Ceramide glucosyltransferase

Abstract: The subject invention provides a novel ceramide glucosyltransferase having catalytic activity of glucose transfer from UDP-Glc to ceramide, and a nucleic acid sequence encoding the ceramide glucosyltransferase.

Description: FIELD OF THE INVENTION The present invention relates to sugar transferases and the genes encoding said enzymes. More specifically, the present invention relates to ceramide glucosyltransferases, i.e., glucosylceramide synthetases catalyzing glucose transfer, and to genes encoding the enzymes. BACKGROUND OF THE INVENTION Glycosphingolipids (GSLs) are a class of membrane components that have the lipid portion embedded in the outer leaflet of the lipid bilayer of plasma membrane and the sugar chains extended to the outer environment. Glycosphingolipids exist essentially in all of the animal cells and are suggested as being important substances responsible for various cellular processes such as differentiation, adhesion, proliferation, and cell-cell recognition (Varki, A., Glycobiology, 3, pp.97-130, 1993). With only a few exceptions, almost all of the glycolipids are synthesized from glucosylceramide as a precursor that is produced by the transfer of glucose to ceramide. Ceramide glucosyltransferase (UDP-glucose: ceramide β 1-1' glucosyltransferase, GlcT-1, EC2.4.1.80: hereinafter in the specification, this enzyme is occasionally referred to simply as "ceramide glucosyltransferase.") catalyzes the first glycosylation step of the glycosphingolipid biosyntheses, i.e., the transfer of glucose from UDP-Glc to ceramide (Basu, S. et al., J. Biol. Chem., 243, pp.5802-5807, 1968). Glucosylceramide (GlcCer), the product of this enzyme, serves as precursors for more than 300 of glycosphingolipid biosyntheses (Radin, N.S., Nuerochem. Res., 5, pp.533-540, 1994). GlcT-1 was first discovered from embryonic chick brain (Basu, S. et al., J. Biol. Chem., 243, pp.5802-5807, 1968). However, the properties of the enzyme have not been fully studied, because of the difficulties in assaying and purifying the enzyme. Only limited data have been published concerning GlcT-1 so far, e.g. successful solubilization of the enzyme from rat Golgi fraction (Durieux, I. et al., Biochem. Biophys. Acta, 1024, pp.263-266, 1990) and discovery of the enzymes in liver and brain each characterized by distinguishable properties (Vunnam, R. et al., Biochem. Biophys. Acta, 573, pp.73-82, 1979). It has recently been revealed that the synthesis of GlcCer occurs at the cytosolic surface of Golgi apparatus, while other glycosylation reactions in biosynthetic pathways of glycosphingolipid take place at the lumenal side of the organelle. However, the most of enzymatic properties of the ceramide glucosyltransferases remain unknown. As for ceramide galactosyltransferase, having a similar catalytic activity, the cloning of the enzyme was reported (Schulte, S. et al., Proc. Natl. Acad. Sci. USA, 90, pp.10265-10269, 1993). SUMMARY OF THE INVENTION An object of the present invention is to provide ceramide glucosyltransferases derived from mammals, preferably from human. Another object of the present invention is to provide the genes encoding said ceramide glucosyltransferases. The inventors of the present invention previously reported the isolation and characterization of a mouse melanoma B16 mutant, GM-95, deficient in ceramide glucosyltransferases (Nozue, M. et al., Int. J. Cancer, 42, pp.734-738, 1988; Ichikawa, S. et al., Proc. Natl. Acad. Sci. USA, 91, pp.2703-2707, 1994). By an expression cloning technique using the deficient mutant cell, GM-95 as a recipient cell, the inventors succeeded in isolating the cDNA encoding human ceramide glucosyltransferase (UDP-glucose; N-acylsphingosine D-glucosyltransferase, EC 2.4.1.80). The present invention provides an enzymatically active domain of the amino acid sequence of the ceramide glucosyltransferase, the ceramide glucosyltransferase being characterized by the amino acid sequence SEQ ID NO. 2: , wherein the enzymatically active domain polypeptide is obtainable by selectively removing a transmembrane domain and ligating a resulting active domain with a signal peptide, and wherein the