US 20050287640 A1
Human spasmolytic polypeptide (HSP) which has the amino acid sequence
1. A method of preparing a glycosylated human spasmolytic polypeptide (hSP) having an amino acid sequence according to SEQ ID NO 1, said method comprising:
(a) culturing a yeast cell containing a DNA sequence encoding said hSP under conditions permitting production and secretion of said hSP from the yeast cell; and
(b) recovering the resulting secreted hSP, wherein the recovered hSP is glycosylated at Asn 15 and is useful in the treatment of peptic ulcers.
2. The method of
3. The method of
4. The method of
This application is a divisional of U.S. Ser. No. 09/528,644 filed 20 Mar. 2000, which is a divisional of U.S. Ser. No. 09/027,893 filed Feb. 23, 1998 which is a continuation of U.S. Ser. No. 08/491,976 filed Aug. 2, 1995, now U.S. Pat. No. 5,783,416, which is a continuation of PCT/DK94/00037 filed Jan. 20, 1994, and claims priority under 35 U.S.C. 119 of Danish application serial no. 0068/93 filed Jan. 21, 1993, which is incorporated herein by reference.
The present invention relates to human spasmolytic polypeptide in glycosylated form, variants of human and porcine spasmolytic polypeptides and a method of producing spasmolytic polypeptides in glycosylated form.
Human spasmolytic polypeptide (HSP) belongs to a family of peptides containing one or more characteristic trefoil domains . The trefoil domain is made up of a sequence of 38 or 39 amino acid residues in which 6 cystein residues are linked in the configuration 1-5,2-4 and 3-6 thus forming a characteristic trefoil structure . The trefoil family of peptides consists of rat intestinal trefoil factor, ITF , human breast cancer associated peptide, pS2 [3, 4, 5], porcine, human and murine spasmolytic polypeptide (PSP, HSP, MSP) [6,7,8] and frog spasmolysins (xP1, xP2 and xP4) [8, 10, 11] all containing 1, 2 or 4 trefoil domains (
The physiological function of the trefoil peptides is poorly understood, and so far only PSP has been studied in any detail. In the porcine pancreas, PSP is found in the acinar cells and to be secreted in large amounts (50-100 mg/ml) into the pancreatic juice upon stimulation with pancreozymin or secretin [12, 13, 14]. PSP is resistant to digestion by intestinal proteases in the gastrointestinal tract , and specific binding of PSP to rat intestinal mucosa cells and membrane preparations from these cells has been demonstrated [15, 16]. In the porcine gastrointestinal tract, specific receptor-like binding to Paneth cells in the duodenum has been found . These results suggest a unique intraluminal function of the peptide. A pharmacological screening has indicated that PSP has spasmolytic and gastric acid secretion inhibitory effects , and studies on mammalian cells have indicated a growth factor-like activity of PSP .
The DNA sequence and derived amino acid sequence of the human counterpart of porcine SP is shown in . Unlike PSP, human SP (
Only very limited amounts of HSP can be prepared by extraction of human tissue. An object of study resulting in the present invention was therefore to prepare recombinant HSP in sufficient amounts for physiological and biochemical studies of the peptide.
It has surprisingly been found that when recombinant HSP is produced in certain host organisms, a proportion of it is produced in glycosylated form by posttranslational modifications. The glycosylated form of HSP has not, to applicant's best knowledge, been described previously.
