Mutant staphylococcus aureus V8 proteases

Abstract: Mutant proteases are obtained with one or more mutation sites in the natural V8 protease protein, and with enzyme activities even in the presence of high urea concentrations. Inactivation of enzyme activity is minimized even in the presence of high concentrations of urea, to thus allow lower amounts of enzyme to be added to urea-containing reaction systems and shorten reaction times. As an additional advantage, the ability to cleave proteins in the presence of high urea concentrations makes it possible to obtain hitherto unobtainable peptide fragments.

Description: The present disclosure relates to mutants of Staphylococcus aureus V8 protease, including those with enzyme activity even under environmental conditions that promote protein denaturation (hereunder Staphylococcus aureus V8 protease will be referred to as "natural V8 protease"), to genes coding for the enzyme proteins, to expression vectors containing such genes, to recombinant cells transformed with such expression vectors, and to methods of producing and using the enzymes. Specific enzyme proteins disclosed herein are mutant V8 proteases with more stable enzyme activity than known natural V8 protease, even under environmental conditions which promote protein denaturation, such as the presence of protein denaturants or high temperature. Natural V8 protease is a serine protease secreted by S. aureus V8 in culture medium. In 1972, G.R. Drapeau et al. isolated and purified it as a serine protease from S. aureus V8 culture medium, which specifically cleaved a C-terminal peptide bond between glutamic acid and aspartic acid (Jean Houmard and Gabriel R. Drapeau (1971), Proc. Natl. Acad. Sci. USA 69, 3506-3509), and in 1987 Cynthia Carmona et al. reported a DNA nucleotide sequence coding for an amino acid sequence of the natural V8 protease (Cynthia Carmona and Gregory L. Gray (1987), Nucleic Acid Res. 15, 6757). After the enzyme is expressed as a precursor consisting of 336 amino acid residues, a prepro sequence consisting of 68 amino acid residues is removed and a mature protein is secreted. The enzyme also has a repeating sequence of proline-aspartic acid-asparagine at the C-terminal region (an amino acid sequence of the amino acid Nos. 221 to 256 from the N-terminus), but the present inventors have already discovered that this sequence is not required for enzymatic activity (Japanese Patent Application No. 6-296028). Although functional analysis of natural V8 protease as an enzyme is still not complete, the enzyme is widely used to determine the amino acid sequence of proteins since it specifically cleaves a C-terminal peptide bond between glutamic acid and aspartic acid. Also, since the natural V8 protease acts on its substrate to some extent even in the presence of urea (about 2M urea), one of the protein denaturation agents which cause protein denaturation, it is used in processes wherein an insoluble fused protein comprising an object peptide expressed in a large amount in a host by recombinant techniques is solubilized with urea, after which the enzyme is allowed to act in the presence of the urea to free the object peptide from the fused protein. The present inventors have succeeded in producing human calcitonin by gene recombination technique at a high yield using the method described above (Japanese Unexamined Patent Publication No. 5-328992). Also, in cases where human glucagon is expressed as a fused protein in an E. coli expression system, natural V8 protease is used to cleave human glucagon from the fused protein (Kazumasa Yoshikawa et al. (1991), Journal of Protein Chemistry 11, 517-525). Thus, natural V8 protease is used in a wide variety of areas of biochemical research and for the production of peptides by gene recombination. Because natural V8 protease carries out some degree of cleavage reaction even in an enzyme reaction mixture containing urea (about 2M urea) which is known to cause protein denaturation, it is used for the production of useful peptides, etc. by recombination methods. However, if it were possible to prepare and use a mutant V8 protease with enzyme activity even under environmental conditions which promote greater protein denaturation, in addition to the properties of the natural V8 protease, then the loss of enzyme activity under such environmental conditions could be minimized. The use of a mutant V8 protease with such properties would provide the advantages of (1) requiring less of the enzyme to be added to the reaction system in the presence of the protein denaturant, (2) allowing the reaction time to be shortened, and (3) making possible protein cleavage in the presence of high concentrations of protein denaturants and at high temperatures to provide hitherto unobtainable peptide fragments, for which reasons such a mutant V8 protease would be highly desirable in research and industry. A general aim herein is to provide new and useful V8 proteases, especially those with good or improved enzyme activity under environmental conditions which promote protein denaturation. Other aspects herein relate to genes coding for such enzyme proteins, to expression vectors containing such genes, recombinant cells transformed with such expression vectors, and methods of producing and using the enzyme proteins. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows (a) the structure of the Staphylococcus aureus V8 protease gene and the annealing sites of the gene, and (b) the base sequences of the PCR primers used for cloning. Fig. 2 shows a process for the construction of plasmid pG97S4DhCT[G]R6 and plasmid pG97S4DhCT[G]R10. Fig. 3 shows a process for construction of plasmid pV8RPT(-). Fig. 4 shows the amino acid sequence (SEQ ID NO: 3) of the fused protein encoded in plasmid pV8RPT(-). The underlined portion is the sequence portion of the wild V8 protease RPT(-) derivative, and the double underlined portion is the amino acid sequence of the R6 linker. Fig. 5 shows (a) nucleotide sequences (SEQ ID NOs: 4 and 5) of the PCR primers used for cloning and (b) the structure of plasmid pV8RPT(-) and the annealing sites of the genes on the plasmid. Fig. 6 illustrates the introduction of PCR mutations into plasmid pV8RPT(-). Fig. 7 shows the changes with time elapse in the reactivity of mutant V8 protease RPT (-) derivatives in the presence of 5M urea. Fig. 8 lists the results of identification of the mutation sites in the mutant V8 protease RPT (-) derivatives. First nucleotide of wild V8 protease RPT(-) derivative gene is designated as nucleotide No. 1. N-terminus of wild V8 protease derivative RPT(-) is designated as amino acid No. 1. Fig. 9 shows a process for the construction of plasmid pV8RPT(-)158. Fig. 10 shows the changes with time elapse in the reactivity of mutant V8 protease RPT(-) derivatives having double and triple mutations, in the presence of 5M urea. Fig. 11 shows a process for the construction of plasmid pV8hCT[G]. Fig. 12 shows a process for the construction of plasmid pV8D. Fig. 13 shows the amino acid sequence (SEQ ID NO: 9) of the fused protein