Methods for improving expression levels of foreign proteins by means of phospholipase fusion expression

Abstract

The invention discloses a method for improving the extracellular expression level of a foreign protein by means of phospholipase fusion expression. Four proteins, PLA.sub.2, MBP, CBD and SUMO, are used as a fusion tag to construct a fusion gene. Compared with an original protein MOH without any fusion tag, the extracellular expression level and enzymatic activity of all the four fusion proteins are increased to some degree. Among them, the fusion protein using PLA.sub.2 as the fusion tag has the highest expression level, which is 7.4 times higher than that of the original protein. Compared with other fusion tags, PLA.sub.2 has a low molecular weight and the fusion protein having PLA2 as the fusion tag has the highest expression level (up to 12 g.Math.L.sup.1 in a 7 L fermentation tank for high-density fermentation). It is shown that the secretory expression of a foreign protein can be effectively increased by using PLA.sub.2 as a fusion tag.

Claims

1. A fusion gene, comprising a gene fragment encoding a fusion tag, a gene fragment encoding a linker peptide, and a gene fragment encoding a foreign protein, wherein the linker peptide has the amino acid sequence of SEQ ID NO:1, wherein the fusion tag is selected from a group of a phospholipase A2 and mutants thereof consisting of (a) a phospholipase A2 having the amino acid sequence of SEQ ID NO: 3; (b) a mutant of the sequence (a), wherein said mutant of the sequence (a) has the first four amino acids deleted from the sequence (a); (c) a mutant of the sequence (a), wherein said mutant of the sequence (a) has the amino acid at position 69, 70, 73, 74, 77 or 90 mutated to alanine in the sequence (a); and (d) a phospholipase A2 mutant having 90% or more homology to the sequence (a).

2. The fusion gene of claim 1, wherein the foreign protein is a prolyl endopeptidase.

3. The fusion gene of claim 2, wherein the prolyl endopeptidase has the amino acid sequence of SEQ ID NO: 4.

4. The fusion gene of claim 1, wherein the fusion gene is inserted into an expression vector.

5. The fusion gene of claim 4, wherein the expression vector with the fusion gene is transformed in a host microorganism to obtain an genetically engineered microorganism expressing the fusion gene.

6. The fusion gene of claim 5, wherein the genetically engineered microorganism is yeast Pichia pastoris comprising the fusion gene of claim 1 inserted into vector pPICZA.

7. A method for expressing a foreign protein, comprising constructing a fusion gene of claim 1 and expressing the fusion gene.

8. The method of claim 7, wherein the foreign protein is a prolyl endopeptidase.

9. The method of claim 8, wherein the fusion tag comprises a phospholipase A2 having the amino acid sequence of SEQ ID NO: 3.

10. The method of claim 8, wherein the fusion tag comprises a mutant of SEQ ID NO:3, wherein said mutant of SEQ ID NO:3 has a point mutation to alanine at position 69, 70, 73, 74, 77 or 90.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1. Schematic diagram of the structure of the fusion gene of the invention.

(2) FIG. 2. Culture indicators of shake flask fermentation of genetically engineered Pichia pastoris GS115 strains, a: OD600; b: protein concentration; c: enzyme activity.

(3) FIG. 3. Western blot results of proteins from genetically engineered Pichia pastoris GS115 strains. a: extracellular protein; b: intracellular protein; M: marker; 1-6 are proteins from MOH, SLMH, CLMH, MLMH, PLMH expressing cells and blank control, respectively.

(4) FIG. 4. SDS-PAGE Image of PLMH extracellular protein expression at different times of the shake flask fermentation. M: marker; 1-7 are shake flask fermentation broths after 0, 24, 48, 72, 84, 96 and 120 hr fermentation, respectively.

(5) FIG. 5. Mass spectrum identification of fusion protein PEP and fusion tag PLA2.

(6) FIG. 6. Comparison of enzymatic properties of PLMH and MOH, a: optimum temperature and temperature stability; b: optimum pH and pH stability.

