEQ ID NO: 82, 124, and 144, respectively, linked via a KL linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 173 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 157, 
     the sequence SEQ ID NO: 175 encoding the fusion protein SEQ ID NO: 176 containing the CT and TMD regions of hST3Gal III corresponding to SEQ ID NO: 66 and 108 respectively, and the 37-74 fragment of the SR region of hST6GalNAc I corresponding to SEQ ID NO: 132, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 175 corresponding to the fusion of the sequence SEQ ID NO: 43 and the sequence SEQ ID NO: 159, 
     the sequence SEQ ID NO: 181 encoding the fusion protein SEQ ID NO: 182 containing the CT region of hST3Gal III, the TMD region of hST8Sia II, and the 36-74 fragment of the SR region of hST6GalNAc I, corresponding to SEQ ID NO: 66, 120, and 130 respectively, linked via a GS linker to the 90-406 C-terminal minimal fragment of the CD domain of hST6Gal I, said sequence SEQ ID NO: 181 corresponding to the fusion of the sequence SEQ ID NO: 43 and sequence SEQ ID NO: 179. 
     The invention also concerns vectors, such as plasmids, viral or bacterial constructs, comprising at least one gene sequence as defined above. 
     The invention also relates to host eukaryotic cells from yeast, fungi, insect, plants, mammalian or human origin, transformed with at least one gene sequence as defined above, using at least one vector as mentioned above. 
     The invention also concerns the use of at least one gene sequence as defined above, or of a vector as mentioned above, for the transformation of cells as defined above, or the use of transgenic animals obtained from such transformed cells, in the frame of the production of recombinant proteins of interest. 
     The invention also relates to a method for the preparation of recombinant proteins of interest comprising the transformation of cells as defined above, with a vector containing at least one nucleotide sequence encoding said recombinant proteins of interest. 
     Preferred recombinant proteins of interest which can be prepared according to a method as mentioned above according to the invention are chosen among hormones, enzymes, clotting factors, carbohydrate antigens/serum biomarkers, cytokines, growth factors, antibodies or receptors. 
     Preferred host cells for the preparation of recombinant proteins of interest as mentioned above, are chosen among drug-approved cells or organisms, preferably rodent, mammalian or human cells. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  represents the membrane topology of glycosyltransferases. Glycosyltransferases are type II membrane proteins including a cytoplasmic N-terminal tail (CT), a transmembrane (TMD) anchor signal followed by a stem region (SR) and a large C-terminal catalytic domain (CD). 
         FIG. 2  Amino acid sequence of sialylmotif L, S, and VS in 20 human sialyltransferases. Consensus amino acid residues in the all sialyltransferases are shown by bold letters. 
         FIG. 3  represents schematic distribution of CT, TMD, SR and CD of the rat ST6Gal I showing key residues which are tyr123 and 7 conserved cys. CT: cytoplasmic tail (9 amino acids); TMD: transmembrane domain (17 aa); SR: stem region (70 aa); CD: catalytic domain (307 aa); L: sialylmotif L; S: sialylmotif L and VS sialylmotif VS 
         FIG. 4  represents DNA constructs to produce chimera and evaluate the role of the cytoplasmic tail, the transmembrane domain in two enzyme isoforms (rST6Gal I Tyr or Cys 123) respectively (Fenteany &amp; Colley, 2005). 
         FIG. 5  represents sequence alignment of the rat and the human ST6Gal I. It can be noticed that the N-terminal sequence comprising the CT (1-9), TMD (10-26) is fully conserved while the juxtamebrane SR portion (27-89) is more variable although the juxtamembrane peptide and especially positively lysine and cysteine residues are wellconserved. 
         FIG. 6  represents the sialic acid pathway in human cells in the context of overall cellular glycosylation. The enzymes involved in this sequential process are: (a and b) UDP-N-acetylglucosamine/2-epimerase/N-acetylmannosamine kinase (UDP-GlcNAc 2-epimerase/ManNAc 6-kinase)—Reactions: (a) epimerase and (b) kinase, (c) N-acetylneuraminic acid phosphate synthase; (NANS SAS; N-acetylneuraminic acid phosphate synthase)—Reaction and Homo sapiens N-acetylneuraminate pyruvate lyase (NPL; N-acetylneuraminate pyruvate lyase)—Reaction, (d) NeuAc 9-phosphatase, (e) Cytidine 5′-monophosphate N-acetylneuramininc acid synthetase, (CMP-NeuAc synthetase), (f) Cytidine monophosphate-sialic acid transporter (Golgi CMP-NeuAc transporter), and (g) sialyltransferases. 
       Of note, the sialic acid derivative shown in this diagram, namely “Neu5Ac,” is widely considered as the ‘human’ form of sialic acid. In all other animals, with the exception of chickens, there is an additional step in the pathway (shown in this diagram) where CMP-Neu5Ac is further hydroxylated and converted to CMP-Neu5Gc by the enzymatic action of CMP-N-Acetylneuraminic Acid Hydroxylase. 
         FIG. 7  represents a schematical overview of the various synthetic chimerical constructs generated by the invention. 
         FIG. 7A  represents the general construction of a synthetic chimera including the N-terminal synthetic domain tagged with the FLAG epitope fused to the minimal catalytic domain (CD) through the addition of a restriction site. 
         FIG. 7B  shows the ligation between the N-terminal synthetic and the catalytic domains using a BamHI restriction site. Note that the constructs are introduced into the vector using restriction sites, namely AflII and XbaI distinct from the ligation site. 
         FIG. 8  shows the topology and characteristic of the pcDNA3.1 (+) vector (Invitrogen). 
         FIG. 9  represents the constructions of the FLAG-hST6Gal I CD and of the chimeric forms of this CD fused to several N-Terminal fragments of other sialyltransferases of variable length. 
         FIG. 9A  corresponds to FLAG-CD. 
         FIG. 9B  corresponds to hST8SiaII-79/CD. 
         FIG. 9C  corresponds to hST8SiaIV-67/CD. 
         FIG. 9D  corresponds to hST6GalNAc I-258/CD. 
         FIG. 9E  corresponds to r/hST3Gal III-46/CD. 
         FIG. 10  represents the digested minimum catalytic domain of hST6Gal I and its amplified PCR product to be used in each construction of the synthetic chimera. Samples were loaded on a 1.5% agarose gel with the SmartLadder (SL) nucleic marker. 
         FIG. 10A  shows the digested hST6Gal I catalytic domain from CMV-vector, issued from the cloning in the laboratory. 
         FIG. 10B  shows the PCR product of the minimum catalytic domain at 966 pb. 
         FIG. 10C  shows the concentrated PCR product of the minimum catalytic domain of hST6Gal I with 5 μL loaded on the gel. 
         FIG. 11  shows an agarose gel (2%) showing the DNA band of the reconstituted synthetic N-terminal region of hST3Gal III. Lane 1 corresponds to a PCR product of 174 pb; lane 2 corresponds to the SmartLadder SF; lane 3 corresponds to 5 μL of the concentrated PCR product of 174 pb size. 
         FIG. 12  represents the total reconstructed enzyme gene of hST3Gal III/CD. 
       In  FIG. 12A , an agarose gel (1.5%) shows the DNA band of the reconstituted synthetic hST3Gal III/CD amplified by PCR. 
         FIG. 12B  corresponds to 5 μL of the concentrated PCR products of around 1200 pb in size. 
         FIG. 12C  represents the product of the digestion of the recombinant vector by the restriction enzymes AflII and XbaI, showing the insertion of the chimera gene (expected size 1200 pb). 
         FIG. 13  shows an agarose gel (2%) showing the DNA band of the reconstituted synthetic N-terminal region of hST6GalNAc I-74. Lane 1 corresponds to a PCR product of 270 pb; lane 2 corresponds to the SmartLadder SF; lane 3 corresponds to 5 μL of the concentrated PCR products of 270 pb size. 
         FIG. 14  represents the complete reconstructed synthetic gene of hST6GalNAc I-74/CD. 
         FIG. 14A  shows an agarose gel (1.5%) with the DNA band of the reconstituted synthetic hST6GalNAc I-74/CD amplified by PCR. 
         FIG. 14B  shows 5 μL of the concentrated PCR products of the chimera of around 1200 pb in size. 
         FIG. 14C  shows the product of the digestion of the recombinant vector by the restriction enzymes AflII and XbaI, showing the insertion of the chimera gene (expected size 1225 pb). 
         FIG. 15  shows an agarose gel (1.5%) showing the DNA band of the reconstituted synthetic N-terminal region of hST6GalNAc I-140. Lane 1: SmartLadder SF; lane 2: PCR product of 468 pb. 
         FIG. 16  shows an agarose gel (1.5%) showing the DNA band of the reconstituted synthetic hybrid gene composed of hST3Gal III-29/37-hST6GalNAc I-74. Lane 1 corresponds to the SmartLadder SF; lane 2 corresponds to a PCR product of 249 pb. 
         FIG. 17  shows an agarose gel (1.5%) showing the DNA band of the reconstituted synthetic hybrid gene composed of hST3Gal III-29/37-hST6GalNAc I-74 ligated with the CD. Lane 1 corresponds to the SmartLadder SF; lane 2 corresponds to a PCR product of 1197 pb. 
         FIG. 18  represents the expression of the minimal catalytic domain (CD) (Panel A) of hST6Gal I in CHO cells, based on a double labelling with FITC-SNA binding 6-linked sialic acid and with an anti-FLAG mAb and using confocal microscopy. Transient expression of the CD was analysed by anti-FLAG labelling (Panel B) and for SNA-FITC binding (Panel C). Panel D shows the superposition of the signal from panels B and C (Donadio et al., 2003) 
         FIG. 19  shows the expression of two chimeric enzymes, hST8SiaIV-67/CD, hST8SiaII-79/CD in CHO cells. Transfected cells were double-labelled with an anti-FLAG mAb (A and D) and with FITC-SNA (B and E). Overlays are represented in C and F. 
         FIG. 20  shows the expression of ST3GalIII/CD and ST3GalIII-hST6GalNAc I-74/CD in CHO cells. Transfected cells were double-labelled with an anti-FLAG mAb (A and D) and with FITC-SNA (B and E). Overlays are represented in C and F. 
         FIG. 21  shows the expression of hST6GalNAc I-74/CD, and 258/CD in CHO cells. Transfected cells were double-labelled with an anti-FLAG mAb (A and D) and with FITC-SNA (B and E). Overlays are represented in C and F. 
         FIG. 22  shows an agarose gel (2%) showing the DNA band of the reconstituted synthetic N-terminal region of hST3Gal III/hST8Sia II/hST6GalNAc I. Lane 1 corresponds to a PCR product of 240 pb; lane 2 corresponds to the SmartLadder SF; lane 3 corresponds to 3 μL of the 240 pb concentrated PCR product. 
         FIG. 23  represents the product of the digestion of the recombinant vector by the restriction enzymes XbaI and BamHI, showing the insertion of the catalytic domain gene (expected size 960 pb). Sample was loaded on a 1.5% agarose gel with the SL nucleic marker. 
         FIG. 24  represents the reconstructed enzyme gene of hST3Gal III/hST8Sia II/hST6GalNAc I/CD. 
       In  FIG. 24   a , an agarose gel (1.5%) shows the product of the digestion of the recombinant vector by the restriction enzymes XbaI and AflII, showing the insertion of the chimera gene (expected size 1200 pb). 
         FIG. 24   b  represents the PCR product obtained after the amplification of a minipreparation sample, confirming the insertion of the chimera gene. 
         FIG. 25  shows the expression of the chimeric enzyme hST3Gal III/hST8Sia II/hST6GalNAc I/CD in CHO cells. Transfected cells were double-labelled with an anti-FLAG mAb (A) and with FITC-SNA (B). Overlay is represented in C. 
     