encoded in plasmid pV8D. The underlined portion is the sequence portion of the wild V8 protease D derivative (V8D), and the double underlined portions are the amino acid sequences of the R6 linkers. The arrows indicate the cleavage sites of OmpT protease. Fig. 14 shows a process for the construction of plasmid pV8F. The V8F gene is the gene coding for the wild V8 protease F derivative (V8F). Fig. 15 shows the amino acid sequence (SEQ ID NO: 15) of the fused protein encoded in plasmid pV8F. The underlined portion is the sequence portion of the wild V8 protease F derivative (V8F), and the double underlined portions are the amino acid sequences of the R6 linkers. The arrows indicate the cleavage sites of OmpT protease. Fig. 16 shows a process for the construction of plasmids pV8D1, pV8D5 and pV8D8. Fig. 17 shows the residual activities of the mutant V8 protease D derivatives (V8D1, V8D5 and V8D8) derived from plasmids pV8D1, pV8D5 and pV8D8, and the wild V8 protease D derivative (V8D) derived from pV8D, in the presence of 3M urea. Fig. 18 shows a process for the construction of plasmids pV8F1, pV8F5, pV8F7, pV8F8 and pV8F158. Fig. 19 shows the changes with time elapse in the reactivity of the mutant V8 protease F derivatives (V8F1, V8F5, V8F7, V8F and V8F158) derived from plasmids pV8F1, pV8F5, pV8F7, pV8F8 and pV8F158 and of the wild V8 protease F derivative (V8F) derived from pV8F, in the presence of 5M urea. Fig. 20 shows the stabilities against 0.1% SDS of the mutant V8 protease F derivatives (V8F1, V8F5 and V8F8) derived from plasmids pV8F1, pV8F5 and pV8F8, and of the wild V8 protease F derivative (V8F). Fig. 21 shows the thermal stabilities at 50°C of the mutant V8 protease F derivatives (V8F1, V8F5 and V8F8) derived from plasmids pV8F1, pV8F5 and pV8F8 and of the wild V8 protease F derivative (V8F). FURTHER EXPLANATIONS We have found that by inducing mutations in wild-type V8 protease gene we can obtain genes for mutant V8 protease derivatives whose V8 protease enzyme activity under denaturing conditions is markedly improved. In this work we used PCR to induce random mutations in a wild V8 protease derivative gene obtained by PCR from natural V8 protease gene. The mutant genes are expressed in a suitable host and one or more mutant V8 proteases which exhibit improved enzymatic activity stability under denaturing conditions are selected. The appropriate protein products encoded by the selected genes are then produced, a method convenient for industrial application. In the present specification, a "mutant V8 protease" refers to an enzyme protein having an enzyme activity even under environmental conditions which promote denaturation of natural V8 protease, a "wild V8 protease derivative" refers to an enzyme protein prepared by deleting a C-terminal portion from natural V8 protease, and a "mutant V8 protease derivative" refers to an enzyme protein prepared by causing a mutation in a wild V8 protease derivative. Three different wild V8 protease derivatives with different C-terminal amino acid sequences were prepared, and these are referred to as (1) wild V8 protease derivative RPT (-) (abbreviated as V8RPT(-)), (2) wild V8 protease D derivative (abbreviated as V8D) and (3) wild V8 protease F derivative (abbreviated as V8F). Also, mutant V8 protease derivatives prepared by introducing mutations in the wild V8 protease derivatives are referred to by attaching the number, for example, "1", "5", "7", "8" or "158" to the end of the derivative name depending on the type of mutation (for example, V8RPT(-) 1, V8D5, V8F158, etc.), indicating the introduced mutation. Environmental conditions under which denaturation of natural V8 protease is promoted are known and include cases where the protein structure changes due to protein denaturation agent or temperature, and the explanation in the present specification deals more specifically with the case of protein denaturation agents, with particular examples of enzyme proteins with sufficient resistance against high concentrations of urea as a protein denaturation agent. As mentioned above, the amino acid sequence and DNA nucleotide sequence of natural V8 protease has already been elucidated, but the three dimensional structure of the protein is not yet fully understood. Thus, it is completely unknown which amino acids may be changed to impart the resistance against high urea concentration. Here, the present inventors devised a method of effecting random mutations in the gene coding for this enzyme to isolate a mutant gene for an enzyme which exhibits urea resistance, incorporating the mutated gene into an expression plasmid, transforming host cells with the recombinant plasmid and expressing the gene, and then screening for recombinant expressing enzymes which exhibited enzyme activity even in the presence of 5M urea (In which natural V8 protease is inactivated). In particular we conducted the following research for the purpose of the present invention. First, a plasmid pV8RPT(-) was constructed incorporating the wild V8 protease derivative RPT(-) gene obtained from the natural V8 protease gene by the PCR. The wild V8 protease derivative RPT(-) is a derivative with a deletion of 48 C-terminal amino acids of natural V8 protease, and plasmid pV8RPT(-) is a plasmid expressing a fused protein of this wild V8 protease derivative RPT(-) with an E. coli β-galactosidase derivative (β-ga197S4D). Next, the wild V8 protease derivative RPT(-) gene on pV8RPT(-) was treated for random mutations by the PCR, to prepare a pool of mutated wild V8 protease derivative RPT(-) genes. The mutated gene pool was substituted for the wild V8 protease derivative RPT(-) gene on pV8RPT(-) and used to transform E. coli JM101, thus obtaining a number of recombinants. The recombinants were cultured, isopropyl-β-D-thiogalactopyranoside (hereunder, IPTG) was added, and the gene for the fused protein was expressed. After adding urea to the culture medium to a final concentration of about 5M, a synthesized substrate for the enzyme, Z-Phe-Leu-Glu-4-nitroanilide was used for the enzyme reaction. As a result of screening about 700 recombinant strains, 4 recombinant strains U1, U5, U7 and U8 were obtained which had enzyme activity under the reaction conditions described above. Upon comparing the enzyme activities of these mutant strains in the presence of 5M urea with the natural V8 protease, all were found to have much higher urea resistance than the natural V8 protease. The plasmids were also isolated from these recombinants and the DNA nucleotide sequences of the mutant V8 protease derivative RPT(-) genes were determined, and as a result, substitution mutations were found of arginine for lysine at the amino acid position 147 in strain U1, of serine for asparagine at the amino acid position 71 in strain U5, of serine for asparagine at the amino acid position 71 and arginine for lysine at the amino acid position 147 in strain U7, and of glutamic acid for aspartic acid at the amino acid position 44 in strain U8. In other words, it became clear that 3 different types of amino acid substitutions had been