(7) FIG. 7. Reduction of sensitive protein in beer fermentation broth by proline proteases; M: marker; 1-4: beer fermentation broth samples with addition of 0, 5, 15 and 25 U.Math.L.sup.1 proline protease; 5: proline protease protein.

EXAMPLES

(8) Materials and Methods:

(9) 1. Method for Determining Protein Concentration

(10) 20 l of supernatant of fermentation broth was added into wells of a 96-well plate, and then 200 l of G250 staining reagent was added. Let it stand at room temperature for 3-5 minutes. The absorbance at 595 nm wavelength was measured with a spectrophotometer, and protein concentration in a sample was calculated according to a standard curve.

(11) 2. Method for Determining Protease Activity

(12) A reaction system was consisted of 10 l of enzyme solution, 10 l of 5 mM substrate (Ala-Ala-Pro-pNA) and 80 l of disodium hydrogen phosphate-citrate buffer. The reaction was carried out at 40 C. for 10 min, and the absorbance at 410 nm wavelength was measured with a spectrophotometer.

(13) Definition of enzyme activity unit (U): under the enzyme activity determination conditions specified above, the amount of enzyme required to catalyze and decompose aforementioned substrate to generate 1 mol of pNA per minute equals to a prolyl endopeptidase enzyme activity unit.

(14) Enzyme activity calculation formula: enzyme activity (U.Math.mL.sup.1)= A*V/(v1rbt), where, A: change in absorbance (OD.sub.testOD.sub.blank); V: total volume of reaction system (mL); v1: sample amount (mL); r: molar extinction coefficient (cm.sup.2.Math.mol.sup.1); b: optical path of a cuvette or a Elisa plate (cm); and t: reaction time (min).

(15) 3. Western Blot Analysis

(16) Western blot analysis was conducted on supernatant and intracellular protein of a fusion protein shake flask fermentation, and the western blot procedure was as follows:

(17) (1) Cut a protein gel to an appropriate size after conventional protein electrophoresis, and immerse the protein gels in a transfer buffer for being balanced.

(18) (2) Cut a PVDF membrane and filter paper of the same size as the gel (8 pieces of filter paper for one PVDF membrane), and soak them in pure methanol first and then in the transfer buffer for 10 min.

(19) (3) Neatly stack the PVDF membrane, the filter paper and the gel in a transfer box, wherein four layers of filter paper, the PVDF membrane, the gel and four layers of filter paper were arranged from bottom to top in sequence. Rolling gently with a roller to get rid of excess air bubbles and buffer every time a layer was applied; finally, absorbing surrounding buffer with a paper towel, closing a lid, and placing the transfer box in a transfer instrument.

(20) (4) Transferring: turn on the power; select List, User Defined (this is an edited program 25 V, 1.0 A, 10 min) and Run in sequence.

(21) (5) Take the PVDF membrane out after transferring, immerse the PVDF membrane in a blocking reagent, and gently shake in a decolorization shaker for 1 hr.

(22) (6) Immerse the confined PVDF membrane in a primary antibody solution diluted in PBST (0.137 M NaCl, 0.0027 M KCl, 0.01 M Na.sub.2HPO.sub.4, 0.0018 M KH.sub.2PO.sub.4, 0.1% (v/v)Tween-20) with a dilution rate of 1:1000 (v/v), conduct shaking and incubation at room temperature for 1 hr. Wash the membrane with PBST 5 times, 10 min for each time.

(23) (7) Immerse the PVDF membrane in a secondary antibody solution diluted with PBST at a ratio of 1:400 (v/v), and add an antibody pre-stained with a Marker at a ratio of 1:10000. Shake in the decolorization shaker for 1 hr. Wash the membrane again with PBST 5 times, 10 min for each time.

(24) (8) Place the washed PVDF membrane in a chemiluminescent baseplate. Conduct elution with a chromogenic agent (A/B liquid mixed based on the ratio of 1:1), and take pictures with a gel imager.

(25) Intracellular protein extraction was conducted with a one-step yeast active protein extraction kit purchased from Sangon Biotech (Shanghai) Co., Ltd.

