Abstract:
The present invention relates techniques for identifying suitable secretion fusion partner (SFP) for hyper-secretory production of recombinant proteins. The SFPs can be obtained from secretome analyzes. Recombinant proteins are produced in a fusion form with a secretion fusion partner (SFP) and can be separated from the SFP by in vitro protease treatment. SFPs of this invention greatly improve the secretion level of target proteins and peptides which are valuable for bio-pharmaceuticals and the bio-industry.

Description:
This application claims priority to U.S. Provisional Appl. No. 61/119,972, filed on Dec. 4, 2008, the entire contents of which are hereby incorporated by reference in their entirety. 
    
    
     REFERENCE TO A SEQUENCE LISTING SUBMITTED ELECTRONICALLY VIA EFS-WEB 
     The content of the electronically submitted sequence listing (Name: 2472_0030001_SubstituteSequenceListing_ascii.txt, Size: 91,872 bytes; and Date of Creation: Oct. 18, 2012) filed herewith with the application is incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention is in the field of recombinant protein expression. In particular, the invention relates to secretion fusion partners (SFPs) and techniques for screening for suitable SFPs. Optimized SFPs for accomplishing high level secretion of target polypeptides are described. The SFPs of the invention are capable of inducing hyper-secretory production of recombinant proteins. 
     Related Art 
     The recombinant expression of proteins of interest is a widely used procedure to produce large quantities of proteins for research purposes or for therapeutic and other commercial uses. A variety of recombinant expression systems are known in the art, including bacterial, yeast, and mammalian host cell systems, and many different proteins have been successfully produced in these systems. However, there are also many proteins that are not easily produced using available expression systems, resulting in little or no protein expression and secretion. Methods for improving the secretion of recombinantly expressed proteins, such as overexpression of molecular chaperones and foldase (Hackel et al.,  Pharm Res  23:790 (2006); Poewer and Robinson,  Biotechnol Prog  23: 364 (2007); Shusta et al.,  Nat Biotechnol  16: 773 (1998)), over-expression of genes related to the secretory pathway ((Carla Fama et al.,  Biochim Biophys Acta  1773: 232 (2007); Wentz and Shusta et al.,  Appl Environ Microbiol  73: 1189 (1998)), engineering of the leader sequence (Clements et al.,  Gene  106: 267 (1991); Kjaerulff and Jensen,  Biochem Biophys Res Commun  336: 974 (2005); Sagiya et al.,  Microbiol. Biotechnol  42: 358 (1994); Li et al.,  Bitechnol Prog  18: 831 (2002)) have had some success with particular proteins of interest. 
     Another way of increasing protein productivity is to link the protein of interest to a fusion partner. Secretory proteins used as a fusion partners, including, human serum albumin (Kang et al.,  Protein Expr Purif  53: 331 (2007); Huang et al.,  J. Pept. Sci  14: 588 (2008)), alpha-lactalbumin (WO1995027782A1), rubredoxin (WO2000039310A1), human glucagon (WO2000053777A1), cathelicidin-related peptide derived from the hagfish (WO2005019242A2), phosphoribulokinase (US6500647B1), protein disulfide isomerase (Kajino et al.,  Appl Environ Microbiol  66: 638 (2000), Staphylococcal Protein A (Moreno et al.,  Protein Expr Purif  18: 242 (2000), Hsp150 protein (Sievi et al.,  Biotechnol. Prog.  19: 1368 (2003), cellulose-binding domain (Ahn et al.,  Appl Microbiol Biotechnol.  64: 833 (2004)) and gold binding peptide (US20050106625A1) have had some success with particular proteins of interest. 
     In an effort to identify secreted proteins and novel signal sequences, several signal sequence trap systems have been developed. U.S. Pat. No. 6,228,590 describes a technique for screening for mammalian signal sequences by transforming reporter protein-deficient yeast with nucleic acids comprising mammalian coding sequences fused to a reporter protein and detecting cells that secrete the reporter protein. A similar system using invertase-deficient yeast and an invertase reporter protein is disclosed in EP0907727. Yeast-based signal sequence traps have been used to identify secreted proteins from human DNA (Klein et al.,  Proc. Natl. Acad. Sci. USA  93:7108 (1996); Jacobs et al.,  Gene  198:289 (1997)), mouse DNA (Gallicioti et al.,  J. Membrane Biol.  183:175 (2001)), zebrafish DNA (Crosier et al.,  Dev. Dynamics  222:637 (2001)),  Arabidopsis  DNA (Goo et al.,  Plant Mol. Biol.  41:415 (1999)), potato DNA (Surpili et al.,  Anais de Academia Brasileira de Ciencias  74:599 (2002)), and  Candida albicans  DNA (Monteoliva et al.,  Eukaryotic Cell  1:514 (2002)). Similar trap systems have been developed using mammalian host cells (Gallicioti et al.,  J. Membrane Biol.  183:175 (2001)) and bacterial host cells (Ferguson et al.,  Cancer Res.  65:8209 (2000). Reporter proteins that have been used in signal sequence traps include invertase (Klein et al.,  Proc. Natl. Acad. Sci. USA  93:7108 (1996)), alpha amylase (U.S. Pat. No. 6,228,590), acid phosphatase (PHO5) (Surpili et al.,  Anais de Academia Brasileira de Ciencias  74:599 (2002)), and β-lactamase Ferguson et al.,  Cancer Res.  65:8209 (2000). 
     A method for identifying translational fusion partners (TFPs) useful for secretion of a target protein is disclosed in WO 2005/068658. The method comprises (i) obtaining a plurality of host cells transformed with a variety of vectors comprising a library of nucleic acid fragments and a target protein-encoding nucleotide sequence fused with a reporter protein-encoding nucleotide sequence, wherein the host cells are deficient in the reporter protein, and (ii) identifying a TFP library from the host cells, wherein the TFP library comprises nucleic acid fragments which individually induce the secretion of the target protein. 
     Translational fusion partner (TFP) technology for secretory production of rarely secretable proteins in yeast was described in WO 2007/015178. In the course of TFP screening from the yeast genome, the YGR106C (Voa1p) gene was discovered. The cellular location of Voa1p protein was recently identified in the ER membrane (Ryan et al., Mol. Biol. Cell, Epub ahead of print, Sep. 17, 2008). Voa1p was proposed to be one of five V0 assembly factors for vacuolar ATPase. 
     There remains a need in the art for additional sequences that enhance expression of proteins, and methods for identifying such sequences. 
     SUMMARY OF THE INVENTION 
     The present invention relates to hyper-secretory production and efficient purification of various recombinant proteins using secretion fusion partners (SFPs), which can be obtained by secretome analysis. Recombinant proteins are extracellularly produced in a fusion form with a secretion fusion partner and can be separated from the SFP by in vitro protease treatment. SFPs described in this invention greatly improve the secretion level of target proteins and polypeptides which are valuable for bio-pharmaceuticals and the bio-industry. Methods for selection/screening of SFPs are also described. Although it is possible to determine or even predict whether a particular protein is secreted, it is not possible to predict whether a secreted protein will act as a SFP. The selection/screening method of the invention allows the selection of proteins, and fragments and derivatives of such proteins, that act as SFPs. The SFPs selected by the present method of screening/selection enhance the recombinant production of proteins that are useful in bio-pharmaceuticals and the bio-industry. Also included in the invention are SFPs and fragments and derivatives thereof that have been identified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  shows (A) the predicted amino sequence and domain of SFP1 protein (SEQ ID NO:1), (B) a schematic diagram of vectors expressing serially deleted SFP1 genes, (C) SDS-PAGE analysis of relative SFP1 protein expression levels. 10% Tris-Tricine SDS-PAGE analysis of 0.6 ml of each culture broth concentrated with 0.4 ml of acetone. Lane 1: Culture broth of 2805 strain transformed with YGaT91 vector; Lane 2: Culture broth of 2805 strain transformed with YGaT92 vector; Lane 3: Culture broth of 2805 strain transformed with YGaT93 vector; Lane 4: Culture broth of 2805 strain transformed with YGaT94 vector; Lane 5: Culture broth of 2805 strain transformed with YGaT95 vector; Lane 6: Culture broth of 2805 strain transformed with YGaT96 vector; Lane 7: Culture broth of 2805 strain transformed with YGaT97 vector; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 2  shows (A) a schematic diagram of vectors expressing SFP1-IL2 fusion proteins, (B) SDS-PAGE analysis of SFP1-IL2 fusion protein expression levels. 10% Tris-Tricine SDS-PAGE analysis of 0.6 ml of each culture broth concentrated with 0.4 ml of acetone. Lane 1: Culture broth of 2805 strain transformed with YGaT92-IL2 vector; Lane 2: Culture broth of 2805 strain transformed with YGaT93-IL2 vector; Lane 3: Culture broth of 2805 strain transformed with YGaT94-IL2 vector; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 3  illustrates (A) a profile for fed-batch fermentation of a recombinant yeast strain containing YGaT92-EXD4 and (B) the results of SDS-PAGE for analyzing proteins secreted into the medium according to fermentation time. 
         FIG. 4  shows SDS-PAGE analysis of purified SFP1-EXD4 fusion protein digested with different concentrations of enterokinase (Invitrogen, USA). Lane 1: Purified SFP1-EXD4 fusion protein; Lane 2: Purified SFP1-EXD4 fusion protein digested with 0.1 μl of enterokinase for 1 hr at 37° C.; Lane 3: Purified SFP1-EXD4 fusion protein digested with 0.2 μl of enterokinase for 1 hr at 37° C.; Lane 4: Purified SFP1-EXD4 fusion protein digested with 0.3 μl of enterokinase for 1 hr at 37° C.; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 5  shows (A) a HPLC analysis of enterokinase digested SFP1-EXD4 fusion protein, (B) SDS-PAGE analysis of HPLC fractions. The numbers above the gel indicate HPLC fraction number. 
         FIG. 6  is a MALDI-TOF analysis of purified EXD4 protein. 
         FIG. 7  shows (A) a schematic diagram of vectors expressing SFP1 variants-EXD4 fusion proteins, (B) SDS-PAGE analysis of SFP1 variants—EXD4 fusion protein expression levels. 10% Tris-Tricine SDS-PAGE analysis of 0.6 ml of each culture broth concentrated with 0.4 ml of acetone. Lane 1: Culture broth of 2805 strain transformed with YGaT92-EXD4 vector; Lane 2: Culture broth of 2805 strain transformed with YGaT921-EXD4 vector; Lane 3: Culture broth of 2805 strain transformed with YGaT922-EXD4 vector; Lane 4: Culture broth of 2805 strain transformed with YGaT923-EXD4 vector; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 8  shows SDS-PAGE analysis of fed-batch fermentation of a recombinant yeast strain containing YGaMKH-EXD4 at the indicated fermentation time. 
         FIG. 9  illustrates (A) a profile for fed-batch fermentation of a recombinant yeast strain containing YGaST6-EXD4-HL and (B) the results of SDS-PAGE for analyzing proteins secreted into the medium according to fermentation time. 
         FIG. 10  shows (A) a profile for fed-batch fermentation of a recombinant yeast strain containing YGaMKH-EGF and (B) the results of SDS-PAGE for analyzing proteins secreted into the medium according to fermentation time. 
         FIG. 11  shows (A) the result of Ni-NTA affinity chromatography of HL-EGF fusion protein. The patched drawing is the SDS-PAGE analysis of indicated fractions and (B) the result of Ni-NTA affinity chromatography of HL-EGF fusion protein after digestion with enterokinase. The patched drawing is the SDS-PAGE analysis of indicated fractions. 
         FIG. 12  illustrates (A) a profile for fed-batch fermentation of a recombinant yeast strain containing YGaMKH-PTH and (B) the results of SDS-PAGE for analyzing proteins secreted into the medium according to fermentation time. 
         FIG. 13  shows SDS-PAGE analysis of purified HL-PTH fusion protein digested with secretion form of recombinant Kex2p (J H Sohn, KRIBB) and enterokinase (Invitrogen, USA). Lane 1: Purified HL-PTH fusion protein; Lane 2: Purified HL-PTH fusion protein digested with Kex2p for 1 hr at 37° C.; Lane 3: Purified HL-PTH fusion protein digested with enterokinase for 1 hr at 37° C.; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 14  shows (A) the growth curve of 2805 strain and arrows indicate the sampling points, (B) the confocal laser scanning microscope of sampled cells after staining with a fluorescent dye hochest. 
         FIG. 15  shows the results of 2D gel electrophoresis of M2 sample. 
         FIG. 16  shows SDS-PAGE analysis for 1-DE/MudPIT 
       (Multidimensional Protein Identification Technology). 
         FIG. 17  shows (A) SDS-PAGE analysis of culture supernatant of Y2805 transformants expressing 19 genes selected from secretome analyses. 10% Tris-Glycine SDS-PAGE analysis of 0.6 ml of each culture broth concentrated with 0.4 ml of acetone. Lane 1: Culture broth of 2805 strain over-expressing BGL2 gene; Lane 2: Culture broth of 2805 strain over-expressing CIS3; Lane 3: Culture broth of 2805 strain over-expressing CRH1; Lane 4: Culture broth of 2805 strain over-expressing CWP1; Lane 5: Culture broth of 2805 strain over-expressing DSE4; Lane 7: Culture broth of 2805 strain over-expressing EGT2; Lane 8: Culture broth of 2805 strain over-expressing EXG1; Lane 9: Culture broth of 2805 strain over-expressing GAS1; Lane 10: Culture broth of 2805 strain over-expressing GAS3; Lane 11: Culture broth of 2805 strain over-expressing GAS5; Lane 12: Culture broth of 2805 strain over-expressing PST1; Lane 13: Culture broth of 2805 strain over-expressing SCW4; Lane 15: Culture broth of 2805 strain over-expressing SIM1; Lane 16: Culture broth of 2805 strain over-expressing TOS1; Lane 17: Culture broth of 2805 strain over-expressing UTH1; Lane 18: Culture broth of 2805 strain over-expressing YGP1; Lane 19: Culture broth of 2805 strain over-expressing YPS1; Lane 20: Culture broth of 2805 strain over-expressing ZPS1; Lane M: Pre-stained protein size marker (Invitrogen). (B) SDS-PAGE analysis of culture supernatant after Endo-H treatment. 