Accordingly, the present invention relates to human spasmolytic polypeptide (HSP) which has the amino acid sequence
In the present context, the term “functionally equivalent” is intended to indicate that the homologous polypeptide has a biological activity (e.g. spasmolytic effect) corresponding to that of native HSP. The term “homologue” is intended to indicate a polypeptide encoded by DNA which hybridizes to the same probe as the DNA coding for HSP under comditions of high or low stringency (e.g. as described in Sambrook et. al., Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989). More specifically, the term is intended to refer to a DNA sequence which is at least 60% homologous to the sequence encoding HSP with the amino acid sequence shown above. The term is intended to include modifications of the DNA sequence such as nucleotide substitutions which do not give rise to another amino acid sequence of the polypeptide, but which correspond to the codon usage of the host organism into which the DNA construct is introduced or nucleotide substitutions which do give rise to a different amino acid sequence and therefore, possibly, a different protein structure which might give rise to a mutant polupeptide with different properties than the native enzyme. Other examples of possible modifications are insertion of one or more codons into the sequence, addition of one or more codons at either end of the sequence, or deletion of one or more codons at either end or within the sequence. The term “glycosylated” is intended to indicate that a carbohydrate moiety is present at one or more sites of the protein molecule. It is at present contemplated that glycosylation of HSP may give rise to differences in the biological activity of the protein, for instance with respect to stability towards proteolytic enzymes in the gastrointestinal tract, solubility at gastric and/or intestinal pH compared to non-glycosylated HSP, antigenicity, half-life, tertiary structure, and targeting to receptors on appropriate cells.
In another aspect, the present invention relates to a variant of a spasmolytic polypeptide (SP) which is a fragment of human spasmolytic polypeptide (HSP) or porcine spasmolytic polypeptide (PSP) comprising at least one trefoil domain.
The variant SP may be provided in both glycosylated and non-glycosylated form. It is at present contemplated that such a variant may be advantageous to use instead of full-length SP because of a higher specific biological activity, increased solubility and stability, longer half-life, easier way of production, or the like.
It is assumed that other spasmolytic polypeptides than HSP will, if provided with a glycosylation site, also be expressed in predominantly glycosylated form. In a further aspect, the present invention therefore relates to a method of preparing a spasmolytic polypeptide in at least 60% glycosylated form, wherein a host cell transformed with a DNA fragment encoding a spasmolytic polypeptide and capable of providing glycosylation of said spasmolytic polypeptide is cultured under conditions permitting production of said spasmolytic polypeptide and recovering the resulting spasmolytic polypeptide from the culture.
It has been found that, at least when recombinant HSP is produced in yeast, the proportion of it that is provided in glycosylated form is in N-glycosylated form. It has further been found that glycosylation takes place at Asn15 of the sequence shown above. In preferred embodiments of glycosylated HSP, the glycosylated side chain contains at least one hexose unit. In particular, the glycosylated side chain may contain at least one mannose unit, preferably at least five mannose units, most preferably at least ten mannose units. In one preferred embodiment of glycosylated HSP of the invention, the glycosylated side chain contains 13-17 mannose units. In other preferred embodiments, the glycosylated HSP is in addition glycosylated with at least one unit of N-acetyl glucosamine (GlcNAc). In the currently preferred embodiment, the glycosylated HSP is glycosylated at Asn15 with (GlcNAc)2(Man)10-15.
It is further contemplated to produce homologues of HSP which are provided with one or more additional glycosylation sites.
Thus, the present invention also relates to HSP homologues, wherein Lys2 is replaced by Asn, Gln7 is replaced by Asn, Arg10 is replaced by Asn, Gly 20 is replaced by Thr or Ser, Gly23 is replaced by Asn, Ile 24 is replaced by Asn, Phe 36 is replaced by Asn, Asp 37 is replaced by Asn, Ser39 is replaced by Asn, Gln53 is replaced by Asn, Glu61 is replaced by Asn, Asp64 is replaced by Asn, Arg66 is replaced by Thr or Ser, Gly69 is replaced by Thr or Ser, Gly72 is replaced by Asn, Ile 89 is replaced by Thr or Ser, Pro98 is replaced by Asn or Val101 is replaced by Thr or Ser, or a combination of two or more of these substitutions. In a currently preferred embodiment of such an HSP homologue, Asp64 is replaced by Asn, and Arg66 is replaced by Thr or Ser.
It is of course understood that HSP homologues of the invention may be glycosylated in the same manner at one or more of these sites as described above for glycosylation at Asn15.