    
    
     LEGENDS OF TABLES 1 TO 5 
     Table 1: Sequences and accession numbers of the 20 sialyltransferases known in human. 
     Table 2: Acceptor substrates and expression sites of human sialyltransferases as described in Harduin-Lepers et al., 2001 and Jeanneau et al, 2003. 
     Table 3: Distribution of the 4 peptide domains shared by human STs. A short N-terminal cytoplasmic tail (CT; around 10 amino-acids), a transmembrane domain (TMD; around 20 amino acids), a stem region (SR), highly variable in length and a catalytic domain (CD; around 300 amino acids), including the sialylmotifs L, S and VS. 
     Table 4: Proposed conserved peptides of around 31-85 amino acids identified within the hypervariable SR region. 5 motifs were found: motif A common to ST6Gal and ST6GalNAc I, motif B common to ST6GalNAc I and ST6GalNAc II, motifs C and D common to all ST8Sia and motif E present in ST3Gal except the ST3Gal I and II (Donadio et al., 2003). 
     Table 5: Design of the four primer pairs used for PCR amplification. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 HUMAN 
                 ACCESSION 
                 ACCESSION N o   
               
               
                 SIALYLTRANSFERASES 
                 N o  GenBank 
                 Swiss Prot 
               
               
                   
               
             
            
               
                 ST6Gal I 
                 A17362 
                 P15907 
               
               
                 ST6Gal II 
                 NM_032528 
                 Q8IUG7 
               
               
                 ST6GalNAc I 
                 NM_018414 
                 Q9NSC7 
               
               
                 ST6GalNAc II 
                 NM_006456 
                 Q9UJ37 
               
               
                 ST6GalNAc III 
                 NM_152996 
                 Q8NDV1 
               
               
                 ST6GalNAc IV 
                 NM_014403 
                 Q9H4F1 
               
               
                 ST6GalNAc V 
                 NM_030965 
                 Q9BVH7 
               
               
                 ST6GalNAc VI 
                 AB035173 
                 Q969X2 
               
               
                 ST3Gal I 
                 L29555 
                 Q11201 
               
               
                 ST3Gal II 
                 U63090 
                 Q16842 
               
               
                 ST3Gal III 
                 L23768 
                 Q11203 
               
               
                 ST3Gal IV 
                 L23767 
                 Q11206 
               
               
                 ST3Gal V 
                 AF105026 
                 Q9UNP4 
               
               
                 ST3Gal VI 
                 AF119391 
                 Q9Y274 
               
               
                 ST8Sia I 
                 L32867 
                 Q92185 
               
               
                 ST8Sia II 
                 U33551 
                 Q92186 
               
               
                 ST8Sia III 
                 AF004668 
                 043173 
               
               
                 ST8Sia IV 
                 L41680 
                 Q92187 
               
               
                 ST8Sia V 
                 U91641 
                 015466 
               
               
                 ST8Sia VI 
                 AJ621583 
                 P61647 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 HUMAN 
                   
                   
               
               
                 SIALYLTRANSFERASES 
                 Acceptor 
                 Expression 
               
               
                   
               
             
            
               
                 ST6Gal I 
                 Galβ1-4GlcNAc 
                 Ubiquitous 
               
               
                 ST6Gal II 
                 Galβ1-4GlcNAc 
                 Brain, testicules, 
               
               
                   
                   
                 thyroid, fetal tissue, 
               
               
                   
                   
                 lymphatic ganglia 
               
               
                 ST6GalNAc I 
                 Galβ1-3GalNAc 
                 Submaxillary and 
               
               
                   
                   
                 mammary 
               
               
                   
                 GalNAcα-O-Ser/Thr 
                 glands, spleen, colon 
               
               
                   
                 Siaα2-3 Galβ1-3GalNAcα- 
               
               
                   
                 O-Ser/Thr 
               
               
                 ST6GalNAc II 
                 Galβ1-3GalNAcα-O- 
                 Lacting mammary glands, 
               
               
                   
                 Ser/Thr 
                 testis 
               
               
                   
                 Siaα2-3Galβ1-3GalNAcα- 
               
               
                   
                 O-Ser/Thr 
               
               
                 ST6GalNAc III 
                 Siaα2-3Galβ1-3GalNAc 
               
               
                 ST6GalNAc IV 
                 Siaα2-3Galβ1-3GalNAc, 
                 Ubiquitous 
               
               
                   
                 GM1b 
               
               
                 ST6GalNAc V 
                 Siaα2-3Galβ1-3GalNAc, 
               
               
                   
                 GM1b 
               
               
                 ST6GalNAc VI 
                 GM1b, GT1b 
               
               
                 ST3Gal I 
                 Galβ1-3GalNAc 
                 Ubiquitous 
               
               
                 ST3Gal II 
                 Galβ1-3GalNAc, GM1, 
                 Heart, liver, skeletal 
               
               
                   
                 asialo- 
                 muscle, 
               
               
                   
                 GM1 
                 thymus, lymph node, 
               
               
                   
                   
                 appendix, salivary 
               
               
                   
                   
                 glands, spleen 
               
               
                 ST3Gal III 
                 Galβ1-3GlcNAc 
                 Skeletal muscle, fetal 
               
               
                   
                 Galβ1-4GlcNAc 
                 tissue 
               
               
                 ST3Gal IV 
                 Galβ1-4GlcNAc-O 
                 Placenta, testis, ovary 
               
               
                   
                 Galβ1-3GalNAc-O 
               
               
                 ST3Gal V 
                 Galβ1-4Glcβ-Cer 
                 Ubiquitous 
               
               
                 ST3Gal VI 
                 Galβ1-4GlcNAc 
                 Heart, placenta, liver 
               
               
                   
                   
                 and most other tissues. 
               
               
                 ST8Sia I 
                 Siaα2-3Galβ1-4Glcβ1-O- 
               
               
                   
                 Cer, GM3 
               
               
                 ST8Sia II 
                 SiaGalβ1-4GalNAc 
                 Embryonic tissues, 
               
               
                   
                   
                 brain 
               
               
                 ST8Sia III 
                 Sia2-3Galβ1-4GlcNAc 
               
               
                 ST8Sia IV 
                 SiaGalβ1-4GalNAc 
                 Brain, fetal tissues, 
               
               
                   
                   
                 adult heart, spleen, 
               
               
                   
                   
                 thymus 
               
               
                 ST8Sia V 
                 GM1b, GD1a, GT1b, GD3 
               
               
                 ST8Sia VI 
                 NeuAcα2,3(6)Galβ 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 ESTIMATION OF LENGTH OF DOMAINS 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Human 
                   
                 Cytoplasmic 
                 Transmembrane 
                 Stem Region 
                 Catalytic Domain 
                 Sialylmotif L 
                 Sialylmotif S 
                 Sialylmotif VS 
               
               
                 Sialyltransferases 
                 Total 
                 Tail (CT) 
                 Domain (TMD) 
                 (SR; +/−10 AA) 
                 (CD; +/−10 AA) 
                 (47 AA) 
                 (24 AA) 
                 (12 AA) 
               
               
                   
               
               
                 ST6Gal I 
                 406 
                 1-9 (9 AA) 
                 10-26 (17 AA)  
                 27-100 (74 AA)  
                 101-406 (306 AA)  
                 181-227 
                 320-343 
                 366-378 
               
               
                 ST6Gal II 
                 529 
                 1-10 (10 AA) 
                 11-30 (20 AA)  
                 31-112 (82 AA)  
                 113-529 (417 AA)  
                 293-339 
                 433-456 
                 479-491 
               
               
                 ST6GalNAc I 
                 600 
                 1-14 (14 AA) 
                 15-35 (41 AA)  
                 36-281 (246 AA) 
                 282-600 (319 AA)  
                 362-408 
                 518-541 
                 563-575 
               
               
                 ST6GalNAc II 
                 374 
                 1-7 (7 AA) 
                 8-28 (21 AA) 
                 29-67 (39 AA) 
                 68-374 (307 AA) 
                 148-194 
                 302-325 
                 347-359 
               
               
                 ST6GalNAc III 
                 305 
                 1-8 (8 AA) 
                 9-28 (20 AA) 
                 29-34 (6 AA)  
                 35-305 (271 AA) 
                  77-123 
                 214-237 
                 273-285 
               
               
                 ST6GalNAc IV 
                 302 
                 1-6 (6 AA) 
                 7-27 (21 AA) 
                 28-30 (3 AA)  
                 31-302 (272 AA) 
                  73-119 
                 210-233 
                 270-282 
               
               
                 ST6GalNAc V 
                 336 
                 1-8 (8 AA) 
                 9-29 (21 AA) 
                 30-50 (21 AA) 
                 51-336 (286 AA) 
                  93-139 
                 230-253 
                 291-303 
               
               
                 ST6GalNAc VI 
                 333 
                 1-43 (43 AA) 
                 44-59 (16 AA)  
                 60-61 (2 AA)  
                 62-333 (272 AA) 
                 105-151 
                 241-264 
                 303-315 
               
               
                 ST3Gal I 
                 340 
                 1-13 (13 AA) 
                 14-34 (21 AA)  
                 35-58 (24 AA) 
                 59-340 (282 AA) 
                 139-185 
                 266-289 
                 312-324 
               
               
                 ST3Gal II 
                 350 
                 1-6 (6 AA) 
                 7-27 (21 AA) 
                 28-68 (41 AA) 
                 69-350 (282 AA) 
                 149-195 
                 276-299 
                 322-334 
               
               
                 ST3Gal III 
                 444 
                 1-8 (8 AA) 
                 9-28 (20 AA) 
                 29-76 (48 AA) 
                 77-444 (368 AA) 
                 157-203 
                 299-322 
                 346-358 
               
               
                 ST3Gal IV 
                 329 
                 1-8 (8 AA) 
                 9-26 (18 AA) 
                 27-36 (10 AA) 
                 37-329 (293 AA) 
                 117-163 
                 258-281 
                 307-319 
               
               
                 ST3Gal V 
                 362 
                 1-5 (5 AA) 
                 6-26 (21 AA) 
                 27-55 (29 AA) 
                 56-362 (307 AA) 
                 136-182 
                 282-305 
                 329-341 
               
               
                 ST3Gal VI 
                 331 
                 1-4 (4 AA) 
                 5-25 (21 AA) 
                 26-34 (9 AA)  
                 35-331 (297 AA) 
                 115-161 
                 256-279 
                 303-315 
               
               
                 ST8Sia 1 
                 356 
                 1-29 (29 AA) 
                 30-48 (19 AA)  
                 49-54 (6 AA)  
                 55-356 (302 AA) 
                 135-182 
                 272-295 
                 318-330 
               
               
                 ST8Sia II 
                 375 
                 1-6 (6 AA) 
                 7-23 (17 AA) 
                 24-73 (50 AA) 
                 74-375 (302 AA) 
                 157-200 
                 292-315 
                 342-354 
               
               
                 ST8Sia III 
                 380 
                 1-9 (9 AA) 
                 10-33 (24 AA)  
                 34-78 (45 AA) 
                 79-380 (302 AA) 
                 159-205 
                 298-321 
                 350-362 
               
               
                 ST8Sia IV 
                 359 
                 1-7 (7 AA) 
                 8-20 (13 AA) 
                 21-58 (38 AA) 
                 59-359 (301 AA) 
                 139-185 
                 277-300 
                 327-339 
               
               
                 ST8Sia V 
                 376 
                 1-17 (17 AA) 
                 18-38 (21 AA)  
                 39-80 (42 AA) 
                 81-376 (296 AA) 
                  93-208 
                 298-321 
                 344-350 
               
               
                 ST8Sia VI 
                 398 
                 1-3 (3 AA) 
                 4-24 (21 AA) 
                 25-102 (78 AA)  
                 103-398 (296 AA)  
                 183-230 
                 320-343 
                 360-378 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 conserved 
                   
                   
                   
                   
                 Motif 
               
               
                 ENZYMES 
                 region 
                 Motif A 
                 Motif B 
                 Motif C 
                 Motif D 
                 E 
               
               
                   
               
             
            
               
                 hST6Gal I 
                 93-165 
                 159-165 
                 X 
                 X 
                 X 
                 X 
               
               
                 hST6GalNAc I 
                 259-331  
                 310-315 
                 324-331 
                 X 
                 X 
                 X 
               
               
                 hST8Sia IV 
                 71-133 
                 X 
                 X 
                 71-90 
                 119-133 
                 X 
               
               
                 hST3Gal IV 
                 75-106 
                 X 
                 X 
                 X 
                 X 
                 84-91 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 PCR product 
               
               
                   
                 Amplified fragments 
                 Primers set 
                 size (Pb) 
               
               
                   
                   
               
             
            
               
                   
                 hST3Gal III 
                 5′ AflII-ST3 
                 186 
               
               
                   
                   
                 3′ST3-BamHI 
               
               
                   
                 hST6GalNAc I-74 
                 5′ AflII-ST6 
                 270 
               
               
                   
                   
                 3′′ST6-BamHI 
               
               
                   
                 hST6GalNAc I-140 
                 5′ AflII-ST6 
                 468 
               
               
                   
                   
                 3′ST6-BamHIn o 2 
               
               
                   
                 hST3Gal III-29/37- 
                 5′ AflII-ST3 
                 246 
               
               
                   
                 hST6GalNAc-74 
                 3′ST6-BamHI 
               
               
                   
                   
               
            
           
         
       
