# Patent

## Title
Protein production method, fusion protein, and antiserum

## Abstract

Disclosed are a highly efficient method for production of heterologous proteins performed by utilizing microorganisms, as well as fusion proteins, and an antiserum. The method includes a method for production of a protein (A) in the form of a fusion protein, comprising the steps of (a) preparing a DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and the C-terminal peptide or its fragment (B) of the Cry proteins produced by Bacillus thuringiensis, and (b) introducing the DNA into a host bacterium to transform the same, and (c) allowing the fusion protein to be expressed in the transformed host bacterium, as well as a method for production of the protein (A) itself comprising a further step of removing the peptide chain (B) from the fusion protein obtained.

## Background
TECHNICAL FIELD
The present invention relates to a method for production of proteins, in particular to a method for production of heterologous proteins using bacteria as a host, and more specifically a highly efficient method for production of heterologous proteins performed by utilizing the characteristic properties of Cry proteins, the proteins formed by Bacillus thuringiensis. The present invention further relates to fusion proteins produced by the method, antiserum and antibodies to such fusion proteins, and reagents containing them, as well as a method for analysis.
BACKGROUND ART
As for production of proteins/enzymes, i.e., major components of various biological products, simple, easy and low-cost methods for their production have been sought, such as introduction of their genes, if they are isolated, into microorganisms, like E. coli, and letting the proteins/enzymes be expressed and accumulate in them. However, there are also many cases in which attempts fail to let a heterologous protein express/accumulate in the cells of microorganisms. The cause of such failures becomes particularly notable when higher production efficiency is sought. That is, the heterologous protein biosynthesized in the cells of a microorganism, which accumulates forming insoluble inclusion bodies, gets inactivated in the process. Generally, it is very difficult to solubilize such insoluble inclusion bodies of a protein and restoring the biological activity of the protein.
Further, in the case where the heterologous protein to be produced has a cytotoxicity, accumulation of that toxic protein adversely affects the proliferation/survival of the very host cells, resulting in lowered production yield, and further, in death of the host cells.
Due to these drawbacks, production of proteins/enzymes, which are major components of various biological products, often has to rely on time-consuming and costly methods, such as utilizing the living organisms that intrinsically produce the protein/enzyme. There seems to be not a small number of cases in which these drawbacks form a factor that hindering development of an efficient method of their industrial production, and this is one of the problems to be solved.
On the other hand, insecticidal proteins produced by an aerobic soil bacterium, Bacillus thuringiensis, has long been known (see as a review e.g., Non-patent Document 1). During its sporulation, Bacillus thuringiensis also produces, separately from spores, generally a single large parasporal inclusion body consisting mainly of a crystal protein (this is called “Cry protein”). Cry proteins consist of about 1000-1200 amino acids, and many of them are known to be produced by various Bacillus thuringiensis strains. About half of them have been found to have an insecticidal activity specific to certain insects, and some of those strains which produce a Cry protein having such an activity have been widely used as BT insecticides. When ingested by larvae of respective target insects, each of those Cry proteins which have an insecticidal activity undergoes cleavage by the digestive fluid at a predetermined position of its amino acid sequence and thus a peptide on its C-terminus side (consisting about 400-500 amino acids: abbreviated to “Cter”) is removed, leaving behind a peptide on the N-terminus side (NB: a short peptide at the N-terminus is also removed), which exhibits a potent insecticidal activity. For example, in the case of a protein named Cry4Aa, an N-terminus region consisting of Met 1 to Gln695 (in particular, a peptide left behind after Met1 to Try57 are removed) of its entire amino acid sequence acts as the very insecticidal component on its target insects (Culex pipiens pallens and the like). Its Cter, which consists of an amino acid sequence starting with Ile696 and is deleted by cleavage, is unnecessary part for the insecticidal activity (see Non-Patent Document 2). And, as for a protein known as Cry1Aa, which is an insecticidal specific to lepidopteran insects (butterflies and moths), the region consisting of Met1 to Lys621 on the N terminal side of its amino acid sequence (in particular, the part of the region left behind after further removal of Met1 to Arg28) works as the very insecticidal component, and its Cter, which consists of an amino acid sequence starting with Ala622, is deleted by cleavage (Non-Patent Document 3). Furthermore, as to Cry1Ac, which is also an insecticidal toxin specific to Lepidopteran insects (butterflies and moths) like Cry1Aa, its very insecticidal component consists of Met1 to Lys623 (in particular, the part of the region left behind after removal of Met1 to Arg28) of its amino acid sequence, and its Cter, which is located on the C-terminal side and consists of amino acid sequence starting with Ala624, does not take part in the generation of insecticidal activity, but is removed by cleavage by the action of the digestive fluid in the larvae (Non-Patent Document 4). Further, in recent years, Cry proteins have been found with which no insecticidal activity is as yet known, and some of them are found to specifically destroy human cancer cells, some specifically kill Tricomonas vaginalis, the pathogen protozoa causing human tricomonasis, and some exhibit a potent worm-killing activity to Nematoda. Though they differ in their biological activity and in their targets at which they exhibit toxic effects, the Cry proteins have a common characteristic property that they occur within Bacillus thuringiensis cells in the form of a large parasporal inclusion body.