It is assumed that the trefoil structure common among spasmolytic polypeptides is important for the function of HSP and PSP. The variant human or porcine SP comprising a fragment of the full-length polypeptide should therefore include at least three disulfide bonds to provide this structure. Consequently, the variant may comprise at least a sequence of amino acids from position 8 to 46 or from position 58 to 95, each of which sequences defines a trefoil domain of HSP and PSP.
As indicated above, the SP variant of the invention may be provided in non-glycosylated form. This may, for instance, be accomplished by substituting Asn15 by another amino acid, e.g. Asp or Glu, or by substituting Thr17 by another amino acid except Ser, e.g. Ala. It is more likely, however, that one or more additional glycosylation sites will be introduced into this domain, for instance by replacing Arg10 by Asn, Gly 20 by Thr or Ser, Gly23 by Asn, Ile 24 by Asn, Phe 36 by Asn, Asp 37 by Asn, or Ser39 by Asn, or a combination of two or more of these substitutions.
On the other hand, it may be desirable to provide the trefoil domain from position 58 to 95 with a glycosylation site lacking in this domain in native HSP and PSP. Thus, Glu61 may be replaced by Asn, Asp64 by Asn, Arg66 by Thr or Ser, Gly69 by Thr or Ser, or Gly72 is replaced by Asn, or a combination of two or more of these substitutions. In a currently preferred embodiment of the variant, Asp64 is replaced by Asn, and Arg66 is replaced by Thr or Ser.
It is of course understood that variants of the invention may be glycosylated in the same manner at one or more of these sites as described above for glycosylation at Asn15 in full-length HSP.
A DNA sequence encoding HSP may suitably be isolated from a human genomic DNA library by PCR (polymerase chain reaction) cloning using primers based on the published cDNA sequence . Alternatively, the DNA sequence may be prepared synthetically by established standard methods, e.g. the phosphoamidite method described by S. L. Beaucage and M. H. Caruthers, Tetrahedron Letters 22, 1981, pp. 1859-1869, or the method described by Matthes et al., EMBO Journal 3, 1984, pp. 801-805. According to the phosphoamidite method, oligonucleotides are synthesized, e.g. in an automatic DNA synthesizer, purified, annealed, ligated and cloned in suitable vectors. The cDNA sequence shown in  may be used as the basis of oligonucleotide synthesis. Alternatively, it is possible to use cDNA coding for HSP obtained by screening a human cDNA library with oligonucleotide probes in accordance with well-known procedures.
Furthermore, the DNA sequence may be of mixed synthetic and genomic, mixed synthetic and cDNA or mixed genomic and cDNA origin prepared by ligating fragments of genomic, synthetic or cDNA origin (as appropriate), the fragments corresponding to various parts of the entire DNA sequence, in accordance with standard techniques.
The SP variant of the invention may be encoded by a fragment of the full-length DNA sequence, prepared by one of the methods indicated above, or by suitably truncating the full-length sequence.
The DNA sequence encoding HSP or an SP variant of the invention may then be inserted in a suitable expression vector. The recombinant expression vector may be any vector which may conveniently be subjected to recombinant DNA procedures, and the choice of vector will often depend on the host cell into which it is to be introduced. Thus, the vector may be an autonomously replicating vector, i.e. a vector which exists as an extrachromosomal entity, the replication of which is independent of chromosomal replication, e.g. a plasmid. Alternatively, the vector may be one which, when introduced into a host cell, is integrated into the host cell genome and replicated together with the chromosome(s) into which it has been integrated.