         FIG. 18  shows SDS-PAGE analysis of culture supernatant of Y2805 transformants expressing 11 genes fused with EXD4 gene, respectively. 10% Tris-Tricine SDS-PAGE analysis of 0.6 ml of each culture broth concentrated with 0.4 ml of acetone. Lane 1: Culture broth of 2805 strain over-expressing BGL2-EXD4 gene; Lane 2: Culture broth of 2805 strain over-expressing GAS3-EXD4; Lane 3: Culture broth of 2805 strain over-expressing GAS5-EXD4; Lane 4: Culture broth of 2805 strain over-expressing PST1-EXD4; Lane 5: Culture broth of 2805 strain over-expressing SCW4-EXD4; Lane 6: Culture broth of 2805 strain over-expressing SCW10-EXD4; Lane 7: Culture broth of 2805 strain over-expressing SIM1-EXD4; Lane 8: Culture broth of 2805 strain over-expressing UTH1-EXD4; Lane 9: Culture broth of 2805 strain over-expressing YGP1-EXD4; Lane 10: Culture broth of 2805 strain over-expressing YPS1-EXD4; Lane 11: Culture broth of 2805 strain over-expressing ZPS1-EXD4; Lane M: Pre-stained protein size marker (Invitrogen). 
         FIG. 19  shows (A) Kyte-Doolittle hydropathy analysis and schematic drawing for the deletion fragments of SCW4 and EXD4 fusion, (B) SDS-PAGE analysis of culture supernatants of each transformant containing gradually deleted SCW4-EXD4 fusion fragments. 
         FIG. 20  shows the results of SDS-PAGE for analyzing proteins secreted into the medium during fed-batch fermentation of a recombinant 2805 yeast strain containing YGa-SCW4-1-EXD4 and YGa-SCW4-3-EXD4, respectively. 
         FIG. 21  shows the results of SDS-PAGE for analyzing the secreted fusion proteins, SCW4-1-EXD4 and SCW4-3-EXD4 before and after treatment of enterokinase. 
         FIG. 22  shows the results of SDS-PAGE of secreted SCW4-hGH into the medium (A) culture broth 10 microliter of cells containing each vector, (B) samples before and after treatment of enterokinase. 
         FIG. 23  shows the results of SDS-PAGE for analyzing proteins secreted into the medium during fed-batch fermentation of a recombinant yeast strain containing YGa-SCW4-2-hGH according to fermentation time. 
         FIG. 24  shows a map of IL-2 expression vector pYGaT92-IL2. 
         FIG. 25  shows a map of exendin-4 expression vector pYGaT923-EXD4. 
         FIG. 26  shows a map of exendin-4 expression vector pYGaMKH-EXD4. 
         FIG. 27  shows a map of exendin-4 expression vector pYGaST6-EXD-HL. 
         FIG. 28  shows a map of EGF expression vector pYGaMKH-EGF. 
         FIG. 29  shows a map of PTH expression vector pYGaMKH-PTH. 
         FIG. 30  shows a map of exendin-4 expression vector pYGaSCW4-1-EXD4. 
         FIG. 31  shows a map of exendin-4 expression vector pYGaSCW4-3-EXD4. 
         FIG. 32  shows a map of hGH expression vector pYGaSCW4-2-hGH. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses the need for high level secretion of target polypeptides and for rapid and efficient screening technique for identification of SFPs applicable for achieving high level secretion of target polypeptides. While the invention is useful to optimize the recombinant expression of any protein, it is particularly useful to enable the production of proteins that cannot be produced on a large scale and/or at low cost due to their low level of expression in known expression systems. Optimized SFPs for accomplishing high level secretion of target polypeptides are described. 
     DEFINITIONS 
     It is to be noted that the term “a” or “an” entity refers to one or more of that entity; for example, “a vector” is understood to represent one or more vectors. As such, the terms “a” (or “an”), “one or more,” and “at least one” can be used interchangeably herein. 
     As used herein, the term “polypeptide” is intended to encompass a singular “polypeptide” as well as plural “polypeptides,” and refers to a molecule composed of monomers (amino acids) linearly linked by amide bonds (also known as peptide bonds). The term “polypeptide” refers to any chain or chains of two or more amino acids, and does not refer to a specific length of the product. Thus, peptides, dipeptides, tripeptides, oligopeptides, “protein,” “amino acid chain,” or any other term used to refer to a chain or chains of two or more amino acids, are included within the definition of “polypeptide,” and the term “polypeptide” may be used instead of, or interchangeably with any of these terms. The term “polypeptide” is also intended to refer to the products of post-expression modifications of the polypeptide, including without limitation glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, or modification by non-naturally occurring amino acids. A polypeptide may be derived from a natural biological source or produced by recombinant technology, but is not necessarily translated from a designated nucleic acid sequence. It may be generated in any manner, including by chemical synthesis. 
     By an “isolated polypeptide” or a fragment, variant, or derivative thereof is intended a polypeptide that is not in its natural milieu. No particular level of purification is required. For example, an isolated polypeptide can be removed from its native or natural environment. Recombinantly produced polypeptides and proteins expressed in host cells are considered isolated for purposed of the invention, as are native or recombinant polypeptides which have been separated, fractionated, or partially or substantially purified by any suitable technique. 
     Also included as polypeptides of the present invention are fragments, derivatives, analogs, or variants of the foregoing polypeptides, and any combination thereof. The terms “fragment,” “variant,” “derivative” and “analog” when referring to polypeptides of the present invention include polypeptides that retain at least some of the biological, antigenic, or immunogenic properties of the corresponding native polypeptide. Fragments of polypeptides of the present invention include proteolytic fragments, as well as deletion fragments, in addition to other specific fragments discussed elsewhere herein. Variants of polypeptides of the present invention include fragments as described above, and also polypeptides with altered amino acid sequences due to amino acid substitutions, deletions, or insertions. Variants may occur naturally or be non-naturally occurring. Non-naturally occurring variants may be produced using art-known mutagenesis techniques. Variant polypeptides may comprise conservative or non-conservative amino acid substitutions, deletions or additions. Derivatives of polypeptides of the present invention, include polypeptides which have been altered so as to exhibit additional features not found on the native polypeptide. Variant polypeptides may also be referred to herein as “polypeptide analogs.” As used herein a “derivative” of a polypeptide refers to a subject polypeptide having one or more residues chemically derivatized by reaction of a functional side group. Also included as “derivatives” are those peptides which contain one or more naturally occurring amino acid derivatives of the twenty standard amino acids. For example, 4-hydroxyproline may be substituted for proline; 5-hydroxylysine may be substituted for lysine; 3-methylhistidine may be substituted for histidine; homoserine may be substituted for serine; and ornithine may be substituted for lysine. 
     By “a reference amino acid sequence” is meant the specified sequence without the introduction of any amino acid substitutions. As one of ordinary skill in the art would understand, if there are no substitutions, the “isolated polypeptide” of the invention comprises an amino acid sequence which is identical to the reference amino acid sequence. 
     Polypeptides described herein may have various alterations such as substitutions, insertions or deletions. Exemplary amino acids that can be substituted in the polypeptide include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). 
     Corresponding fragments of polypeptides at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identical to the polypeptides and reference polypeptides described herein are also contemplated. 
     Sequence identity is calculated by comparing two optimally aligned sequences over that region of comparison, determining the number of positions at which the identical amino acid residue or nucleotide occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the region of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. In one aspect, percent identity is calculated as the percentage of amino acid residues or nucleotides in the smaller of two sequences which align with an identical amino acid residue or nucleotide in the sequence being compared, when four gaps in a length of 100 amino acids or nucleotides may be introduced to maximize alignment (Dayhoff, in Atlas of Protein Sequence and Structure, Vol. 5, p. 124, National Biochemical Research Foundation, Washington, D.C. (1972), incorporated herein by reference). A determination of identity is typically made by a computer homology program known in the art. An exemplary program is the Gap program (Wisconsin Sequence Analysis Package, Version 8 for UNIX, Genetics Computer Group, University Research Park, Madison, Wis.) using the default settings, which uses the algorithm of Smith and Waterman (Adv. Appl. Math., 1981, 2: 482-489, which in incorporated herein by reference in its entirety). 
     In certain embodiments, substitutions are conservative amino acid substitutions. A “conservative amino acid substitution” is one in which the amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined within the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). 
     In one embodiment, the invention relates to a method of identifying a secretion fusion partner (SFP), said method comprising: (i) transforming a first host cell with a heterologous promoter operably linked to a polynucleotide encoding a secreted polypeptide; (ii) determining whether said secreted polypeptide is over-secreted from said first host cells as compared to the secretion level of said polypeptide if assayed when said polypeptide&#39;s natural promoter is linked to said polynucleotide encoding said secreted polypeptide; (iii) transforming a second host cell with a polynucleotide construct comprising a polynucleotide encoding a target polypeptide and a polynucleotide encoding a polypeptide determined to be over-secreted in step (ii), wherein said target polypeptide and said over-secreted polypeptide are fused in any order; (iv) culturing said second host cell under conditions where said polynucleotide construct expresses a fusion polypeptide; and (v) determining whether said fusion polypeptide is secreted into the extra-cellular culture medium; thereby identifying whether said over-secreted polypeptide is a secretion fusion partner. 
     In the methods of the present invention, SFPs may be identified from a “secretome” or “total secreted polypeptides.” The secretome includes polypeptides secreted into and collected from the extracellular culture medium. Secretome are encoded by the DNA of any eukaryotic or prokaryotic organism, including bacteria, fungi (e.g., yeast), plants, and animals (e.g., mammals). Suitable bacteria include, but are not limited to  Escherichia  and  Bacillus  species. Suitable yeasts include, but are not limited to  Candida, Debaryomyces, Hansenula, Kluyveromyces, Pichia, Schizosaccharomyces, Yarrowia, Saccharomyces, Schwanniomyces , and  Arxula  species. Examples of specific species include  Candida utilis, Candida boidinii, Candida albicans, Kluyveromyces lactis, Pichia pastoris, Pichia stipitis, Schizosaccharomyces pombe, Saccharomyces cerevisiae, Hansenula polymorpha, Yarrowia lipolytica, Schwanniomyces occidentalis , and  Arxula adeninivorans . Other fungi that may serve as a source of DNA include, but are not limited to  Aspergillus, Penicillium, Rhizopus , and  Trichoderma  species. Plants that may serve as a source of DNA include, but are not limited to  Arabidopsis , maize, tobacco, and potato. Suitable animals include, but are not limited to humans, mice, rats, rabbits, dogs, cats, and monkeys. In one embodiment, secretome can be derived from yeast, bacteria, plants or animals. 
     Secretome analysis for selecting abundantly secreted polypeptides can be accomplished using the techniques available in the art. For example, total secreted polypeptides isolated by concentrating culture supernatant can be analyzed using 2-D gel electrophoresis and/or Multidimensional Protein Identification Technology (1-DE/MudPIT). Polypeptides from the secretome can be analyzed by any kinds of protein purification columns, such as ion exchange columns, hydrophobic interaction columns, gel filtration columns, affinity columns, and reverse phase columns. 
     In one embodiment, yeast total secreted polypeptides (yeast secretome) produced during normal yeast cell growth are analyzed. Normal cell growth means cells that were cultured in minimal media (e.g., 0.67% yeast nitrogen base without amino acids, 0.5% casamino acid, 2% glucose and 0.002% uracil). Altered conditions could be used, which may include different carbon sources instead of glucose, e.g., galactose, xylose, fructose, mannose, sucrose, raffinose, and cellobiose. The altered conditions can also include limiting the level of any component of the media, e.g., nitrogen or phosphate. 
     The term “abundantly secreted” refers to polypeptides that are at least in the top 40%, 45%, 50%, 55%, 60%, 65%, or 70% in level of the secreted polypeptides from the secretome. Abundantly secreted polypeptides can be determined by PAI (protein abundance index) (Rappsilber et al.,  Genome Res.  12:1231-45 (2002)) which may be proportional to the number of proteins secreted. Examples of abundantly secreted proteins are shown in Table 1. 
     The term “over-secreted” is defined as the secretion of a polypeptide from a host cell at a level of at least 5×, 6×, 7×, 8×, 9× or 10× over the level of secretion of the polypeptide when expressed from the polypeptide&#39;s natural promoter. Over-secretion can also be assayed by comparing the secretion level of the abundantly secreted polypeptides compared to wild-type protein secretion levels. For example, wild-type yeast secreted proteins do not exceed secretion levels of about 20 mg/L during normal cell growth, however, when linked with a strong heterologous promoter, some of these proteins are over-secreted and exceed the secretion level of 20 mg/L. 