    [Non-Patent Document 1] M. Ohba, H. Hori and H. Sakai, “Bacillus thuringiensis: Science of Insecticidal Proteins”, Industrial Publishing & Consulting, Inc., Feb. 28, 2005.
    [Non-Patent Document 2] M. Yamagiwa et al., Appl. Environ. Microbiol. 65: 3464-3469 (1999)
    [Non-Patent Document 3] P. Grochulski et al., J. Mol. Biol. 254: 447-464 (1995)
    [Non-Patent Document 4] J. N. Aronson and H. C. Arvidson, Appl. Environ. Microbiol. 53: 416-421 (1999)


DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
Against the above background, the objective of the present invention is to provide a method for production of heterologous proteins/enzymes utilizing microorganisms, with improved efficiency compared with conventional methods.
Means to Solve the Problem
The present inventors found that when brought to express in host bacterium cells, such as E. coli cells, a fusion protein consisting of the Cter of one of the Cry proteins and some other protein which was fused with it, an insoluble inclusion bodies containing the fusion protein (hereinafter referred to as a “crystal”) formed in the cells, and thus this heterologous protein accumulated in a great amount in the cells, and that those crystals had following excellent properties distinguished from those of other insoluble inclusion bodies so far known:
a. that the heterologous protein/enzyme has been contained in the crystals, with its biological activities kept intact,
b. that crystals are efficiently solubilized in an alkaline buffer solution with a pH of 10-11,
c. that after solubilized, the heterologous protein/enzyme can be easily recovered in the supernatant, with its biological activities kept intact,
d. that even a cytotoxic heterologous protein can be efficiently produced through crystal formation, and
e. that crystals can be stored as they are for a long time, with their stability being kept uncompromised.
The present inventors further found that the full-length Cter is not indispensable for such crystals to be formed, but a part consisting of some 140-160 amino acids on its N-terminal side suffices. In particular, examination was performed, based on 4AaCter(696-851) of Cry4Aa2, which is a Cter's N-terminal 1⁄3-long fragment and with which it was first discovered that a fusion protein consisting of it and some other protein forms crystals, and by selecting fragments of various other Cter's amino acid sequences according to their similarity to the former fragment based on the alignment technique. As a result, formation of similar crystals was found with them. It was also found that an antiserum which is created by immunizing an animal with a fusion protein obtained according to the present invention is reactive with the original protein, i.e., the protein prior to fused with the Cter employed. The present invention has been completed through further studies on the basis of these findings.
Thus, the present invention provides what follows:
1. A method for production of a protein (A) in the form of a fusion protein, comprising the steps of
(a) preparing a DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and other peptide chain (B), on the N- or C-terminal side of the latter the former being combined, wherein the peptide chain (B) is a C-terminal peptide chain included in the amino acid sequence of one of the Cry proteins produced by Bacillus thuringiensis listed in Table 1 or 2 and including in itself a corresponding partial sequence identified in the tables,