obtained, and the gene derived from strain U7 was a double mutant with a combination of the amino acid substitutions of strains U1 and U5. The present inventors have demonstrated for the first time that amino acid mutations at these particular positions enable an otherwise wild-type V8 protease derivative RPT(-) to exhibit urea resistance. These amino acid positions are assumed to be important positions from the standpoint of maintaining the protein structure of V8 protease. Thus, the introduction of other amino acids at these positions alters the protein structure of the enzyme of the present invention, thus adequately raising the urea resistance. Since the protein from the U7 strain with double mutations had the highest urea resistance of the obtained mutant V8 protease derivatives RPT(-), the present inventors expected that if the above-mentioned 3 types of mutations were combined and introduced into the wild V8 protease derivative RPT(-) gene, the resistance might be further increased beyond the urea resistance exhibited by each individual mutant V8 protease derivative RPT(-), and therefore a mutant V8 protease RPT(-) derivative gene with a triple mutation was prepared, and its fused protein was expressed and studied for urea resistance. As a result, the mutant V8 protease derivative RPT(-) with a triple mutation clearly exhibited higher urea resistance than any individual mutant V8 protease derivative RPT(-). The present inventors then investigated production of this mutant V8 protease derivative. Production was attempted by a method in which the mutant V8 protease D or F derivative, each having a different C-terminal amino acid sequence from mutant V8 protease derivative RPT(-), was first expressed as an inactive inclusion body, and then the active mutant V8 protease D or F derivative was obtained. This was because of the low amount of expression with the mutant V8 protease derivative RPT(-), which is disadvantageous from the standpoint of production. The wild V8 protease D and F derivatives used for preparation of the mutant V8 protease D and F derivatives are derivatives lacking, respectively, 56 and 53 amino acids from the C-terminus of natural V8 protease. The mutant V8 protease derivative gene was incorporated into plasmids pV8D and pV8F used to express the wild V8 protease D derivative and the wild V8 protease F derivative, and.upon culturing, an insoluble fused protein consisting of the E. coli β-galactosidase derivative, the mutant V8 protease D or F derivative and an aminoglycoside 3'-phosphotransferase derivative (tAPT) was recovered, the fused protein was cleaved with the endogenous E. coli ompT protease in the presence of urea, and the mutant V8 protease D or F derivative was cut out from the fused protein. Then, the mutant V8 protease D or F derivative was purified through a refolding step and purifying chromatography step, showing that production was possible on an industrial scale. Each purified mutant V8 protease D or F derivative was then used for a comparison test with their wild forms, under environmental conditions which promote the protein denaturation of each type, e.g. in the presence of urea, in the presence of SDS and at high temperature. As a result, the mutants had lower inactivation rates in the presence of urea or SDS compared to the wild forms, clearly demonstrating that resistance was exhibited against these denaturation agents. Since the mutant V8 protease F derivative with the triple mutation, i.e., V8F158, has more stable enzyme activity than natural V8 protease even in the presence of 5M urea, a peptide cleavage experiment was conducted using a fused protein of this mutant protein. The fused protein used was β-ga197S4DhCNP-22R5-3, having a structure wherein a protective peptide (β-ga197S4D) and human C-type atrial natriuretic peptide (CNP-22) are fused via a linker peptide, and CNP is released by the V8 protease. As a result of studying the cleavage efficiency of V8F158 and natural V8 protease on the fused protein of human C-type atrial natriuretic peptide in the presence of 5M urea, it was conclusively shown that V8F158 was able to cleave the fused protein with a much higher cleavage efficiency. From the examples conducted by the present inventors it is apparent that the mutant V8 protease F derivatives (V8F1, V8F5 and V8F8) exhibit greater resistance against denaturants with enzyme reaction in the presence of 5M urea concentration and 0.1% SDS, than the wild V8 protease F derivatives. However, upon comparison of the enzyme reactions at high temperature (50°C), the mutant V8 protease F8 derivative (V8F8) had lower thermal stability than the wild V8 protease F derivative (V8F). Different results were obtained for resistance of the enzymes under denaturing conditions, depending on the different protein denaturing conditions of urea and temperature, and the results may be interpreted as further suggestion that the amino acid residues of the mutated sites are important for maintaining the higher structure of the proteins. Thus, since a person skilled in the art can easily infer that the resistance against temperature will increase if the amino acid at the mutation site of the mutant V8 protease F8 derivative (V8F8) is replaced with another amino acid, the usefulness of the present invention is in no way diminished. The present inventors have proven the usefulness of the mutant V8 protease derivatives by comparing the enzyme activities of mutant V8 protease derivatives, wild V8 protease derivatives and natural V8 protease under environmental conditions which promote protein denaturation. From the results obtained by the present inventors, it is easily possible for a person skilled in the art to infer that introduction of such mutants into natural V8 protease will result in enzyme activity equal to or greater than that of the natural form, even under environmental conditions which promote protein denaturation. Consequently, the enzyme proteins of the present invention are mutant V8 proteases which exhibit enzyme activity even under environmental conditions which promote protein denaturation. These proteases are ones characterized by having one or more mutation sites in the natural V8 protease. More specifically, proteases of this invention are characterized in that the mutation sites are the aspartic acid at position 44, the asparagine at position 77 and/or the lysine at position 147 from the N-terminus of the natural V8 protease, in which case there may be one, two or three mutation sites. The most preferred embodiments are: (1) a case of one mutation site wherein, counting from the N-terminus of the natural V8 protease, the mutation site is a substitution of glutamic acid for aspartic acid at the position 44, a substitution of serine for asparagine at the position 71 or a substitution of arginine for lysine at the position 147; (2) a case of two mutation sites wherein, counting from the N-terminus of the natural V8 protease, the mutation sites are aspartic acid at the position 44 and asparagine at the position 71 (in which case glutamic acid is preferably