     In one embodiment, the methods of the invention further comprise determining an optimal size of a SFP for secretion of a fusion polypeptide. The optimal size of an SFP can be determined by deletion analysis of said SFP, wherein the level of secretion of fusion polypeptides, each containing different deletion constructs of the SFP, are compared. Some SFPs may have an optimal size that allows for even higher expression of fusion polypeptides than the expression obtained with the initially identified SFP. The optimal size of a SFP may allow for increased secretion level of a target polypeptide compared to the secretion level of the target polypeptide when fused to a sub-optimal SFP. The optimal size of an SFP may vary between target polypeptides, and can be determined using the methods disclosed herein or known in the art once the SFP has first been identified. 
     In one embodiment, SFP deletion fragments ending with hydrophilic sequences are selected. The hydrophilic domain of a protein is usually located near the surface of protein. Thus, the junction of the SFP and target polypeptide can be easily exposed between two polypeptides, which may make it easier for a protease to cleave the junction to release target polypeptides in vitro. 
     The term “fragment thereof,” as applied to a SFP, refers to a polypeptide comprising any portion of the amino acid sequence of the SFP, wherein the fragment substantially retains the ability to induce the secretion of a target polypeptide to which it is fused. 
     The term “substantially retains the ability to induce the secretion of a target polypeptide to which it is fused,” as used herein, refers to a fragment or derivative of a SFP which retains at least 50% of the ability of the parent SFP to induce secretion of a target polypeptide to which it is fused. In some embodiments, at least 60, 65, 70, 75, 80, 85, 90, or 95% of the ability to induce the secretion of a target polypeptide to which it is fused is retained. The ability to induce the secretion of a target polypeptide may be determined by routine techniques well known in the art and described above. 
     The term “derivative thereof,” as applied to a SFP, refers to a polypeptide consisting of an amino acid sequence that is at least 70% identical to the amino acid sequence of the SFP, wherein the polypeptide substantially retains the ability to induce the secretion of a target polypeptide to which it is fused. In some embodiments, the derivative comprises an amino acid sequence that is at least 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identical to the amino acid sequence of the SFP. The derivative may comprise additions, deletions, substitutions, or a combination thereof to the amino acid sequence of the SFP. A derivative may include a mutant polypeptide with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11-15, 16-20, 21-25, or 26-30 additions, substitutions, or deletions. Additions or substitutions also include the use of non-naturally occurring amino acids. 
     Examples of derivatives of SFPs include, but are not limited to, deletion mutants (e.g., unidirectional deletion), addition of functional sequences (e.g., glycosylation sites, restriction enzyme sites), and deletion or addition (e.g., swapping) of pro-sequences or pre-sequences identified within SFPs. One of skill in the art can prepare derivatives of SFPs or nucleic acids encoding SFPs using routine mutagenesis techniques, such as those described in the references cited above, and identify derivatives that substantially retain the ability to induce the secretion of a target polypeptide to which it is fused. 
     In one embodiment, the SFP or a derivative or a fragment thereof is identified by the methods of the invention. In another embodiment, the nucleotide sequence encoding a SFP is selected from BGL2 (SEQ ID NO: 62), GAS3 (SEQ ID NO: 63), GAS5 (SEQ ID NO: 64), PST1 (SEQ ID NO: 65), SCW4 (SEQ ID NO: 66), SCW10 (SEQ ID NO: 67), SIMI (SEQ ID NO: 68), UTH1 (SEQ ID NO: 69), YGP1 (SEQ ID NO: 70), YPS1 (SEQ ID NO: 71), and ZPS1 (SEQ ID NO: 72). In another embodiment, a SFP is selected from BGL2 (SEQ ID NO: 80), GAS3 (SEQ ID NO: 81), GAS5 (SEQ ID NO: 82), PST1 (SEQ ID NO: 83), SCW4 (SEQ ID NO: 84), SCW10 (SEQ ID NO: 85), SIMI (SEQ ID NO: 86), UTH1 (SEQ ID NO: 87), YGP1 (SEQ ID NO: 88), YPS1 (SEQ ID NO: 89), and ZPS1 (SEQ ID NO: 90). 
     The methods of the present invention may be used with a “target polypeptide” or derivative thereof which is a polypeptide for which there is a desire for high level recombinant expression. The term “derivative thereof,” as applied to a target polypeptides, refers to a polypeptide consisting of an amino acid sequence that is at least 70% identical to the amino acid sequence of the target polypeptide, wherein the polypeptide substantially retains its biological activity. In some embodiments, the derivative comprises an amino acid sequence that is at least 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identical to the amino acid sequence of the target polypeptide. The derivative may comprise additions, deletions, substitutions, or a combination thereof to the amino acid sequence of the target polypeptide. A derivative may include a mutant polypeptide with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11-15, 16-20, 21-25, or 26-30 additions, substitutions, or deletions. Additions or substitutions also include the use of non-naturally occurring amino acids. 
     Examples of derivatives of target proteins or polypeptides include, but are not limited to deletion mutants (e.g., unidirectional deletion), addition of functional sequences (e.g., glycosylation sites, restriction enzyme sites), and deletion or addition (e.g., swapping) of pro-sequences or pre-sequences identified within the target polypeptide. One of skill in the art can prepare derivatives of target polypeptides or nucleic acids encoding target polypeptides using routine mutagenesis techniques, such as those described in the references cited above, and identify derivatives that substantially retain biological activity of the target polypeptide. 
     Where the target polypeptide is fused to a SFP, the target polypeptide and the SFP are not polypeptides of the same naturally occurring protein. The target polypeptide may be one that is being studied for research purposes or one that is being produced for commercial purposes, e.g., therapeutic or industrial use. The target polypeptide may be from any plant, animal, or microorganism, and may be naturally occurring or modified, as long as it can be encoded by a nucleic acid. In one embodiment the target polypeptide is a human protein. In another embodiment, the target polypeptide is a cytokine, serum protein, colony stimulating factor, growth factor, hormone, or enzyme. 
     For example, the target polypeptide may be selected from an interleukin, coagulation factor, interferon-α, -β or -γ, granulocyte-colony stimulating factor, granulocyte macrophage-colony stimulating factor, tissue growth factor, epithelial growth factor, TGFα, TGFβ, epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, follicle stimulating hormone, thyroid stimulating hormone, antidiuretic hormone, pigmentary hormone, parathyroid hormone, luteinizing hormone-releasing hormone, carbohydrate-specific enzymes, proteolytic enzymes, lipases, oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, immunoglobulins, cytokine receptors, lactoferrin, phospholipase A2-activating protein, insulin, tumor necrosis factor, calcitonin, calcitonin gene related peptide, enkephalin, somatomedin, erythropoietin, hypothalamic releasing factor, prolactin, chorionic gonadotropin, tissue plasminogen activator, growth hormone releasing peptide, thymic humoral factor, anticancer peptides, or antibiotic peptides. Specific examples include, but are not limited to human interleukin-2 (hIL-2), exendin-3, exendin-4 (EXD4), glucagon-like-peptide-1 (GLP-1), parathyroid hormone (PTH), human interleukin-1β, human interleukin-6, human interleukin-32α, -32β or -32γ, Factor VII, Factor VIII, Factor IX, human serum albumin, human interferon-α, -β or -γ, human granulocyte-colony stimulating factor, human granulocyte macrophage-colony stimulating factor, human growth hormone (hGH), human platelet-derived growth factor, human basic fibroblast growth factor, human epidermal growth factor (EGF), human insulin-like growth factor, human nerve growth factor, human transforming growth factor β-1, human follicle stimulating hormone, glucose oxidase, glucodase, galactosidase, glucocerebrosidase, glucuronidase, asparaginase, arginase, arginine deaminase, peroxide dismutase, endotoxinase, catalase, chymotrypsin, uricase, adenosine diphosphatase, tyrosinase, bilirubin oxidase, bovine galactose-1-phosphate uridyltransferase, jellyfish green fluorescent protein,  Candida antarctica  lipase B,  Candida rugosa  lipase, fungal chloroperoxidase, β-galactosidase, resolvase, α-galactosidase, β-glucosidase, trehalose synthase, cyclodextrin glycosyl transferase, xylanase, phytase, human lactoferrin, human erythropoietin, human paraoxonase, human growth differentiation factor 15, human galectin-3 binding protein, human serine protease inhibitor, Kunitz type 2, human Janus kinase 2, human fms-like tyrosine kinase 3 ligand, human YM1 &amp; 2, human CEMI, human diacylglycerol acyltransferase, human leptin, human mL259, human proteinase 3, human lysozyme, human DEAD box protein 41, human etoposide induced protein 24, mouse caspase1, bovine angiogenin, and earthworm lumbrokinase. 
     In one embodiment, the target polypeptide is a polypeptide that is difficult to produce using conventional recombinant production methods, that is, a polypeptide that is not produced at all or is only produced at low levels. In another embodiment, the target polypeptide is one that is readily produced using known expression systems, but for which there is a desire to achieve higher levels of expression. 
     In one embodiment, a fusion polypeptide of the invention refers to a polypeptide comprising a secreted polypeptide fused to a target polypeptide in any order. In another embodiment, the invention relates to an isolated fusion polypeptide comprising a SFP of the invention fused to a target polypeptide. 
     As used herein the term “fused” refers to a fusion polypeptide produced recombinantly. In one embodiment, the fusion polypeptide comprises a secreted polypeptide fused to a target polypeptide, wherein the secreted polypeptide and target polypeptide are fused in any order. In another embodiment, the SFP is fused at the N-terminus or C-terminus of the target polypeptide. The SFP and target polypeptide can be fused with or without intervening amino acids, such as those encoded by linker DNA. In some embodiments, the distance between the SFP and target polypeptide can be 0 to 10; 0 to 20; 0 to 30; 0 to 40; or more amino acids. In some embodiments, the fusion polypeptide comprises a protease recognition sequence and/or an affinity tag. 
     In one embodiment, the isolated fusion polypeptide comprises a SFP or a derivative thereof comprising a hydrophilic (HL) domain comprising amino acids 176-213 of SEQ ID NO: 1, and a target polypeptide. In one embodiment, a modified HL domain is encoded by SEQ ID NO: 45. 
     The present invention further relates to methods of recombinantly producing a target polypeptide using the SFPs of the invention. In one embodiment, the method comprises preparing a construct comprising a nucleotide sequence encoding a target polypeptide operably linked to a nucleotide sequence encoding a SFP or a derivative or fragment thereof, transforming a host cell with the construct, culturing the host cell under conditions in which a fusion polypeptide is produced and secreted from the host cell, and separating said SFP from said target polypeptide. 
     The target polypeptide may be recombinantly produced using any expression system known in the art. Preferably, the target polypeptide is recombinantly expressed, e.g., in bacterial, yeast, or mammalian cell cultures. Recombinant expression may involve preparing a vector comprising a polynucleotide encoding the target polypeptide, delivering the vector into a host cell, culturing the host cell under conditions in which the target polypeptide is expressed, and separating the target polypeptide. Methods and materials for preparing recombinant vectors and transforming host cells using the same, replicating the vectors in host cells and expressing biologically active foreign polypeptides and proteins are discussed herein and described in Sambrook et al., Molecular Cloning, 3rd edition, Cold Spring Harbor Laboratory, 2001 and Ausubel et al., Current Protocols in Molecular Biology, John Wiley &amp; Sons, New York 3rd edition, (2000), each incorporated herein by reference. 
     The target polypeptide may be isolated from the medium in which the host cells are grown, by purification methods known in the art, e.g., precipitation from the media, conventional chromatographic methods including immunoaffinity chromatography, receptor affinity chromatography, hydrophobic interaction chromatography, lectin affinity chromatography, size exclusion filtration, cation or anion exchange chromatography, high pressure liquid chromatography (HPLC), reverse phase HPLC, and the like. Still other methods of purification include those methods wherein the desired polypeptide is expressed and purified as a fusion polypeptide having a specific affinity peptide, tag, label, or chelating moiety that is recognized by a specific binding partner or agent. The purified polypeptide can be cleaved to yield the desired polypeptide, or can be left as an intact fusion polypeptide. Cleavage of the affinity tag component may produce a form of the desired polypeptide having additional amino acid residues as a result of the cleavage process. In one embodiment, the affinity tag is GST, MBP, NusA, thioredoxin, ubiquitin, FLAG, BAP, 6HIS, STREP, CBP, CBD, S-tag, or any combination thereof. 
     The target polypeptides of the invention may be extracellularly produced in a fusion form with a secretion fusion partner and can be separated from the SFP by in vitro protease treatment. If the isolated target polypeptide is not biologically active following the isolation procedure employed, various methods for “refolding” or converting the polypeptide to its tertiary structure and generating disulfide linkages, can be used to restore biological activity. Methods known to one of ordinary skill in the art include adjusting the pH of the solubilized polypeptide to a pH usually above 7 and in the presence of a particular concentration of a chaotrope. The selection of chaotrope is very similar to the choices used for inclusion body solubilization but usually at a lower concentration and is not necessarily the same chaotrope as used for the solubilization. It may be required to employ a reducing agent or the reducing agent plus its oxidized form in a specific ratio, to generate a particular redox potential allowing for disulfide shuffling to occur in the formation of the protein&#39;s cysteine bridge(s). Some of the commonly used redox couples include cysteine/cystamine, glutathione (GSH)/dithiobis GSH, cupric chloride, dithiothreitol (DTT)/dithiane DTT, 2-mercaptoethanol (bME)/dithio-b(ME). To increase the efficiency of the refolding, it may be necessary to employ a cosolvent, such as glycerol, polyethylene glycol of various molecular weights, and arginine. 