     
     DESCRIPTION OF THE INVENTION 
     The invention will be further described in the detailed description which follows. 
     I—Introduction 
     Bases of the Invention: 
     The inventors have focused on the study of hST6Gal I because this enzyme activity is missing in all host cells used so far for heterologous protein production, especially in drug-approved cell systems. The human enzyme has been cloned by the inventors in Ronin, 2001. 
     In contrast to the literature, the inventors were initially able to show that the full-length enzyme can differentially sialylate acceptor glycoproteins of bi-, tri- and/or tetraantennary glycan structure (Ronin 2001) and at that time, the inventors hypothesized that within the transferase structure, a steric control located in the hypervariable SR region should be exerted on the CD to regulate and naturally constrain enzymatic specificity. For the purpose of producing sialylated proteins of biomedical interest, this regulatory control should be abolished and as a result, the specificity may be enlarged. Inside the conserved region (93-165) of hST6Gal I, an hydrophobic sequence has been noticed: 93-QVWxKDS-100. The importance of this sequence has been studied by progessively deleting hST6Gal I of its N-terminal part within the conserved region newly identified. The Δ35, Δ48, Δ60 and Δ82 deleted mutants show an increasing transfer efficiency. Deletions carried out before and after the conserved domain (93-165 of ST6GalI) showed that the Δ100 mutant lose its catalytic activity, whereas the Δ89 (containing the hydrophobic sequence QVWxKDS) possesses an optimal catalytic activity. The results clearly indicate that this short sequence is crucial to promote activity. This hydrophobic sequence may contribute to local conformational changes essential for the active site to promote sialic acid transfer. 
     A second study has been realized by the inventors aimed at elucidating the molecular and cellular basis that govern the acceptor preference of STs (Ronin 2003). As it was difficult to delineate the CD from the hypervariable region of the SR, they hypothesized that the SR of STs should contain structural features related to an acceptor preference. 53 animal and human enzymes of known specificity were analyzed by bioinformatics to determine if STs may share a peptidic portion to restrict the acceptor recognition to an enzyme subfamily. A highly conserved region of around 31-85 amino acids has been identified and inside this region, 5 motifs were found: motif A common to ST6Gal and ST6GalNAc I, motif B common to ST6GalNAc I and ST6GalNAc II, motifs C and D common to all ST8Sia and motif E present in ST3Gal except the ST3Gal I and II (Table 4). 
     Those 5 motifs are typical of a STs subgroup sharing a similar catalytic activity and thus involved in the same way to transfer sialic acid. They are located at the end of the SR closed to the sialylmotif L and can be considered as part of the CD as key feature for acceptor recognition. 
     Of special interest also, was also the finding that the Δ89 mutant is extremely efficient in CHO cells and that it follows the intracellular pathway from the early golgi to the trans golgi/trans golgi network. Δ89 is expressed along the intracellular routing of the glycoproteins and glycolipids in the entire stacks of the golgi apparatus (Ronin, 2003). This work made it possible to delineate the minimum CD for hST6Gal I containing the crucial hydrophobic sequence (QVWxKDS) and displacing the acceptor recognition from intracellular resident acceptors to cell surface glycoconjugates. These findings gave the ground of designing novel membrane-anchored chimera which may display a similar intracellular trafficking to encounter neosynthesized glycoproteins within the secretory pathway as they are packed in engineered host cells when produced as drugs. 
     The minimum CD of hST6Gal I Δ89 have been used in the invention as an “optimized CD” of enlarged specificity and increased transfer efficiency and will be further named CD. 
     The Distribution of the Sialyltransferases in the Golgi Apparatus 
     Golgi localization of GTs is not strictly organized, enzymes are not distributed and isolated in a single compartment of the golgi apparatus, most of them overlap and co-compartmentalized (Colley, 1997; Berger et al., 1998; Berger, 2002). No clear retention signals have been identified yet but enzymes form overlapping gradients across the stacks (Opat et al., 2001), and only crucial regions have been identified. 
     There are two hypothesis concerning retention mechanisms in the golgi: i) the length of the TMD drives the golgi retention (bilayer thickness model; Bretscher &amp; Munro, 1993; Colley, 1997; Munro, 1998) and ii) the proteins oligomerization leeds to golgi retention (Oligomerization/kin recognition hypothesis; Machamer, 1991; Colley, 1997), in which authors postulate that enzymes interact in golgi cisterna to form too large structures to enter in transport vesicules (Munro, 1998). 
     The length of the TMD seems also crucial as a retention signal. TMD represents a key factor in the retention process, in most of cases it is sufficient to promote a golgi localization i.e. throughout the cis-, medial and trans compartments. Lengthening the TMD of STs results in reduced retention and synthetic TMD (creates by mutagenesis) of 17 leucine gives golgi retention whereas one of 23 leucines does not (Munro, 1991, 1995, 1998). Concerning the retention signals of the STs, several regions are involved in an independent manner to retain enzymes in the golgi apparatus. The TMD with its flanking regions are sufficient for golgi retention (Colley, 1997). Other work suggest that no specific sequence in the TMD are required for golgi retention, especially in the presence of appropriate spaced cytoplasmic and luminal flanking sequences and/or SR (Colley, 1997). The presence of negatively charged amino acid sequences on STs, particularly close to the membrane anchor, was found to mislocalize the proteins in the golgi apparatus. The presence of FLAG or MYC epitope, in the CT, disturbs the golgi localization of ST6Gal I, whereas using an additional spacer sequence, to space out the negative charges from the TMD, reveals strong advantage for golgi retention (Yang et al., 1996). The SR sequences alone appear to function as an other type of golgi retention (Colley, 1997). 
     There is probably more than one retention mechanism, explaining the colocalization of the STs inside the trans golgi and the trans golgi network. Moreover the localization of the enzymes also depends of the cell type where they are expressed (Colley, 1997). The TMD of STs alone may be sufficient for golgi retention in MDCK cells but the same TMD and its flanking sequences are clearly required for golgi retention in CHO cells (Colley, 1997). Of interest, available data reveal that changing the CT, the TMD and the SR does not disturb the catalytic activity of STs, but allow to delocalize them inside the golgi apparatus (Grabenhorst et Conradt, 1999). 
     The most studied enzyme for the golgi retention signal, is the ratST6Gal I (rST6Gal I), that has been localized in the trans golgi and the trans golgi network of hepatocytes (Roth et al., 1985; Opat et al., 2001) and in post golgi compartments in other cells (Colley, 1997). Two natural isoforms of rST6Gal I exist, they differ only by one amino acid at position 123 in the CD. This is due to a single A to G nucleotide change, resulting of Tyr to Cys amino acid change. No catalytic activity differences were found between the two isoforms of the enzyme (Chen et al., 2003). The STcys form is found in the golgi in moderately expressing cells and never at the cell surfaces or cleaved or secreted into the media of COS-1 or HeLa cells, whereas the STtyr isoform is found in the golgi, at low levels on the cell surface and is cleaved and secreted from COS-1 and HeLa cells (Ma et al., 1997). To explain the two different localizations of the proteins, authors have proposed two hypothesis on the retention mechanism: bilayer thickness and oligomerization. First, analysis of lengthening TMD of STcys and tyr suggests that the bilayer thickness is not a predominant process for the golgi retention for ST6Gal I. Second, analysis of oligomers reveals that 100% and 13% of STcys and STtyr respectively are insoluble at pH 6.3 (late golgi pH), but at pH 8.0 both isoforms are soluble. The results suggest that conformational changes and disulfide bounds formation in the CD represent the basis for the increased ability for STcys to form oligomers and its stable localization in the golgi apparatus. The nature of the amino acid 123 influences the oligomers formations (Chen et al., 2000). Moreover, the oligomerization process and the catalytic activity depend also on the other seven conserved cystein residues (C24, 139, 181, 332, 359, 361 and 403). The Cys24, inside the TMD is required for dimerization, while the cystein residues present in the CD are required for trafficking and catalytic activity. Cys181 and Cys332 (in sialylmotif L and S, respectively) enzymes are retained in the endoplasmic reticulum and are minimally active or inactive respectively. Cys359 and 361 (between sialylmotifs S and VS) are inactive enzymes without compromising their localization and trafficking. Cys139 or Cys403 mutated enzymes produce no change of the catalytic activity and of the golgi localization. The replacement of these two cystein residues decreases cleavage and secretion suggesting that they are necessary for signal cleavage (Qian et al., 2001) ( FIG. 3 ). 
     Mutants of STtyr enzymes, sharing deletion in the SR have been characterized: STtyrΔ4 and STtyrΔ5 (deleted of amino acids 32 to 104 and 86 to 104, respectively) are not active and/or secreted, STtyrΔ1, Δ2 and Δ3 (deleted between amino acids 32 to 86) represent not cleavable forms and show an increasing cell surface expression (Kitazume et al., 1999). 
     A recent work (Fenteany &amp; Colley, 2005) show that multiple signals are indeed required for rST6Gal I oligomerization and for the golgi localization. Authors aimed at reevaluating the role of the CT, the TMD of the two enzyme isoforms (Tyr or Cys 123). They realised several DNA constructs to produce chimera as represented in  FIG. 4 . Lengthening the transmembrane domain (with more than 17 amino acids) does not enhance the golgi exit, it does not change trafficking or golgi localization. Concerning the role of the CT and the TMD in the STcys isoform, there is a signal composed of 3 Lysine residues in the CT that are recognized as export signal out of the ER. Authors show that altering both CT and TMD disturb the routing of the enzyme. The only difference between the isoforms of rST6Gal I is their capacity to form oligomers according to the pH. For STcys, the oligomers formation, according to the pH, is compromised only when CT and TMD are altered. For STtyr, it increases slightly the rate of golgi exit (Fenteany &amp; Colley, 2005). 
     All these recent findings concerning the rST6Gal I can be considered applicable to the human ST6Gal I since both enzymes share 85% homology and that both CDs are virtually identical. Indeed, the two sequences alignment ( FIG. 5 ) clearly show that the seven cysteins previously described are conserved in the hST6Gal I, and the Tyrosine 123 is also present. 
     Despite considerable efforts from scientists, no signal in the sequence of STs or FuTs could be demonstrated. It is believed that a multifactorial process may slow down the enzymes in their way to the surface to target them in appropriate compartments which are the Trans Golgi Network for ST6Gal. Accordingly, the inventors hypothesized that CT, TMD and SR may be alternatively exchanged to anchor a defined CD in host cells which lack the relevant gene and fail to express the relevant transferase activity. Thus, it is therefore possible to generate a wide array of combination as these 3 portions can be selected out of 55 animal known genes. Each of them can be potentially fused to 20 known distinct catalytic domains to display the ability of transferring sialic acid. This is of definite interest for the humanisation of yeast ( Pichia pastoris  or  Shyzosaccharomyces piombae ), plant and insects which do not express these activities and yet are used to produce recombinant drugs. Although not tested, this reasoning may also well apply to FuTs, GalTs, GalNAcTs and GlcNAcs as they govern the biosynthesis of glycotopes of high clinical relevance. 
     Importance of Several Residues 
     As previously described, a fourth motif (motif 3) has been identified in the ST family, between the S and the VS motifs. It is composed of four highly conserved amino acids with the following consensus sequence: (H/y)Y(Y/F/W/h)(E/D/q/g). Site directed mutagenesis on the hST3Gal I showed the functional importance of two amino acids in this motif: His299 and Tyr300. Results suggest the importance of aromatic residues, their possible involvement in acceptor recognition and their contribution for an optimal catalytic efficiency. Particularly the invariant Tyr300 plays a major conformational role. Mutational analysis showed that mutants H299A and Y300A display no catalytic activities, whereas mutant Y300F restore partially the activity (Jeanneau et al., 2004). This motif is also present in the catalytic domain of hST6Gal I. Moreover several cystein residues are of major importance in dimerization and catalytic activity as previously evoked (Qian et al., 2001). At least one disulfide bound has been evidenced between two conserved cystein residues inside the sialylmotifs L and S. This link is essential to maintain the protein conformation. The same observations revealed the existence of this binding in the PST (Angata et al., 2001), moreover, a second disulfide bound exists between sialylmotifs and the C-terminal region. Dimerization of the ST6Gal I, via disulfide bound, has been demonstrated. This enzyme inside the golgi apparatus is for around 20 to 30% in dimer form which has a lower activity because its affinity is reduced for the CMP-NeuAc (Jeanneau, 2003). 
     “Autoglycosylations” 
     GTs are often themselves glycosylated. Few data are available concerning the state of STs glycosylation and its importance on the biological function of the enzymes. It seems that the N-glycosylation on the ST6Gal I is not required for the biological activity of the enzyme in vivo. On the other hand, when tested in vitro on mutant, enzymatic activity is only observed for the the protein mutated on the first glycosylation site. These results suggest the existence of two N-glycans (Asn 146 and Asn 158) that can stabilise and/or prevent the protein against degradation. The mutation on the second glycosylation site may leading to aggregation or degradation of the protein (Chen &amp; Colley, 2000; Chen et al., 2000). However, the N-glycosylation on the ST8Sia I can affect its activity and its subcellular localization (Martina et al., 1998). The elimination of the N-glycans reduced its in vivo activity with less than 10% of the initial activity. It has been recently shown that ST8SiaII and ST8SiaIV possess, in addition to their classical activity, an autopolysialylation activity (Muhlenhoff et al., 2001). This autopolysialylation occurs on the N-glycans at the position Asn 74 in the PST and at the positions Asn 69 and 219 in the STX. Site directed mutagenesis on these sites inactive the enzyme both in vivo and in vitro. As a result, care has been taken to maintain the Asn 74 and Asn 69 glycosylation sites when using SR of ST8SiaIV and ST8SiaII respectively to construct chimerical genes. 
     II—Use of the Sequences of the Invention for Introducing New Transferase Activity in Expression Systems 
     IIa—The Role of Glycans, their Importance in the Immune System 
     Glycans are recognition signals in most living organisms but they are also structural key determinants for protein biopotency. 
     They play a pivotal role in protein folding oligomerization, quality control, sorting and transport (Helenius &amp; Aebi, 2001). Glycans represent oligosidic epitopes that can be considered as carbohydrate antigens. The nature of the glycans modifies the immunoreactivity. Several families of oligosidic antigens exist: ABH blood groups determinants, Lewis tissue groups, antigens members of the T family and antigens specific of the polysialylated or sulfated cellular adhesion (Legaigneur et al., 1999). Glycans have multiple roles: they are also involved in mechanisms of interactions between cells and between cells and matrix. Some cell proteins named Lectins specifically recognize glycanic structures and act as specific receptors (Gabius et al., 2004). In most cases, they are components of cell surface glycoconjugates. 
     The carbohydrate moieties act as recognition signals in the immune system and influence the immune recognition in at least two ways: i) the conformation of the protein is altered and it modulates their biological function; ii) the oligosaccharides serve as recognition determinants. Particularly, sialic acids contribute greatly to both of these effects because their highly electronegative and hydrophilic nature may influence the conformation of sialylated macromolecules, and this is particularly relevant for glycoproteins. Sialic acids possess the ability to act as biological masks to prevent or reduce the accessibility and play a significant role on the cell surface in the recognition process of self/non self discrimination (Pilatte et al., 1993, Glycobiology 2006 to be added). 