(26) 4. Purification of Fusion Protein

(27) A fermentation broth was centrifuged at 6000g for 30 min after 84 hr of shake flask fermentation, pellets were discarded, a supernatant was collected, ultrafiltration concentration was conducted with a 30-kDa ultrafiltration tube, and the fermentation broth was precipitated with 60% ammonium sulfate (NH.sub.4).sub.2SO.sub.4 for 4 hr after concentration. Protein precipitation was dissolved in a 20 mM phosphate buffer (pH 5.0) and dialyzed overnight. The whole process was operated on ice at 4 C. A dialyzed protease solution was subjected to Ni column purification. Ni-NTA nickel column purification comprised the steps of: centrifuging 500 mL of fermented broth obtained by shake flask fermentation at 4 C. and 6300 rpm for 30 min, taking a supernatant, conducting filtration with a 0.22 m aqueous phase microfiltration membrane, and then conducting concentration to 50 mL with a 30-kDa ultrafiltration tube. Protein purification was conducted with an AKTA purifier protein purification system by means of a conventional method. Before loading a sample, the system and a 5 mL Ni-NTA Superflow Cartridge chromatographic column were pre-equilibrated using a solution A, then the sample was loaded, and collection of flow through peaks needed to be noted. The system and the Ni chromatographic column were washed again with an equilibration solution for being equilibrated. Flushing was conducted with a 5% solution B prior to formally eluting with the solution B and collecting the protein, so as to elute off impurity proteins attached to the column. Linear elution was conducted with a 0-0.05 mol.Math.L.sup.1 eluant, collection of elution peaks needed to be noted, the flow rate was controlled to be 2 mL.Math.min.sup.1, and SDS-PAGE gel electrophoresis was conducted on the elution peaks for purity detection. SDS-PAGE analysis was conducted on the purified protein.

(28) 5. Mass Spectrum Identification of Fusion Protein PLMH

(29) A recombinant protein sample was separated by conventional SDS-PAGE, stained and decolorized, then a target band to be identified was cut off, and enzyme protein verification was conducted through peptide mass fingerprinting. The method comprised the steps of:

(30) (1) cutting off the target band on a gel with a scalpel blade and placing the target band in an EP tube (cutting a gel block into pieces in the size of about 1 mm.sup.3);

(31) (2) adding 200-400 L of 100 mM NH.sub.4HCO.sub.3/30% for decolorization, washing the gel, and removing the supernatant;

(32) (3) adding 90 L of 100 mM NH.sub.4HCO.sub.3 and 10 L of 100 mM DTT into each tube, incubating at 56 C. for 30 min to reduce the protein;

(33) (4) removing the supernatant, adding 100 L of 100% CAN into each tube, and absorbing the 100% CAN after 5 min;

(34) (5) adding 70 L of 100 mM NH.sub.4HCO.sub.3 and 30 L of 200 mM IAA into each tube, and keeping it dark for 20 min;

(35) (6) removing the supernatant, adding 100 L of 100 mM NH.sub.4HCO.sub.3 into each tube, and standing at room temperature for 15 min;

(36) (7) removing the supernatant, adding 100 L of 100% ACN, absorbing the 100% ACN after 5 min, and conducting lyophilization;

(37) (8) after lyophilization, adding 5 L of 2.5-10 ng.Math.L.sup.1 trypsin, and standing at 4 C. for 30-60 min for full imbibition of the gel blocks;

(38) (9) adding about 20-30 L of 25 mM NH.sub.4HCO.sub.3 buffer for reaction overnight at 37 C., lasting about 20 hr;

(39) (10) sucking out an enzymatic hydrolysate and transferring to a new EP tube for lyophilization; and

(40) (11) after completing sample preparation, adding 0.1% TFA for redissolving, conducting sample application, and conducting mass spectrum analysis.

(41) 6. Property Study of Fusion Protein PLMH

(42) (1) Method for determining optimum temperature: according to a method for determining enzyme activity, the reaction system was placed at 25-80 C. for reaction for 5 minutes.

(43) (2) Method for determining temperature stability: a purified fusion protease solution was placed at 25-80 C. for 120 hr, and remaining enzyme activity was measured according to a conventional enzymatic activity determination method.