     The term “polynucleotide” is intended to encompass a singular nucleic acid as well as plural nucleic acids, and refers to an isolated nucleic acid molecule or construct, e.g., messenger RNA (mRNA), virally-derived RNA, or plasmid DNA (pDNA). A polynucleotide may comprise a conventional phosphodiester bond or a non-conventional bond (e.g., an amide bond, such as found in peptide nucleic acids (PNA)). The term “nucleic acid” refers to any one or more nucleic acid segments, e.g., DNA or RNA fragments, present in a polynucleotide. By “isolated” nucleic acid or polynucleotide is intended a nucleic acid molecule, DNA or RNA, which has been removed from its native environment. For example, a recombinant polynucleotide encoding a therapeutic polypeptide contained in a vector is considered isolated for the purposes of the present invention. Further examples of an isolated polynucleotide include recombinant polynucleotides maintained in heterologous host cells or purified (partially or substantially) polynucleotides in solution. Isolated RNA molecules include in vivo or in vitro RNA transcripts of the present invention, as well as positive and negative strand forms, and double-stranded forms, of pestivirus vectors disclosed herein. 
     Isolated polynucleotides or nucleic acids according to the present invention further include such molecules produced synthetically. In addition, a polynucleotide or a nucleic acid may be or may include a regulatory element such as a promoter, ribosome binding site, or a transcription terminator. 
     As used herein, a “coding region” is a portion of nucleic acid which consists of codons translated into amino acids. Although a “stop codon” (TAG, TGA, or TAA) is not translated into an amino acid, it may be considered to be part of a coding region, if present, but any flanking sequences, for example promoters, ribosome binding sites, transcriptional terminators, introns, 5′ and 3′ non-translated regions, and the like, are not part of a coding region. Two or more coding regions of the present invention can be present in a single polynucleotide construct, e.g., on a single vector, or in separate polynucleotide constructs, e.g., on separate (different) vectors. Furthermore, any vector may contain a single coding region, or may comprise two or more coding regions, e.g., a vector of the present invention may encode one or more polypolypeptides, which are post- or co-translationally separated into the final polypeptides via proteolytic cleavage. In addition, a vector, polynucleotide, or nucleic acid of the invention may encode heterologous coding regions, either fused or unfused to a first or second nucleic acid encoding of the invention, or variant or derivative thereof. Heterologous coding regions include without limitation specialized elements or motifs, such as a secretory signal peptide or a heterologous functional domain. 
     In certain embodiments, the polynucleotide or nucleic acid is DNA. In the case of DNA, a polynucleotide comprising a nucleic acid, which encodes a polypeptide normally may include a promoter and/or other transcription or translation control elements operably associated with one or more coding regions. An operable association is when a coding region for a gene product, e.g., a polypeptide, is associated with one or more regulatory sequences in such a way as to place expression of the gene product under the influence or control of the regulatory sequence(s). Two DNA fragments (e.g., a polypeptide coding region and a promoter associated therewith) are “operably associated” if induction of promoter function results in the transcription of mRNA encoding the desired gene product and if the nature of the linkage between the two DNA fragments does not interfere with the ability of the expression regulatory sequences to direct the expression of the gene product or interfere with the ability of the DNA template to be transcribed. Thus, a promoter region would be operably associated with a nucleic acid encoding a polypeptide if the promoter was capable of effecting transcription of that nucleic acid. The promoter may be a cell-specific promoter that directs substantial transcription of the DNA only in predetermined cells. Other transcription control elements, besides a promoter, for example enhancers, operators, repressors, and transcription termination signals, can be operably associated with the polynucleotide to direct cell-specific transcription. Suitable promoters and other transcription control regions are disclosed herein. 
     A variety of transcription control regions are known to those skilled in the art. These include, without limitation, transcription control regions, which function in vertebrate cells, such as, but not limited to, promoter and enhancer segments from cytomegaloviruses (e.g., the immediate early promoter, in conjunction with intron-A), simian virus 40 (e.g., the early promoter), and retroviruses (such as, e.g., Rous sarcoma virus). Other transcription control regions include those derived from vertebrate genes such as actin, heat shock protein, bovine growth hormone and rabbit β-globin, as well as other sequences capable of controlling gene expression in eukaryotic cells. Additional suitable transcription control regions include tissue-specific promoters and enhancers as well as lymphokine-inducible promoters (e.g., promoters inducible by interferons or interleukins). 
     Similarly, a variety of translation control elements are known to those of ordinary skill in the art. These include, but are not limited to ribosome binding sites, translation initiation and termination codons, and elements derived from viral systems (particularly an internal ribosome entry site, or IRES, also referred to as a CITE sequence). 
     A polynucleotide of the present invention may include RNA, for example, in the form of messenger RNA (mRNA). RNA of the present invention may be single stranded or double stranded. 
     Polynucleotide and nucleic acid coding regions of the present invention may be associated with additional coding regions which encode secretory or signal peptides, which direct the secretion of a polypeptide encoded by a polynucleotide of the present invention. According to the signal hypothesis, proteins secreted by mammalian cells have a signal peptide or secretory leader sequence which is cleaved from the mature protein once export of the growing protein chain across the rough endoplasmic reticulum has been initiated. Those of ordinary skill in the art are aware that polypeptides secreted by vertebrate cells generally have a signal peptide fused to the N-terminus of the polypeptide, which is cleaved from the complete or “full length” polypeptide to produce a secreted or “mature” form of the polypeptide. In certain embodiments, the native signal peptide, e.g., an immunoglobulin heavy chain or light chain signal peptide is used, or a functional derivative of that sequence that retains the ability to direct the secretion of the polypeptide that is operably associated with it. Alternatively, a heterologous mammalian signal peptide, or a functional derivative thereof, may be used. For example, the wild-type leader sequence may be substituted with the leader sequence of human tissue plasminogen activator (TPA) or mouse β-glucuronidase. 
     The term “construct” refers to a non-naturally occurring nucleic acid molecule. A construct is a polynucleotide that encodes a fusion polypeptide. In one embodiment, the construct encodes a fusion polypeptide comprising a SFP or a candidate SFP and a target polypeptide. A construct can further comprise a circular or linear vector and can be combined with other polynucleotides, for example by homologous recombination. 
     As used herein, the term “vector” refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. One type of vector is a “plasmid”, which refers to a circular double stranded DNA loop into which additional DNA segments can be ligated. Another type of vector is a viral vector, wherein additional DNA segments can be ligated into the viral genome. Certain vectors are capable of autonomous replication in a host cell into which they are introduced (e.g., bacterial vectors having a bacterial origin of replication and episomal mammalian vectors). Other vectors (e.g., non-episomal mammalian vectors) are integrated into the genome of a host cell upon introduction into the host cell, and thereby are replicated along with the host genome. The vectors of the present invention are capable of directing the expression of genes encoding target polypeptides to which they are operatively linked. Such vectors are referred to herein as “expression vectors”. In general, expression vectors of utility in recombinant DNA techniques are often in the form of plasmids. In the present specification, “plasmid” and “vector” can be used interchangeably as the plasmid is the most commonly used form of vector. However, the invention is intended to include such other forms of expression vectors, such as viral vectors (e.g., replication defective retroviruses, adenoviruses and adeno-associated viruses), that serve equivalent functions. 
     Vector DNA can be introduced into prokaryotic or eukaryotic cells via conventional transformation or transfection techniques. As used herein, the terms “transformation” and “transfection” are intended to refer to a variety of art-recognized techniques for introducing foreign nucleic acid (e.g., DNA) into a host cell, including calcium phosphate or calcium chloride co-precipitation, DEAE-dextran-mediated transfection, lipofection, or electroporation. Suitable methods for transforming or transfecting host cells can be found in Sambrook, et al. (MOLECULAR CLONING: A LABORATORY MANUAL. 2nd ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989), and other laboratory manuals. 
     For stable transfection of mammalian cells, it is known that, depending upon the expression vector and transfection technique used, only a small fraction of cells may integrate the foreign DNA into their genome. In some embodiments, in order to identify and select these integrants, a gene that encodes a selectable marker (e.g., resistance to antibiotics) is generally introduced into the host cells along with the gene of interest. Various selectable markers include those that confer resistance to drugs, such as G418, hygromycin and methotrexate. Nucleic acid encoding a selectable marker can be introduced into a host cell on the same vector as that encoding the target polypeptide or can be introduced on a separate vector. Cells stably transfected with the introduced nucleic acid can be identified by drug selection, auxotrophic marker selection, media composition, carbon source selection, or other methods known in the art (e.g., cells that have incorporated the selectable marker gene will survive, while the other cells die). 
     In one embodiment, a nucleotide sequence encoding a polypeptide or a fragment or a derivative thereof used in the methods of the present invention may further comprises at the 5′ end and 3′ end, DNA that is used for in vivo homologous recombination with a linear vector of the invention. The 5′ end and 3′ end DNA provides sufficient homologous sequence to allow in vivo recombination between the nucleotide sequence encoding a polypeptide or a fragment or a derivative thereof and the linear vector when they are co-transformed into the host cell. In one embodiment, the 5′ end and 3′ end DNA each comprise at least 20 base pairs that overlap with sequence of the linear vector, e.g., at least 30 or 40 base pairs. The addition of the 5′ and 3′ DNA may be carried out using routine recombinant DNA techniques, e.g., PCR and/or restriction enzyme cleavage and ligation. 
     The polynucleotide of the present invention may further encode an affinity tag, e.g., GST, MBP, NusA, thioredoxin, ubiquitin, FLAG, BAP, 6HIS, STREP, CBP, CBD, or S-tag. In some embodiments, the affinity tag may be encoded by a linker DNA or may be encoded by another portion of the polynucleotide of the invention such as the portion 5′ or 3′ to the region encoding the fusion protein. 
     The polynucleotide of the present invention may further include a linker DNA. In one embodiment the linker DNA encodes a linker peptide. 
     The linker DNA of the invention may be of sufficient length and have sufficient sequence identity to a portion of the nucleotide sequence of a linear vector to allow in vivo recombination between a polypeptide-encoding nucleotide sequence and the linear vector when they are co-transformed into a host cell. In one embodiment, the linker DNA is more than 20 base pairs in length, e.g., more than 30 or 40 base pairs in length. In a further embodiment, the linker DNA is at least 80% identical to the corresponding sequence on the linear vector, e.g., at least 85%, 90%, 95%, or 99% identical. 
     In one embodiment, the linker DNA encodes a protease recognition sequence thereby allowing cleavage at the junction of the SFP and the target polypeptide. For example, the linker DNA may encode a yeast kex2p or Kex2-like protease recognition sequence (e.g., an amino acid sequence comprising Lys-Arg, Arg-Arg, or Leu-Asp-Lys-Arg (SEQ ID NO: 74)), a mammalian furin-recognition sequence (e.g., an amino acid sequence comprising Arg-X-X-Arg), a factor Xa-recognition sequence (e.g., an amino acid sequence comprising Ile-Glu-Gly-Arg (SEQ ID NO: 75)), an enterokinase-recognition sequence (e.g., an amino acid sequence comprising Asp-Asp-Lys), a subtilisin-recognition sequence (e.g., an amino acid sequence comprising Ala-Ala-His-Tyr (SEQ ID NO: 76)), a tobacco etch virus protease-recognition sequence (e.g., an amino acid sequence comprising Glu-Asn-Leu-Tyr-Phe-Gln-Gly (SEQ ID NO: 77)), a ubiquitin hydrolase-recognition sequence (e.g., an amino acid sequence comprising Arg-Gly-Gly) or a thrombin-recognition sequence (e.g., an amino acid sequence comprising Arg-Gly-Pro-Arg (SEQ ID NO: 78)). 