     Since N-glycans are well represented in serum glycoproteins and many other tissue proteins of human body, it is highly desirable that they should not be antigenic when present in recombinant glycosylated drugs. However since HuEPO produced in CHO cells has been developed as 1 st  approved drug, it appeared that several terminal sugars may be antigenic in humans: polyLacNAc, alpha1,3 Gal and N-Glycolylneuraminic acid. According to the most recent regulatory bodies, they should not be present any further in recombinant therapeutics. 
     IIb—The Clearance 
     Aside from their influence on the physico-chemical properties and the biological functions of proteins, glycans also possess a prevailing role on duration of glycoproteins in blood. This phenomenon is named metabolic clearance and represents the rate at which a natural compound or a drug is removed from the body by the liver or the kidney. It is defined as the plasmatic volume purified according to time (mL/min). This mechanism allows the organism to eliminate drugs. 
     Most of the circulating plasmatic proteins belong to the glycoproteins family sharing N-linked oligosaccharides and all of them are terminated in sialic acid, particularly in humans. Removal of sialic acid by a neuraminidase drastically decreases the plasmatic lifespan of the serum glycoproteins to minutes and promotes their uptake by the liver (van den Hamer et al., 1970; Morell et al., 1971). For example, when the pregnancy hormone (more recently recombinant HuEPO) is lacking sialic acid, its half-life is reduced to 2 minutes, whereas usually, its lifespan is around 48 hours. It is widely known at present that sialic acid is required to keep glycoproteins in blood because they prevent the circulating proteins from elimination by a sialoglycoprotein receptor (ASGPR or hepatic Lectin) composed of two subunits and included in the membranes of hepatocytes (Hudgin et al., 1974; Kawasaki &amp; Ashwell, 1976; Bianucci &amp; Chiellini, 2000). This receptor binds galactose or N-acetylglucosamine residues of the desialylated N-glycans (Meier et al., 2000). The recognized glycoproteins are then internalized in endocytic vesicles covered of clathrin and redirected into lysosomes where they are degraded (Ashwell &amp; Hardford, 1982). 
     This step appeared crucial for pharmacokinetic properties of therapeutic recombinant glycoproteins as lower organisms are not capable of sialylating proteins and in CHO cells, the addition of sialic acid is extremely sensitive to the energetic status of the cells in culture (NeuAc is synthesized by condensation of pyruvate and lactic acid) and is barely complete. All the sialylated blockbusters (EPO, IFN, GM-CSF, FSH, Ab . . . ) should be purified for the manufacturer to present a high sialic acid content and be controlled for batch-to-batch consistency. 
     IIc—Control of Glycosylation &amp; Allergenicity 
     Sialic acid residues at the terminal position of N-glycans are of major importance for therapeutic proteins as the sialylation of the proteins confers important properties to glycoproteins. The machinery required for the synthesis, the activation and the introduction of sialylated residues is poorly represented in the different recombinant proteins expression systems. Then, the recombinant human proteins produced are most often under or even not-sialylated compared to their native counterparts. Moreover, if they are expressed in mammalian cells, the sialylation may occur through N-glycolylneuraminic acid (NeuGc) which significantly differs from the N-Acetylated derivative (NeuAc) as it is present in mammalian cells but not in human cells. 
     Because there is an important issue with sialic acid in the market of recombinant drugs, many research teams are working on the modification of the N-glycosylation process in the different existing expression systems. Intensive work is aiming at humanizing the glycosylation pattern of the recombinant proteins to approach the pattern found in the natural glycoproteins as closely as possible and improve pharmacokinetics as well as safety of the product. Meanwhile, the procedure will tend to reduce the presence of immunogenic determinants. In this respect, a very recent study confirmed that 6-linked sialic acid and 3-linked sialic acid prevent glycoproteins to bind receptors of the immune system and generate activation (Glycobiology 2006). Adding 6-linked sialic acid to recombinant glycosylated proteins would therefore largely benefit to the safety of the drug and be major advance in the field. 
     Proteins of therapeutic interest were first extracted from natural sources such as blood, placenta, human or animal tissues. However, this approach is limited by the quantity of human tissues available and may bring in important risks of contamination (viruses, prions, oncogenes) and/or generate allergic reactions due to traces of animal proteins or toxins. With the rise of molecular biology, new approaches have been developed to produce proteins using quite different expression systems. A large range of heterologous expression systems are available (Andersen &amp; Krummen, 2002) and each of them possess advantages and disadvantages with a particular attention given to the N-glycosylation pattern of the recombinant proteins produced in these systems. 
     IId—Biosynthesis of N-Glycans in Known Expression Systems: Needs for Humanization of the Glycosylation Pathway 
     Bacteria 
     One of the most used system is the bacteria  Escherichia coli  ( E. coli ) (Swartz, 2001; Baneyx, 1999), but its main inconvenient is that the human post-traductionnal modifications, particularly glycosylations, are not realised by this prokaryote because no such glycosyltranferases are expressed in  E. coli . This can lead to misfolding and subsequent reject of the therapeutic proteins of interest by the immune system, reduction in their lifespan and biological activity. 
     No N-glycosylation of the human type has been found so far in this microorganism. 
     Yeasts and Filamentous Fungi 
     Yeasts and filamentous fungi are also well-established eukaryote expression systems and they possess a cellular machinery approaching those of human cells. Yeast produces complex proteins and realizes several post-traductional modifications including glycosylations. Both yeasts and mushrooms typically produce mannose-rich glycans by adding until 100 mannose residues (concerning yeast) on top of the pentasaccharidic core (Tanner &amp; Lehele, 1987; Herscovics &amp; Orlean, 1993). Those hypermannosylation definitely foster immune response in human. Moreover, until now, no complex oligosaccharide containing sialic acid, galactose, fucose and N-acetylgalactosamine has been found inside the glycoproteins produced by these organisms (Blanchard, 2004). 
     The N-glycosylation process realized by yeasts and mushrooms is similar to the mammalian process with respect to the initial steps in the ER but the presence of polymannans added in the golgi apparatus prevent them to get approval from regulatory bodies. 
     Insect Cells 
     Proteins expressed in insect cells are properly folded, secreted and may receive post-traductional modifications. The post-traductional modifications carry out by these cells are similar to those realised by mammalian cells, in particular concerning the N-glycosylation of the protein. The glycan structures obtained in this case, are however incomplete and designated as paucimannose due to the presence of an undesirable N-acetylglucosaminidase activity which degrades the neoglycoproteins expressed during Baculovirus expression (Blanchard, 2004). 
     The lack of neuraminic acid in insects is still actively debated (Marchal et al., 2001, Lerouge et al 2005). In some insect cells lines, α1,3 linked fucose residues are found and this may trigger off an immune responds in human. At present, the use of this system is therefore restricted to produce vaccinal antigens. 
     Transgenic Plants 
     As other eukaryotic cells, plants exhibit a complex and sophisticated cellular machinery which may be used to produce therapeutic proteins. Recombinant proteins possess a very good pharmacological quality because plants express the enzymes required for the maturation of the proteins. 
     However, the glycosylation process needs several adjustments not to produced allergenic proteins. Indeed, N-glycosylations in plants (Lerouge et al., 2000) are similar to those realized in humans as far as core glycosylation is concerned. The glycosylations are still lacking sialylated antennae and there is an addition of β1,2-xylose and α1,3-fucose. Both residues are highly immunogenic for human and currently considerably compromise the approval of transgenic plants as expression systems for therapeutics. 
     Mammalian Cells 
     Mammalian cells expression system, namely CHO cells, is currently the only drug-approved system to produce recombinant therapeutics. Such cells show a major advantage because they are able to synthesis complex proteins, achieve complex type N-glycosylation of high molecular mass. Mammalian cells naturally use, not only the enzymes involved in the synthesis and the transport of the nucleotides-sugars, but also the glycosyltransferases required to guarantee a complex glycosylation of heterologous recombinant proteins with a high level of α2,3-sialylation. However, there is lack of other enzymes such as the α1,3/4-fucosyltransferases and α2,6-sialytransferases, that transfer glycosidic motifs specific of the N-glycans of human tissues. In rodents, N-glycolylneuraminic acid is substantially preferred in N-glycans, O-acetylation of N-Acetyl neuraminic acid at position 4, 7, 8 and most often 9 also occurs frequently and each of these derivatives may be potentially immunogenic in human. This represents a limitation for the use of mouse cells or lactating mice/rabbits to express recombinant proteins (Blanchard, 2004). 
     IIe—Glycoengineering 
     As discussed above, terminal sialylation of approved glycoproteins is the most difficult step to obtain in all the expression systems available so far. As an example, no α2,6-liked sialic acid could be added while it is the prevalent linkage in human blood. In all organisms, the enzymatic machinery required is simply missing ( FIG. 6 ). 
     The ultimate goal of research in this field is improving the glycosylation process, the yield of production and the quality of the recombinant proteins. The use of expression systems genetically modified for glycosylation would allow to produce glycoproteins with an exceptional homogeneity in their glycanic structures. Such systems could be then used to develop a high level production for proteins of biomedical interest of well-defined structures. The glycosylation engineering in yeast (Hamilton et al., 2003; Roy et al., 2000) has first prevented the addition of polymannosidic chains. Then, enzymes which are necessary to galactosylation and N-acetylglucosaminylation have been added in the systems (Maras et al., 1999; Bretthauer, 2003; Vervecken et al., 2004). However, the addition of the terminal sialic acid is still difficult to realise, due to the number and location enzymes involved in this process but is currently being done in Japan ( FIG. 6 ). 
     In insect cells, the GTs catalyzing the transfer of immunogenic sugars have been removed and the sialylation has been realised by adding 3 genes encoding for the N-acetylglucosamine 2-epimerase, N-acetylneuraminyl lyase and CMP-Neu5Ac synthase (Jarvis et al., 1998; Aumiller et al., 2003). Authors have created new insect cell lines (SfSWT-3) designed to synthesize their own CMP-sialic acid. The resulting cells express all the 7 mammalian genes necessary to form an N-glycan, produce CMP-sialic acid and sialylated a recombinant protein when cultured in a serum-free growth medium (Aumiller et al., 2003). This work has been patented. 
     In plants, the first strategy used aimed to prevent the addition of allergenic sugars by stocking proteins in the ER. But in this case, the glycans can not be of the complex type. An other strategy was based on the inhibition of several GTs inside the golgi apparatus. This inhibition can be completed and/or can enter in competition with the endogeneous machinery for maturation. The addition of the sialyl machinery is also in process. 
     In the case of mammalian cells, work has been realized through the over-expression of an α2,3-ST and a β1,4Gal (Weikert et al., 1999), these enzymes are both present in the genome but their activities vary upon culture conditions. This leads to a wide variability concerning the presence of the terminal Gal and sialic acid and also to extensive microheterogeneity in glycan structures of secretory proteins. 
     Researches have been mostly oriented in the optimization of the galactosylation and sialylation (Granbenhorst et al., 1999) by introducing an α2,6-ST (Bragonzi et al., 2000). A CHO cell line stably expressing full length α2,6-ST has been established long ago and has also been disappointing. The ratio observed between α2,6 and α2,3-linked terminal sialic acid residues carried was of 40.4% of α2,6- and 59.6% α2,3-sialic acid residues improving pharmacokinetics in clearance studies (Bragonzi et al., 2000). Despite improvement in humanization of cells, the ratio between α2,6 and α2,3-linked terminal sialic acid residues cannot be controlled and is not even favorable to the 6-activity. 
     In summary, the sialylation is a critical step to control glycan structures in proteins produced by genetic engineering. The main difficulty resides not so much in expressing the lacking activity but in getting the heterologous production system to work successfully. Indeed, 3 main objectives should be met to humanize glycosylation of current expression systems: 1) getting the donor substrate and the relevant transporter needed for this enzyme to work within the Golgi. 2) getting the enzyme properly compartmentalized to eventually compete with endogeneous sialyltransferase activity. 3) getting the enzyme able to catalyze sialic transfer to any relevant acceptor substrate. 
     IIf—Background of the Invention: 
     The strategy of the inventors is based on the consideration that the full length ST6 enzyme has been so far unable to meet the above objectives, especially the competition with the endogenous ST3 activity existing in most mammalian cells, probably because the gene product is not inserted in intracellular compartments involved with the secretory pathway. The inventors thus developed a procedure which may afford the best opportunity for a transferase CD to reach the golgi compartments where the neoglycoproteins are sorted into secretory vesicles and/or where they meet the engineered ST before/instead of the existing ST3 
     III—Design of Synthetic Membrane STs 
     The present invention consists in a procedure which delivers a panel of chimeric STs of known catalytic activity, possessing a membrane anchor to target them to the Golgi apparatus of eukaryotic cells. None of them exist in living cells and are considered as “synthetic STs” because their construction does not need any of the DNA sequence coding for CT, TMD or SR. The relevant oligonucleotides are within 200 pb (60 amino acids) in length and could be designed by informatics and obtain commercially. Using PCR methods, a tagged synthetic anchor is constructed to code for the N-terminal half of the ST protein and fused to the optimized catalytic domain of the hST6GalI. 
     The invention describes various molecular methods to construct 3 types of membrane chimeric STs: 
     homologous construct: ST anchor fused to the CD Δ89 of ST6Gal. Synthetic or eventually copied by PCR. In this case, CT+TMD+SR portions are from the same ST: ST3Gal/ST6GalNAc/ST8Sia. The example describes a 200 pb sequence 
     hybrid construct: CT+TMD are from a ST and the SR from another ST. Here too, the minimal size is 200 pb. 
     Heterologous construct: CT, TMD and SR fragments are from different STs. Here too, the minimal size is 200 pb. 
     long construct: a CT+TMD+SR1 construct of 200 pb is prepared and fused to a second SR2 construct of at most 200 pb in such a way that SR2 is downstream from SR1. The duplicated constructed can be further ligated at the 3′ end to any other SR construct as as repeatedly as needed. 
     The size of 200 pb has been based on the observation that the shortest N-terminal portion of all human STs is ST6GalNAcIII with CT+TMD+SR=200 pb. This enzyme has virtually no stem. 
     At present, the inventors have validated a method to construct synthetic fragment of DNA and fuse them to a minimal catalytic domain to generate a novel transferase enzyme of desired activity. Several representative constructs are available for further expression. The synthetic chimeras are active and are being studied in CHO cells using transient expression and confocal microscopy. 
     Examples 
     Example 1 
     Producing the Minimal Catalytic Domain (CD) of hST6Gal I (SEQ ID NO: 43 Coding for SEQ ID NO: 44) 
     The human ST6Gal I (hST6Gal I) cloned by Legaigneur et al., 2001 was used to amplify the minimal CD cloned into the pFLAG-CMV-vector (Sigma) (Donadio et al., 2003) that was used in all the chimera constructs. 
     The recombinant vector was first digested to verify the presence of the CD of hST6Gal I. The running agarose gel reveals the presence of a nucleotidic band at around 1000 pb, corresponding to the expected size for the minimal catalytic domain ( FIG. 10A-C ). 
     The CD corresponding to the amino acids 90 to 406 of hST6Gal I was obtained by PCR using the following primers: 
     