enzymatically active domain is capable of catalyzing glucose transfer from UDP-Glc to ceramide. The present invention further provides a soluble polypeptide having catalytic activity of glucose transfer from UDP-Glc to ceramide which comprises the active domain as desribed above. The present invention further provides a nucleic acid sequence encoding the active domain as desribed above or the soluble polypeptide as desribed above. The present invention further provides an extracellularly releasable protein comprising the enzymatically active domain together with one or more signal peptides. The human ceramide glucosyltransferase is characterized by the amino-acid sequence (amino acids from No.1 to 394) of SEQ. ID. No.2 disclosed in the sequence listing. In another aspect, the present invention provides the gene encoding the active domain of said human ceramide glucosyltransferase. The ceramide glucosyltransferase gene is characterized by from nucleotide No.1 to 1185 (including the terminal codon) of the nucleic acid sequence of SEQ. ID. No.1 disclosed in the sequence listing. The enzyme of the present invention has activity of catalyzing glucose transfer from UDP-Glc to ceramide. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the synthetic pathway of glycosphingolipids. Open letters indicate enzymes and glycosphingolipids deficient in GM-95 cells. Broken arrows represent reactions that do not proceed in GM-95 cells due to the deficiency of substrates. Cer, ceramide; GalT, UDP-galactose: glucosylceramide galactosyltransferase; and SAT, CMP-sialic acid: lactosylceramide sialyltransferase. Fig. 2 shows the amino acid sequence of the human GlcT-1 as a preferred embodiment of the ceramide glucosyltransferase of the present invention. Fig. 3 shows hydropathy plot of the amino acid sequence of the human GlcT-1 as a preferred embodiment of the ceramide glucosyltransferase of the present invention. In the figure, positive values represent increased hydrophobicity and negative values increased hydrophilicity. DETAILED DESCRIPTION OF THE INVENTION A. Preferred Embodiments Besides the naturally derived human enzyme described above, any one of the enzymes, which has activity of catalyzing glucose transfer from UDP-Glc to ceramide and is characterized by a modified amino acid sequence in which the amino acid sequence defined by SEQ. ID. No.2 is inserted, deleted, and/or substituted with one or more amino acid residues, may be used to prepare the active domain of the ceramide glucosyltransferase of the present invention. Among these enzymes, naturally derived enzymes that are being expressed in mammals such as humans, monkeys, dogs, cats, cows, horses, rats, and mice are preferred. The present invention provides a class of polypeptides which has activity of catalyzing glucose transfer from UDP-Glc to ceramide and comprises, as a part or the whole thereof, an amino acid sequence that is a portion of the amino acid sequence of the above-described enzyme and responsible for glucose transfer from UDP-Glc to ceramide (i.e., so called an enzymatically active domein). These polypeptides comprise, as a part or the whole thereof, the active domein derived from the above-described enzyme as a part thereof. A polypeptide, which is obtainable by selectively removing a transmembrane domain (hydrophobic domein) and ligating a resulting active domain with a signal peptide, is expected to be useful as a soluble enzyme in an extracellularly releasable form. For a determination of the hydrophobic domain, methods well known to one of ordinary skilled artisan may be applied, which include, for example, the method of Kyte et al. (Kyte. J. et al., J. Mol. Biol., 157, pp.105-132, 1982) and the method of Hopp & Woods. According to another aspect of the present invention, nucleotide sequences encoding the active domain of any one of the above-described ceramide glucosyltransferase and those encoding any one of the above polypeptides. Examples include, any one of genes encoding the above-described human ceramide glucosyltransferase and, as a preferred example thereof, the gene characterized by from nucleotide No.1 to 1185 (including the terminal codon) of the nucleic acid sequence