substituted for aspartic acid at the position 44 and serine for asparagine at the position 71), aspartic acid at the position 44 and lysine at the position 147 (in which case glutamic acid is preferably substituted for aspartic acid at the position 44 and arginine for lysine at the position 147), or asparagine at the position 71 and lysine at the position 147 (in which case serine is preferably substituted for asparagine at position 71 and arginine for lysine at position 147); and (3) a case of three mutation sites wherein, counting from the N-terminus of the natural V8 protease, the mutation sites are aspartic acid at the position 44, asparagine at the position 71 and lysine at the position 147 (in which case glutamic acid is preferably substituted for aspartic acid at the position 44, serine for asparagine at the position 71 and arginine for lysine at the position 147). The method of preparing the genes coding for these mutant V8 proteases may be a method of isolating the genes from S. aureus, for example, a method of designing a primer from the reported natural V8 protease gene sequence followed by isolating them from the gene bank of S. aureus, or the PCR method used by the present inventors. Needless to mention, it is also possible to chemically synthesize the genes from the known natural V8 protease gene sequence of S. aureus. In the present specification, the mutant V8 protease derivative genes were prepared by the PCR method, but the mutant genes may be prepared using any conventionally known mutation method, such as an in vivo method (mutagenic agent treatment, ultraviolet or radiation treatment, etc.) or one of a variety of in vitro mutation methods. Mutant strains may also be selected according to the present invention by the spontaneous mutation selection method. For enzyme reaction conditions which promote protein denaturation, the enzyme reaction may be conducted in a reaction medium containing urea or SDS, or at high temperature (for example, 45°C or higher), or with other protein denaturation agents such as guanidine hydrochloride and various surfactants. Enzyme reaction conditions which have a protein denaturing effect include 2 to 5M urea, 0.01 to 6M guanidine hydrochloride, 0.01 to 10% SDS and temperatures of 45 to 65°C. The following methods may be used to express the genes for the mutant V8 proteases according to the invention which have enzyme activity even under environmental conditions which promote protein denaturation. In Examples in the present specification, E. coli cells were used as the host cells, but the host cells may also be prokaryotic cells such as Staphylococci, Salmonella, Actinomycetes, Bacillus subtilis, etc. or eukaryotic cells such as filamentous bacteria, yeast, insect cells, animal cells or the like. The mutant V8 protease genes may be introduced into these host cells by any commonly used transformation technique. The expression plasmid used for the transformation is preferably an expression plasmid with a promoter capable of functionally controlling the expression of a gene downstream of the promoter in the host cells, and by which the direct expression method or so-called fusion protein expression method may be carried out. Also, the vector used may be one capable of homologous gene recombination, with the gene of interest incorporated into the chromosome of a suitable host cell to express the relative protein. By another method, a virus or phage incorporating the gene may be used to infect host cells for expression of the gene of interest. As a method of producing a mutant V8 protease according to the invention with enzyme activity even under environmental conditions which promote protein denaturation, the present inventors have presented in Examples a very highly efficient production method, but any known common gene recombination technique may be used for the production, without limitation to the method described here. Examples of possible product methods employing the expression method described above include (1) a method for production by direct expression of the mature protein of interest in the host cells followed by separation and purification of the protein in soluble or insoluble form, (2) a method for production by expression of the protein of interest in the host cells as a fused protein in either soluble or insoluble form, cleavage of the fused protein under conditions which allow cleavage by a processing enzyme, followed by separation and purification of the protein of interest, (3) a method for production by extracellular secretion of the protein of interest followed by separation and purification of the protein, and (4) a method for production by separation and purification of the protein of interest in soluble or insoluble form from the periplasm of the host cells. Needless to mention, when the protein of interest is expressed by direct expression or by the fused protein method, the separation and purification are accomplished through a suitable refolding step in cases where an insoluble protein has been obtained. In Examples in the present specification, hydrophobic chromatography after refolding is employed for purification of the mutant V8 protease derivatives of the invention, but other methods of purification normally used for protein purification, such as gel filtration, ion chromatography, etc. can provide high degrees of purity. Also, since the enzyme protein is activated once the refolding reaction has been completed and decomposes any contaminating proteins, resulting in the mutant V8 protease derivative as the major protein component after completion of the reaction, the purification is, of course, very easily accomplished. EXAMPLES The present invention will now be more fully explained by way of the following examples. Example 1. Isolation of wild V8 protease derivative RPT(-) gene The wild V8 protease RPT(-) derivative gene was isolated by the PCR method. Two different PCR primers were designed with the sequences (SEQ ID NOs: 1 and 2) shown in Fig. 1(b), and were synthesized with a DNA synthesizer (Model 392, product of Applied Biosystems Co.). Primers A and B correspond to regions of the V8 protease gene shown in Fig. 1(a), and they contain sequences at their 5' ends recognized by the restriction endonucleases XhoI and SalI, respectively. The PCR was conducted using S. aureus V8 (ATCC27733) chromosomes isolated and prepared according to the method of Jayaswal, R.K. et al. (J. Bacteriol. 