       
         
           
               
               
            
               
                 5′BamHI-Δ4: 5′-GAGCCCGGATCCGAG 
                 (SEQ ID NO: 204) 
               
               
                 GCCTCCTTC-3′ and 
                   
               
               
                   
               
               
                 3′Δ4-XbaI: 5′TAACCCTCTAGATTAGC 
                 (SEQ ID NO: 205) 
               
               
                 AGTGAATGGTCCGGAAGC-3′. 
                   
               
            
           
         
       
     
     These primers contain BamH1 (5′BamHI-Δ4) and XbaI (3′Δ4-XbaI) restriction sites and the natural stop codon of the sequence. The BamH1 restriction site was used to ligate the CD of hST6Gal 90-406 to the synthetic N-terminal part of other STs to form chimeric enzymes. The XbaI restriction site at the 3′ end was used to introduce the chimera inside the pcDNA3.1 vector (Invitrogen) ( FIG. 11 ). 
     PCR was performed on an I-Cycler apparatus (Biorad), using 2.5 units of ProofStart DNA polymerase (Qiagen), 300 μM of each dNTP (Sigma Aldrich), 1 μM each primer, 1× of ProofStart manufacturer buffer (Qiagen), 1× of manufacturer Q-Solution (Qiagen) and 1.5 mM of Mg 2+  in a final volume of 50 μl with 200 ng of pFlag-CMV-hST6Gal I-90-406. The reaction was performed as follows: 95° C. for 5 min, followed by 40 cycles of 30 s at 94° C., 30 s at 55° C. and 1 min at 72° C. The amplified PCR product was analyzed by electrophoresis on a 1.5% agarose gel (amplification of one PCR product at the expected size of 1000 pb;  FIG. 17B ) and purified using the Gel Extraction kit Qiagen according manufacturers&#39; instructions. 
     Several PCR amplification products were pooled and concentrated to estimate the amount of DNA in an agarose gel ( FIG. 10C ). Purified PCR products were kept at −20° C. until used. 
     Example 2 
     Assembling Synthetic N-Terminal Domains 
     To create a wide array of STs as sorted by bioinformatic analysis, CT, TMD and SR from distinct STs were synthesized and assembled. 
     All the selected N-terminal parts of the chimera contain the 3 typical regions of the ST family: the CT, the TMD and the SR. Of note, the SR may be of variable length and could originate from synthetic gene duplication. All of these fragments i.e. CT+TMD+SR have been entirely reconstituted using complementary hybridization and phosphorylation steps as described below. 
     a—Construction of the Non-Catalytic Domain of hST3Gal III (SEQ ID NO: 151 Encoding SEQ ID NO: 152) 
     Six sense oligonucleotides and five antisense oligonucleotides corresponding to part of the N-terminal region (1 to 138 pb) sequence of hST3Gal III were designed and synthesized (Eurogentec). This example shows the method of synthesizing a naturally occurring sequence of interest. A FLAG epitope (in Bold and underlined) was added between the initiating codon ATG and the second codon (GGA) of the hST3Gal III sequence: 
     
       
         
           
               
               
            
               
                 ATG    GACTACAAAGACGATGACGACAAG    GGA 
                 (SEQ ID NO: 206). 
               
            
           
         
       
     
     One microgram of each internal nucleotide was phosphorylated using 1 unit of polynucleotide kinase (Eurogentec) in a kinase buffer containing 2 mM of ATP (Sigma). Reaction was performed for 60 min at 37° C., and the incubation finally inactivated for 10 min at 65° C. All the oligonucleotides were separately denatured for 10 min at 80° C. and then mixed for matching. Matching was performed overnight with a decreasing temperature gradient from 80° C. to 20° C. The resulting fragment was then subjected to PCR. 
     b—Construction of the Non-Catalytic Domain of hST6GalNAc I-74 (SEQ ID NO: 145 Encoding SEQ ID NO: 146) 
     One of the shortest stem within the ST family is the stem of the hST6GalNAc III, which is composed of around 6 amino acids. A possible N-terminal domain of this ST is of 74 amino acids (CT, TMD and SR) which correspond to 222 pb. Therefore to delineate a construct with the shortest SR, a synthetic N-terminal domain of 222 pb corresponding to the 74 first amino acids of the hST6GalNAc I was reconstituted. 
     Nine sense oligonucleotides and eight antisense oligonucleotides corresponding to the complete N-terminal region (1 to 222 pb) sequence of hST6GalNAcI were designed and synthesized (Eurogentec). A FLAG sequence (in Bold and underined) was added between the initiating codon ATG and the second codon (GGA) of the hST6GalNAc I sequence: 
     
       
         
           
               
               
            
               
                 ATG    GACTACAAAGACGATGACGACAAG    GGA 
                 (SEQ ID NO: 206). 
               
            
           
         
       
     
     The phosphorylation and the matching reactions were performed as previously described. 
     c—Synthetic Oligonucleotide Duplication to Assemble the Non Catalytic Domain hST6GalNAc I1-140 (SEQ ID NO: 148) 
     hST6GalNAc I was selected as the sialyltransferase exhibiting the longest SR (246 aas). In this example, the length of the synthetically reconstituted CT+TMD+SR portion (1- to 35 amino acids residues) was joint to another SR stretch (36-140 amino acids residues) to duplicate the length and be able to add a membrane anchor of 140 amino acids residues as reported in Donadio et al., 2003. 
     d—Construction of a Hybrid Synthetic Domain: hST3Gal III 1-28-hST6GalNAc I 37-74 (SEQ ID NO: 160) 
     An hybrid N-terminal region containing first the CT and the TMD of the hST3Gal III (1 to 28 amino acids residues) and second the begin of the SR of hST6GalNAc I (amino acids 37 to 74) was constructed. 
     The total length of this synthetic hybrid is of 66 amino acids corresponding to 198 nucleotides (SEQ ID NO: 159). Nine sense oligonucleotides and eight antisense oligonucleotides corresponding to the hybride N-terminal region described above (1 to 225 pb), using the sequences of hST3Gal III/hST6GalNAc I, were designed and synthesized (Eurogentec). A FLAG epitope (in Bold) was added between the initiating codon ATG and the second codon (GGA) of the hST3Gal III sequence: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 206) 
               
               
                   
                 5′-ATG    GACTACAAAGACGATGACGACAAG    GGA-3′. 
               
            
           
         
       
     
     The phosphorylation and the matching reaction were performed as previously described. 
     e—PCR Amplification of the Synthetic Constructs 
     After the synthetic reconstitution step, the product was amplified by PCR technique using specific primers that bring in the desired restriction sites at each end of the reconstituted DNA fragment in order to ligate them first with the selected CD and then into the vector pcDNA3.1. 
     The following primers were designated: 
     
       
         
           
               
               
            
               
                 5′AflII-ST3: 5′-GAGCCCCTTAAGAT 
                 (SEQ ID NO: 207) 
               
               
                 GGACTACAAAGACGACGATGACG-3′ 
                   
               
               
                   
               
               
                 and 
                   
               
               
                   
               
               
                 3′ST3-BamHI: 5′-TAAGGGGGATCCGC 
                 (SEQ ID NO: 208) 
               
               
                 TAGAGTGACTATACTTACTGGA-3; 
                   
               
               
                   
               
               
                 5′AflII-ST6: 5′-GAGCCCCTTAAGAT 
                 (SEQ ID NO: 209) 
               
               
                 GGACTACAAAGACGACGATGACG-3′ 
                   
               
               
                   
               
               
                 and 
                   
               
               
                   
               
               
                 3′ST6-BamHI: 5′-TAAGGGGGATCCGG 
                 (SEQ ID NO: 210) 
               
               
                 TTGTGGAGGAACGGGA-3′; 
                   
               
               
                   
               
               
                 3′ST6-BamHIn°2: 5′-TAAGGGGGATC 
                 (SEQ ID NO: 211) 
               
               
                 CTCTGGGTGACAGTGTGTTCAC-3′. 
                   