     It is a preference to avoid unwanted cleavage of the fusion polypeptide by endogenous host proteases, either within the protease site in the linker or within the secreted polypeptide or the target polypeptide. Likewise, it is preferred to avoid cleavage within the target polypeptide or secreted polypeptide or SFP or fragment or derivative thereof by the protease used to cleave the secreted polypeptide from the target polypeptide. Thus, where a linker DNA encoding a protease recognition sequence is transformed into a host cell as part of a polynucleotide encoding a fusion polypeptide, the host cell preferably does not express the protease that recognizes the protease sequence in the linker. The host cell can either naturally not express the protease or the host cell can be modified to not express the protease (e.g., kex2 mutant host cells, kex2-like proteases mutant host cell, and furin mutant host cell). In certain embodiments, where the fusion polypeptide comprises a secreted polypeptide and a target polypeptide, the secreted polypeptide or SFP or fragment or derivative thereof and/or the target polypeptide can either naturally not comprise the host protease recognition sequence or the secreted polypeptide or SFP or fragment or derivative thereof and/or target polypeptide can be modified so that they do not contain sequences that are recognized by the host protease. Where the fusion polypeptide comprises a secreted polypeptide or SFP or fragment or derivative thereof, a target polypeptide, and a peptide linker comprising a protease recognition sequence, the secreted polypeptide or SFP or fragment or derivative thereof and/or the target polypeptide can either naturally not comprise the protease recognition sequence or the secreted polypeptide or SFP or fragment or derivative thereof and/or the target polypeptide can be modified so that they do not contain sequences that are recognized by the protease that recognizes the protease recognition sequence of the peptide linker. 
     In another embodiment, the linker DNA encodes an affinity tag, e.g., GST, MBP, NusA, thioredoxin, ubiquitin, FLAG, BAP, 6HIS, STREP, CBP, CBD, or S-tag. 
     In a further embodiment, the linker DNA encodes a restriction enzyme recognition site and a protease recognition sequence (e.g., kex2p-like protease- or kex-2p-recognition sequence). 
     Expression of polypeptides in prokaryotes may be carried out with vectors containing constitutive or inducible promoters directing the expression of the target polypeptide-reporter polypeptide fusion. Examples of suitable  E. coli  expression vectors include pTrc (Amrann et al.,  Gene  69:301-315 (1988)) and pET (Studier et al., GENE EXPRESSION TECHNOLOGY: METHODS IN ENZYMOLOGY 185, Academic Press, San Diego, Calif. (1990) 60-89). 
     For expression in yeast cells, suitable yeast expression vectors include, but are not limited to pYepSec1 (Baldari et al.,  EMBO J.  6:229-234 (1987)), pMFa (Kurjan et al.,  Cell  30:933-943 (1982)), pJRY88 (Schultz et al.,  Gene  54:113-123 (1987)), pYES2 (Invitrogen Corporation, San Diego, Calif.), and picZ (Invitrogen Corp, San Diego, Cal.). 
     For expression in insect cells, baculovirus expression vectors may be used. Examples of baculovirus vectors available for expression of polypeptides in cultured insect cells (e.g., SF9 cells) include the pAc series (Smith et al.,  Mol. Cell. Biol.  3:2156-2165 (1983)) and the pVL series (Lucklow et al.,  Virology  170:31-39 (1989)). 
     In another embodiment, the host cells are mammalian cells and the vector is a mammalian expression vector. Examples of mammalian expression vectors include, but are not limited to, pCDM8 (Seed,  Nature  329:840 (1987)) and pMT2PC (Kaufman et al.,  EMBO J.  6: 187-195 (1987)). When used in mammalian cells, the expression vector&#39;s control functions are often provided by viral regulatory elements. For example, commonly used promoters are derived from polyoma, adenovirus 2, cytomegalovirus and Simian Virus 40. For other suitable expression systems for both prokaryotic and eukaryotic cells, see, e.g., Chapters 16 and 17 of Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL. 2nd ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989. 
     Preferred vectors include, but are not limited to, plasmids, phages, cosmids, episomes, viral particles or viruses, and integratable DNA fragments (i.e., fragments integratable into the host genome by homologous recombination). Preferred viral particles include, but are not limited to, adenoviruses, baculoviruses, parvoviruses, herpesviruses, poxviruses, adenoassociated viruses, Semliki Forest viruses, vaccinia viruses, and retroviruses. Preferred expression vectors include, but are not limited to, pcDNA3 (Invitrogen) and pSVL (Pharmacia Biotech). Other expression vectors include, but are not limited to, pSPORT™ vectors, pGEM™ vectors (Promega), pPROEXvectors™ (LTI, Bethesda, Md.), Bluescript™ vectors (Stratagene), pQE™ vectors (Qiagen), pSE420™ (Invitrogen), and pYES2™ (Invitrogen). 
     In one embodiment, expression vectors are replicable DNA constructs in which a DNA sequence encoding the target polypeptide is operably linked or connected to suitable control sequences capable of effecting the expression of the target polypeptide in a suitable host. DNA regions are operably linked or connected when they are functionally related to each other. For example, a promoter is operably linked or connected to a coding sequence if it controls the transcription of the sequence. Amplification vectors do not require expression control domains, but rather need only the ability to replicate in a host, usually conferred by an origin of replication, and a selection gene to facilitate recognition of transformants. The need for control sequences in the expression vector will vary depending upon the host selected and the transformation method chosen. Generally, control sequences include, but are not limited to a transcriptional promoter, enhancers, an optional operator sequence to control transcription, polyadenylation signals, a sequence encoding suitable mRNA ribosomal binding and sequences which control the termination of transcription and translation. Such regulatory sequences are described, for example, in Goeddel; GENE EXPRESSION TECHNOLOGY: METHODS IN ENZYMOLOGY 185, Academic Press, San Diego, Calif. (1990). Regulatory sequences include those that direct constitutive expression of a nucleotide sequence in many types of host cell and those that direct expression of the nucleotide sequence only in certain host cells (e.g., tissue-specific regulatory sequences). It will be appreciated by those skilled in the art that the design of the expression vector can depend on such factors as the choice of the host cell to be transformed, the level of expression of polypeptide desired, etc. 
     The expression vectors of the invention can be introduced into host cells to thereby produce proteins or peptides, e.g., fusion proteins or peptides, encoded by nucleic acids as described herein. Preferred vectors contain a promoter that is recognized by the host organism. 
     In one embodiment, the promoter of the present invention is a strong heterologous promoter which is used for the recombinant production of foreign polypeptides. The heterologous promoter may be inducible or may be constitutive. Preferred heterologous promoters are those used for commercial production of proteins, such as those described below. The heterologous promoter of the invention is distinguishable from the natural or wild-type SFP promoter. 
     In certain embodiments, the promoter sequences of the present invention may be prokaryotic, eukaryotic or viral. Examples of suitable prokaryotic sequences include the PR and PL promoters of bacteriophage lambda (The bacteriophage Lambda, Hershey, A. D., Ed., Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1973), which is incorporated herein by reference in its entirety; Lambda II, Hendrix, R. W., Ed., Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1980), which is incorporated herein by reference in its entirety); the trp, recA, heat shock, and lacZ promoters of  E. coli  and the SV40 early promoter (Benoist et al.,  Nature,  290:304-310 (1981)), which is incorporated herein by reference in its entirety). For yeast, examples of suitable promoters include, but are not limited to GAPDH, PGK, ADH, PHO5, TEF, GAL1, and GAL10. Additional promoters include, but are not limited to, mouse mammary tumor virus, long terminal repeat of human immunodeficiency virus, maloney virus, cytomegalovirus immediate early promoter, Epstein Barr virus, Rous sarcoma virus, human actin, human myosin, human hemoglobin, human muscle creatine, and human metallothionein. 
     Additional regulatory sequences can also be included in preferred vectors. Examples of suitable regulatory sequences are represented by the Shine-Dalgarno sequence of the replicase gene of the phage MS-2 and of the gene cII of bacteriophage lambda. 
     Moreover, suitable expression vectors can include an appropriate marker that allows the screening of the transformed host cells. The transformation of the selected host is carried out using any one of the various techniques well known to the expert in the art and described in Sambrook et al., supra. 
     An origin of replication can also be provided either by construction of the vector to include an exogenous origin or may be provided by the host cell chromosomal replication mechanism. If the vector is integrated into the host cell chromosome, the latter may be sufficient. Alternatively, rather than using vectors which contain viral origins of replication, one skilled in the art can transform mammalian cells by the method of co-transformation with a selectable marker and target polypeptide DNA. An example of a suitable marker is dihydrofolate reductase (DHFR) or thymidine kinase (see, U.S. Pat. No. 4,399,216). 
     Nucleotide sequences encoding the target polypeptide may be recombined with vector DNA in accordance with conventional techniques, including blunt-ended or staggered-ended termini for ligation, restriction enzyme digestion to provide appropriate termini, filling in of cohesive ends as appropriate, alkaline phosphatase treatment to avoid undesirable joining, and ligation with appropriate ligases. Techniques for such manipulation are disclosed by Sambrook et al., supra and are well known in the art. Methods for construction of mammalian expression vectors are disclosed in, for example, Okayama et al.,  Mol. Cell. Biol.  3:280 (1983), Cosman et al.,  Mol. Immunol.  23:935 (1986), Cosman et al.,  Nature  312:768 (1984), EP-A-0367566, and WO 91/18982, each of which is incorporated herein by reference in its entirety. 
     The host cells used in the present invention may be any host cells known to those of skill in the art. In certain embodiments, suitable host cells include bacterial, fungal, (e.g., yeast), plant, or animal (e.g., mammalian or insect) cells. In some embodiments, suitable yeast cells include  Candida, Debaryomyces, Hansenula, Kluyveromyces, Pichia, Schizosaccharomyces, Yarrowia, Saccharomyces, Schwanniomyces , and  Arxula  species. Specific examples include  Candida utilis, Candida boidinii, Candida albicans, Kluyveromyces lactis, Pichia pastoris, Pichia stipitis, Schizosaccharomyces pombe, Saccharomyces cerevisiae, Hansenula polymorphs, Yarrowia lipolytica, Schwanniomyces occidentalis , and  Arxula adeninivorans . Other suitable fungi include  Aspergillus, Penicillium, Rhizopus , and  Trichoderma  species. In some embodiments, bacteria that may be used as host cells include  Escherichia, Pseudomonas , and  Bacillus  species. In some embodiments, suitable plant host cells include  Arabidopsis , maize, tobacco, and potato. In some embodiments, animal cells include cells from humans, mice, rats, rabbits, dogs, cats, monkeys, and insects. Examples include CHO, COS 1, COS 7, BSC 1, BSC 40, BMT 10, and Sf9 cells. In a particular embodiment, the host cells are yeast cells. 
     Polynucleotides of the invention may be introduced into the host cell as part of a circular plasmid, or as linear DNA comprising an isolated polypeptide coding region or a viral vector. Methods for introducing DNA into the host cell that are well known and routinely practiced in the art include transformation, transfection, electroporation, nuclear injection, or fusion with carriers such as liposomes, micelles, ghost cells, and protoplasts. 
     A reporter protein that is rapidly and efficiently detectable may be used in the present invention. In one embodiment, the reporter protein has an activity that can be positively selected for in order to automate the screening process. In an additional embodiment, the reporter protein is a protein that is secreted into the extracellular space, e.g., invertase, sucrase, cellulase, xylanase, maltase, amylase, glucoamylase, galactosidase (e.g., alpha-galactosidase beta-galactosidase, melibiase), phosphatase (e.g., PHO5), beta-lactamase, lipase or protease. In a particular embodiment, the secreted protein permits a cell to grow on a particular substrate. As an example of reporter system in mammalian cell, CD2/neomycin-phosphotransferase (Ceo) gene can be used as a secretion reporter in the media containing antibiotics G418 to trap the secretion pathway genes in mouse embryonic stem cells (De-Zolt et al., Nucleic Acid Res. 34:e25 (2006)). 
     In one embodiment, the host cells are yeast, the reporter protein is invertase and the transformed yeast cells are selected for their ability to grow on sucrose or raffinose. In another embodiment, the host cells are yeast, the reporter protein is melibiase and the transformed yeast cells are selected for their ability to grow on melibiose. In a further embodiment, the host cells are yeast, the reporter protein is amylase (e.g., an endoamylase, exoamylase, β-amylase, or glucoamylase), the yeast cells are non-amylolytic, and the transformed cells are screened for their ability to degrade starch. In an additional embodiment, the step of identifying cells showing an activity of the reporter protein occurs by using a reporter protein which provides resistance to a growth inhibitor, e.g., an antibiotic. In another embodiment, the reporter protein is a protein that can be detected visually, e.g., green fluorescent protein or luciferase. In one embodiment, the step of identifying cells showing an activity of the reporter protein occurs by using two or more reporter proteins, e.g., lipase and invertase. 