TABLE 1






C-terminal peptide, and


Cry proteins
partial sequence (SEQ ID NO:)







Cry1Aa1(SEQ ID NO: 50), Cry1Aa2(SEQ ID NO: 51),
C-terminal peptide: the part starting


Cry1Aa3(SEQ ID NO: 52), Cry1Aa4(SEQ ID NO: 53),
from Ala622


Cry1Aa5(SEQ ID NO: 54), Cry1Aa8(SEQ ID NO: 55),
Partial sequence: Ala622-Pro777


Cry1Aa9(SEQ ID NO: 56), Cry1Aa10(SEQ ID NO: 57),
(SEQ ID NO: 1)


Cry1Aa11(SEQ ID NO: 58), Cry1Aa12(SEQ ID NO: 59),



Cry1Aa13(SEQ ID NO: 60), Cry1Aa14(SEQ ID NO: 61)



Cry1Ab3(SEQ ID NO: 62), Cry1Ab4(SEQ ID NO: 63),
C-terminal peptide: the part starting


Cry1Ab8(SEQ ID NO: 64), Cry1Ab9(SEQ ID NO: 65),
from Ala623


Cry1Ab10(SEQ ID NO: 66), Cry1Ab12(SEQ ID NO: 67),
Partial sequence: Ala623-Pro778


Cry1Ab13(SEQ ID NO: 68), Cry1Ab15(SEQ ID NO: 69),
(SEQ ID NO: 2)


Cry1Ab16(SEQ ID NO: 70), Cry1Ab17(SEQ ID NO: 71),



Cry1Ab21(SEQ ID NO: 72)



Cry1Ab2(SEQ ID NO: 73)
C-terminal peptide: the part starting



from Ala624



Partial sequence: Ala624-Pro779



(SEQ ID NO: 3)


Cry1Ac1(SEQ ID NO: 74), Cry1Ac4(SEQ ID NO: 75),
C-terminal peptide: the part starting


Cry1Ac7(SEQ ID NO: 76), Cry1Ac8(SEQ ID NO: 77),
from Ala624


Cry1Ac9(SEQ ID NO: 78), Cry1Ac10(SEQ ID NO: 79),
Partial sequence: Ala624-Pro779


Cry1Ac11(SEQ ID NO: 80), Cry1Ac16(SEQ ID NO: 81),
(SEQ ID NO: 4)


Cry1Ac19(SEQ ID NO: 82)



Cry1Ac5(SEQ ID NO: 83), Cry1Ac12(SEQ ID NO: 84),
C-terminal peptide: the part starting


Cry1Ac14(SEQ ID NO: 85), Cry1Ac15(SEQ ID NO: 86),
from Ala623


Cry1Ac20(SEQ ID NO: 87)
Partial sequence: Ala623-Pro778



(SEQ ID NO: 5)


















TABLE 2







C-terminal peptide, and



Cry proteins
Partial sequence(SEQ ID NO:)








Cry4Aa1(SEQ ID NO: 88),
C-terminal peptide: the part



Cry4Aa2(SEQ ID NO: 89),
starting from Ile696



Cry4Aa3(SEQ ID NO: 90)
Partial sequence: Ile696-Pro851




(SEQ ID NO: 6) or




Ile801-Ser829 (SEQ ID NO: 7)



Cry4Ba1(SEQ ID NO: 91),
C-terminal peptide: the part



Cry4Ba2(SEQ ID NO: 92),
starting from Val652



Cry4Ba5(SEQ ID NO: 93)
Partial sequence: Val652-Pro807




(SEQ ID NO: 8) or




Ile757-Ser785 (SEQ ID NO: 9)