(44) (3) Method for detecting optimum pH: buffers with different pH values were prepared, including 0.02 mol.Math.L.sup.1 citrate phosphate buffer (pH 2.0-8.0), 0.05 mol.Math.L.sup.1 Tris-HCl buffer (pH 9.0-10.5) and glycine/NaOH buffer (pH 11.0-12.0), and the enzyme activity of the prolyl endopeptidase in these pH ranges was measured with 100% relative enzyme activity as the highest enzyme activity.

(45) (4) Method for detecting pH stability: the enzyme solution was placed in the above buffer for incubation at room temperature for 120 hr, and remaining enzyme activity was detected at pH 5.0 and 40 C.

(46) (5) Using Ala-Ala-Pro-pNA as the substrate, a substrate solution with final substrate concentration of 0.1-1.0 mM was prepared, the activity of the prolyl endopeptidase was measured at 40 C., and corresponding kinetic parameters were calculated by means of double-reciprocal plot. All experiments have three repeats.

Example 1. Preparation of Fusion Genes

(47) (1) Gene source: a phospholipase A2 gene was derived from Streptomyces violaceoruber with the amino acid sequence of SEQ ID NO: 3, and the sequence was amplified from a pPIC9K-PLA2 plasmid previously constructed in our laboratory; a prolyl endopeptidase (PEP) gene MO was derived from Aspergillus oryzae with the amino acid sequence of SEQ ID NO: 4, and the sequence was amplified from a pPIC9K-MO plasmid previously constructed in the laboratory; and the amino acid sequences of a maltose binding protein (MBP), a cellulose binding domain (CBD) and a small ubiquitin-related modifier (SUMO) gene were shown in SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7 respectively. The above genes were optimized according to Pichia pastoris codon preference and synthesized by Sangon Biotech (Shanghai) Co., Ltd.

(48) (2) A fusion tag fragment comprising a phospholipase A2, an MBP, a CBD or a SUMO gene, and a PEP gene fragment were obtained through PCR amplification or chemical synthesis. After column purification, overlap extension PCR was conducted to obtain a fusion gene, wherein the fusion gene sequentially comprised a fusion tag fragment, a linker peptide and a PEP gene fragment, wherein the nucleotide sequence of the linker peptide (GGGGSGGGGS) was shown in SEQ ID NO. 1. The fusion genes comprising a phospholipase A2, an MEP, a CBD and a SUMO gene sequence are named as PLMH, MLMH, CLMH and SLMH, respectively. The nucleotide sequence of the fusion gene PLMH containing the fusion tag phospholipase A2 (nucleotide sequence shown in SEQ ID NO: 8) was shown in SEQ ID NO. 2. Primers used in the invention were shown in Table 1.

(49) TABLE-US-00001 TABLE1 Primersusedintheinvention Primers Sequence(5-3') Serialnumber PLA2-F ATCAGAATTCGCTCCACCTCAGGCTGC SEQIDNO:9 PLA2R ACAATCCTAAAGAACCACCACCACCAGAACCACCACCACCA AGAATTTTC SEQIDNO:10 CBD-F AGAATTCCAGCAGACTGTCTGGGGACA SEQIDNO:11 CBD-R AGAACCACCACCACCAATGCATTGGGCATA SEQIDNO:12 SUMO-F AGAATTCGGTCACCATCATCATCATCA SEQIDNO:13 SUMO-R AGAACCACCACCACCGGGATCAAACTCACC SEQIDNO:14 MBP-F AGAATTCGCTAGTAAGATCGAAGAAGG SEQIDNO:15 MBP-R AGAACCACCACCACCTCCAGCACCATCGCC SEQIDNO:16 PEP-F GGTGGTGGTGGTTCTGGTGGTGGTGGTTCTTTAGGATTGT SEQIDNO:17 PEP-R AGCGGCCGCCATAACTGCACCCTTAG SEQIDNO:18 Among them, the underlined nucleotides were a restriction enzyme site, and the black bold nucleotides refer to the nucleotide sequence for the linker peptide.