of SEQ. ID. No.1 disclosed in the sequence listing; and a modified nucleic acid sequence encoding an active domain polypeptide capable of catalyzing glucose transfer from UDP-Glc to ceramide in which the the nucleic acid sequence of SEQ. ID. No.1 is inserted, deleted, and/or substituted with one or more nucleic acids, all of which fall within the scope of the genes of the present invention. DNA sequences comprising any one of the aforementioned nucleotide sequences as a part thereof also fall within the scope of the present invention. The gene encoding the human ceramide glucosyltransferase can readily be obtained by one of ordinary skilled in the art according to the processes detailed in the examples set out below in the specification or by referring to the disclosed processes, or alternatively, by optionally applying appropriate alterations or modifications thereto. In addition, ceramide glucosyltransferases derived from mammals other than human can also be obtained according to the exemplified processes by suitably choosing readily available experimental materials. The genes of the present invention are useful for diagnostic probes to evaluate expression of the above enzyme in a living body, as well as for the manufacture of the aforementioned enzymes and polypeptides. In further aspect, the present invention provides a recombinant vector comprising any one of the aforementioned genes or the DNA sequences, and a transformant cell that is transformed with said recombinant vector. Types of vectors are not particularly limited and any vectors may be used so far as they are available in the art. Preferably, types of vectors may be appropriately chosen depending on a type of a host cell to be transformed. Types of host cells are also not particularly limited and any types of cells may be used, such as, procaryotic cells including bacteria, e.g. Escherichia coli, or eucaryotic cells including animal cells or plant cells. An expression system using vaccuro virus may also be used. The present invention also provides a process for preparing any one of the aforementioned ceramide glucosyltransferases or any one of the polypeptides catalyzing glucose transfer from UDP-Glc to ceramide, which comprises the step of separating and isolating said enzyme or polypeptide from a cultivation mixture obtained by culturing the above-described transformant. Methods of such cultivations as well as methods for separation and isolation of the desired enzyme or polypeptide are well known in the art. The present invention will be further detailed by referring to the following examples. However, the scope of the present invention is not limited to these examples. B. Examples (1) Materials and Methods (a) Materials Monoclonal antibody M2590 (anti-GM3; IgM, Hirabayashi, Y. et al., J. Biol. Chem., 260, pp.13328-13333, 1985) was purchased from Meiji Seika Co. Ltd. Tokyo, Japan. Anti-mouse IgM goat IgG fraction (µ chain specific) was obtained from Organon Technika Corporation. High fluorescent anti-mouse gamma globulin produced in goat was purchased from Antibodies Incorporated (CA, USA). Lipofectin and G418 were obtained from Life Tech. Oriental (Tokyo, Japan). The pET system was from Takara Shuzo Co., LTD. (Kyoto, Japan; Studier, F. et al., Method in Enzymol., 185, pp.60-89, 1990). Bis-(sulfosuccinimidyl) suberate, a bifunctional cross-linker, was obtained from PIERCE (IL, USA). 6-(((N-7-nitrobenz-2-oxa-1,3-diazol-4-yl)-amino)-caproyl)-sphingosine, C 6 -NBD-Cer, was from Molecular Probes Inc. (OR, USA). pPSVE-PyE plasmid which carries the early region of polyoma virus, was generous gift from Dr. M. Fukuda (La Jolla Cancer Research Foundation, La Jolla, CA; Bierhuizen, M.F. et al., Genes and Dev., 7. pp.468-478, 1993). All other reagents were of analytical grade. (b) cDNA library A cDNA library of a human melanoma cell line SK-Mel-28 was a gift from Dr. M. Fukuda (La Jolla Cancer Research Foundation, La Jolla, CA). The library was prepared from poly (A) + RNA by the method of Seed et al. (Seed, B. et al., Proc. Natl. Acad. Sci. USA, 84, pp.3365-3369, 1987) using pcDNAI (Invitrogen, CA) as a mammalian expression vector. The library contained 2.5×10 6 independent clones