               
            
           
         
       
     
     These primers contain AflII (5′AflII-ST3 and 5′AflII-ST6) and BamHI (3′ ST3-BamHI, 3′ ST6-BamHI and 3′ ST6-BamHI no. 2) restriction sites to ensure the further ligations. 
     PCRs were carried out with the primer pairs set described in Table 5. They were performed on a PCR apparatus (I-Cycler, Biorad) with 5 μL of the each synthetic fragments in a solution containing 2.5 units of ProofStart DNA polymerase (Qiagen), 300 μM of each dNTP (Sigma Aldrich), 1 μM each primer, 1× of ProofStart manufacturer buffer (Qiagen), 1× of manufacturer Q-Solution (Qiagen) and 1.5 mM of Mg 2+  in a final volume of 50 μL. a 
     The reaction was performed as follows: 95° C. for 5 min, followed by 40 cycles of 30 s at 94° C., 30 s at 55° C., 53° C., 53° C. and 55° C. respectively for hST3Gal III (SEQ ID NO: 151), hST6GalNAc I-74 (SEQ ID NO: 145), hST6GalNAc I-140 (SEQ ID NO: 147) and hST3Gal III-28/37-hST6GalNAc I-74 (SEQ ID NO: 159), and 1 min at 72° C. The amplified PCR product was analyzed by electrophoresis on a 2% agarose gel, according to the PCR product size (Table 5): 
     hST3 Gal III: only one PCR product was amplified at an estimated size of 170 pb, which corresponds to the expected size of 174 pb ( FIG. 11 ). 
     hST6 Gal NAc I:  FIG. 13  shows one amplified PCR product with an estimated size of 270 pb, which corresponds to the expected size of 270 pb. 
     hST6GalNAc I-140: a 468 pb nucleotidic sequence was amplified ( FIG. 15 ). 
     hST3Gal III-28-hST6GalNAc I37-74: a 249 pb nucleotide sequence ( FIG. 17 ) was amplified. 
     The PCR products were purified using the Gel Extraction kit Qiagen according manufacturers&#39; instructions. 
     The PCR product of hST6GalNAc I-140 was submitted to direct both strand DNA sequencing by Genome Express (Meylan, France). 
     The PCR product of hST3Gal III and hST6GalNAc I-74 were ligated to the CD (SEQ ID NO: 43) before being introduced in the pcDNA3.1 vector (Invitrogen) to be also further sequenced by Genome Express (Meylan, France). 
     The hST3Gal III-28-hST6GalNAc I37-74 synthetic fragment was ligated with the CD (SEQ ID NO: 43) and directly sequenced in both strand prior its introduction into the pcDNA3.1 vector (Invitrogen). 
     f—Quantification of the PCR Products 
     All the PCR products were pooled and concentrated according to each synthetic construct. Two volumes ethanol and 0.1 volume of 3 M sodium acetate, pH 5.2 were added to PCR product samples. The mix was incubated for 30 min at −20° C. and centrifuged 20 min at 10000 g at 4° C. The pellets were washed with 100 μL of 70% ethanol and a centrifugation was run for 10 min at 10000 g at 4° C. Then supernatants were removed and the pellets dried and resuspended in 50 μL of purified water. Samples were conserved at −20° C. until use. Five microliters of the four concentrated synthetic fragments were loaded on a 2% agarose gel to estimate their quantity ( FIGS. 11 ,  12 ,  13  and  14 ). 
     Example 3 
     Assembling Non-Catalytic N Terminal Membrane Domains with a Sialyltransferase Activity 
     a—Digestions and Purification 
     All the PCR products (either the synthetic N-terminal constructs or the catalytic domain) were digested by the BamHI restriction enzyme. 
     200 nanograms of the synthetic N-terminal constructs were digested in a solution containing 3 units of the BamHI restriction enzyme, 0.1 μg.μL −1  of Bovin Serum Albumin and 1× of the appropriate manufacturer&#39;s buffer E (Promega) in a final volume of 20 μL. 
     500 nanograms of the CD were digested with 5 units of BamHI, 0.1 μg.μL −1  of Bovin Serum Albumin and 1× of buffer E (Promega) in a final volume of 20 μL. 
     Digestions were performed for 90 min at 37° C. and the incubation was finally inactivated for 15 min at 65° C. Digested DNA was purified using the PCR purification kit (Qiagen). 
     b—Ligation 
     Each of the N-terminal fragments were ligated to the CD fragment in a solution containing 1.5 units of T4 DNA ligase, 1× of the ligation buffer, both commercialized (Promega), 62.5 ng, 78.2 ng and 75 ng, respectively for hST3Gal III, hST6GalNAc I-74, and the hybrid hST3Gal III-28-hST6GalNAc I37-74 fragment and 100 ng of CD in a final volume of 20 μL. The mix was incubated at 15° C. overnight and followed by an inactivation step for 10 min at 70° C. ( FIG. 17 ). 
     c—Amplification of the Tagged Synthetic Insert 
     To verify the proper ligation, the ligation products were directly submitted to PCR using the following primers pairs: 5′AflII-ST3/3′Δ4-XbaI, 5′AflII-ST6/3′Δ4-XbaI and 5′AflII-ST3/3′Δ4-XbaI respectively for the ST3/CD, the ST6GalNAc-74/CD and the hybrid/CD. 
     Reactions were carried out using 2.5 units of ProofStart DNA polymerase (Qiagen), 300 μM of each dNTP (Sigma Aldrich), 1 μM each primer, 1× of ProofStart manufacturer buffer (Qiagen), 1× of manufacturer Q-Solution (Qiagen) and 3 mM of Mg 2+  in a final volume of 50 μL. The reaction was performed as follows: 95° C. for 5 min, followed by 40 cycles of 1 min at 94° C., 1 min at 57° C., and 1 min 30 s at 72° C. 
     The amplified PCR products were analyzed by electrophoresis on a 1.5% agarose gel:
     hST3Gal III/CD synthetic insert (SEQ ID NO: 167): a 1200 pb PCR product was amplified ( FIG. 12 ), which corresponds to the expected size of the ligated DNA fragment: ST3Gal III plus CD.   hST6GalNAc I-74/CD insert (SEQ ID NO: 161): a 1200 pb PCR product was amplified ( FIG. 14 ), which corresponds to the expected size of the ligated DNA fragment: ST6GalNAc I plus CD that is of 1225 pb exactly.   hST3GalIII-28/37hST6GalNAc-74/CD insert (SEQ ID NO: 175): a 1200 pb PCR product was amplified ( FIG. 17 ) which corresponds to the expected size of the ligated DNA fragment hST3GalIII-29/hST6GalNAc37-74 plus CD.   

     The amplified PCR products were purified using the Gel Extraction kit Qiagen according manufacturers&#39; instructions. 
     Amplified PCR products of the same synthetic ST were pooled, concentrated and quantified on a 1.5% agarose gel ( FIGS. 12 ,  13  and  16 ), as previously described above, to obtained enough quantity of DNA to ligate the chimera into the pcDNA3.1 vector. 
     d—Cloning the Synthetic ST Insert into an Expression Vector 
     1—Digestion and Purification 
     Synthetic inserts such as the hST3Gal III/CD or hST6GalNAc I-74/CD insert were ligated into the pcDNA3.1 vector into AflII and XbaI restriction sites. 
     The synthetic insert and the pcDNA3.1 vector ( FIG. 12 ) (Invitrogen) were first digested by the restriction enzyme XbaI: 
     for each construction, 500 ng of vector were digested using 2.5 units of XbaI (Promega), 0.1 μg.μL −1  of BSA and 1× of the appropriate buffer D, supplied by manufacturer (Promega), in a final volume of 20 μL, following the manufacturer&#39;s instructions; 
     the insert was digested with 2 to 2.5 units of XbaI (Promega), depending on the quantity of insert available after PCR (e.g. 200 ng hST3Gal III/CD, hST3Gal III-28/hST6GalNAc I 37-74/CD and 500 ng of hST6GalNAc I-74/CD were respectively digested with 2 and 2.5 units of XbaI), 0.1 μg.∥L −1  of BSA and 1× of Buffer D (Promega), in a final volume of 20 μL. 
     The digestions were performed at 37° C. during 60 min and inactivated at 65° C. for 15 min. 
     The digested products were then submitted to a digestion by the restriction enzyme AflII following the manufacturer&#39;s instructions (Biolabs): using the appropriate number enzyme units, depending on the DNA quantity (2, 2, 2.5 and 2.5 units, respectively for hST3Gal III/CD, hST3Gal III-28/hST6GalNAc I 37-74/CD, hST6GalNAc I-74/CD and pcDNA3.1), 0.1 μg.μL −1  of BSA and 1× of buffer 2 (Biolabs) in a final volume of 50 μL. The digestions were carried out at 37° C. for 60 min and inactivated at 65° C. for 20 min. The digestions were purified with the PCR purification kit (Qiagen). 
     2—Ligation 
     The ligation conditions were calculated according to the following formula: 
               X   ⁢           ⁢   ng   ⁢           ⁢   insert     =           (     Y   ⁢           ⁢   pb   ⁢           ⁢   insert     )     ⁢           ⁢     (     x   ⁢           ⁢   ng   ⁢           ⁢   vecteur   ⁢           ⁢   pcDNA   ⁢           ⁢   3.1     )           size   ⁢           ⁢   in   ⁢           ⁢   pb   ⁢           ⁢   vecteur   ⁢           ⁢   pcDNA   ⁢           ⁢   3.1     ≈   5428       ×           ⁢   molar   ⁢           ⁢   ratio   ⁢           ⁢     Insert   Vecteur             
where the size of the insert is around 1200 pb for the synthetic construct (e.g. hST3Gal III/CD, hST3Gal III-28/hST6GalNAc I 37-74/CD or hST6GalNAc I-74/CD), the size of the vector pcDNA3.1 is of 5428 pb, the quantity of vector used is of 50 ng or 100 ng, and finally the molar ratio of Insert/Vector is of 3/1.
 