Example 2. Construction of Recombinant Expression Vector

(50) Example of the fusion gene PLMH:

(51) The fusion gene fragment PLMH obtained from the Example 1 after column purification was subjected to A tail addition (aKaRa rTaq 0.5 L, 10PCR Buffer 5 L, dNTP Mixture 4 L and purified product 40.5 L reacted for 15-20 min at 72 C.), then ligated to a pMD-19T vector, and then transformed into Escherichia coli. The plasmids of a transformant were extracted and sequenced, and the obtained validated recombinant plasmid was named pMD-19T-PLMH.

(52) The plasmid pMD-19T-PLMH and pPICZA were digested with EcoR I and Not I at the same time and then ligated to obtain a recombinant expression vector pPICZA-PLMH containing the fusion gene PLMH (in the construction process of the recombinant plasmid, a 6His tag was added to the C-terminal of the prolyl endopeptidase to facilitate isolation and purification).

(53) In a similar manner, recombinant expression vectors pPICZA-MLMH, pPICZA-CLMH, pPICZA-SLMH and pPICZA-MOH were obtained. Among them, MOH was obtained by adding a 6His tag to the C-terminal of an original prolyl endopeptidase during the construction of the recombinant plasmid.

Example 3. Construction of Genetically Engineered Bacteria

(54) The recombinant expression vector pPICZA-PLMH was used as an example to illustrate the process of constructing the genetically engineered bacteria.

(55) The pPICZA-PLMH was linearized with Sac I and then transformed into a P. Pichia GS115/pPIC9K yeast competent cell (pPIC9K was introduced into a host to overcome the histidine defect of the host for ease of culture).

(56) Electrotransformation: transferring and mixing a tube of competent cells with the linearized plasmid, keeping it on ice for 3-5 min, and then transferring it into a 2 mm pre-cooled electrotransformation cup for electroporation. The parameters of an electroporation instrument were: voltage 1500 V, resistance 200, and capacitance 25 F. Immediately after electrotransformation, 1 mL of 1 mol.Math.L.sup.1 sorbitol solution was added, evenly mixed and transferred to a 1.5 mL EP tube for resuscitation at 30 C. for 1 hr. The supernatant was removed after centrifugation with 100 L remaining. A YPD+Zeocin plate was coated with the re-suspending bacterial cells, and single-colony transformants were obtained after culturing at 30 C. for 3 days in an incubator. Monoclonal strains were selected for validation. The validated strain was identified as GS115/pPIC9K/pPICZA-PLMH P. Pichia strain that expresses the fusion gene PLMH, whose secreted proline protease was named PLMH.

(57) In a similar manner, recombinant strains GS115/pPIC9K/pPICZA-MLMH, GS115/pPIC9K/pPICZA-CLMH and GS115/pPIC9K/pPICZA-SLMH expressing fusion genes MLMH, CLMH, and SLMH and a recombinant strain GS115/pPIC9K/pPICZA-MOH expressing the non-fusion/control gene MOH were prepared. Proline proteases secreted by the above strains were named MLMH, CLMH, SLMH and MOH proteins, respectively.

Example 4. Shake Flask Fermentation of Recombinant Strains Expressing Four Proline Proteases

(58) The recombinant strains were streaked and cultured on a YPD plate for about 3 days. Monoclonal strains were selected and placed in a 250 ml triangular flask containing 25 mL BMGY medium and cultured for 16-18 hr at 200 rpm, 30 C. The fermentation broth was centrifuged after OD600 reached 2-6, and yeast cells were re-suspended in 50 mL BMMY medium and cultured at 28 C., 250 rpm, and 1% methanol was added every 24 hr to induce exogenous protein expression. Sampling was conducted periodically to determine cell concentration, protein concentration and enzyme activity.