Cry4Ba4(SEQ ID NO: 94)
C-terminal peptide: the part




starting from Val651




Partial sequence: Val651-Pro806




(SEQ ID NO: 10) or




Ile756-Ser784 (SEQ ID NO: 11)



Cry4Ba3(SEQ ID NO: 95)
C-terminal peptide: the part




starting from Val652




Partial sequence: Val652-Pro807




(SEQ ID NO: 12) or




Ile756-Ser784 (SEQ ID NO: 13)



Cry8Ca1(SEQ ID NO: 96)
C-terminal peptide: the part




starting from Lys672




Partial sequence: Lys672-Pro829




(SEQ ID NO: 14)









(b) introducing the DNA into a host bacterium to transform the same, and
(c) allowing the fusion protein to be expressed in the host bacterium which has been transformed.
2. The method for production according to 1 above, wherein in the step of preparing the DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and the other peptide chain (B), on the N- or C-terminal side of the latter the former being combined, a DNA coding for an amino acid sequence which provides a specific cleavage site for a proteolytic enzyme is interposed between the DNA which codes for the peptide chain forming protein (A) and the DNA which codes for the peptide chain (B).
3. The method for production according to 1 or 2 above comprising a further step of fracturing the host bacterium containing the fusion protein thus expressed to collect the fusion protein.
4. The method for production according to 3 above comprising a further step of purifying the collected fusion protein through solubilization of the same in an alkaline aqueous solution.
5. The method for production of the protein (A) comprising a step of removing the peptide chain (B) from the fusion protein obtained according to 4 above.
6. The method for production according to 5 above, wherein the removal of the peptide chain (B) is done by treating a specific cleavage site for a proteolytic enzyme, which the fusion protein has between the peptide chain forming the protein (A) and peptide chain (B), with the proteolytic enzyme.
7. A fusion protein produced by the method according to 3 or 4 above.
8. An antiserum reactive to the protein (A) which antiserum is obtained by immunizing a mammalian animal with the fusion protein according to 7 above, and then collecting the serum of the animal.
9. The antiserum according to 7 or 8 above, wherein the protein (A) is C-reactive protein.
10. An antibody to the protein (A) isolated from the antiserum according to 8 or 9 above.
11. A testing reagent comprising the antiserum according to 8 or 9 above.
12. A testing reagent comprising the antibody according to 10 above.
13. A method for analysis of a sample for protein (A), comprising the steps of bringing the sample into contact with the antiserum according to 8 above, or the antibody according to 9 above, to form an antigen-antibody complex, and detecting the antigen-antibody complex.
14. The method according to 13 above, wherein the protein (A) is C-reactive protein.
Effect of the Invention
According to the present invention, utilizing a bacterium, such as E. coli, as a host, a heterologous protein can be produced in the form of a fusion protein in a great amount within the cells of the bacterium, in the form of insoluble crystals, maintaining (but in a potential manner) activities of the original protein. As these insoluble crystals, in the situation where they are, do not exhibit the activities of the original heterologous protein in the host bacterium, even proteins otherwise harmful or lethal to the host can be produced in a large scale. Furthermore, the crystals can be easily isolated, and then solubilized in an alkaline condition, collected and purified. The solubilized fusion protein has the activities of the original, heterologous protein, and further it is also possible to readily recover the original heterologous protein through removal, by any of proper methods, of the portion which has been derived from the Cter employed. Therefore, the present invention enables production of heterologous proteins utilizing a bacterium, such as E. coli, as a host, with dramatically improved efficiency.