172:5783-5788(1990)) and these PCR primers. To 50 µl of a reaction solution containing 1.0 µM of primer, 1 µg of chromosomal DNA, 50 mM KCl, 10 mM Tris-HCl (pH 8.3), 1.5 mM MgCl 2 , 0.01% gelatin and 200 µM of dNTP (mixture of dATP, dGTP, dCTP and dTTP) there was added 2.5 units of TaqDNA polymerase, and the PCR was conducted with 30 cycles of 94°C for 1 minute, 72°C for 2 minutes and 55°C for 2 minutes. As a result there was obtained the wild V8 protease RPT(-) derivative gene lacking the prepro sequence and 48 C-terminal amino acids. The gene was then subjected to agarose gel electrophoresis and purified using SUPREP-2 (Takara Shuzo, KK.), and then cleaved with the restriction enzymes XhoI and SalI, to prepare wild V8 protease derivative RPT(-) gene fragment containing the XhoI and Sail cohesive ends. Example 2. Construction of expression vector pV8RPT(-) and expression of wild V8 protease derivative RPT(-) The pG97S4DhCT[G]R6 used in this Example (Appl. Microbiol. Biotechnol. (1995) 42, 703-708) is a plasmid with high expression of a fused protein of an E. coli β-galactosidase derivative with human calcitonin precursor (hCT[G]), and this plasmid was constructed from plasmid pBR322 and plasmid pG97S4DhCT[G] (Fig. 2). E. coli strain W3110 containing the plasmid pG97S4DhCT[G] has been deposited at the National Institute of Bioscience and Human Technology Agency of Industrial Science and Technology 1-3, Higashi 1-chome Tsukuba-shi Ibaraki-ken 305. Japan, on August 8, 1991 as Escherichia coli SBM323, according to the Budapest Treaty, and has been assigned the deposit number FERM BP-3503. In order to express the wild V8 protease derivative RPT(-) gene obtained by the PCR, pG97S4DhCT[G]R6 was treated with XhoI and SalI, and a DNA fragment (3.1 kb) lacking the human calcitonin precursor gene portion was prepared by agarose gel electrophoresis. This DNA fragment was linked with the previously obtained wild V8 protease gene fragment containing the XhoI and SalI cohesive ends using T4 DNA ligase, and JM101 (this strain is available from Takara Shuzo, KK. and elsewhere) was transformed therewith to construct pV8RPT(-) (Fig. 3). Fig. 4 shows the amino acid sequence (SEQ ID NO: 3) for this fused protein (βG97VBRPT(-)) of the wild V8 protease RPT(-) derivative and the β-galactosidase derivative, expressed by the plasmid. After cultivation of JM101/pV8RPT(-) in 100 ml of LB medium (0.5% yeast extract, 1.0% tryptone, 0.5% Nacl) at 37°C to an OD660 of 1.0, IPTG was added to a final concentration of 2 mM to induce expression. Cultivation was continued for 2 hours after the addition, and then the cells were collected by centrifugation and suspended in a TE buffer (10 mM Tris-HCl (pH 8.0), 1 mM EDTA) to an OD660 of 5. The suspension was disrupted with an ultrasonic disrupter (Cellruptor: Toso Electric, KK.), and then the insoluble fraction was removed by centrifugation for 5 minutes and the supernatant fraction was used as the crude enzyme solution. A synthetic substrate (Z-Phe-Leu-Glu-4-nitroanilide; product of Boehringer-Mannheim) was used to measure the activity of the V8 protease. After mixing 20 µl of a 10 mM Z-Phe-Leu-Glu-4-nitroanilide solution (DMSO solution) with 940 µl of a 100 mM Tris-HCl (pH 8.0) buffer solution, 40 µl of the crude enzyme solution was added, and the increase in absorbance at 405 nm upon reaction at room temperature for 5 minutes was measured. A Hitachi spectrophotometer Model U-3200 was used for the measurement. As a result, an activity corresponding to 8 µg/ml of natural V8 protease was found in the crude enzyme solution prepared from JM101/pVBRPT(-), thus demonstrating that activity is exhibited in the form of a fused protein with the β-galactosidase derivative, and lacking the prepro sequence and the C-terminal repeating sequence. Example 3. Mutation of V8 protease derivative RPT(-) gene by PCR The wild V8 protease derivative RPT(-) gene was subjected to mutation by the PCR to obtain V8 proteases with resistance against urea. The pV8RPT(-) obtained in Example 2 (Fig. 3) is a plasmid expressing a fused protein of the wild V8 protease derivative RPT(-) lacking 48 amino acids at the C-terminus (V8RPT(-)) with an E. coli β-galactosidase derivative (β-ga197S4D), and the fused protein is expressed in the cellular soluble fraction (this fused protein will hereunder be referred to as βG97V8RPT(-)) has V8 protease activity. The primers shown in Fig. 5(a) (SEQ ID NOs: 4 and 5) were used for a PCR reaction with the βG97V8RPT(-) gene on the aforementioned plasmid (Fig. 5(b)). To 50 µl of a reaction solution containing 1 µmol of primer, 50 ng of pV8RPT(-), 50 mM KCl, 10 mM Tris-HCl (pH 8.3), 1.5 mM MgCl 2 , 0.01% gelatin, 10% dimethylsulfoxide (DMSO), 1 mM each of dGTP, dCTP and dTTP and 200 µM of dATP there was added 2.5 units of TaqDNA polymerase, and the PCR was conducted with 30 cycles of 94°C for 1 minute, 72°C for 2 minutes and 55°C for 2 minutes. The resulting PCR product (1 Kbp) was subjected to chloroform treatment and ethanol precipitation and then dissolved in 50 µl of a TE buffer (10 mM Tris-HCl (pH 8.0), 1 mM EDTA). Example 4. Screening for mutant V8 protease derivatives RPT(-) with urea resistance (primary screening) The PCR product obtained in Example 3 was cleaved with restriction enzymes BglII and SalI, and the 0.8 Kbp fragment containing the mutated wild V8 protease derivative RPT(-) gene was isolated by 0.8% agarose gel electrophoresis, after which a ligation kit (product of Takara Shuzo, KK.) was used to link it with a 3.0 Kbp pV8RPT(-)-derived BglII-SalI fragment (Fig. 6). After completion of the reaction, it was used to transform E. coli JM101 (available from In Vitrogen, Catalog No. c660-00) by the calcium chloride method, and transformants were obtained in a 10 µg/ml tetracycline-containing LB agar medium (1% tryptone, 0.5% yeast extract, 0.5% NaCl, 1.5% agar). The restriction enzyme reaction, agarose gel electrophoresis and transformation were all conducted according to conventional methods. Next, a medium was prepared (pH 7.4) containing 5 mg/ml glycerine, 6 mg/ml Na 2 HPO 4 , 3 mg/ml KH 2 PO 4 , 0.5 mg/ml NaCl, 1.0 mg/ml NH 4 Cl, 2 mM MgSO 4 •7H 2 O, 0.1 mM CaCl 2 , 40 µg/ml of each amino acid (20 types), 1 µg/ml thiamine chloride, and 5 µg/ml tetracycline, and 50 µl thereof was dispensed into each well of a 96-well culturing plate (Product No. 25860 of Corning Co.). Each of the transformant strains obtained earlier was seeded in the medium and cultured at 37°C. After one night of static culturing, 50 µl of fresh medium with the same composition was dispensed into each well and static culturing was continued for 3 hours at 37°C. A 10 µl portion of 50 mM IPTG was then added to induce gene expression at 37°C for one hour. Next, 30 µl of a 10 mg/ml aqueous lysozyme solution was added, and after standing for 10 minutes, 30 µl of a solution containing 0.1% triton X-100 and 5 mM EDTA (pH 8.0) was added for bacteriolysis. After subsequently adding 160 µl of 0.1M Tris-HCl (pH 8.0) containing 10M urea (to a final urea concentration of 4.85M) and allowing the mixture to stand at 30°C for 30 minutes, 10 µl of a DMSO solution containing 20 mM Z-Phe-Leu-Glu-4-nitroanilide (product of Boehringer-Mannheim Co.) was added for reaction overnight at 30°C. Upon screening about 700 transformants, 4 strains with the strongest degree of yellow coloration produced by decomposition of the enzyme substrate Z-Phe-Leu-Glu-4-nitroanilide were obtained and assigned the names U1, U5, U7 and U8. Example 5. Screening for mutant V8 protease derivatives RPT(-) with urea resistance (secondary screening) After culturing the 4 strains (U1, U5, U7 and U8) in 10 ml of LB medium (0.5% yeast extract, 1.0% tryptone, 0.5% NaCl) at 37°C to an OD660 