in E. coli strain MC1061/P3. (c) Cell lines and culture conditions GM-95, a B16 melanoma mutant that does not express GlcT-1, was established according to reported methods (Nozue, M. et al., Int. J. Cancer, 42, pp. 734-738, 1988; Ichikawa, S. et al., Proc. Natl. Acad. Sci. USA, 91, pp.2703-2707, 1994). GM-95-PyT, a recipient cell line for transient expression, was established by cotransfection of GM-95 with pSV2neo plasmid and pPSVE-PyE plasmid (Bierhuizen, M.F. et al., Genes and Dev., 7, pp.468-478, 1993), followed by the selection with G418. The cells were ordinarily maintained in Dulbecco's modified Eagle medium, or in DMEM (Gibco Laboratories, NY) supplemented with 10% fetal calf serum (FCS) under 5% CO 2 . (d) Isolation of a cDNA by complementation. Plasmids that carries cDNAs were prepared from an amplified portion of the cDNA library. GM-95-PyT cells were transfected with the plasmid DNA by electroporation (Chu, G. et al., Nucleic Acids Res., 15, pp.1311-1326, 1987). Outlines of the procedures were as follows: 2×10 7 GM-95-PyT cells were washed with K-PBS - (30.8 mM NaCl, 120.7 mM KCl, 8.1 mM Na - / 2HPO 4 , and 1. 46 mM KH 2 PO 4 ), and suspended in 400 µl of K-PBS - supplemented with 5 mM MgCl 2 (K-PBS + ). Plasmid DNA (100 µg) dissolved in 400 µl K-PBS + was added to the cell suspension and the mixture was incubated on ice for 10 min. Cells were transferred to 0.4 cm cubette and exposed to a 300 V pulse with a capacitance of 960 µF by Gene Pulsar (Bio Rad). Then, the cells were again cooled on ice. After 10 min, the cell suspension was diluted with 5 ml of cold serum-free DMEM and incubated for 30 min at 25°C. The cells were then inoculated in culture dishes and cultured at 37°C in DMEM supplemented with 20% FCS. A total of 1 × 10 8 cells was transfected as above, and cultured in five 15 cm culture dishes. After sixty hours, the cells were detached from the plate by incubation with 5 ml per plate of PBS containing 5 mM EDTA. The cells were resuspended at a concentration of 5×10 7 cells/ml in PBS-EDTA-NaN 3 (PBS containing 5% FBS, 0.02% sodium azide, and 0.5 mM EDTA) and then subjected to a reaction with M2590 mAb (20 µg/ml) for 2 hrs on ice. After the reaction, cells were washed twice with ice-cooled PBS, resuspended in PBS containing 50 mM HEPES buffer (pH 8.3) and 0.2 mM BS 3 at a concentration of 1×10 7 cells/ml, and incubated for 30 min on a ice-bath. After then, the cells were washed twice with ice-cooled PBS, resuspended in 10 ml of PBS-EDTA-NaN 3 , and divided into five 6 cm panning plate coated with anti-mouse IgM goat IgG (µ chain specific, Wysocki, L. et al., Proc. Natl. Acad. Sci. USA, 75, pp.2844-2848, 1978). After incubation for 4 h at 25 °C, non-adherent cells were removed by gentle washing with PBS-EDTA-NaN 3 . The plasmids were extracted from the adherent cells by the method of Hirt et al. (Hirt, B., J. Mol. Biol., 26, pp.365-369, 1967) and introduced into E. coli., MC1061/P3 by electroporation. The plasmids were prepared from 500 ml of 2 × YT (Maniatis, T. et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, Plainview, NY), and another round of panning was performed as described above. After the second round of panning, 500 E. coli clones were divided into 32 pools and plasmids were prepared from each pool by mini preparation (Maniatis, T. et al. as described above). The plasmids were introduced into GM-95-PyT cells cultured in 24 well plates by the DEAE dextran method (Sussman, D.J. et al., Mol. Cell Biol., 4, pp.1641, 1984) and the cells were cultured for 60 h. The cells were then harvested from each of the wells and the GlcT-1 activity was measured. Two pools were found to be positive. The 62 individual E. coli clones from the positive pools were examined as described above and a ceramide glucosyltransferase cDNA was isolated. The clone was designated as pCG-1. (e) Stable expression of GlcT-1 in GM-95 cells pCG-1 or pcDNAI was co-transfected with pSV2neo using lipofection (Felgner, P. L. et al., Proc. Natl. Acad. Sci. USA, 84, pp.7413-7417, 1987) as described below. By using 10 cm tissue culture dishes, GM-95 cells were cultured in 10 ml DMEM supplemented with 10% FCS until they reached 50% confluency. The