Furthermore, the present invention also enables to obtain, in a large amount, even such proteins as have been obtainable so far only in a trace amount, in the form in which the proteins are fused to a C-terminal peptide chain of a Cry protein. Thus, it makes it easier to obtain antisera to original proteins, through immunization of mammalian animals (e.g., rabbit, goat) with their fusion proteins and collection of the sera. Such antisera can be used, e.g., as testing reagents, for detection or measurement of the original proteins in given samples (e.g., a biological sample such as body tissues, blood, plasma, or serum). Further, antibodies (polyclonal antibodies) to the original proteins can also be isolated from such antisera, and can be used for the same purpose.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates the primer sets used for preparation of synthetic DNAs which then were used for construction of the gene coding for 4AaCter (696-851) by repetitive PCR. The following primers were used:
C1-1-f (SEQ ID NO: 15), C1-1& 2-r (SEQ ID NO: 16), C1-2&3-f (SEQ ID NO: 17), C1-3&4-r (SEQ ID NO: 18), C1-4&5-f (SEQ ID NO: 19), C1-5&6-r (SEQ ID NO: 20), C1-6&7-f (SEQ ID NO: 21), C1-7&8-r (SEQ ID NO: 22), C1-8&9-f (SEQ ID NO: 23) and C1-9-r (SEQ ID NO: 24).
FIG. 2 is a schematic illustration of the process for construction of the DNA coding for the entire 4AaCter(696-851) by repetitive PCR.
FIG. 3 illustrates the order of combination of GST and 4AaCter (696-851) in their fusion protein.
FIG. 4 is a schematic diagram illustrating the flow of the steps from the construction of the DNA coding for 4AaCter (696-851) up to the construction of an expression vector for the fusion protein.
FIG. 5 illustrates the primer pair which was employed to amplify the DNA fragment coding for 4AaCter(696-851). The following primers were used: X-Syn4A-C1-f (SEQ ID NO: 27) and X—S-Syn4A-C1-r (SEQ ID NO: 28).
FIG. 6 illustrates a map of expression vector pGEX-6P-1. Shown are the nucleic acid and amino acid sequences in the pGEX-6P-1 cloning site (SEQ ID NO: 29 and SEQ ID NO: 30, respectively).
FIG. 7 is a photograph showing crystal formation in E. coli wells which were transformed with pGST-4AaCter and induced for expression of it.
FIG. 8 is a photograph showing localization of crystals in the insoluble fraction in E. coli cells which were transformed with pGST-4AaCter and induced for expression of it. In the figure, M: size marker, 1: whole cell following induction of expression, 2: supernatant following fracturing the cells, 3: precipitate following fracturing the cells, 4: supernatant following solubilization.
FIG. 9 is a diagram showing the results of examination for crystal forming property of fusion proteins which were produced using various fragments of Cter from Cry4Aa. Shown is a “Block 7” amino acid sequence which corresponds to amino acids 801 to 855 of SEQ ID NO: 88.
FIG. 10 is a diagram showing the two different orders of combination of MM29 kD and 4AaCter (696-851) in their fusion proteins.
FIG. 11 is a schematic diagram illustrating the flow of the steps of construction of the expression vector for production of 4AaCter-MM29 kD.
FIG. 12 illustrates a map of pGEX-4T-3. Shown are amino nucleic acid and amino acid sequences in the pGEX-4T-3 multi cloning site (SEQ ID NO: 38 and 39, respectively).
FIG. 13 illustrates the primer pair which was employed to remove the entire open reading frame (ORF) of GST except ATT. The following primers were used: p-delta-GST-4T-3 (CAGGAAACAG TATTCATGGG ATCCCCGAAT TCCCGG; SEQ ID NO: 40); Forward primer (GGATCCCCGA ATTCCCGG; SEQ ID NO: 41); and Reverse primer (CATGAATACT GTTTCCTG; SEQ ID NO: 42).
FIG. 14 is a diagram showing the nucleotide sequence near the site where “ATG” is combined with the 5′-end of the DNA coding for MM29 kD. The nucleic acid sequence around the 5′ end of MM29 kD is shown in SEQ ID NO: 43.
FIG. 15 is a schematic diagram illustrating the flow of construction of the expression vector for production of MM29 kD-4AaCter.