of 1.0, IPTG was added to a final concentration of 2 mM, and culturing was continued for 2 hours, after which the cells were collected by centrifugal separation. The cells were then suspended in a TE buffer (10 mM Tris-HCl (pH 8.0), 1 mM EDTA) to an OD660 of 5, and the cells were disrupted with an ultrasonic disruptor (Cellruptor: Toso Electric, KK.). The disrupted solution was centrifuged at 12,000 rpm for 5 minutes to remove the insoluble fraction, and the supernatant fraction was used as the crude enzyme solution. For measurement of the protease activity, 20 µl of a 20 mM Z-Phe-Leu-Glu-4-nitroanilide solution was mixed with 940 µl of a 50 mM Tris-HCl (pH 8.0) buffer solution, 40 µl of the crude enzyme solution was added, and the increase in absorption at 405 nm upon reaction at room temperature for 5 minutes was measured using a Hitachi spectrophotometer Model U-3200. After measuring the V8 protease activity of the crude enzyme solutions from the 4 strains, an amount of each crude enzyme solution corresponding to 0.2 µg enzyme activity of natural V8 protease (for example, endoproteinase Glu-C, product of Boehringer-Mannheim Co.) was used to determine the change in reactivity over time in the presence of urea. The experiment was conducted with a reaction solution containing 5M urea, 50 mM Tris-HCl (pH 8.0), 0.4 mM Z-Phe-Leu-Glu-4-nitroanilide and 2% DMSO, measuring the change in the reaction with time based on the increase in absorbance at 405 nm (using a Hitachi spectrophotometer Model U-3200). The results are shown in Fig. 7. As Fig. 7 clearly shows, the fused proteins of the mutant V8 protease derivatives RPT(-) produced by mutant strains U1, U5, U7 and U8 (hereunder referred to respectively as βG97V8RPT(-)1, βG97V8RPT(-)5, βG97V8RPT(-)7 and βG97V8RPT(-)8) maintained decomposition activity on the substrate, exhibiting resistance against urea, in contrast with the wild form βG97V8RPT(-). Example 6. Identification of mutation sites and preparation of mutant V8 protease derivative RPT(-) fused protein with combined mutations The plasmids were isolated and purified from the mutant strains U1, U5, U7 and U8 by conventional methods. These plasmids will hereunder be referred to as pVBRPT(-)1, pV8RPT(-)5, pV8RPT(-)7 and pV8RPT(-)8. The DNA nucleotide sequences of the mutant V8 protease derivative genes on each of the plasmids were then determined using a DNA.sequencer (A.L.F. DNA Sequencer), manufactured by Pharmacia. Determination of the DNA nucleotide sequences was accomplished by the fluorescent label method employing fluoro-dUTP, using an AutoRead Sequencing Kit, also manufactured by Pharmacia, and the following primers were used. Primer A (a sense primer annealing with a portion from nucleotide Nos. 1 to 22, nucleotide No. 1 being the first base of the gene coding for the wild V8 protease RPT(-) derivative shown in Fig. 4); Primer D1 (a sense primer annealing with a portion from nucleotide Nos. 266 to 294 of the same); Primer D2 (an antisense primer annealing with a portion from nucleotide Nos. 266 to 294 of the same); The DNA sequencing procedure described in the manufacturer's laboratory manual was followed. As a result it was demonstrated that the mutations shown in Fig. 8 had occurred in the mutant V8 protease derivatives RPT(-) produced by the 4 mutant strains. In particular, the mutant V8 protease derivative on pV8RPT(-)7 was a double mutation, which was shown to be a combination of the mutations of pVBRPT(-)1 and pV8RPT(-)5. Thus, it was concluded that the double mutation was the reason for the higher activity of βG97V8RPT(-)7 shown in Fig. 7 compared to βG97V8RPT(-)1 and βG97V8RPT(-)5. This shows the possibility of creating a highly urea-resistant enzyme by combining the mutations. In order to confirm this, plasmid pV8RPT(-)158 was constructed, having all 3 of the obtained mutations. Restriction enzyme sites (DraI, EcoRI) present on the V8 protease gene were used for the construction according to the procedure outlined in Fig. 9. The 0.4 Kbp DraI-EcoRI fragment from pV8RPT(-)7 was exchanged for that of pV8RPT(-)8 to construct pV8RPT(-)158. The triple mutant V8 protease derivative RPT(-) derived from this plasmid (βG97V8RPT(-)158) was studied for changes in reactivity with time elapse in the presence of 5M urea, according to the procedure described above (see Fig. 10). As a result, βG97V8RPT(-)158 was demonstrated to have a more prolonged decomposition reaction on the synthetic substrate than βG97V8RPT(-)7, by which it was understood that the 3 mutations impart urea resistance in a cumulative manner. Example 7. Construction of expression vector pV8D Plasmid pV8D expressing the wild V8 protease D derivative (V8D) as an inactive inclusion body was constructed according to the procedure outlined in Figs. 11 and 12. First, a BglII-SalI fragment (3.0 kb) and EcoRV-BglII fragment (0.7 kb) were prepared from pV8RPT(-), and were linked with a NarI-SalI fragment (0.2 kb) prepared from pG97S4DhCT[G]R10 to obtain pV8hCT[G] (Fig. 11). The plasmid pG97S4DhCT[G]R10 (Appl. Microbiol. Biotechnol. (1995) 42, 703-708) may be constructed from plasmid pBR322 and plasmid pG97S4DhCT[G] in the same manner as pG97S4DhCT[G]R6 in Example 2. Next, the hCT[G] portion of the obtained pV8hCT[G] (the 0.1 kb BstE-SalI fragment) was replaced with a 0.8 kb SmaI-SalI fragment containing the aminoglucoside 3'-phosphotransferase gene (APT) region of pUC4K (Vieira, J. and Messing, J., Gene 19, 259 (1982); readily available as Product No. 27-4958-01 of Pharmacia Biotech), to construct pV8D (Fig. 12). Fig. 13 shows the amino acid sequence (SEQ ID NO: 9) of the wild V8 protease D derivative (V8D) fused protein expressed by this plasmid. The wild V8 protease D derivative is a derivative lacking 56 of the C-terminal amino acids from the natural protease. The fused protein was prepared using the portion of natural V8 protease to the 212th amino acid from the N-terminus (the EcoRV site). That is, the fused protein has a structure wherein the β-galactosidase derivative and part of the aminoglucoside 3'-phosphotransferase (tAPT) are fused at the N-terminal and C-terminal portions of the wild V8 protease D derivative (V8D), respectively, via R6 linkers. The R6 linker has the amino acid sequence: and has a structure wherein the peptide bond between RR in the sequence is cleaved by ompT protease of E. coli . Example 8. Construction of expression vector pV8F Plasmid pV8F expressing the wild V8 protease F derivative (V8F) as an inactive inclusion body is a plasmid which expresses a fused protein which is a derivative of the aforementioned wild V8 protease D derivative (V8D) having a V8 protease portion 3 amino acids longer at the C-terminal end, and it was constructed in the following manner by the PCR method and gene cloning. First, primer IV (SEQ ID NO: 11): and primer V (SEQ ID NO: 12): were synthesized, and 0.1 µg of pV8RPT(-) constructed in Example 2 was used as the template DNA for an amplification reaction at the wild V8 protease derivative gene end, after which it was cleaved with EcoRI and SacI to prepare a 0.1 kb gene