In the vector, the DNA sequence encoding HSP or an SP variant of the invention should be operably connected to a suitable promoter sequence. The promoter may be any DNA sequence which shows transcriptional activity in the host cell of choice and may be derived from genes encoding proteins either homologous or heterologous to the host cell. Examples of suitable promoters for directing the transcription of the DNA encoding the inhibitor of the invention in mammalian cells are the SV 40 promoter (Subramani et al., Mol. Cell Biol. 1, 1981, pp. 854-864), the MT-1 (metallothionein gene) promoter (Palmiter et al., Science 222, 1983, pp. 809-814) or the adenovirus 2 major late promoter. Suitable promoters for use in yeast host cells include promoters from yeast glycolytic genes (Hitzeman et al., J. Biol. Chem. 255, 1980, pp. 12073-12080; Alber and Kawasaki, J. Mol. Appl. Gen. 1, 1982, pp. 419-434) or alcohol dehydrogenase genes (Young et al., in Genetic Engineering of Microorganisms for Chemicals (Hollaender et al, eds.), Plenum Press, New York, 1982), or the TPI1 (U.S. Pat. No. 4,599,311) or ADH2-4c (Russell et al., Nature 304, 1983, pp. 652-654) promoters. Suitable promoters for use in filamentous fungus host cells are, for instance, the ADH3 promoter (McKnight et al., The EMBO J. 4, 1985, pp. 2093-2099) or the tpiA promoter.
The DNA sequence encoding HSP or an SP variant may also be operably connected to a suitable terminator, such as the human growth hormone terminator (Palmiter et al., op. cit.) or (for fungal hosts) the TPI1 (Alber and Kawasaki, op. cit.) or ADH3 (McKnight et al., op. cit.) promoters. The vector may further comprise elements such as polyadenylation signals (e.g. from SV 40 or the adenovirus 5 E1b region), transcriptional enhancer sequences (e.g. the SV 40 enhancer) and translational enhancer sequences (e.g. the ones encoding adenovirus VA RNAs).
The recombinant expression vector may further comprise a DNA sequence enabling the vector to replicate in the host cell in question. An examples of such a sequence (when the host cell is a mammalian cell) is the SV 40 origin of replication, or (when the host cell is a yeast cell) the yeast plasmid 2μ replication genes REP 1-3 and origin of replication. The vector may also comprise a selectable marker, e.g. a gene the product of which complements a defect in the host cell, such as the gene coding for dihydrofolate reductase (DHFR) or one which confers resistance to a drug, e.g. neomycin, hygromycin or methotrexate, or the Schizosaccharomyces pombe TPI gene (described by P. R. Russell, Gene 40, 1985, pp. 125-130.
The procedures used to ligate the DNA sequences coding for HSP or the SP variant, the promoter and the terminator, respectively, and to insert them into suitable vectors containing the information necessary for replication, are well known to persons skilled in the art (cf., for instance, Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor, N.Y., 1989).
The host cell into which the expression vector is introduced may be any cell which is capable of producing the inhibitor of the invention and is preferably a eukaryotic cell, such as a mammalian, yeast or fungal cell.
The yeast organism used as the host cell may be any yeast organism which, on cultivation, produces large quantities of the inhibitor of the invention. Examples of suitable yeast organisms are strains of the yeast species Saccharomyces cerevisiae, Saccharomyces kluyveri, Schizosaccharomyces pombe or Saccharomyces uvarum. The transformation of yeast cells may for instance be effected by protoplast formation followed by transformation in a manner known per se.
Examples of suitable mammalian cell lines are the COS (ATCC CRL 1650), BHK (ATCC CRL 1632, ATCC CCL 10) or CHO (ATCC CCL 61) cell lines. Methods of transfecting mammalian cells and expressing DNA sequences introduced in the cells are described in e.g. Kaufman and Sharp, J. Mol. Biol. 159, 1982, pp. 601-621; Southern and Berg, J. Mol. Appl. Genet. 1, 1982, pp. 327-341; Loyter et al., Proc. Natl. Acad. Sci. USA 79, 1982, pp. 422-426; Wigler et al., Cell 14, 1978, p. 725; Corsaro and Pearson, Somatic Cell Genetics 7, 1981, p. 603, Graham and van der Eb, Virology 52, 1973, p. 456; and Neumann et al., EMBO J. 1, 1982, pp. 841-845.