     The host cells of the present invention do not exhibit reporter protein activity. In one embodiment, the host cells naturally do not express the reporter protein. In other embodiments, the gene(s) encoding the reporter protein have been deleted in whole or in part or have been mutated such that the reporter protein is not expressed or is expressed in an inactive form. Methods for rendering a cell deficient in a particular protein are well known in the art and any such method may be used to prepare the host cells of the present invention (Sambrook et al., supra). For yeast, a reporter gene deficiency can be introduced using well known gene replacement techniques (Rothstein,  Meth. Enzymol.  194:281 (1991)). 
     Nucleic acids encoding a target polypeptide may be obtained from any source using routine techniques well known in the art, including isolation from a genomic or cDNA library, amplification by PCR, or chemical synthesis. 
     A library of nucleic acids or fragments thereof may be obtained from DNA of any type, including genomic DNA, cDNA, synthetic DNA, and recombinant DNA. Nucleic acids other than DNA may also be used, including RNA and non-naturally occurring nucleic acids. A library of pre-selected nucleic acid fragments may be obtained by diversifying previously identified nucleic acid fragments, e.g., by unidirectional deletion, mutation, addition of functional sequences (e.g., glycosylation sites) or swapping of pre- and pro-signal sequences between nucleic acid fragments. In one embodiment, the nucleic acid fragments have a size of less than 1000 base pairs, e.g., less than 700, 500, or 300 base pairs. A library of nucleic acid fragments may be constructed by enzymatic cleavage of the DNA, by cDNA synthesis, or by recombinant DNA technology (e.g., unidirectional deletion, mutagenesis). 
     The nucleic acid fragments may be derived from the entire genome of an organism, e.g., an entire genomic or cDNA library. The fragments may also be derived from any subset of the entire genome, e.g., a subtracted library or a sized library. 
     The following examples are illustrative, but not limiting, of the methods and compositions of the present invention. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered and which are obvious to those skilled in the art are within the spirit and scope of the invention. 
     EXAMPLES 
     Example 1 
     Determination of Optimal Size of YGR106C Gene for Extracellular Secretion 
     This example demonstrates the optimal regions of YGR106 that are needed for extracellular secretion. As shown in  FIG. 1A , the YGR106C (hereafter Secretion Fusion Partner1, SFP1) protein (SEQ ID NO: 1) consists of 265 amino acid residues containing signal peptide, three glycosylation sites, one hydrophilic domain (HL) and one trans-membrane domain (TM). 
     Over-expression of intact YGR106C gene under the control of GAL10 promoter produced no YGR106C protein in culture medium. However, a truncated SFP1 (amino acids 1-213 of SEQ ID NO: 1) was highly secreted into the culture medium using a C-terminally truncated form of YGR106C under the control of yeast GAL10 promoter. 
     Further identification of the optimal domains of the SFP1 gene were determined for secretion. Several functional domains of SFP1 protein such as secretion signal (amino acids 1-19 of SEQ ID NO: 1), hydrophilic domain (HL) (amino acids 176-213 of SEQ ID NO: 1) and transmembrane domain (TM) (amino acids 220-247 of SEQ ID NO: 1) were determined by Kyte-Doolittle hydropathy analysis ( FIG. 1A ). 
     Recombinant yeast  Saccharomyces cerevisiae  2805 (Mat a ura3 INV2 pep4::HIS3 can1) strains containing different vectors with serially deleted SFP1 genes were constructed and the secretion of SFP1 related proteins from each vector was compared ( FIG. 1B ). Initially, to express the intact SFP1 protein, the open reading frame (ORF) of SFP1 was amplified from  S. cerevisiae  2805 genomic DNA with PCR primers, a sense primer T9F (SEQ ID NO: 2) containing BamHI site and an anti-sense primer H159 (SEQ ID NO: 3) containing SalI site. PCR was carried out with Pfu polymerase (Stratagene, USA) or Ex-Taq DNA polymerase (TaKaRa Korea Biomedical Inc., Seoul, Korea). PCR conditions included one denaturing step of 94° C. for 5 min, and 25 amplification cycles of 94° C. for 30 sec, 55° C. for 30 sec and 72° C. for 1 min, followed by a final extension of 72° C. for 7 min. The amplified SFP1 ORF was digested with BamHI-SalI and subcloned into BamHI-SalI sites of YEGα-HIR525 (Sohn et al.,  Process Biochem.  30:653 (1995)), and the resulting plasmid was named YGaT91. 
     In order to express a truncated SFP1 protein which deleted C-terminus to TM domain, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H160 (SEQ ID NO: 4). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT92. 
     To express another truncated SFP1 protein which deleted from C-terminus to half of HL domain, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H161 (SEQ ID NO: 5). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT93. 
     To express another truncated SFP1 protein which deleted from C-terminus to HL domain, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H162 (SEQ ID NO: 6). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT94. 
     To express another truncated SFP1 protein which deleted from C-terminus to the 3 rd  glycosylation site, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H205 (SEQ ID NO: 7). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT95. 
     To express another truncated SFP1 protein which deleted from C-terminus to the 2 nd  glycosylation site, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H204 (SEQ ID NO: 8). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT96. 
     To express another truncated SFP1 gene which deleted from C-terminus to the 1 st  glycosylation site, a partial SFP1 gene was amplified from YGaT91 vector with a sense primer T9F (SEQ ID NO: 2) and an anti-sense primer H203 (SEQ ID NO: 9). The amplified partial SFP1 gene was cloned into the YEGα-HIR525 by using the same method of YGaT91 construction and the resulting plasmid was named YGaT97. 
     Yeast  S. cerevisiae  2805 strain (Mat a ura3 INV2 pep4::HIS3 can1) was transformed with the constructed vectors (YGaT91, YGaT92, YGaT93, YGaT94, YGaT95, YGaT96, and YGaT97). Single colonies selected from UD plates (0.67% yeast nitrogen base without amino acids, 0.77 g/l amino acid mixture, 2% glucose and 2% agar) of different transformations were cultured in YPDG broth media (1% yeast extract, 2% Bacto-peptone, 1% glucose, 1% galactose) for 40 hours at 30° C. Secreted proteins in the 0.6 ml of each culture broth were concentrated with 0.4 ml of acetone and separated by SDS-PAGE. As shown in  FIG. 1C , SFP1 related proteins were detected only in cells harboring YGaT92, YGaT93 and YGaT94 (lanes 2, 3, and 4, respectively). Two bands, one glycosylated form and the other non-glycosylated, were detected in all three positive strains. But the other cells, YGaT91, YGaT95, YGaT96, and YGaT97, showed no such bands (lane 1, 5, 6, and 7, respectively). These results show that removal of the TM domain and retension of the domain containing all three glycosylation sites allows for SFP1 extracellular secretion. 
     Example 2 
     Determination of Optimal Size of SFP1 Gene as a Fusion Partner for Secretion of a Target Protein 
     This example demonstrates the use of SPF1 derivatives as fusion partners. In order to test SFP1 derivatives as fusion partners for the secretion of an exemplary target protein, human interleukin-2 (hIL-2), three vectors were constructed to express hIL-2 as fusion proteins with three SFP1 derivatives (SFP1-92 (SEQ ID NO: 39), SFP1-93 (SEQ ID NO: 40), and SFP1-94 (SEQ ID NO: 41)) of YGaT92, YGaT93 and YGaT94, respectively ( FIG. 2A ). A hIL-2 fusion with YGaT91 was also generated, SFP1-91 (SEQ ID NO: 38), data not shown. To fuse hIL2 gene with SFP1-92 of YGaT92, a partial SFP1 gene was amplified with a sense primer GAL100 (SEQ ID NO: 10) which recognize GAL10 promoter and an anti-sense primer H121 (SEQ ID NO: 11) from YGaT92 vector. To facilitate the fusion with hIL2 gene and to induce in vivo cleavage of the hIL2 fusion proteins by yeast dipeptidyl protease Kex2p (Mizuno K et al.,  Biochem. Biophys. Res. Commun.  156:246 (1988)), H121 primer (SEQ ID NO: 11) was designed to contain Kex2p cleavage sequence and N-terminal hIL2 sequence. Human IL-2 gene was amplified with a sense primer IL2F (SEQ ID NO: 12) which contains a part of SFP1 sequence complementary to H121 primer (SEQ ID NO: 11) and an anti-sense primer IL2R (SEQ ID NO: 13). IL2R primer contains a part of GAL7 terminator sequence. The amplified PCR fragment containing a SFP1-92 and hIL-2 gene was fused by overlap-extension PCR with GAL100 and GT50R (SEQ ID NO: 14) primer. GT50R primer is an anti-sense primer recognizing GAL7 terminator. The resulting PCR product was flanked with 100 bp of GAL10 promoter sequence and 50 by of GAL7 terminator sequence. One of the merits of  S. cerevisiae  as an expression host is the possibility to use an efficient and correct homologous recombination strategy. It is well known in the art that a linearized vector and a DNA fragment that shares DNA sequence overlap on either side of the fragment ends can undergo recombination that restores circular topology of plasmid (Kunes et al.,  Genetics.  115: 73 (1987)). This feature of  S. cerevisiae  was used for the construction of an expression host system. 
     To use YGaT92 vector for in vivo recombination backbone, YGaT92 vector was digested with BamHI/SalI. The linearized vector fragment was isolated from agarose gel using a gel extraction kit (Bioneer, Korea). The PCR product amplified with GAL100/GT50R primer set shared more than 50 nucleotides with the linearized vector. The minimum requirement for in vivo recombination is about a 30 nucleotide overlap (Oldenberg et al.,  Nucleic Acids Res.  25: 451 (1997). A fifty nucleotide overlap is sufficient for plasmid re-construction in  S. cerevisiae . Recombinant  S. cerevisiae  2805 strain was directly constructed by co-transformation with the above-described PCR product and vector fragment. The resulting plasmid constructed by recombination was named YGaT92-IL2 ( FIG. 24 ). For the construction of  S. cerevisiae  2805 strain transformed with YGaT93-IL2 vector, we used the same procedure as used for YGaT92-IL2 plasmid construction except a H120 primer (SEQ ID NO: 15) was used instead of H121 primer (SEQ ID NO: 11). The H120 primer was an antisense primer recognizing the 3′ terminus of SFP1 gene of the YGaT93 vector and containing a Kex2p cleavage sequence and a N-terminal hIL2 sequence. To transform the  S. cerevisiae  2805 strain with YGaT94-IL2 vector, the H119 primer (SEQ ID NO: 16) was used instead of the H121 primer (SEQ ID NO: 11) and otherwise, the same procedure as described for the YGaT92-IL2 plasmid construction was used. The H119 primer is an antisense primer recognizing 3′ terminus of SFP1 gene of YGaT94 vector and contains Kex2p cleavage sequence and N-terminal hIL2 sequence. 
     Single colonies selected from UD plate (0.67% yeast nitrogen base without amino acids, 0.77 g/l amino acid mixture, 2% glucose and 2% agar) were cultured in YPDG broth media (1% yeast extract, 2% Bacto-peptone, 1% glucose, 1% galactose) for 40 hours at 30° C. Secreted proteins in the 0.6 ml of each culture broth were concentrated with 0.4 ml of acetone and separated by SDS-PAGE. As shown in  FIG. 2B , the SFP1 derivative protein and hIL2 was secreted from the  S. cerevisiae  cells harboring YGaT92-IL2 (SEQ ID NO: 58) and YGaT93-IL2 (lane 1 and 2, respectively) but not for YGaT94-IL2 cells (lane 3). This result show that the HL domain is important for the secretion of SPF1 derivative proteins when expressed in a fusion form. 
     Example 3 
     Expression of Target Protein Fused with SFP1 Derivatives 
     The SFP1-92 (SEQ ID NO: 39) constructed in Example 2 from YGaT92 was used for the secretory production of Exendin-4 (EXD4), a 39 amino acids peptide analogue of glucagons-like peptide-1 (GLP1). For simple and efficient purification of intact EXD4 proteins, 6-Histidine tag and enterokinase cleavage site (DDDDK (SEQ ID NO: 79), D: aspartic acid, K: Lysine) were added to the C-terminus of SFP1. Therefore the fusion protein from N-terminus to C-terminus included a SFP1 fragment, a 6-Histidine tag, an enterokinase cleavage site and an EXD4 sequence. To construct YGaT92-EXD4 vector that expressed SFP1-92 EXD4 fusion protein, the SFP1-92 gene was amplified from YGaT92 vector with GAL100 primer (SEQ ID NO: 10) and anti-sense primer HDK-R (SEQ ID NO: 17) that recognize the HL sequence and contains 6 Histidine codons. The EXD4 gene was amplified with sense primer HDK-F (SEQ ID NO: 18) that contains 18 nucleotides complementary to HDK-R primer and DDDDK codons and anti-sense primer EXD-R (SEQ ID NO: 19) which contains 18 nucleotide of GT50R (SEQ ID NO: 14) primer sequence. The amplified SFP1-92 and EXD4 gene was fused by overlap-extension PCR with the GAL100/GT50R primer set. The recombinant  S. cerevisiae  2805 strain harboring YGaT92-EXD4 vector was directly constructed by in vivo recombination through the co-transformation of the fused fragment and BamHI/SalI digested YGaT92 vector fragment as described in Example 2. 
     A recombinant yeast strain transformed with the YGaT92-EXD4 was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of SFP1-92-EXD4 fusion protein. A seed culture to be inoculated in the fermentor was cultured in a flask using a seed culture medium (6.7% yeast nitrogen base without amino acids, 0.5% casamino acids and 2% glucose). When the culture using a fermentation culture medium (4% yeast extract, 1% peptone, 2% glucose) as an initial fermentation medium reached an OD600 of about 15, a fed-batch medium (15% yeast extract, 30% glucose, 30% galactose) was supplied with various amounts according to cell growth rates. After a culture period of about 48 hrs, the culture reached an OD600 of about 160. 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIG. 3A-B ). Compared to a standard protein bands, the secreted SFP1-EXD4 was estimated to be about 500 mg/L. Supernatant was recovered by centrifugation to remove yeast cells, and concentrated and desalted by ultrafiltration (Quickstand, Amersham). 
     The fusion protein, SFP1-92-EXD4 was purified with Ni-NTA affinity column (QIAGEN, USA) ( FIG. 4 , lane 1). To recover EXD-4 from SFP1-92 fusion protein, the purified fusion protein was digested with different concentrations of enterokinase (Invitrogen, USA). The samples were dissolved in enterokinase buffer [20 mM Tris-HCl (pH8.0), 50 mM NaCl, 2 mM CaCl 2 ]. Equal amount of protein samples were digested with 0.1, 0.2 and 0.3 μl of enterokinase for 1 hr at 37° C. The resulting proteins were analyzed by SDS-PAGE ( FIG. 4 , lanes 2, 3, and 4, respectively). 