medium was replaced with 10 ml serum-free DMEM, and then, 300 µl of water mixed with pCG-1 (60 µg), pSV2neo (6 µg), and Lipofectin (150 µg) was added to the culture. As a control, pcDNAI vector being lack of the insert was used instead of pCG-1. The next day, the cells were subjected to selection in a medium containing 800 µ g/ml of G418. After two weeks, more than 100 colonies were obtained from each of the plates. The cells were detached from the plates, mixed and replaced on new plates. The cells were maintained for two months in the medium containing G418 and used for flow-cytometry or lipid analyses without cloning. (f) Expression of GlcT-1 in E. coli The cDNA from pCG-1 was cloned into the EcoRI site of Bluescript KS vector to locate BamHI site at the 3' end (pCG-2). An NdeI site was introduced at the ATG codon of the GlcT-1 open-reading frame by standard PCR methods (Feigner. P.L. et al., Proc. Natl. Acad. Sci. USA, 84, pp.7413-7417, 1987) using primers, i.e. 5'-ATCATATGGCGCTGGACCTGGC-3' and 5'-CAATCTAGCATCAACATTTGGATA. The resulting 300 bp-fragment was digested with HindIII and cloned into the SmaI-HindIII region of pUC119. After amplification, the fragment was recovered by digestion with KpnI and HindIII. The fragment was then ligated to 1.1kb KpnI-HindIII fragment of pCG-2 to afford pCG-3. Then, the full-length cDNA having NdeI and BamHI sites, that was derived from pCG-3, was cloned into E. coli expression vector pET-3a (Studier, F. et al., Method in Enzymol., 185, pp.60-89, 1990). The resulting plasmid pET-CG1 was transformed into the E. coli strain BL21 (DE3) (Studier, F. et al., ibid., 185, pp.60-89, 1990). For the expression of the cloned GlcT-1, E. coli cells harboring the plasmids were grown in NZCYM medium at 37°C (Maniatis, T. et al. as described above). When cell density was reached 0.25 OD 600 , isopropyl-β-thiogalactopyranoside (IPTG) was added to a final concentration of 1 mM and the cells were incubated for an additional 5 hours. After the incubation, the cells were harvested and then disrupted by sonication. 150 µg of cell lysate was used for an enzyme assay, and an incubation was performed for 7 h. Other conditions were in accordance with the standard assay conditions. (g) Nucleotide sequence determination The plasmid containing human GlcT-1 cDNA was treated with exonucleaseIII and mung bean nuclease to construct nested deletion clones of various lengths (Heinkoff, S., Gene, 28, pp.351-359, 1984). Nucleotide sequences were determined in both directions by either the dideoxynucleotide chain termination method using BucaBEST (Takara Shuzo, Co., LTD, Kyoto, Japan; Sanger, F., et al., Proc. Natl. Acad. Sci. USA, 74, pp.5463-5467, 1977) or the cycle sequencing methods using Taq DNA polymerase. Applied Biosystems model 373 A and Pharmacia A.L.F.DNA sequencers were used for the analyses. Some analyses were performed at the Takara Shuzo customer service center. (h) DNA manipulation DNA manipulation was carried out according to the method of Maniatis et al. (Maniatis, T., as described above). (i) Flow cytometry Cells were detached from culture dishes using PBS containing 5 mM EDTA and then reacted with M2590 mAb (20µg/ml in PBS-EDTA-NaN 3 ) for 2 h on an ice bath. The cells were washed with PBS and reacted with FITC labeled anti-mouse immunoglobulin for 3 h on ice. After washing with PBS, the cells were analyzed by FACStar (product of Backton Dickinson). (j) Lipid analyses Cells were collected by scraping at subconfluency, washed twice with PBS and lyophilized. Total lipids were extracted with 20 volumes of chloroform : methanol (2 : 1, v/v), filtered, and concentrated to dryness under reduced pressure. The total lipids were then again dissolved in a small volume of chloroform : methanol (2 : 1, v/v) and chromatographed on a precoated silica-gel TLC plate (E. Merck) using chloroform : methanol : water (65:25:4, v/v). Glycosphigolipids (GSLs) were visualized with orcinol-HCl reagent. (k) Enzyme assay GlcT-1 activity was assayed according to the method of Lipsky et al. (Lipsky, N.G. et al., J. Cell. Biol., 100, pp.27-34, 1986) with slight modifications. C 6 -NBD-Cer, a synthetic fluorescent substrate, in a form of liposome was