Alternatively, fungal cells may be used as host cells. Examples of suitable fungal cells are cells of filamentous fungi, e.g. Aspergillus spp. or Neurospora spp., in particular strains of Aspergillus oryzae or Aspergillus niger. The use of Aspergillus spp. for the expression of proteins is described in, e.g., EP 238 023.
According to the present method, yeast cells are currently preferred for producing HSP and other SPs (such as those shown in
The medium used to cultivate the cells may be any conventional medium suitable for growing mammalian cells or fungal (including yeast) cells, depending on the choice of host cell. The spasmolytic polypeptide will be secreted by the host cells to the growth medium and may be recovered therefrom by conventional procedures including separating the cells from the medium by centrifugation or filtration, precipitating the proteinaceous components of the supernatant or filtrate by means of a salt, e.g. ammonium sulfate, purification by a variety of chromatographic procedures, e.g. ion exchange chromatography or affinity chromatography, or the like.
The present invention also relates to a pharmaceutical composition comprising HSP or a variant spasmolytic polypeptide of the invention together with a pharmaceutically acceptable carrier or excipient. In the composition of the invention, the variant may be formulated by any of the established methods of formulating pharmaceutical compositions, e.g. as described in Remington's Pharmaceutical Sciences, 1985. The composition may typically be in a form suited for oral or rectal administration and may, as such, be formulated as tablets or suppositories.
HSP or an SP variant of the invention is contemplated to be useful for the prophylaxis or treatment of gastrointestinal disorders. More specifically, it is contemplated for the treatment of gastric or peptic ulcers, inflammatory bowel disease, Crohn's disease or injury to the intestinal tract caused by radiation therapy, bacterial or other infections, etc.
The invention is further illustrated in the following example with reference to the appended drawings in which
Material and Methods
Standard DNA techniques were used as previously described . Synthetic oligonucleotides were prepared on an automatic DNA synthesizer (380B, Applied Biosystems) using commercially available reagents. DNA sequence determinations were performed by the dideoxy chain-termination technique . Polymerase chain reactions (PCR) were performed on a DNA Thermal Cycler (Perkin Elmer Cetus) using a commercial kit (GeneAmp, Perkin Elmer Cetus).
PCR Cloning of HSP
The first trefoil domain of HSP was isolated by a PCR reaction in which 1 μg human genomic DNA (Clontech, Palo Alto, Calif., USA) was used as a template. The reaction mixture contained 100 pmole each of the forward primer 1 (GGCTGAGCCCCCATAACAG) (SEQ ID NO:4) and reverse primer 2 (TGGAAACACCAGGGGAC) (SEQ ID NO:5) (
The 115 bp PCR fragment was digested with DdeI and then ligated to a 31 bp duplex formed from the oligonucleotides (GAGAAACCCTCCCCCTGCCAGTGCTCCAGGC) (SEQ ID NO:6) and (TCAGCCTGGAGCACTGGCAGGGGGAGGGTTTCTC). The ligation product was amplified by PCR using forward primer 3 (GCTGAGAGATTGGAGAAGAGAGAGAAACCCTCCCCCT) (SEQ ID NO:7) and reverse primer 2. The 3′ part of primer 3 is identical to the N-terminal encoding part of the HSP gene and the 5′ part of primer 3 is identical to the C-terminal encoding part of the hybrid leader gene (
The second trefoil domain of HSP was PCR-cloned from human genomic DNA as described for the first domain by replacing primers 1 and 2 with forward primer 4 (TGCGTCATGGAGGTCTC) (SEQ ID NO:8) and reverse primer 5 (AGCACCATGGCACTTCAAAG) (SEQ ID NO:9) (
A DNA construct encoding the hybrid leader fused in-frame to the entire HSP gene was obtained by ligation of the 360 bp EcoRI-AvaII fragment and the 195 bp AvaII-XbaI fragment described above to the 2.7 kb EcoRI-XbaI fragment from vector pTZ19R . This construct was then transformed into E. coli strain MT-172 (r−, m+) by selection for resistance to ampicillin. DNA sequencing of the resulting plasmid, KFN-1843, showed that the correct construction had been obtained.