(59) The results of shake flask fermentation of four recombinant strains expressing the fusion proteins and a control MOH strain were shown in FIG. 1. The results showed that the cell concentrations (OD600) of the five strains were almost identical during the 120-hour fermentation period (FIG. 1a). In terms of protein concentration, the extracellular protein concentrations of bacterial cells reached the peak at 96 hr (FIG. 1b), with the concentrations of PLMH, MLMH, CLMH, SLMH and MOH being 0.220.05 mg.Math.ml.sup.1, 0.200.05 mg.Math.ml.sup.1, 0.170.03 mg.Math.ml.sup.1, 0.150.05 mg.Math.ml.sup.1 and 0.120.04 mg.Math.mF.sup.1, respectively, and the highest enzyme activities appeared at 84 hr (FIG. 1c), which were 2807.1 U.Math.L.sup.1, 1876.9 U.Math.L.sup.1 and 1465.5 U.Math.L.sup.1, 1056.5 U.Math.L.sup.1 and 387.2 U.Math.L.sup.1, respectively. During shake flask fermentation, PLMH was higher than those of other three fusion proteins and the MOH protein in terms of total protein concentration and enzyme activity.

Example 5. Influences of Different Linker Peptides on Fusion Gene Expression

(60) A linker peptide GGGGSGGGGSKR (SEQ ID NO: 19) containing a KEX2 protease cleavage site and a linker peptide LEVLFQGPENLYFQS (SEQ ID NO: 20) containing two protease cleavage sites were used to replace the linker peptide GGGGSGGGGS in the previous fusion gene PLMH. Similar methods as described above were used to construct recombinant strains expressing the two fusion genes with different linker peptides and similar fermentation methods were used for culturing the same.

(61) The expression level of the fusion gene with the GGGGSGGGGS as the linker peptide was shown in FIG. 1, and extracellular fermentation liquid enzyme activity reached 280 U.Math.L.sup.1, whereas secretory expression of fusion enzymes using the other two linker peptides could not be achieved in Pichia pastoris, and enzyme activity was undetectable in a fermentation supernatant. These results showed that the selection of linker peptides can play a significant role in the expression of the fusion proteins.

Example 6. Western Blot Detection of Fusion Proteins

(62) Western blot detection analysis was conducted on intracellular and extracellular proteins of the five recombinant strains. The results showed that a target band in the extracellular western blot (FIG. 2a) appeared in the vicinity of 80 kDa, and only one single band existed. At the same time, the results of western blot were consistent with extracellular protein content and enzyme activity, and proteins contents of the fusion proteins were higher than that of the MOH protein. The results of intracellular western blot (FIG. 2b) showed that the PEP protein synthesized by Pichia pastoris cells was completely secreted extracellularly, and there was no intracellular accumulation of the PEP protein.

Example 7. SDS-PAGE Analysis and Mass Spectrum Identification of Fusion Protein PLMH

(63) The GS115/pPIC9K/pPICZA-PLMH recombinant cells were cultured in a shake flask. The fermentation broth was collected at different times during the fermentation and analyzed using a SDS-PAGE. The SDS-PAGE analysis showed that the target bands in the vicinity of 80 kDa became brighter with the increase of fermentation time, and the target bands at 24, 18 and 14 kDa also became brighter. It indicated that with the increase of methanol induction time, the secretion of target proteins also increases. The four bands labeled as a, b, c and d in FIG. 3 were cut out from the SDS-PAGE gel and extracted for mass spectrum identification. The mass spectrum identification results showed that the protein at the position a in FIG. 3 was PEP, proteins at the positions b, c, and d are different forms of PLA2 protein, where the protein at the position d was the original PLA2 protein, and proteins at positions b and c were glycosylated PLA2 proteins (only the mass spectrum results of the protein band a and c were shown in FIG. 4.

Example 8. Purification of Fusion Protein PLMH and MOH Protein

(64) PLMH and MOH proteins were purified using ultrafiltration, ammonium sulfate precipitation, dialysis, and Ni column purification. Yield and specific activity after each purification step were shown in Table 2. After ultrafiltration concentration, ammonium sulfate precipitation, dialysis, Ni column purification and other steps, PLMH and MOH proteins of electrophoretic purity were obtained.