     Construction of pcDNA3.1/ST3/CD recombinant vector was performed using 33 ng of the digested chimera hST3Gal III/CD (SEQ ID NO: 167), 50 ng of pcDNA3.1 digested vector, 3 units of T4 DNA ligase enzyme (Promega) and 1× of the ligation buffer in a final volume of 15 μL. The mix was incubated overnight at 4° C. and the reaction was inactivated for 10 min at 70° C. 
     The second construction of pcDNA3.1/ST6/CD recombinant vector was performed using 66.5 ng of the digested chimera hST6GalNAc I-74/CD (SEQ ID NO: 161), 100 ng of pcDNA3.1 digested vector, 3 units of T4 DNA ligase enzyme (Promega) and 1× of the ligation buffer in a final volume of 15 μL. The mix was incubated overnight at 15° C. and the reaction was inactivated for 10 min at 70° C. 
     The third construction of pCDNA3.1/HYB/CD recombinant vector was performed using 66.5 ng of the digested chimera hST3Gal III-28/hST6GalNAc I 37-74/CD (SEQ ID NO: 175), 100 ng of pCDNA3.1 vector, 3 units of T4DNA ligase enzyme (Promega) and 1× of ligation buffer in a final volume of 15 μL. The mix was incubated overnight at 15° C. and the reaction was inactivated for 10 min at 70° C. 
     3—Cloning 
     Competent Cells Chemical Transformation 
     One Shot® TOP10 chemically competent  E. coli  (Invitrogen) were transformed with the recombinant vector following the manufacturer&#39;s instructions. Two microliters of each ligation reaction were added to 25 μL of chemical competent cells and mixed by taping gently. The vials were incubated on ice for 30 min. The chemical transformations were performed for exactly 40 s in a 42° C. water bath. Vials were removed from the 42° C. bath and place on ice during 2 min. Two hundred and fifty microliters of pre-warmed S.O.C. medium, provided by Invitrogen, were added into each vial under sterile conditions. Then, the mixtures were shaked at 37° C. for exactly 1 h at 225 rpm in a shaking incubator. Each transformations were spread on LB agar plates (10 g of bacto-tryptone, 5 g of bacto-yeast extract, 10 g of NaCl and 15 g of agar per liter of solution from GibcoBRL) containing 50 μg/mL of ampicillin under sterile conditions. Plates were inverted, incubated at 37° C. overnight and maintained at 4° C. the next day until the transformants selection. 
     Amplification 
     At the end of the day, single colonies were isolated and inoculated into 3 mL of LB (10 g of bacto-tryptone, 5 g of bacto-yeast extract, 10 g of NaCl from Difco) containing 50 μg/mL of ampicillin. The growth was ensured at 37° C. in a shaking incubator overnight. The next day, glycerol stocks of cultures were prepared by mixing 0.85 mL of cultures with 0.15 mL of sterile glycerol and transferred to a cryovial. Glycerol stocks were stored at −80° C. until sequence verification and further use. 
     Minipreparation of Plasmids 
     The recombinant vectors were purified by Minipreparation procedure (Maniatis), 1.5 mL of cultures were isolated and centrifuged for 5 min at 10000 g to pellet the bacteria. Cold Solution I (5 mM glucose, 25 mM Tris-HCl pH 8.0, 10 mM EDTA pH 8.0) was added (100 μL) to resuspend the pellet, 200 μL of Solution II (0.2 N NaOH and 1% SDS) and 150 μL of Solution III (3 M potassium and 5 M acetate) were also added. The mixtures were gently mixed by vortexing, placed on ice 5 min, and centrifuged at 4° C., 5 min at 15000 g. The supernatants were taken and 1 volume of isopropanol was added. The minipreparations were incubated 5 min at room temperature and centrifuged at 4° C., 5 min at 15000 g. Then, the supernatants were removed. The DNA pellet was dried and resuspended into 50 μL of water. The quantity of DNA was measured using a spectrophotometer DO apparatus (Biophotometer, Eppendorf). At least 30 minipreparations are performed to screen the presence of the interest insert for each construction. 
     One microgram of each of the 30 minipreparations was double digested using AflII and XbaI restriction enzyme in order to verify the presence of the insert. First, the digestion solution contained 10 units of XbaI (Promega), 0.1 μg.μL −1  of BSA and 1× of the appropriate manufacturer buffer D (Promega) in a final solution of 20 μL. Digestions were performed in a 37° C. water bath for 1 h and inactivated at 65° C. for 15 min. Samples were secondly digested using 10 units of AflII restriction enzyme (Biolabs), 0.1× of BSA and 1× of Buffer 2 (Biolabs) in a final volume of 50 μL. Reactions were carried out in a 37° C. water bath for 1 h and inactivated at 65° C. for 20 min. The digested samples were all purified using the PCR purification kit (Qiagen). The digestion products were loaded on a 1.5% agarose gel to detect the presence of the chimeric insert. The electrophoresis was run at 100 V during around 35 min. 
     In the case of the pcDNA3.1/ST3/CD and pcDNA3.1/ST6/CD, an insert of 1200 pb was detected ( FIG. 19C  or  23 C). The size of this insert corresponds exactly to the expected size of the reconstituted chimera ST3Gal III/CD or ST6GalNAc I-74/CD. 
     The positive clones were fully sequenced to assess the expected inserted DNA sequence. 
     Sequencing 
     The cloning steps were verified by both strands DNA sequencing by Genome Express (Meylan, France) using universal primers present inside the vector: T7 Promoter and BGH Reverse. 
     The final expected sequences of the three chimeras ST3Gal III/CD (SEQ ID NO: 167), hST3Gal III-28/hST6GalNAc I 37-74/CD (SEQ ID NO: 175), and ST6GalNAc I-74/CD (SEQ ID NO: 161). The nucleotidic sequence obtained was aligned to the expected sequence and the alignment revealed 100% identity between these three sequences. The deduced amino acid sequence was also aligned with the expected sequence of the chimera (http://www.infobiogen.fr/services/analyseq/cgi-bin/alignp_in.pl) and it shows 100% identity between two of theses amino acid sequences. 
     The nucleotide sequence obtained for hST6GalNAc I-140 showed 84% identity with the theoretical sequence. The missing parts of the sequence are the 36 first and the 39 last nucleotides, they both correspond to the primers designed to sequence the DNA fragment in both strands. Moreover the amino acids alignment between the expected and the resulting sequences showed 76.9% identity. The 12 first and the 24 last amino acids are not found due to their correspondences with the specific primers used for the sequencing. 
     Minipreparation 
     After the DNA insert sequence verification, the recombinant plasmids were amplified in 100 mL cultures to obtain high quantity of each construction (pcDNA3.1/ST3/CD and pcDNA3.1/ST6/CD and pcDNA3.1/HYB/CD). A single colony of each plasmid from a freshly streaked selective plate was picked and inoculated into a starter 3 mL culture of LB medium containing the selective antibiotic (ampicillin 50 μg.μL −1 ). The cultures were incubated overnight at 37° C. with vigorous shaking. The started cultures were diluted into 100 mL of selective LB medium and incubated once again overnight at 37° C. with vigorous shaking. The bacterial cells were harvested and the recombinant vectors were purified using the QIAGEN® Plasmid Midi Kit (Qiagen) following the manufacturer&#39;s instructions. The yield was determined with an UV spectrophotometry to quantify the DNA concentration (biophotometer, Eppendorf). Each construct was stored at −20° C. until used. 
     Example 4 
     Functional Expression of the Synthetic Constructs in CHO Cells 
     The expressed inserts are represented in  FIG. 15 . 
     A—Transient Expression of Synthetic ST6s in CHO Cells 
     The CHO-K1 cell line was used to express constructions as described by Donadio et al. (2003). Briefly, cells were grown in Ham medium supplemented with 10% of Fetal Calf Serum (FCS), fungizone (2.5 μg/mL) and gentamicin (50 μg/mL) at 37° C. in a 5% CO 2  incubator. Transfections with FLAG-CMV vector constructs were carried out using the LipoFectamine reagent following the recommended procedure of manufacturer, with 3 μg of recombinant plasmid DNA. Immunofluorescence experiments were run after 36-48 h of transfection. 
     Double labelling with FITC-SNA and anti-FLAG mAb was performed as described by Donadio et al. (2003). Cells were fixed in a 1% para-formaldehyde (PFA solution, saturated with 1% BSA, washed with phosphatase-buffered saline (PBS) and incubated with FITC-SNA. Cells expressing a ST6 activity are labeled in green. Samples were washed twice with PBS and fixed in 1% PFA, further washed with PBS, incubated in 0.05 M NH4Cl and washed with PBS. Cells were permeabilized with 0.1% Triton in PBS and further incubated in PBS containing 10% goat serum. Anti-FLAG labeling was carried out using the M2 anti-FLAG mAb (1:600) in PBS containing 5% inactivated goat or horse serum followed by incubation with a secondary anti-mouse IgG antibody labeled with Alexa Fluor 568 (1:200) in the same buffer. Cells expressing the FLAG-tagged sialyltransferase are labeled in red. After washings, cells were mounted in Mowiol and kept at −20° C. in dark. Confocal microscopy was performed with an Olympus or Leica instrument. Confocal images were processed with a Metamorph Imaging System. Volumes were originally traced as 24-bit TrueColor Images and transferred to Adobe Photoshop as RGB TIFF or JPG files. Under such conditions, untransfected CHO-K1 cells are not labeled with either of the above reagents. 
     B—Enzymatic Activity of Synthetic ST6s in CHO Cells 
     B1 Activity of the Optimized Catalytic Domain of hST6Gal I 
     The hST6Gal I CD, was cloned into the pFLAG-cytomegalovirus vector (pFLAG-CMV1) from Sigma, using a preprotrypsinogen signal peptide. This construction and these plasmids were previously characterized in vitro by Legaigneur et al. (2001) and shown to give an enzymatic protein of the expected size released into the cell medium. This soluble CD was found equally active on known exogeneous glycoprotein acceptors of various degree of branching. 
     To generate the minimal CD soluble form of hST6Gal I, a PCR fragment encoded between the 
     5′ (5′-TAATA AAGCTT GAGGCCTCCTTCCAG-3′) (SEQ ID NO: 212) introducing a HindIII restriction site and the 
     3′ (5′-CTATT GGATCC TTAGCAGTGAATGGT-3′) (SEQ ID NO: 213) encoding a BamHI site was generated. The fragment was then HindIII/BamHI digested and inserted into the pFLAg-CMV1 mammalian expression vector and subcloned into the pBluescript II KS vector (Stratagene) for further constructions. 
     The minimal ST6Gal I CD (SEQ ID NO: 44) tagged with the FLAG epitope ( FIG. 18 ) is expressed at a high level in CHO cells ( FIG. 18B , D). Its activity and localization was characterized using double labeling with anti-FLAG monoclonal Antibody and FITC-SNA. Interestingly, the mutant FLAG-CD was highly efficient in sialylating cell surface acceptors as shown in  FIGS. 18C  and D. 
     B2—Functional Expression of ST8/CD 
     CHO-K1 cells were transiently transfected with chimeric forms of hST8SiaIV (1-67)/CD (SEQ ID NO: 173), hST8SiaII (1-79)/CD (SEQ ID NO: 171). Their activity and localization were characterized using double labeling with anti-FLAG monoclonal Antibody and FITC-SNA, as shown in  FIG. 19 . 
     Both hST8SiaIV (1-67)/CD (SEQ ID NO: 174) and hST8SiaII (1-79)/CD (SEQ ID NO: 172) chimeras were found active on endogeneous cell acceptors as the SNA lectin reveals an intense cell surface labeling ( FIGS. 19B , C, E and F), indicating that the CD is active independently of the origin of N-Terminal part fused to the catalytic domain. The α2,6-sialyltransferase activity is maintained for the both chimeras, indicating that in the chimerical enzyme, the information contained in the minimal CD is necessary and sufficient for acceptor recognition and transfer efficiency. 
     B3—Functional Expression of ST3/CD and Hybrid ST3-ST6GalNAc/CD 
     CHO-K1 cells were transiently transfected with chimeric constructs of hST3Gal III/CD (SEQ ID NO: 167), and ST3 1-28-ST6GalNAc37-74/CD (SEQ ID NO: 175). Their activity and localization were characterized using double labeling with anti-FLAG monoclonal Antibody and FITC-SNA, as shown in  FIG. 19 . 
     Again, both chimeras were found active on endogeneous acceptors as SNA binding reveals an intense cell surface labeling ( FIGS. 19B , C, E and F). This indicates that the CD is active independently of the origin of CT+TMD+SR added. Since α2,6-sialyltransferase activity is maintained for both chimeras, it can be also concluded that the information contained in the minimal CD is necessary and sufficient for full transfer efficiency. 
     Therefore, fusing ST3/8 sequences upstream from the CD, does not alter the expression of a ST6 catalytic activity. Since rat and human ST3 GalIII and more generally rodent and mammalian ST3Gal lIII are identical in their N-terminal (CT+TMD+juxtamembrane SR) portion, it also appears that exchanging any of this peptide portion with non-human heterologous has no effect on ST6 catalytic activity. Furthermore, substitution of CT, TMD and SR with heterologous portions selected among enzymes of O-glycosylation like ST GalNAcI does not alter recognition of intracellular acceptors nor enzymatic activity. 
     B4—Functional Expression of hST6GalNAc I-74/CD and hST6GalNAc 258/CD 
     A similar approach was used to estimate the expression and activity of hST6GalNAc I (1-74)/CD (SEQ ID NO: 162) and hST6GalNAc I-258/CD (SEQ ID NO: 166) enzymes. Both chimeric transferases are active: SNA binding revealed a similar intense cell surface sialylation and the FLAG labeling shows identical localization within the Golgi ( FIGS. 21G , H and I). 
     It can thus be concluded that the length of the SR region can be widely variable and extended up to 258 residues without affecting the functional expression and intracellular localization of a chimerical ST6Gal catalytic activity. This also further confirms that the length of the heterologous N-terminal membrane anchor does not affect the biological active conformation of the newly designed sialyltransferase. 
     Based on the above examples, it can be stated that the process described in the invention can abolish the regulatory control of the SR region over the CD activity. As a result, any possible combination of CT+TMD+SR among glycosyltransferases (sialyltransferases) of the N- or O-glycosylation pathway can be selected to equip host cells with a needed transferase activity through a relevant CD. Furthermore, the process described in the invention is also able to target a needed activity in intracellular compartments in which neosynthesized glycoproteins migrate to the cell surface. This finding is of crucial importance for properly sorting and secreting sialylated proteins of therapeutic interest in cells (from yeast to human) engineered as described in the invention. 
     Example 5 
     Construction of a Hybrid Synthetic Domain: hST3Gal III/hST8Sia II/hST6GalNAc I, and Fusion to the CD of hST6Gal I 
     An hybrid N-terminal region (SEQ ID NO: 180) containing first the CT of hST3Gal III (1 to 8 amino acids residues; SEQ ID NO: 66), then the TMD of hST8Sia II (7 to 23 amino acids residues; SEQ ID NO: 120) and finally the SR of hST6GalNAc I (amino acids 36 to 74; SEQ ID NO: 130) was constructed. A FLAG epitope (in Bold) was added between the initiating codon ATG and the second codon (GGA) of the hST3Gal III sequence: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 206) 
               
               
                   
                 5′-ATG    GACTACAAAGACGATGACGACAAG     GGA-3′   
               
            
           
         
       
     
     The total length of this hybrid is 72 amino acids (SEQ ID NO: 180) corresponding to 216 nucleotides (SEQ ID NO: 179). Eight sense oligonucleotides and seven anti-sense oligonucleotides corresponding to the hybrid N-terminal region described above (1 to 216 nucleotides), using the sequences of hST3Gal III/hST8Sia II/hST6GalNAc I, were designed and synthesized (Eurogentec). 
     The phosphorylation and the matching reaction were performed as previously described. 
     PCR Amplification of the Synthetic Construct 
     After the synthetic reconstitution step, the product was amplified by a high fidelity PCR using specific primers that bring in the desired restriction sites at each end of the reconstituted DNA fragment in order to ligate it into the pcDNA3.1 vector. The AflII and BamHI restriction sites were respectively introduced on the 5′ and the 3′ extremities. 
     The following primers were designated: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 207) 
               
               
                   
                 5′AflII-ST3: 5′-GAGCCCCTTAAGATGGACTACAAA 
               
               
                   
                 GACGATGACG-3′ 
               
               
                   
               
               
                   
                 and 
               
               
                   
               
               
                   
                 (SEQ ID NO: 214) 
               
               
                   
                 3′ST6-BamHIn°3: 5′-TAAGGGGGATCCGGTTGTC 
               
               
                   
                 CTCCTTGCCCT-3′ 
               
            
           
         
       
     
     The PCR was performed on a PCR apparatus (I-cycler, Biorad) with 5 μL, of the synthetic fragment in a solution containing 1× of ProofStart manufacturer buffer and 1× of manufacturer Q-Solution, 1.5 mM of Mg 2+ , 300 μM of each dNTP (Sigma Aldrich), 1 μM forward primer AflII-ST3, 1 μM reverse primer ST6-BamHI no. 3, 2.5 units of ProofStart DNA polymerase (Qiagen) in a final volume of 50 μL. The thermocycling profile used was: 95° C. for 5 minutes, followed by 40 cycles of 30 seconds at 94° C., 30 seconds at 55° C. and 1 minute at 72° C. 
     The amplified PCR products were analysed by electrophoresis on a 2% agarose gel. A 240 pb nucleotidic sequence was amplified which corresponds to the expected size of the fragment ( FIG. 22  lane 1). The PCR products were purified using the Gel Extraction Kit (QIA GEN) according to manufacturer&#39;s instructions. 
     Quantification of the PCR Product 
     All PCR products were pooled and concentrated. Two volumes of ethanol and 0.1 volume of 3 M sodium acetate, pH 5.2, were added to the PCR products. The mix was incubated for 30 min at −20° C. and centrifuged 20 min at 10000 g at 4° C. The pellet was washed with 100 μL of 70% ethanol, centrifuged again 10 min at 10000 g at 4° C. Then, the supernatant was removed and the pellet dried and resuspended in 30 μL double distilled water. DNA sample was conserved at −20° C. until use. 
     Three microliters of the concentrated fragment was loaded on a 2% agarose gel to estimate its quantity ( FIG. 22  lane 3). 
     Ligation of the Catalytic Domain into an Expression Vector, pcDNA3.1(+) 
     1—Digestions and Purifications 
     The nucleotide sequence of the catalytic domain CD of hST6Gal I (SEQ ID NO: 43) was ligated into the pcDNA3.1 vector into BamHI and XbaI restriction sites. 
     Both pcDNA3.1(+) vector and CD were first digested by the BamHI restriction enzyme:
     500 ng of the CD were digested with 5 units of BamHI (Promega), 0.1 μg.μL −1  of Bovin Serum Albumin and 1× of buffer E (Promega) in a final volume of 20 μL.   1 μg of the pcDNA3.1 vector was digested with 10 units of BamHI (Promega), 0.1 μg.μL −1  of Bovin Serum Albumin and 1× of buffer E (Promega) in a final volume of 20 μL.   