used for the assay. C 6 -NBD-Cer (50 µg) and lecithin (500 µg) were mixed in 100 µl ethanol and the solvent was removed to dryness. After 1 ml of water was added to the residue, liposomes were prepared by sonication. A standard reaction mixture [100 µl; 20 mM Tris-HCl (pH 7.5)/500 µ MUDP-Glc/20 µl liposomes/50 µg cell protein as enzyme source] was incubated for 4 h at 30°C. After then, lipids were extracted and applied on silica-gel 60 plates. NBD lipids were separated by using CHCl 3 /CH 3 OH/H 2 O (65:25:4, v/v) and the lipids were visualized by UV illumination. (1) Protein assay Proteins were assayed using Micro BCA Protein Assay Reagent Kit (PIERCE, IL, USE) according to the method of Smith et al. (Smith, P.K., et al., Anal. Biochem., 150, pp.76-85, 1985). (m) Isolation of mRNA and Northern blot analysis Poly A + RNA from each cell line was isolated by Fast Track mRNA Isolation Kit (Invitrogen Corporation) according to the manufacture's instruction. Poly A + RNA (2 µg) was subjected to electrophoresis on 1% agarose gel containing formaldehyde and transblotted onto nylon membrane (Hybond-N, Amersham, Maniatis, T. et al., as described above). A pre-made membrane was used for the analysis of mRNA from various tissues (Human Multiple Tissue Northern Blot, Clontech). The 1.1 kb HindIII-XhoI cDNA fragment of pCG-1 was labeled with [α- 32 P] dCTP (6000 Ci/mmol, Amersham) by Multiprime DNA labelling system (Amersham) and used as a probe. Hybridization was carried out at 42°C for 24 h in 5×SSPE [1×SSPE contains 0.18 M NaCl, 10 mM NaPO 4 (pH7.7) and 1 mM EDTA] containing 50% formaldehyde, 2% SDS, 10×Denhart's solution, 100 µg/ml of salmon sperm DNA, and 32 P labeled probe. After hybridization, the membrane was washed with 2×SSC containing 0.5% SDC and 0.1×SSC each for 40 min at 50 °C. (2) Results (a) cDNA cloning By using a mammalian expression cloning system developed by Seed et al. (Seed, B. et al., Proc. Natl. Acad. Sci. USA, 84, pp.3365-3364, 1987), cDNAs from a SK-Mel-28 library in the expression vector peDNAI were introduced in GM-95-PyT that lacks GlcT-1 activity. The recipient cell line GM-95-PyT, GM-95 stably expressing polyoma large T antigen, allows episomal replication of transfected plasmids. Although GM-95-PyT are deficient in GlcT-1, other enzymes involved in glycosphingolipid syntheses retained their enzymatic activities. Thus, the complementation of the GlcT-1 cDNA restores the expression of GlcCer, LacCer, and GM3 that are expressed in the parental cell line. Three days after the transfection, cells expressing GM3 were selected by panning with the anti-GM3 mAb and petri dishes coated with anti-mouse-IgM. The binding between the mAb and GM3 was not strong enough to hold the cells on the plate, and accordingly, the bound mAb and cell surface proteins were cross-linked before the panning. Plasmids were isolated from the adherent cells and elecroporated into E. coli for further amplification. After the second round of transfection, selection, and amplification, E. coli colonies were divided into subpools and screened by a sibling selection based on the enzyme assay until a single clone pCG-1 was isolated. (b) Transient expression of GlcT-1 activity in the mutant cells The mutant cells transfected with pCG-1 were assayed for the GlcT-1 activities using C 6 -NBD-ceramide as a substrate. SK-Mel-28 cells; SK-Mel-28 cells with no UDP-Glc addition; MEB-4 cells; MEB-4 cells with no UDP-Glc addition; GM-95-PyT cells; GM-95-PyT cells with no UDP-Glc addition; GM-95-PyT cells transfected with pcDNA I; GM-95-PyT cells transfected with pcDNA I with no UDP-Glc addition; GM-95-PyT cells transfected with pCG-1; GM-95-PyT cells transfected with pCG-1 with no UDP-Glc addition; C 6 -NBD-Cer (100 pmoles); C 6 -NBD-GlcCer (100 pmoles); and C 6 -NBD-SM (100 pmoles) were subjected to simultaneous analysis. As a result, GlcT-1 activity was detected in GM-95-PyT cells transfected with pCG-1, but not detected in the cells transfected with the pcDNAI vector. The production of C 6 -NBD-GlcCer was found to be UDP-Glc dependent. GlcT-1 activities were also detected in SK-Mel-28 cells from which the library was derived as well as in the parental cell line MEB-4. A