(65) TABLE-US-00002 TABLE 2 Purification of PLMH and MOH Specific activity Purification Yield Purification steps (U .Math. mg.sup.1) factor (%) PLMH Fermentation supernatant 2.9 1 100 Ultrafiltration (30 kDa) 5.8 2 84 (NH.sub.4).sub.2SO.sub.4 precipitation 10.2 3.5 17.5 (60%), dialysis Ni-NTA affinity purification 43.2 14.9 8.9 MOH Fermentation supernatant 3.2 1 100 Ultrafiltration (30 kDa) 5.8 1.8 77 (NH.sub.4).sub.2SO.sub.4 precipitation 10.2 3.2 16 (60%), dialysis Ni-NTA affinity purification 49.3 15.4 4.6

Example 9. Comparison of Enzymatic Properties of Fusion Protein PLMH and MOH Protein

(66) Enzymatic property study was conducted on purified PLMH and MOH enzymes to determine the enzymatic properties such as optimum temperature, temperature stability, optimum pH, pH stability, km and kcat. As shown in FIG. 5, PLMH and MOH proteins had the same enzymatic properties. The optimum temperatures for both enzymes were 40 C. However, only 30% of enzyme activity was left after 120 min of incubation at 40 C., and when temperature reached 45 C., remaining enzyme activity was almost zero after 120 min of incubation. The optimum pH for both enzymes were 5.5, and the best stable phase was obtained at pH 6.0. After incubation at pH 5.0-7.5 for 120 min, remaining enzyme activity was still above 40%, indicating that PEP was relatively stable at a neutral pH.

(67) In addition, Km, kcat and kcat/Km of the fusion protein PLMH were 0.230.01 mM, 112.510.02 S.sup.1 and 489.17 s.sup.1.Math.mM.sup.1, respectively. Km, kcat and kcat/Km of MOH were 0.280.01 mM, 139.40.02 S.sup.1 and 496.4 s.sup.1.Math.mM.sup.1, respectively. There was little difference between the two enzymes.

Example 10. Applications of Prolyl Endopeptidase PLMH

(68) (1) The Effect of Prolyl Endopeptidase/Proline Protease on the Reduction of Sensitive Proteins in Beer

(69) 500 ml fermented beer was filtered, added with a certain amount of PEP, and incubated in a water bath at 40 C. for 1 hr. It was then precipitated overnight with 100% saturated ammonium sulfate at 4 C., and centrifuged at 10,000 rpm for 30 min to collect the precipitate. The precipitate was dissolved by citrate buffer and the total protein of beer was obtained after dialysis at 4 C. Different concentrations of PLMH protein (5 U/L, 15 U/L, 25 U/L) was added to the beer total protein. After glycolysis for 3 hr at 40 C., SDS-PAGE electrophoresis was performed to detect the sensitive proteins.

(70) From FIG. 6, it can be found that the sensitive proteins that cause turbidity of the beer fermentation broth are approximately 40 kDa and 12 kDa in size. Compared with the blank control, the proline protease can reduce the amount of sensitive proteins in the beer. With the increasing amount of proline protease added, the amount of large sensitive proteins in the beer was gradually decreased.

(71) (2) Effects of Proline Proteases on Non-Biological Stability During Storage of Fermented Beer

(72) Prolyl endopeptidase PLMH of different concentrations (5 U.Math.L.sup.1, 15 U.Math.L.sup.1, and 25 U.Math.L.sup.1) was added to filtered and sterilized beer fermentation broth and refrigerated at 4 C. Samples were collected every other week for turbidity determination. Three samples in each group were collected for 6 weeks.

(73) The effect of the proline protease on the non-biological stability of the beer fermentation broth was determined. The accumulation of protein in beer during storage resulted in the increase of turbidity (EBC). The addition of the proline protease could reduce the turbidity during storage of beer, and this effect was increased with the increase of added proline protease. EBC was reduced to 0.3 after 15 U.Math.L.sup.1 proline protease PLMH was added, indicating that the proline protease of the invention can be effectively applied to reduce the turbidity of beer.