     Digestions were performed for 60 min at 37° C. and the incubations were finally inactivated for 15 min at 65° C. Digested products were purified using the PCR purification Kit (QIAGEN). 
     The digested products were then submitted to a digestion by the restriction enzyme XbaI (Promega), following the manufacturer&#39;s instructions: using the appropriate enzyme units number, depending on DNA quantity (5 and 6, respectively for CD and pcDNA3.1), 0.1 μg.μL −1  of BSA and 1× of buffer D (Promega) in a final volume of 50 μL. The digestions were carried out at 37° C. for 60 min. The digestions were purified using the PCR purification Kit (QIAGEN). 
     2—Ligation 
     The ligation conditions were calculated according to the formula previously mentioned. The size of the insert CD is around 960 pb, the size of the vector pcDNA3.1 is 5428 pb, the quantity of vector used is 100 ng, and finally the molar ratio of Insert/Vector is 3/1. 
     The construction of pcDNA3.1/CD recombinant vector was performed using 53 ng of the double digested CD, 100 ng of pcDNA3.1 double digested vector, 6 units of T4 DNA ligase enzyme (Promega) and 1× ligation buffer in a final volume of 20 μL. The mix was incubated overnight at 15° C. and the reaction was inactivated for 10 min at 70° C. 
     3—Cloning 
     Competent Cells Chemical Transformation 
     One Shot® TOP10 Chemically Competent  E. coli  (Invitrogen, Life Technologies) were transformed with the recombinant vector following manufacturer&#39;s instructions. Five microliters of the ligation product were added to 50 μL aliquot of TOP 10 cells and mixed by tapping gently. The mix was incubated on ice for 30 minutes, then heat-shocked for exactly 30 seconds in a 42° C. water bath and placed on ice for 2 min. Two hundred and fifty microliters of preheated SOC medium (Invitrogen) were added to the transformation reaction and incubated at 37° C. for one hour with shaking at 250 rpm. The transformation reaction was spread on LB agar plates (composition mentioned previously) containing 50 μg.μml −1  of ampicillin. Plates were incubated overnight at 37° C. 
     Amplification 
     At the end of the next day, single colonies were picked and incubated, with 3 ml of LB broth (composition mentioned previously) containing 50 μg.ml −1  of ampicillin, overnight at 37° C. with shaking at 250 rpm. 
     Minipreparation of Plamsids 
     The recombinant vectors were purified from 1.5 mL aliquots of the previous cultures using the QIAprep Spin Miniprep Kit provided by QIAGEN and following manufacturer&#39;s instructions. Twenty-four minipreparations were performed to screen the presence of the insert. 
     Five microliters of each of the 24 minipreparations were double digested using XbaI and BamHI restriction enzymes in order to verify the presence of the insert. 
     First, the digestion solution contained 12 units of XbaI (Promega), 0.1 μg.μL −1  of BSA and 1× of buffer D (Promega) in a final volume of 10 μL. Digestions were performed for 2 hours at 37° C. Samples were secondly digested using 20 units of BamHI (Promega) and 1× of buffer E (Promega) in a final volume of 20 μL. Reactions were carried out overnight at 37° C. The digestion products were loaded on a 1.5% agarose gel to detect the presence of the CD. An insert of around 1000 pb was detected ( FIG. 23 ) which corresponds to the expected size of the CD (960 pb). 
     One positive clone has been selected for the next steps of the construction. 
     Ligation of the N-Terminal Region into pcDNA3.1/CD Recombinant Vector 
     1—Digestions and Purifications 
     The nucleotide sequence of the N-terminal region (SEQ ID NO: 179) was ligated into the pcDNA3.1/CD recombinant vector into BamHI and AflII restrictions sites. Both N-terminal fragment and pcDNA3.1/CD recombinant vector were first digested by the BamHI restriction enzyme:
     300 ng of the N-terminal fragment were digested with 5 units of BamHI (Promega), 0.1 μg.μL −1  of BSA and 1× of buffer E (Promega) in a final volume of 20 μL.   2 μg of pcDNA3.1/CD recombinant vector were digested with 20 units of BamHI (Promega), 0.1 μg.μL −1  of BSA and 1× of buffer E (Promega) in a final volume of 20 μL.
 
Digestions were performed for 100 min at 37° C. and the incubations were finally inactivated for 15 min at 65° C. Digested products were purified using the PCR purification Kit (QIAGEN).
   

     The digested products were then submitted to a digestion by the restriction enzyme AflII (New England BIOLABS), following the manufacturer&#39;s instructions: using the appropriate enzyme units number, depending on DNA quantity (10 and 20, respectively for N-terminal fragment and pcDNA3.1/CD recombinant vector), 1× of BSA and 1× of buffer 2 (New England BIOLABS) in a final volume of 50 μL. The digestions were carried out at 37° C. for 60 min and the incubations were finally inactivated for 20 min at 65° C. The digestions were purified using the PCR purification Kit (QIAGEN). 
     2—Ligation 
     The ligation conditions were calculated according to the formula previously mentioned. The size of the insert N-terminal fragment is 240 pb, the size of the pcDNA3.1/CD recombinant vector is 6388 pb, the quantity of vector used is 100 ng, and finally the molar ratio of Insert/Vector is 1/1. 
     The construction of pcDNA3.1/N-terminal fragment/CD recombinant vector was performed using 5 ng of the digested N-terminal fragment, 100 ng of pcDNA3.1/CD digested recombinant vector, 6 units of T4 DNA ligase enzyme (Promega) and 1× of ligation buffer in a final volume of 20 μL. The mix was incubated overnight at 15° C. and the reaction was inactivated for 10 min at 70° C. 
     3—Cloning 
     Competent Cells Chemical Transformation 
     One Shot® TOP10 Chemically Competent  E. coli  (Invitrogen) were transformed with the ligation product following manufacturer&#39;s instructions and as previously described. 
     Amplification 
     At the end of the next day, single colonies were picked and incubated, with 3 ml of LB broth (composition mentioned previously) containing 50 μg.ml −1  of ampicillin, overnight at 37° C. with shaking at 250 rpm. 
     Minipreparation of Plasmids 
     The recombinant vectors were purified from 1.5 mL aliquots of the previous cultures using the QIAprep Spin Miniprep Kit provided by QIAGEN following manufacturer&#39;s instructions. Twenty four minipreparations were performed to screen the presence of the insert. 
     Five microliters of each of the 24 minipreparations were double digested using XbaI and AflII restriction enzymes in order to verify the presence of the insert (N-terminal fragment+CD; SEQ ID NO: 181). 
     The digestion solution contained 20 units of XbaI (BIOLABS), 20 units of AflII (BIOLABS), 1× of BSA (BIOLABS) and 1× of buffer 2 (BIOLABS) in a final volume of 20 μL. Digestions were performed for 2 hours and 30 min at 37° C. The digestion products were loaded on a 1.5% agarose gel to detect the presence of both N-terminal region and CD. An insert of around 1200 pb was detected ( FIG. 24   a ) which corresponds to the expected size of the insert. 
     A PCR was finally performed on five minipreparation samples to verify the presence of the insert DNA (N-terminal fragment followed by the CD: SEQ ID NO: 181). The PCR was performed on a PCR apparatus (I-cycler, Biorad) with 1 μL of each of the minipreparation samples in a solution containing 1× of ProofStart manufacturer buffer and 1× of manufacturer Q-Solution, 3 mM of Mg 2+ , 300 μM of each dNTP (Sigma Aldrich), 1 μM forward primer AflII-ST3, 1 μM reverse primer Δ4-XbaI, 2.5 units of ProofStart DNA polymerase (Qiagen) in a final volume of 50 μL. 
     The oligonucleotides included in the reaction were as follows: 
                            (SEQ ID NO: 207)           5′AflII-ST3: 5′-GAGCCCCTTAAGATGGACTACAAA           GACGATGACG-3′                   and                   (SEQ ID NO: 205)           3′Δ4-XbaI: 5′-TAACCCTCTAGATTAGCAGTGAATGG           TCCGGAAGC-3′            
The thermocycling profile used was: 95° C. for 5 minutes, followed by 40 cycles of 1 min at 94° C., 1 min at 57° C. and 1 minute 30 s at 72° C. The amplified PCR products were analysed by electrophoresis on a 1.5% agarose gel: a 1200 pb nucleotidic sequence was amplified for each minipreparation sample which corresponds to the expected size of the total insert ( FIG. 24   b ).
 
Sequencing
 
     The cloning steps were verified by both strands DNA sequencing by GENOME express (Meylan, France) using universal primers present inside the vector: T7 promoter and BGH reverse. 
     The nucleotidic sequence obtained was aligned to the expected sequence and the alignment revealed 100% identity. The deduced amino acid sequence was also aligned with the expected sequence SEQ ID NO: 181 of the chimera and it shows 100% identity. 
     Midipreparation 
     After the DNA insert sequence verification, the recombinant plasmid was amplified in 100 mL culture to obtain high quantity of the construction. Four hundred microliters of the corresponding 3 mL LB broth culture mentioned previously were inoculated into 100 mL LB broth containing 50 μg.ml −1  of ampicillin, and incubated overnight at 37° C. with shaking at 250 rpm. The bacterial cells were harvested and the recombinant vectors were purified using the QIAGEN Plasmid Midi Kit (Qiagen) following manufacturer&#39;s instructions. The yield was determined with an UV spectrophotometer to quantify the DNA concentration. The construct was stored at −20° C. 
     Cell Culture: Expression of the Chimeric Enzyme hST3Gal III/hST8Sia II/hST6GalNAc I/CD (SEQ ID NO: 182) in CHO Cells 
     CHO-K1 cells were routinely cultured at 37° C. with 5% of CO 2  in DMEM supplemented with 10% FBS, 2.5 μg/mL fungizone and 50 μg/mL gentamicin. The day before transfection, cells were seeded at a density of 100 000 cells/mL in a 6-well plate containing 3 glass coverslips per well. CHO-K1 cells were transiently transfected with FuGENE 6 transfection reagent (Roche) using a 3:1 ratio. 
     Following one day of culture, cells were fixed with 1% paraformaldehyde for 15 minutes at room temperature (RT). Cells were washed three times with PBS, blocked for 30 minutes at RT with 1% BSA and then incubated with 10 μg/mL of SNA-FITC (vector laboratories) for one hour at 4° C. Cells were successively washed three times with PBS, fixed with 3% paraformaldehyde for 20 minutes at RT, further washed with PBS and incubated with 0.05 M NH 4 Cl for 10 minutes at RT. After washings with PBS, cells were permeabilized with 0.25% Triton X-100 for 5 minutes at RT and blocked with 10% inactivated goat serum and 1% BSA for 30 minutes at RT. Cells were incubated for one hour at RT with 1 μg/mL monoclonal anti-FLAG M2 antibody (Sigma) in PBS containing 5% inactivated goat serum and 0.5% BSA. Cells were washed three times with PBS and incubated for 45 minutes with 5.7 μg/mL of Alexa Fluor® 594-coupled secondary goat anti-mouse antibody (Molecular Probes) in PBS containing 5% inactivated goat serum and 0.5% BSA. After washings with PBS, cells were mounted in Mowiol and analysed using an Olympus microscope ( FIG. 25 ). 
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