Construction of the HSP Secreting Yeast Strain
Plasmid KFN-1843 described above was digested with EcoRI and XbaI. The resulting 558 bp fragment was isolated and ligated to the 9.3 kb NcoI-XbaI fragment and the 1.6 kb NcoI-EcoRI fragment both from the yeast expression vector pMT-636. Plasmid pMT-636 is derived from the S. cerevisiae-E. coli shuttle vector CPOT [25, 33] by deletion of the 0.4 kb HpaI-NruI fragment from the Leu-2 gene. The ligation mixture was transformed into E. coli strain MT-172, and the HSP expression plasmid, KFN-1847, was isolated (
The transformant described above was cultivated at 30° C. for 3 days in yeast peptone dextrose (YPD) medium  supplied with additional yeast extract (60 g/l). An OD 650 nm value of 52 was reached at the end of the fermentation.
Purification of r-HSP
The concentration of r-HSP in the yeast fermentation broth and fractions obtained during the purification was measured by analytical HPLC. Aliquots (usually 50-200 μl) were injected onto a Vydac 214TP54 reverse-phase C4 HPLC column (0.46×25 cm) equilibrated at 30° C. at a flow rate of 1.5 ml/min with 0.1% (v/v) TFA in 5% (v/v) acetonitrile. The concentration of acetonitrile in the eluting solvent was raised to 65% (v/v) over 30 min. Absorbance was measured at 280 nm. The peaks eluting at 15.6 min. and 16.1 min. (
From a 10 litre fermentor, 8 litres of fermentation broth was isolated by centrifugation at 3,000 rpm for 10 min. The supernatant was concentrated to 0.9 litre using an Amicon ultrafiltration unit (RA 2000) equipped with an Amicon spiral ultrafiltration cartridge type S1Y3, MW cutoff 3,000 (Product No. 540620). The pH was adjusted to 1.7 and the conductivity in the resulting concentrated sample was measured to 4.7 mS.
The sample was pumped onto a Fast Flow S-Sepharose (Pharmacia) column (5×11 cm) with a flow rate of 40 ml/h. Previous to the application, the column was equilibrated in 50 mM formic acid buffer, pH=3.7. After application of the sample, the column was washed with 500 ml of 50 mM formic acid buffer, pH=3.7. The peptides were eluted from the column by a linear gradient between 1.5 litres of 50 mM formic acid buffer, pH=3.7 and 1.5 litres of 50 mM formic acid buffer, pH=3.7 containing 0.6 M NaCl. Fractions of 10 ml was collected at a flow rate of 40 ml/h and the absorbance was measured at 280 nm. Fractions were assayed for the content of r-HSP and glycosylated r-HSP in the HPLC-system previously described. The elution profile is shown in
Glycosylated r-HSP and r-HSP were further purified by preparative HPLC chromatography. Pooled fractions (approx. 200 ml) were pumped onto a Vydac 214TP1022 C4 column (2.2×25 cm) equilibrated in 0.1% (v/v) TFA. The column was washed with 100 ml of 0.1% (v/v) TFA in 10% (v/v) MeCN. The peptides were eluted at 25° C. and at a flow rate of 5 ml/min with a linear gradient (650 ml) formed from MeCN/H2O/TFA (10.0:89.9:0.1 v/v/v) and MeCN/H2O/TFA (60.0:39.9:0.1 v/v/v). UV-absorption was monitored at 280 nm, and fractions corresponding to 10 ml were collected and analysed for the content of r-HSP or glycosylated r-HSP.