fragment. Meanwhile, primer VI (SEQ ID NO: 13): and primer VII (SEQ ID NO: 14): were also synthesized and 0.1 µg of pV8D was used as a template DNA for an amplification reaction of the R6 linker sequence and the aminoglucoside 3'-phosphotransferase gene portion, after which it was cleaved with EcoT22I and SacI to prepare a 0.3 kb gene fragment. The PCR was conducted under the same conditions as Example 1. The 0.1 kb and 0.3 kb gene fragments obtained as described above were linked with an EcoRI-EcoT22I fragment (4.2 kb) from pV8D to construct pV8F (Fig. 14). Fig. 15 shows an amino acid sequence (SEQ ID NO: 15) of the wild V8 protease F derivative (V8F) fused protein expressed by this plasmid. The wild V8 protease F derivative is a derivative lacking 53 C-terminal amino acids from the natural protease. Example 9. Production of mutant V8 protease D (V8D) and F derivatives (V8F) and confirmation of urea resistance Plasmids pV8D and pV8F were used to attempt high expression of mutant V8 protease D (V8D) and F (V8F) derivatives. (1) Mutant V8 protease D derivative As shown in Fig. 16, the 0.7 kbp BglII-EcoRI fragments derived from pV8RPT(-)1, pV8RPT(-)5 and pV8RPT(-)8 were each inserted into the 3.9 kbp BglII-EcoRI fragment to construct pVDl, pV8D5 and pV8D8. After culturing JM101 having each of the plasmids in a fermenter (30L Kit Fermenter, product of Komatsugawa Chemical Instruments) in a medium (20 L, pH 7.0) containing 4 g/L K 2 HPO 4 , 4 g/L KH 2 PO 4 , 2.7 g/L Na 2 HPO 4 , 0.2 g/L NH 4 Cl, 1.2 g/L (NH 4 ) 2 SO 4 , 4 g/L yeast extract, 2 g/L MgSO 4 •7H 2 O, 40 mg/L CaCl 2 •2H 2 O, 40 mg/L FeSO 4 •7H 2 O, 10 mg/L MnSO 4 •nH 2 O, 10 mg/L AlCl 3 •6H 2 O, 4 mg/L CoCl 2 •6H 2 O, 2 mg/L ZnSo 4 •7H 2 O, 2 mg/L Na 2 MoO 4 •2H 2 O, 1 mg/L CuCl 2 •2H 2 O, 0.5 mg/L H 3 BO 4 and 10 mg/L tetracycline, at 37°C to an OD660 of 10 while successively adding glycerine, IPTG was added to a final concentration of 2 mM and culturing was continued for 3 hours. The culture medium truth obtained was subjected to homogenization with a MantonGaullin homogenizer (Model 15M-8TBA, from MantonGaullin Co.) under conditions of 600 Kg/cm 2 , and the precipitating fraction was recovered by centrifugation at 7000 rpm for 30 minutes. After adding deionized water until the OD660 of the precipitate reached 100, 15 ml thereof was taken, and 2.5 ml of 1M Tris-HCl (pH 8.0), 250 µl of 1M dithiothreitol (DTT) and 12g of urea were added to dissolve the inclusion bodies, after which deionized water was added to prepare 50 ml and the solution was incubated at 37°C for 6 hours. After subsequent 21-fold dilution with a 20 mM potassium phosphate buffer (pH 7.5) containing 0.4M (NH 4 ) 2 SO 4 , the solution was allowed to stand on ice overnight. This refolding procedure yielded about 80 µg/ml of each of the mutant V8 protease D derivatives, i.e. the mutant V8 protease D derivatives V8D1, V8D5 and V8D8 derived from pV8D1, pV8D5 and pV8D8. For purification of each of the proteases, (NH 4 ) 2 SO 4 was added to a final concentration of 1.8M, and then 300 ml of the mixture was purified using a butyl Toyopearl 650M (product of Toso, KK.). The sample was added to a Φ16 mm × 62 mm column equilibrized with a 10 mM potassium phosphate buffer (pH 7.5) containing 1.8M (NH 4 ) 2 SO 4 , and purification was conducted with a linear concentration gradient from 1.8M to 0M concentration of (NH 4 ) 2 SO 4 . Each of the proteases eluted near the 0.9M (NH 4 ) 2 SO 4 concentration, and about 20 mg of each purified enzyme was obtained. The activities of the purified enzymes were measured by the method described in Example 3. The purified V8D1, V8D5 and V8D8 were used to reexamine the resistance against urea. Each of the mutant enzymes (V8D1, V8D5 and V8D8) was added at 40 µl (concentration: 150 µg/ml) to 960 µl of a reaction mixture containing 3M urea, 50 mM Tris-HCl (pH 8.0) and 2% DMSO, and the mixtures were allowed to stand at 30°C for 30 minutes, after which a synthetic substrate (Z-Phe-Leu-Glu-4-nitroanilide) was added to a final concentration of 0.4 mM, and the residual activity of each enzyme was measured with the activity immediately after addition of the enzyme defined as 100%. The wild V8 protease D derivative (V8D) produced from pV8D by the same procedure was used as a control. As a result, V8D1, V8D5 and V8D8 had higher residual activities than V8D, demonstrating that the introduction of their mutations produce mutant V8 protease D derivatives with improved resistance to urea (Fig. 17). (2) Mutant V8 protease F derivatives Mutations introduced into pV8F were also studied. Following the procedure outlined in Fig. 18, 0.7 kbp BglII-EcoRI fragments derived from pV8RPT(-)1, pV8RPT(-)5, pV8RPT(-)7, pV8RPT(-)8 and pV8RPT(-)158 were inserted into the 3.9 kbp BglII-EcoRI fragment of pV8F to construct pV8F1, pV8F5, pV8F7, pV8F8 and pV8F158. JM101 strains possessing these plasmids were used to separate mutant V8 protease F derivatives having each of the mutations, i.e. the mutant V8 protease F derivatives V8F1, V8F5, V8F7, F8F8 and V8F158 derived from pV8F1, pV8F5, pV8F7, pV8F8 and pV8F158, by the method described above. The purified V8F1, V8F5, V8F7, V8F8, F8F158 and the wild V8 protease F derivative (V8F) derived from pV8F as a control, which were produced by the same procedure, were used to reexamine the resistance against urea. As a result, each of the mutant enzymes was reconfirmed to have excellent urea resistance in the presence of 5M urea (Fig. 19), thus demonstrating the ability to produce mutant V8 protease F derivatives with excellent resistance to urea. Example 10. Study of stability of V8F1, V8F5 and V8F8 against sodium dodecyl sulfate (0.1%) and heat (50°C) V8F1, V8F5 and V8F8 obtained in Example 9 were used to study their stability against sodium dodecyl sulfate (hereunder, SDS) and heat. For SDS stability, a solution containing 0.1% SDS, 50 mM Tris-HCl (pH 8.0) and an amount of each enzyme (V8F1, V8F5, V8F8) corresponding to 4.0 µg/ml enzyme activity of natural V8 protease was incubated at 30°C, and 900 µl thereof was taken at prescribed intervals, after which 100 µl of a solution containing 4 mM Z-Phe-Leu-Glu-4-nitroanilide and 20% DMSO was added and the residual activity was measured. For the thermal stability at 50°C, a solution containing 50 mM Tris-HCl (pH 8.0) and an amount of each enzyme (V8F1, V8F5, V8F8) corresponding to 4.0 µg/ml enzyme activity of natural V8 protease was incubated at 50°C, and 900 µl thereof was taken at prescribed intervals, cooled on ice, and measured for residual activity. As shown in Fig. 20, the mutant enzymes V8F1, V8F5 and V8F8 had lower inactivation rates than the wild form used as the control, and increased stability against 0.1% SDS. This demonstrated that the introduction of the 3 urea-resistant mutations Lys147Arg (V8F1), Asn71Ser (V8F5) and Asp44Glu (V8F8) is also effective against denaturation by SDS. Also, upon thermal inactivation at 50°C, V8F1 and V8F5 had lower inactivation rates than the wild V8F, showing that the mutations Lys147Arg and Asn71Ser impart thermal resistance (Fig. 21). Example 11. Cleavage of fused