low level of NBD-GlcCer synthesis was detected due to endogenous UDP-Glc in the lysates from these two cell lines without the addition of UDP-Glc. (c) Restoration of glycosphingolipid expression in the mutant cells introduced with the GlcT-1 cDNA. Expression of glycosphingolipid was first analyzed by flow cytometry using anti-GM3 mAb. Although a significant shift of main peak and a small population of strongly stained cells were observed in GM-95-PyT cells transfected with pCG-1, the ratio of positive population was relatively low. This was mainly due to the low transfection efficiency of GM-95-PyT cells. Accordingly, in order to produce GM-95 cells expressing GlcT-1 stably, GM-95 cells were co-transfected with pCG-1 and pSV2neo and selected with G418 for neomycin resistance, and as a result, more than 100 colonies were obtained. The cells were mixed and expanded for glycosphingolipid analysis without cloning. 75% of the cells were positively stained with GM3, and 15% were stained stronger than the parental cells. Glycosphingolipids of the neomycin resistant cells were further analyzed by TLC followed by orcinol/H 2 SO 4 reaction. The total lipids from 10 6 cells were spotted onto silica-gel plate and developed with a solvent (chloroform : methanol : water = 65:25:4, v/v). SK-Mel-28 cells; MEB-4 cells; GM-95 cells; GM-95 cells cotransfected with pcDNA I and pSV2neo; GM-95 cells cotransfected with pCG-1 and pSV2neo; GlcCer (2µg); LacCer (1µg); and GM3 (1µ g) were simultaneously analyzed, and glycosphingolipids were visualized with orcinol/H 2 SO 4 reagent. GlcCer and GM3 were detected in the total lipids extracted from GM-95 cells carrying pCG-1 but not from the cells carrying pcDNAI. (d) Expression of enzymatically active GlcT-1 in E. coli. In order to demonstrate that the isolated cDNA encodes the desired ceramide glucosyltransferase, per se, and that it does not encode a regulatory protein required for the expression of GlcT-1 activity, enzymatically active GlcT-1 was expressed in E. coli that is not capable of expressing GlcT-1 and sphingolipids including glycosphingolipids. pET3a/BL21 (DE3); pET3a/BL21 with no UDP-Glc addition; pET3a-CG1/BL21 (DE3); pET3a-CG1/BL21 (DE3) with no UDP-Glc addition; C 6 -NBD-Cer (40 pmoles); and C 6 -NBD-GlcCer (40 pmoles) were simultaneously analyzed. As a result, the GlcT-1 activity was detected in the cells transformed with pET-CG-1. The production of C 6 -NBD GlcCer was observed as being UDP-Glc dependent, and no activity was detected in the cells that carried pET3a vector alone. From these results, it is apparent that the above-mentioned cDNA encodes GlcT-1. (e) Primary structure of GlcT-1. Fig. 2 shows the primary amino acid sequence of GlcT-1 deduced from nucleotide sequence of the cDNA. GlcT-1 consists of 394 amino acids and has a calculated molecular mass of 44,853 dalton. No significant homologies with GlcT-1 were found by searches of the DNA and protein data bases (GenBank and PIR), which revealed that GlcT-1 is a novel protein. The GC-rich sequence that precedes to the initiation codon is presumably a part of CpG island sequence that surrounds the promoters of house-keeping genes (Bird, A.P., Nature, 321, pp.209-213, 1986). The genes that contain the above sequence are usually essential for cell viability and are expressed in most cells. The enzyme with a similar catalytic activity, ceramide galactosyltransferase (CGT), has recently been cloned (Schulte, S. et al., Proc. Natl. Acad. Sci. USA, 90, pp.10265-10269, 1993). The sequence analysis of this enzyme revealed that the enzyme was homologous to glucuronyltransferases that are participated in drug metabolic pathways. Since GlcT-1 and CGT catalyze similar reactions, it may be worth considering a possibility that these enzymes are homologous. However, no significant sequence homology was observed, which suggests that these two enzymes have different evolutional origins. The glucuronyltransferases are expressed at the lumenal side of endoplasmic reticulam (Drake, R.R. et al., J. Biol. Chem., 267, pp.11360-11365, 1992). In contrast, GlcT-1 is a single enzyme that catalyzes the above reaction on the cytoplasmic surface, which may also support