Example 11. Production of Prolyl Endopeptidase Using the Genetically Engineered Strain

(74) A 7 L fermentation tank high-density fermentation was performed to produce prolyl endopeptidase PLMH using the genetically engineered strain GS115/pPIC9K/pPICZA-PLMH containing the fusion gene PLMH. The enzyme activity of PLMH was measured after fermentation, and the highest enzyme activity reached 1800 U.Math.L.sup.1.

(75) The fermentation method was as follows:

(76) The recombinant strain was cultured in the YPD medium for 16-18 hr, and the recombinant strain was inoculated into a 7 L fermentation tank for glycerol growth phase culture at 30 C. and pH 5.5, wherein the fermentation tank contained 2.1 L of BSM basal salt medium. When glycerol in the BSM basal salt medium was consumed and ran out and the dissolved oxygen value increased sharply, the glycerol transition phase started. 50% (V/V) glycerol (containing 12 mL.Math.L.sup.1 of PTM1) was fed to the cells until the cell OD600 value reached 90-110, and glycerol feeding was then stopped. The induction phase started after starving the cells for 0.5 hr during which methanol was fed and maintained at the concentration of 0.08-0.12%, and temperature was controlled at 26-28 C.

(77) The materials used in this example were as follows: YPD medium: protein powder 20 g.Math.L.sup.1, yeast powder 10 g.Math.L.sup.1, glucose 20 g.Math.L.sup.1; BSM basal salt medium: CaSO.sub.4(cp) 1.1 g.Math.L.sup.1, K.sub.2SO.sub.4 (AR) 18.2 g.Math.L.sup.1, anhydrous magnesium sulfate AR 7.27 g.Math.L.sup.1, KOH(AR) 4.128 g.Math.L.sup.1, glycerol 40 g.Math.L.sup.1, 85% H3PO4 26.7 ml.Math.L.sup.1; PTM1: copper sulfate pentahydrate 6 g.Math.L.sup.1, potassium iodide 0.089 g.Math.L.sup.1, sulfate monohydrate 3.0 g.Math.L.sup.1, sodium molybdate 0.2 g.Math.L.sup.1, boric acid 0.02 g.Math.L.sup.1, zinc sulfate heptahydrate 42.2 g.Math.L.sup.1, ferrous sulfate septihydrate 65 g.Math.L.sup.1, biotin 0.2 g.Math.L.sup.1, cobalt chloride hexahydrate 0.5 g.Math.L.sup.1, sulfuric acid 5 ml.Math.L.sup.1.

Example 12. Effects of Different PLA2 Mutants on the Production of the Fusion Proteins Having PEP and PLA2 Mutants

(78) Since phospholipase A2 has phospholipase activity, the phospholipase activity of the enzyme can have a negative effect on the fermentation and the expression of the fusion gene containing phospholipase A2. While the fusion of phospholipase A2 to a foreign protein (e.g. PEP) facilitates the secretion of the fusion protein, the phospholipase activity of the enzyme is neither needed nor desirable for the expression and activity of the foreign/target protein. Therefore, two groups of phospholipase A2 mutations were designed to study the effects of mutations on the phospholipase A2 activity and the expression level of the fusion protein.

(79) The first group of mutation was a truncation mutation, that is, four amino acids (from position 1 to position 4) were deleted from the amino acid sequence of phospholipase A2. The results showed that phospholipase activity could still be detected from the recombinant strain expressing the fusion gene with the PLA2 truncation mutation. The expression of the fusion protein was not affected, and the proline protease activity of the fusion protein with the truncated PLA2 mutant did not change compared to that of the fusion protein with the PLA2.

(80) The second group of mutation was point mutations in which an amino acid at position 69, 70, 73, 74, 77 or 90 of the sequence of phospholipase A2 (SEQ ID NO: 3) was mutated into an alanine. The results showed that phospholipase activity could not be detected in a fermentation supernatant from the recombinant strain expressing the fusion gene containing one of the six PLA2 point mutations. But the activity of proline protease was not affected, and the proline protease activity of the fusion protein with a mutated phospholipase did not change compared to that of the fusion protein with the wild-type PLA2.

(81) Based on the above findings, it was suggested that the fusion protein with a proline protease fused to one of the six point mutant PLA2 can achieve the best results.