protein by V8F158 V8F158 (the triple mutant V8 protease F derivative) was used for a protein cleavage experiment in the presence of urea. A fused protein derived from pG97S4DhCNP-22R5-3 was used as the substrate for the experiment. The fused protein β-gal97S4DhCNP-22R5-3 has a structure wherein a protective peptide (β-ga197S4D) and human C-type atrial natriuretic peptide (hCNP) are fused via a linker, and hCNP is released by the V8 protease (Japanese Unexamined Patent Publication No. 5-328992). The natural V8 protease and V8F158 were compared in their ability to cleave hCNP from the fused peptide in the presence of 5M urea. The type of fused peptide derived from pG97S4DhCNP-22R5-3, the expression of the fused peptide, the recovery of the expressed inclusion body, the reaction conditions for V8 protease and the analysis of the fused protein and released hCNP were all according to the conditions described in Japanese Unexamined Patent Publication No. 5-328992. However, the urea concentration was adjusted to 5M for the reaction, and V8F158 was added in an amount corresponding to 4 µg/ml activity in terms of natural V8 protease. After 30 minutes of reaction, the cleavage efficiency (proportion of fused protein cleaved) was calculated from the peak of HPLC, resulting in 60% for the natural V8 protease compared to 98% for V8F158, and thus the effectiveness of V8F158 in the presence of a high urea concentration was also demonstrated in the fused protein cleavage reaction. Publicly known natural V8 protease is commonly used for the production of useful peptides, etc. by gene recombination methods because it still carries out some degree of cleavage reaction even in enzyme reaction solutions containing about 2M urea; however, as mentioned above, the enzymes of the present invention may add even greater urea resistance to the properties of the natural enzyme. Thus, an enzyme according to the invention may be used to minimize inactivation of enzyme activity even in the presence of high concentrations of urea, to thus require lower amounts of enzyme to be added to urea-containing reaction systems and shorten reaction times. An additional advantage is that the ability to cleave proteins in the presence of high urea concentrations makes it possible to obtain hitherto unobtainable peptide fragments. It should be understood that sequence variations other than the mutations mentioned above may be tolerated (or even desirable, with other technical effects in mind) provided that the V8 protease activity of the Staphylococcus aureus - derived enzyme is retained. In particular there may be deletions, additions or substitutions relative to the wild-type sequence, as has already been illustrated by the use of C-terminal-truncated proteases. The deliberate adoption or toleration of such sequence variations is of course a matter of routine for the skilled person, since what matters in relation to the present invention is determinable by simple tests, namely those for V8 protease activity and its maintenance under denaturing conditions. SEQUENCE LISTING SEQ ID NO: 1 Sequence Length: 31 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 2 Sequence Length: 31 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 3 Sequence Length: 344 Sequence Type: Amino acid Topology: Linear Molecular Type: Protein Sequence SEQ ID NO: 4 Sequence Length: 28 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 5 Sequence Length: 31 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 6 Sequence Length: 31 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 7 Sequence Length: 29 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 8 Sequence Length: 29 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 9 Sequence Length: 532 Sequence Type: Amino acid Topology: Linear Molecular Type: Protein Sequence SEQ ID NO: 10 Sequence Length: 24 Sequence Type: Amino acid Topology: Linear Molecular Type: Peptide Sequence SEQ ID NO: 11 Sequence Length: 31 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 12 Sequence Length: 33 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 13 Sequence Length: 33 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 14 Sequence Length: 27 Sequence Type: Nucleic acid Strandedness: Single Topology: Linear Molecular Type: Synthetic DNA Sequence SEQ ID NO: 15 Sequence Length: 537 Sequence Type: Amino acid Topology: Linear Molecular Type: Protein Sequence

Claims: A mutant Staphylococcus aureus V8 protease comprising at least an N-terminal amino acid sequence from the first Valine to the 212th Aspartic acid of the amino acid sequence of the natural Staphylococcus aureus V8 protease, wherein at least one of the 44th Aspartic acid, the 71st Asparagine and the 147th Lysine is replaced with another amino acid. A mutant protease according to claim 1, comprising an amino acid sequence from the 125th Val to the 336th Asp in SEQ ID No. 3, wherein at least one of the 168th Asp, the 195th Asn and the 271st Lys is replaced with another amino acid. A mutant protease according to claim 2, wherein one of the 168th Asp, the 195th Asn and the 271st Lys is replaced with another amino acid. A mutated protease according to claim 2, wherein (a) the 168th Asp and the 195th Asn (b) the 168th Asp and the 271st Lys, or (c) the 195th Asn and the 271st Lys are replaced with other amino acids. A mutated protease according to claim 2, wherein all of the 168th Asp, the 195th Asn and the 271st Lys are replaced with other amino acids. A mutated protease according to any one of claims 2 to 5, wherein the total length of the amino acid sequence corresponds to the amino acid sequence from the 105th Arg to the 345th Gln of SEQ ID No. 3. A mutated protease according to any one of claims 2 to 5, wherein the total amino acid sequence corresponds to the amino acid sequence from the 105th Arg to the 340th Arg of SEQ ID No. 9. A mutated protease according to any one of claims 2 to 5, wherein the total amino acid sequence corresponds to the amino acid sequence from the 105th Arg to the 345th Arg of SEQ ID No: 15. A mutated protease according to any one of claims 2 to 8, wherein the 168th Asp is replaced with Glu, the 195th Asn is replaced with Ser; and/or the 271st Lys is replaced with Arg. A gene coding for a mutant Staphylococcus aureus V8 protease according to any one of claims 1 to 9. An expression vector containing a gene according to claim 10. Recombinant cells transformed with an expression vector according to claim 11. A method for producing a mutant Staphylococcus aureus V8 protease with enzyme activity even under environmental conditions which promote protein denaturation, which method comprises culturing recombinant cells according to claim 12 and then recovering the desired protease protein from the cultured product. The use of a mutant V8 protease according to any one of claims 1 to 9 for protein cleavage.

Inventors: OHSUYE KAZUHIRO, YABUTA MASAYUKI, OHSUYE, KAZUHIRO, YABUTA, MASAYUKI
Applicants: DAIICHI SUNTORY PHARMA CO LTD, DAIICHI SUNTORY PHARMA CO., LTD.
Application year: 1996
Main classification: C12N15
IPC codes: C07K14, C12N9