this hypothesis. The hydropathy plot analysis (Kyte, J. et al., J. Mol. Biol., 157, pp.105-132, 1982) proved the presence of a hydrophobic segment near the N terminus that is the potential membrane-anchoring domain. In addition, the considerable hydrophobicity was detected in the regions close to the C terminus, that may locate in the membrane. (f) Northern blot analysis of GlcT-1 mRNA The expression of GlcT-1 mRNA in melanoma cells and various human tissues was analyzed by Northern blot analysis (poly A + RNA, 2 µg). (A) the melanoma cell lines: SK-Mel-28; MEB-4; and GM-95; and (B) human tissues: heart; brain; placenta; lung; liver; skeletal muscle; kidney; and pancreas were subjected to a simultaneous analysis. Hybridization with human glyceraldehyde 3-phosphate dehydrogenase (G3PDH) cDNA was carried out as control experiments. A single transcript of 3.5 kb was detected in all of the cells and tissues examined. Strong expressions of GlcT-1 mRNA were detected in SK-Mel-28 and MEB-4 cells. Due to the deficiency of the GlcT-I, a very faint band was observed in GM-95 cells. As for human tissues, GlcT-1 mRNA expressions were observed in all of the tissues examined. In control experiments, strong expression of G3PDH mRNA was observed in skeletal muscle and heart as reported previously (Vincent, S. et al., Nucleic Acids Res., 21, pp.1498, 1993). The ceramide glucosyltransferases of the present invention are responsible for the initial glycosylation step of glycosphingolipid biosynthesis in mammals, preferably in human. They have properties to catalyze the transfer of glucose from UCP-Glc to ceramide to afford glucosylceramide. Accordingly, the enzymes of the present invention are essential for life maintenance of mammals and are extremely valuable enzymes. In addition, the enzymes of the present invention are also useful as laboratory usable agents in the fields of biochemistry and biotechnology, as well as preparatory agents for the manufacture of pharmacologically active substances. Furthermore, they are expected to be useful as medicament for therapeutic and preventive treatment for various diseases based on reduced expression of said enzyme. The genes of the present invention are useful for a mass preparations of the above enzymes in highly purified forms, and also useful as probes that can be used for clinical tests and diagnosis of diseases based on abnormal expression of the above enzyme. An example of the gene and the expression product thereof according to the present invention will be described in the sequence listings set out below. Sequence Listing (1) Information for SEQ ID No.1: (i) Sequence Characteristics: (A) length: 1185 bases (B) Types : nucleic acid (C) Topology : linear (Xi) Sequence Description: SEQ ID No.1: (2) Information for SEQ ID No.2: (i) Sequence Characteristics: (A) length: 394 amino acids (B) Type : amino acid (C) Topology : linear (Xi) Sequence Description: SEQ ID No.2:

Claims: An enzymatically active domain of the amino acid sequence of the ceramide glucosyltransferase, the ceramide glucosyltransferase being characterized by the amino acid sequence SEQ ID NO. 2: , wherein the enzymatically active domain polypeptide is obtainable by selectively removing a transmembrane domain and ligating a resulting active domain with a signal peptide, and wherein the enzymatically active domain is capable of catalyzing glucose transfer from UDP-Glc to ceramide. A soluble polypeptide having catalytic activity of glucose transfer from UDP-Glc to ceramide which comprises the active domain according to claim 1. A nucleic acid sequence encoding the active domain according to claim 1 or the polypeptide according to claim 2. An extracellularly releasable protein comprising the enzymatically active domain according to claim 1 together with one or more signal peptides.

Inventors: HIRABAYASHI YOSHIO, ICHIKAWA SHIN-ICHI, HIRABAYASHI, YOSHIO, ICHIKAWA, SHIN-ICHI, Hirabayashi, Yoshio, The Inst. Phys. Chem. Res., Ichikawa, Shin-ichi, The Inst. Phys. Chem. Res.
Applicants: RIKAGAKU KENKYUSHO, THE INSTITUTE OF PHYSICAL AND CHEMICAL RESEARCH
Application year: 1996
Main classification: C12N15
IPC codes: C07K14, C07H21, C12N1, C12R1, C12N9, C12N15