Characterization of r-HSP and Glycosylated r-HSP
Amino acid composition analysis were carried out by hydrolysis of 50 μg peptide with 6M HCl for 24 h at 110° C. as previously described ; no correction for loss during hydrolysis was carried out. Amino acid sequence analysis was determined by automated Edman degradation using an Applied Biosystems Model 470A gas-phase sequencer . Carbohydrate composition analysis was carried out by hydrolysis of 50 μg peptide with 2M HCl for 1 h, 2h and 4h at 100° C. and monosaccharides were separated on a CarboPac PAI (Dionex, Sunnyvale, Calif.) column (4×250 mm) eluted with 14 mM NaOH. The monosaccharides were detected by pulsed amperometric detection (Dionex PAD-detector). The amount of monosaccharides was corrected to zero time of hydrolysis and calculated as nmol of monosaccharide per nmol of peptide.
Mass spectrometry analysis was performed using an API III LC/MS/MS system (Sciex, Thornhill, Ontario, Canada). The triple quadrupole instrument has a mass-to-charge (m/z) range of 2400 and is fitted with a pneumatically assisted electrospray (also referred to as ion-spray) interface [23, 24]. Sample introduction was done by a syringe infusion pump (Sage Instruments, Cambridge, MA) through a fused capillary (75 μm i.d.) with a liquid flow-rate set at 0.5-1 μl/min. The instrument m/z scale was calibrated with the singly-charged ammonium adduct ions of poly(propylene glycols)(PPG's) under unit resolution.
The accuracy of mass measurements was generally better than 0.02%.
Expression and Purification
DNA fragments encoding the two trefoil domains of HSP were isolated by PCR from human genomic DNA using primers based on the published cDNA sequence . The full length HSP gene was obtained from the PCR cloned fragments by addition of synthetic DNA fragments. The HSP gene was fused in-frame to a hybrid yeast leader sequence by overlay extension PCR  (
The yeast expression plasmid pKFN-1847 contains the leader-HSP gene inserted between the S. cerevisiae triose phosphate isomerase promoter and terminator . The expression vector (
The plasmid was transformed into the yeast strain MT-663, carrying a deletion in the TPI gene, by selecting for growth on glucose.
The expression level of r-HSP in the present yeast system is approx. 120 mg/l. As can be seen from
After the initial concentration of the yeast supernatant by ultrafiltration, the first purification step was cationic exchange chromatography on a Fast Flow S column.
The fractions from the Fast Flows S column were pooled as indicated in
Characterization of r-HSP and Glycosylated r-HSP
Table 2 shows the amino acid sequencing results obtained on r-HSP and glycosylated r-HSP. The average repetitive yield was 94.4% (r-HSP) and 94.6% (glycosylated r-HSP), respectively. In both cases the first 40 residues of the two peptides were confirmed by the sequence analysis. In the glycosylated HSP, no PTH-a.a. was found in Edman degradation cycle No. 15. The HSP sequence from residue 15-17 (Asn-Arg-Thr) corresponds to a consensus sequence for N-glycosylation of Asn-15.
The carbohydrate composition analysis of glysocylated r-HSP showed the presence of 12.8 nmol mannose (Man) and 1.6 nmol of N-acetyl glucoseamine (GlcNAc) per nmol of r-HSP. By peptide mapping of r-HSP and glycosylated r-HSP in combination with mass spectrometry and sequencing analysis (results not shown), no other residue besides Asn-15 of the glycosylated r-HSP was found to be modified, i.e. no O-glycosylation was found.
Molecular weights corresponding to two series of carbohydrate side chains can be deduced from the combination of carbohydrate composition data and ISMS-data, namely (GlcNAc)2(Hex)10-15 and (Hex)13-17 (Table 3). As mannose is the only hexose in the glyco-sylated r-HSP, and as Asn-15 is the only glycosylated residue, it seems reasonable to conclude that the structure of the glycosylation site is Asn-(GlcNAc)2-(Man)10-15. The observed Asn-(Hex)13-17 forms are thus most likely to arise from fragmentation in the mass spectrometer, by which the two GlcNAc residues lose an acetyl group and are converted into two hexoses.
The structure of Asn-(GlcNAc)2-(Man)10-15 has previously been reported as high mannose type of N-glycosylation for other peptides and proteins expressed in yeast .