     Several small protein bands were generated rater than two bands. Those small fragments were likely the result of non-specific digestion of SFP1 by enterokinase. SFP1 protein contains DDK (137th amino acid) and EDK (168th amino acid) residues, which are possible substrate of enterokinase. 
     For the further analyses of EXD-4 recovered from SFP1-92-EXD4, enterokinase treated sample ( FIG. 4 , lane 3) was fractionated by HPLC ( FIG. 5A ). Proteins detected as peaks in HPLC chromatogram were analyzed by SDS-PAGE ( FIG. 5B ). HPLC fraction number 41 showed as a single band expected to be EXD-4. The protein was further analyzed to determine its molecular weight (MW) by MALDI-TOF (Korea Basic Science Institute, Daejeon, Korea) ( FIG. 6 ). The MW of EXD-4 produced from the SFP1-92 fusion was 4187.8 Da which is was matched with the MW calculated by its amino acid sequence. 
     To construct robust SFP1-92 fusion partner that is resistant to enterokinase, DDK and EDK residue were changed to DGK and EGK residue, respectively ( FIG. 7A ). To change DDK residue to DGK residue, the 5′ SFP1-92 fragment was amplified from YGaT92-EXD4 with GAL100 primer (SEQ ID NO: 8) and anti-sense mutagenic primer H307 (SEQ ID NO: 20) that contains a glycine codon rather than an aspartic acid codon of the DDK residue. The 3′ SFP1-92-EXD4 fragment was also amplified with sense primer H306 (SEQ ID NO: 21) that is complementary to H307 (SEQ ID NO: 20) and GT50R primer (SEQ ID NO: 14) from the YGaT92-EXD4 vector. These fragments were fused by overlap extension PCR with the GAL100/GT50R primer set. After digestion with BamHI/SalI, the fused fragment was cloned into the BamHI/SalI site of the YGaT92-EXD4 vector. The nucleotide sequence of the resulting plasmid was confirmed and named YGaT921-EXD4 containing SFP1-921 (SEQ ID NO: 42). 
     In order to change EDK residue to EGK residue, the 5′ SFP1 fragment was amplified from YGaT92-EXD4 with GAL100 primer (SEQ ID NO: 10) and anti-sense mutagenic primer H309 (SEQ ID NO: 22) that contains a glycine codon rather than an aspartic acid codon of the EDK residue. The 3′ SFP1-92-EXD4 fragment was also amplified with sense primer H308 (SEQ ID NO: 23) that is complementary to H309 (SEQ ID NO: 22) and GT50R primer (SEQ ID NO: 14) from the YGaT92-EXD4 vector. These fragments were fused by overlap extension PCR with the GAL100/GT50R primer set. After digestion with BamHI/SalI, the fused fragment was cloned into the BamHI/SalI site of the YGaT92-EXD4 vector. The nucleotide sequence of the resulting plasmid was confirmed and named YGaT922-EXD4 containing SFP1-922 (SEQ ID NO: 43). 
     In order to change both DDK and EDK residues to DGK and EGK, respectively, the 5′ SFP1 fragment was amplified from YGaT921-EXD4 with GAL100 primer (SEQ ID NO: 10) and anti-sense mutagenic primer H309 (SEQ ID NO: 22) that contains a glycine codon rather than an aspartic acid codon of the EDK residue. The 3′ SFP1-EXD4 fragment was also amplified with sense primer H308 (SEQ ID NO: 23) that is complementary to H309 (SEQ ID NO: 22) and GT50R primer (SEQ ID NO: 14) from YGaT92-EXD4 vector. These fragments were fused by overlap extension PCR with the GAL100/GT50R primer set. After digestion with BamHI/SalI, the fused fragment was cloned into BamHI/SalI site of YGaT92-EXD4 vector. The nucleotide sequence of the resulting plasmid was confirmed and named YGaT923-EXD4 ( FIG. 25 ) containing SFP1-923 (SEQ ID NO: 44). 
     The  S. cerevisiae  2805 strain was transformed with the vectors: YGaT92-EXD4, YGaT921-EXD4, YGaT922-EXD4, and YGaT923-EXD4. Single colonies selected from UD plate (0.67% yeast nitrogen base without amino acids, 0.77 g/l amino acid mixture, 2% glucose and 2% agar) were cultured in YPDG broth media (1% yeast extract, 2% Bacto-peptone, 1% glucose, 1% galactose) for 40 hours at 30° C. Proteins contained in the 0.6 ml of culture supernatants were precipitated with 0.4 ml of acetone and dissolved in enterokinase buffer [20 mM Tris-HCl (pH8.0), 50 mM NaCl, 2 mM CaCl 2 ]. Equal amount of protein samples were digested with 0.1 μl of enterokinase for 1 hr at 37° C. and separated by SDS-PAGE. 
     As shown in  FIG. 7B , SFP1 produced from the YGaT92-EXD4 transformant was digested to around 15 kDa fragments ( FIG. 7B , lane 1) but SFP1 produced from YGaT921-EXD4 and YGaT922-EXD4 transformants ( FIG. 7B , lanes 2 and 3, respectively) were more resistant to internal SFP1 enterokinase digestion than SFP1 from YGaT92-EXD4. Finally, most of the SFP1 fragment produced from the YGaT923-EXD4 (SEQ ID NO: 59) transformant was intact ( FIG. 7B , lane 4). Therefore, the results show that the SFP1 variant from YGaT923-EXD4 was successfully applied for expression and purification of target protein. 
     Example 4 
     Secretion of Target Proteins Fused with the HL Domain of SFP1 
     As shown in Example 2, the HL domain plays an important role for the secretion of target protein. The function of HL in secretion of target proteins may be due to the acidic charged amino acids within the HL domain because the solubility of protein is closely related to the net charge of protein. To investigate the function of the HL domain as a fusion partner, we used the HL domain for the secretion of EXD4. 
     The HL domain was fused to the N-terminus of the target protein. The HL-EXD4 gene was amplified from YGaT923-EXD4 vector with the H221 (SEQ ID NO: 24)/GT50R (SEQ ID NO: 14) primer set and the pre-pro leader peptide of mating factor α(MFα) was amplified with GAL100/LNK-R (SEQ ID NO: 25) primer set. Because H221 and LNK-R primer (SEQ ID NO: 25) contains complementary linker sequence, these two fragments were fused with GAL100 (SEQ H) NO: 8)/GT50R (SEQ ID NO: 14) primer set by overlap-extension PCR. The YGaMKH-EXD4 ( FIG. 26 ) transformant was directly constructed by co-transformation with the fused fragment and BamHI/SalI digested YGaT92 vector fragment as described in Example 2. The YGaMKH-EXD4 plasmid contains a linker peptide (AASASAGLALDKR) (SEQ ID NO:91) between the pre-pro leader peptide of MFα and peptide for in vivo processing by Kex2p, A recombinant yeast strain transformed with the YGaMKH-EXD4 was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of HL-EXD4. A seed culture to be inoculated in the fermentor was cultured in a flask using a seed culture medium (6.7% yeast nitrogen base without amino acids, 0.5% casamino acids and 2% glucose). When the culture using a fermentation culture medium (4% yeast extract, 1% peptone, 2% glucose) as an initial fermentation medium reached an OD600 of about 15, a fed-batch medium (15% yeast extract, 30% glucose, 30% galactose) was supplied with various amounts according to cell growth rates. After a culture period of about 48 hrs, the culture reached an OD600 of about 150. 10 □l of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIG. 8 ). When compared to standard protein bands, the secreted HL-EXD4 was estimated to be about 200 mg/L. 
     To test the effect of C-terminal fusion of HL peptide to the target protein, a plasmid, YGaST6-EXD-HL ( FIG. 27 ) was constructed. The EXD4 gene was amplified with sense primer H412 (SEQ ID NO: 26) and anti-sense primer H413 (SEQ ID NO: 27) from YGaMKH-EXD4 and HL peptide was amplified with HL-F (SEQ ID NO: 28) and HL-GT50R (SEQ ID NO: 29) from YGaMKH-EXD4. Because the H413 primer (SEQ ID NO: 27) contains a complementary sequence to HL-F primer, these two fragments were fused with H412 (SEQ ID NO: 26)/GT50R primer set by overlap-extension PCR. The H412 primer (SEQ ID NO: 26) contains linker sequence and can fuse to pre-pro leader of MFα amplified with GAL100 (SEQ ID NO: 10)/LNK-R (SEQ ID NO: 25) primer set. Each of amplified fragments was fused by overlap-extension PCR with GAL100/GT50R primer set in order of pre-pro leader of MFα, EXD4 and HL domain gene. YGaST6-EXD-HL transformant was directly constructed by co-transformation with the fused fragment and BamHI/SalI digested YGaT92 vector fragment as described in Example 2. A recombinant yeast strain transformed with the YGaST6-EXD4-HL was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of EXD4-HL. After a culture period of about 48 hrs, the culture reached an OD600 of about 160. 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIGS. 9A  and B). Compared to standard protein bands, the secreted EXD4-HL was estimated to be about 500 mg/L. In the case of HL fusion to EXD4, the C-terminal fusion showed much higher secretion level of EXD4 than N-terminal fusion. Thus, the results show that the HL domain is useful for the secretion of proteins in fusion form at both the N-terminus and C-terminus of the target protein. However, C-terminal fusion showed increased secretion of target protein. 
     To further test the HL domain as a fusion partner, the HL domain was used for expression of human epidermal growth factor (hEGF). The YGaMKH-EGF plasmid ( FIG. 28 ) was constructed. In YGaMKH-EGF, the HL domain is fused to the N-terminus of hEGF, the MFα pre-pro peptide-HL fusion peptide gene was amplified from the YGaMKH-EXD4 vector with the GAL100 (SEQ ID NO: 10)/DDK-R (SEQ ID NO: 30) primer set and the hEGF gene was amplified with sense primer H410 (SEQ ID NO: 31) which contains complementary sequence to DDK-R primer and anti-sense primer H411 (SEQ ID NO: 32) which contains the same sequence as GT50R (SEQ ID NO: 14). Each of the amplified fragments was fused by overlap-extension PCR with the GAL100/GT50R primer set. The YGaMKH-EGF transformant was co-transformed with the fused fragment and the BamHI/SalI digested YGaT92 vector fragment as described in Example 2. 
     A recombinant yeast strain transformed with the YGaMKH-EGF was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of HL-EGF. After a culture period of about 48 hrs, the culture reached an OD600 of about 155. 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIGS. 10A  and B). Compared to standard protein bands, the secreted HL-EGF was estimated to be about 400 mg/L. 
     The HL-hEGF fusion protein was directly purified by Ni-NTA affinity chromatography ( FIG. 11A ). To separate hEGF and HL peptide, the purified fusion protein was digested with enterokinase and the resulting fragments were fractionated by Ni-NTA affinity chromatography again. As shown in  FIG. 11B , intact and pure hEGF (6 kD) was efficiently purified. 
     The HL domain was also applied for the secretory production of human parathyroid hormone (hPTH). The YGaMKH-PTH ( FIG. 29 ) vector was constructed by fusing the HL domain to N-terminus of hPTH. The hPTH gene was amplified with sense primer H310 (SEQ ID NO: 33) which contains complementary sequence to DDK-R primer (SEQ ID NO: 30) and anti-sense primer H311 (SEQ ID NO: 34) which contains the same sequence as GT50R (SEQ ID NO: 14). This fragment was fused with the MFα pre-pro peptide-HL fusion peptide gene by overlap-extension PCR with GAL100 (SEQ ID NO: 10)/GT50R (SEQ ID NO: 14) primer set. The YGaMKH-PTH transformant was directly constructed by co-transformation with the fused fragment and the BamHI/SalI digested YGaT92 vector fragment as described in Example 2. A recombinant yeast strain transformed with the YGaMKH-PTH was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of HL-PTH. After a culture period of about 48 hrs, the culture reached an OD600 of about 120. 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIGS. 12A  and B). Two major bands related to HL-PTH were detected. The majority of hPTH was detected in the fusion form of MFα pro-HL-PTH at 60 kD due to non-complete in vivo cleavage by Kex2p. The band showing Kex2p cleavage of HL-PTH was also detected. Overall the secreted proteins related to PTH were estimated to be more than 500 mg/L. The His tagged proteins in the fermentation supernatant were directly purified by Ni-NTA affinity chromatography. The purified proteins were separated into two kinds of band in SDS-PAGE as expected ( FIG. 13 , lane 1). The larger band (Pro-HL-PTH) was disappeared after in vitro processing with Kex2p ( FIG. 13 , lane 2). The fusion protein (HL-PTH) was correctly separated to HL peptide and hPTH peptide (lane 3) by enterokinase digestion. 
     Examples 1-4 show that identification and modification of optimal regions of the YGR106C gene resulted in construction of efficient multi-functional fusion partners derived from SFP1 for the secretory production and isolation of recombinant proteins. 
     Example 5 
     Selection of Secretion Fusion Partners from the Yeast Secretome 
     This example demonstrates a technique for identifying abundantly secreted proteins useful as fusion partners. 
     First, yeast total secreted proteins (yeast secretome) produced during normal yeast cell growth were analyzed. For yeast secretome isolation, yeast  S. cerevisiae  2805 strain was cultivated in minimal media (0.67% yeast nitrogen base without amino acids, 0.5% casamino acid, 2% glucose and 0.002% uracil) for 20 hours (M1) and 40 hours (M2). Five hundred milliliter of culture supernatant was concentrated using membrane filtration and the total secreted proteins were recovered. Yeast cells were confirmed to be intact using a confocal laser scanning microscope after staining the cells with a fluorescent dye hochest ( FIGS. 14A  and B). 
     The M2 secretome sample was analyzed by 2-D gel electrophoresis ( FIG. 15 ). Most of the secretome proteins were identified in acidic regions, except the RNase A which was added to remove the ribonucleic acid contamination in the total protein samples. As shown in  FIG. 15 , the 2-D gel electrophoresis was not enough to identify all secreted proteins present in sample M2. Accordingly, 1-DE/MudPIT (Multidimensional Protein Identification Technology) method was also applied for a more complete identification of the yeast secretome ( FIG. 16 ). As a result, 57 and 83 proteins were identified from M1 and M2, respectively. Taken together, 98 unique proteins were identified. Among them, 42 proteins were commonly detected in M1 and M2 samples. To confirm the proteins that were most likely secreted proteins, two programs, WoLF PSORT and pTARGET, for predicting protein localization and for signal prediction were used. Among the 42 proteins, 35 proteins (representing 80%) were predicted as secreted proteins (Table 1). 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Thirty five genes identified by yeast secretome analysis and their 
               
               
                 protein abundance index (PAI) determined by MASS analysis. 
               
             
          
           
               
                   
                 Gi Number 
                 Standard Name 
                 Systematic Name 
                 PAI 
               
               
                   
                   
               
             
          
           
               
                 1 
                 6320260 
                 PST1 
                 YDR055W 
                 15.4 
               
               
                 2 
                 6323331 
                 EXG1 
                 YLR300W 
                 9.9 
               
               
                 3 
                 6321718 
                 SCW4 
                 YGR279C 
                 9.1 
               
               
                 4 
                 6324169 
                 YGP1 
                 YNL160W 
                 7.2 
               
               
                 5 
                 6321721 
                 BGL2 
                 YGR282C 
                 5.8 
               
               
                 6 
                 6324419 
                 ZPS1 
                 YOL154W 
                 5.1 
               
               
                 7 
                 6319552 
                 ECM33 
                 YBR078W 
                 4.2 
               
               
                 8 
                 6323964 
                 SCW10 
                 YMR305C 
                 3.4 
               
               
                 9 
                 6323871 
                 GAS3 
                 YMR215W 
                 3.4 
               
               
                 10 
                 6323967 
                 GAS1 
                 YMR307W 
                 2.8 
               
               
                 11 
                 6322895 
                 UTH1 
                 YKR042W 
                 2.5 
               
               
                 12 
                 6323150 
                 YPS3 
                 YLR121C 
                 2.2 
               
               
                 13 
                 6319638 
                 TOS1 
                 YBR162C 
                 2.2 
               
               
                 14 
                 6321628 
                 CRH1 
                 YGR189C 
                 2.2 
               
               
                 15 
                 6322754 
                 CWP1 
                 YKL096W 
                 1.5 
               
               
                 16 
                 6324002 
                 EGT2 
                 YNL327W 
                 1.5 
               
               
                 17 
                 6324395 
                 DSE4 
                 YNR067C 
                 1.5 
               
               
                 18 
                 6322303 
                 CIS3 
                 YJL158C 
                 1.5 
               
               
                 19 
                 6322068 
                 SIM1 
                 YIL123W 
                 1.2 
               
               
                 20 
                 6324543 
                 GAS5 
                 YOL030W 
                 1.0 
               
               
                 21 
                 6323014 
                 BPT1 
                 YLL015W 
                 1.0 
               
               
                 22 
                 6322864 
                 PRY2 
                 YKR013W 
                 0.7 
               
               
                 23 
                 6319568 
                 PHO3 
                 YBR092C 
                 0.7 
               
               
                 24 
                 6321906 
                 BZZ1 
                 YHR114W 
                 0.7 
               
               
                 25 
                 6323288 
                 HSP60 
                 YLR259C 
                 0.7 
               
               
                 26 
                 6323139 
                 CCW12 
                 YLR110C 
                 0.6 
               
               
                 27 
                 6323423 
                 CCW14 
                 YLR390W-A 
                 0.6 
               
               
                 28 
                 6323009 
                 KNS1 
                 YLL019C 
                 0.6 
               
               
                 29 
                 6321410 
                 SCW11 
                 YGL028C 
                 0.5 
               
               
                 30 
                 6322290 
                 N/A 
                 YJL171C 
                 0.5 
               
               
                 31 
                 6322287 
                 KRE9 
                 YJL174W 
                 0.3 
               
               
                 32 
                 6322684 
                 PIR1 
                 YKL164C 
                 0.3 
               
               
                 33 
                 6324263 
                 SUN4 
                 YNL066W 
                 0.2 
               
               
                 34 
                 6321496 
                 SPR3 
                 YGR059W 
                 0.2 
               
               
                 35 
                 6322753 
                 CWP2 
                 YKL096W-A 
                 0.2 
               
               
                   
               
             
          
         
       
     
     Many of the secreted proteins were identified as cell wall proteins and proteins with GPI (glycosylphosphatidyl inositol) anchor. Abundantly secreted proteins were determined by PAI (protein abundance index) (Rappsilber et al.,  Genome Res.  12:1231-45 (2002)) which could be proportional to the number of proteins secreted. Based on this analysis, twenty of the abundantly secreted proteins were selected. 
     The genes of the 19 abundantly secreted proteins were amplified from genomic DNA using 19 different sense primers (SEQ ID NO: 35) and antisense primers (SEQ ID NO: 36). 5′ and 3′ ends of the amplified DNA fragments contained a stretch of homologous sequence with a part of the GAL10 promoter and the GALT terminator, respectively, for in vivo recombination with a EcoRI-SalI digested YEGα-HIR525, as described above. Yeast transformants were easily obtained by the transformation of both linearized vector and PCR fragment through in vivo recombination. Twenty different transformants obtained from 19 different PCR fragments were cultivated in YPDG (1% yeast extract, 2% peptone, 1% glucose, and 1% galactose) media. Three hundred microliters of each culture supernatant was concentrated with acetone. Each acetone-concentrated culture supernatant was analyzed in SDS-PAGE as shown in  FIG. 17A . 
     To distinguish poor candidates from good candidate SFPs, the secretion level of the abundantly secreted proteins from a strong promoter was determined compared to wild-type protein secretion levels. Compared to the wild-type protein secretion levels shown in lane WT of  FIG. 17A , a subset (eleven) of the tested proteins, expressed using the strong GAL10 promoter, showed extraordinary strong bands, suggesting over-secretion into the culture supernatant. Glycosidase, Endo-H treatment of each sample resulted in the correct protein sizes expected from the amino acid sequence of each protein ( FIG. 17B ) demonstrating that most of the over-secreted proteins were glycosylated. Eleven (11) of the 19 selected abundantly secreted proteins BGL2 (SEQ ID NO: 80), GAS3 (SEQ ID NO: 81), GAS5 (SEQ ID NO: 82), PST1 (SEQ ID NO: 83), SCW4 (SEQ ID NO: 84), SCW10 (SEQ ID NO: 85), SIMI (SEQ ID NO: 86), UTH1 (SEQ ID NO: 87), YGP1 (SEQ ID NO: 88), YPS1 (SEQ ID NO: 89), and ZPS1 (SEQ ID NO: 90) were tested as candidate SFPs for the secretion of heterologous proteins. The 11 proteins were encoded by the following polynucleotides: BGL2 (SEQ ID NO: 62), GAS3 (SEQ ID NO: 63), GAS5 (SEQ ID NO: 64), PST1 (SEQ ID NO: 65), SCW4 (SEQ ID NO: 66), SCW10 (SEQ ID NO: 67), SIM1 (SEQ ID NO: 68), UTH1 (SEQ ID NO: 69), YGP1 (SEQ ID NO: 70), YPS1 (SEQ ID NO: 71) and ZPS1 (SEQ ID NO: 72). 
     Vectors for expression of fusion proteins were constructed using open reading frames (OFRs) of polynucleotides encoding the 11 over-secreted proteins each fused to EXD4. Eleven fusion proteins were tested for their level of secretion into the culture supernatant. YGa-ORF vectors were recovered from each transformant producing the respective proteins in  FIG. 17 . For the construction of each fusion protein expression vector, 11 PCR fragments were amplified from eleven YGa-ORF vectors containing different ORFs using primer GAL100 (SEQ ID NO: 10) and 11 different antisense primers (SEQ ID NO: 37). The 5′ and 3′ ends of amplified DNA fragments contained a stretch of homologous sequence with the GAL10 promoter and exendin-4, respectively. The eleven PCR fragments and the exendin-4 amplified from YGaT92-EXD4 with primers EXD-F (SEQ ID NO: 46) and GT50R (SEQ ID NO: 14) were used as templates for the 11 different overlap extension PCRs using primers, GAL100 (SEQ ID NO: 10) and GT50R (SEQ ID NO: 14), respectively. Each extended PCR fragment was transformed with an EcoRI-SalI digested YEGα-HIR525, as described above. Two transformants from each transformation were cultivated in YPDG (1% yeast extract, 2% peptone, 1% glucose and 1% galactose) for 40 hours. A 0.6 ml sample from supernatant was concentrated using 0.4 ml of acetone and analyzed by SDS-PAGE, as shown in  FIG. 18 . Six fusion proteins (GAS3-EXD4, GAS5-EXD4, PST1-EXD4, SCW4-EXD4, YGP1-EXD4, and YPS1-EXD4) were found to be efficiently secreted into extracellular medium. 
     Example 5 showed that abundantly secreted proteins selected from yeast secretome were effective as secretion fusion partners for the secretory production of recombinant proteins. Although Example 5 used yeast secreted proteins, the secreted polypeptides of any organism, such as those described throughout the specification, may be used. As shown in this example, the screening method of the invention is an efficient way to identify SFPs, as it narrowed the possible candidate SFPs from 35 secreted proteins to 11, six of which proved to be effective SFPs 
     Example 6 
     Determination of the Optimal Size of SCW4 Gene as a Fusion Partner 
     This example demonstrates the determination of the optimal size of SCW4 as a fusion partner for the secretion of a target proteins, e.g., exendin-4. Eight SCW4 deletion clones were constructed based on Kyte-Doolittle hydropathy analysis ( FIG. 19A ). The eight SCW4 fragments were amplified with GAL100 (SEQ ID NO: 10) and eight different antisense primers H453-H460 (SEQ ID NOs: 47-54) which each contained a 6 Histidine sequence. The amplified fragments were fused with EXD4 gene amplified from YGaT92-EXD4 with sense primer (SEQ ID NO: 55) and GT50R (SEQ ID NO: 14) by overlap extension PCRs using primers, GAL100 (SEQ ID NO: 10) and GT50R (SEQ ID NO: 14), respectively. Each extended PCR fragment was transformed with an EcoRI-SalI digested YEGα-HIR525 as described in the previous examples. Three colonies of 8 different transformants were cultivated in YPDG (1% yeast extract, 2% peptone, 1% glucose, and 1% galactose) media. Ten (10) microliter of culture broth for each sample was directly analyzed in SDS-PAGE (without concentration). As shown in  FIG. 19B , SCW4-1, SCW4-2, SCW4-3 and SCW4-4 containing different C-terminal fragments of SCW4 showed strong activities as fusion partners for the secretion of EXD4. The optimal size of SCW4 as a fusion partner for EXD4 was shown to be less than 169 amino acids of the whole SCW4 protein (380 amino acids). 
     A recombinant yeast strain transformed with the YGaSCW4-1-EXD4 ( FIG. 30 ) and YGaSCW4-3-EXD4 ( FIG. 31 ) were cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for the ability to induce the secretory production of fusion proteins. After a culture period of about 48 hrs, the culture reached an OD600 of about 130. 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIG. 20 ). Compared to standard protein bands, the secreted SCW4-1-EXD4 (SEQ ID NO: 60) and SCW4-3-EXD4 (SEQ ID NO: 61) were estimated to be over 3 grams per liter. 
     To test the robustness of SCW4 protein against enterokinase, the fermentation broths were digested with enterokinase for 1 hr at 37° C. without purification. Fusion proteins were correctly divided into SCW4 protein and exendin-4 peptide, as shown in  FIG. 21 . Therefore, these results show that the modified SCW4 fusion partners considerably increased the yield of exendin-4 protein and simplified the purification process. 
     The effectiveness of SCW4 as a general fusion partner for other proteins was tested. SCW4-1, SCW4-2, SCW4-3 and SCW4-4 were applied for secretory production of human growth hormone (hGH). The hGH gene was amplified with sense primer (SEQ ID NO: 56) and antisense primer (SEQ ID NO: 57). This fragment was flanked with a stretch of 6 histidine and a GALT terminator sequence. PCR amplified SCW4-1, -2, -3 and -4 fragments were fused with the hGH gene by overlap extension PCRs using primers, GAL100 (SEQ ID NO: 10) and GT50R (SEQ ID NO: 14), respectively. Each extended PCR fragment was transformed with an EcoRI-SalI digested YEGα-HIR525 as described above. Two colonies of 4 different transformations were cultivated in YPDG (1% yeast extract, 2% peptone, 1% glucose, and 1% galactose) media. Ten (10) microliters of culture broth of each sample was directly analyzed in SDS-PAGE (without concentration). As shown in  FIG. 22A , different sized SCW4-hGH fusion protein bands were detected for each sample. To confirm the fusion protein, the culture supernatants were incubated with enterokinase for 1 hr at 37° C. to cleave the fusion proteins. The correct size hGH was retrieved from SCW4-1-hGH, SCW4-2-hGH and SCW4-4-hGH ( FIG. 22B ). Thus, the N-terminal fragments of SCW4 showed strong activities as fusion partners for the secretion of hGH, as well as EXD4. 
     A recombinant yeast strain transformed with the YGaSCW4-2-hGH ( FIG. 32 ) was cultured in a 5-L jar fermentor by fed-batch culture to be evaluated for its ability to induce the secretory production of fusion proteins. After a culture period of about 48 hrs, 10 μl of the medium was collected at the given time points and assessed for secreted proteins by SDS-PAGE ( FIG. 23 ). Compared to standard protein bands, the secreted SCW4-2-hGH (SEQ ID NO: 73) was estimated to be over 3 grams per liter. 
     Thus, the results of Example 6 show that SCW4 and fragments thereof are effective as fusion partners for recombinant expression of target proteins, and may be used to produce large quantities of target proteins. 
     Having now fully described the invention, it will be understood by those of skill in the art that the same can be performed within a wide and equivalent range of conditions, formulations, and other parameters without affecting the scope of the invention or any embodiment thereof. All patents, patent applications and publications cited herein are fully incorporated by reference herein in their entirety.