FIG. 16 presents photographs showing crystal formation in the E. coli cells which were transformed with pAGST-4AaCter-MM29 kD or pAGST-MM29 kD-4AaCter, respectively, and induced for their expression.
FIG. 17 present photographs showing the results of SDA-PAGE performed with 4AaCter-MM29 kD and MM29 kD-4AaCter.
FIG. 18 is a schematic diagram illustrating the flow of construction of expression vector pGST-1AaCter.
FIG. 19 presents photographs showing crystal formation in E. coli cells which were transformed with pGST-1AaCter and induced for expression of it.
FIG. 20 presents photographs showing the result of SDS-PAGE demonstrating the insoluble-fraction localization of crystals obtained from the E. coli cells which were transformed with pGST-1AaCter and induced for its expression. In the figure, M: size marker, 1: IPTG(−) total proteins, 2: IPTG(+), 3: IPTG(+) centrifugation supernatant (soluble protein fraction), 4: IPTG(+) centrifugation precipitate (insoluble protein fraction).
FIG. 21 presents photographs showing the result of SDA-PAGE carried out following alkaline solubilization of the crystals obtained from E. coli cells were transformed with pGST-1AaCter and induced for expression. In the figure, M: size marker, 1: centrifugation supernatant (soluble protein fraction), 2: centrifugation precipitate (insoluble protein fraction).
FIG. 22 presents photographs showing crystal formation in E. coli cells which were transformed with pGST-1AcCter and induced for expression.
FIG. 23 present photographs showing the result of SDS-PAGE demonstrating the insoluble-fraction localization of crystals obtained from E. coli cells which were transformed with pGST-1AcCter and induced for expression. In the figure, M: size marker, 1: IPTG(−) total proteins, 2: IPTG(+) total proteins, 3: IPTG(+) entrifugation precipitate (insoluble protein fraction), 4: IPTG(+) centrifugation supernatant (soluble protein fraction).
FIG. 24 presents photographs showing the results of SDA-PAGE following alkaline solubilization of crystals obtained from E. coli cells which were transformed with pGST-1AcCter and induced for expression, indicating the result of solubility test of the crystals formed with 1AcCter. In the figure, M: size marker, 1: centrifugation precipitate (insoluble protein fraction), 2: centrifugation supernatant (soluble protein fraction).
FIG. 25 is a photograph showing the result of SDS-PAGE for confirmation of expression of CRP and 4AaCter-CRP.
FIG. 26 is a photograph showing the result of Western blotting of 4AaCter-CRP and human-derived CRP.
FIG. 27 is a photograph showing the result of Western blotting of 4AaCter and 4AaCter-CRP.



BEST MODE FOR CARRYING OUT THE INVENTION
In the present specification, amino acid numbers are determined by assigning “1” to the methionine (Met) residue corresponds to the start codon in the original Cry protein.
The ability of a fusion protein to form crystals is thought to be the property that is common to the Cter region of the Cry proteins, which are characterized by formation of a large parasporal inclusion body, for as shown in the Examination section, crystal formation was alike confirmed in the experiments employing combinations of different Cter's and different heterologous proteins, and since the structure of Cter's are highly conserved among insecticidal toxins.
A large number of Cry proteins are known (Bacillus thuringiensis Toxin Nomenclature: http://www.lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt/). Of the full-length amino acid sequence of a Cry protein, the N-terminal side portion which determines such biological activities as an insecticidal activity, is not needed for the purpose of the present invention, and therefore the remaining portion, the C-terminal peptide (Cter), may be utilized for formation of fusion proteins with the protein of interest. Further, in utilizing a Cter, full length of it is not necessary, but its N-terminal part consisting of about 140-160 amino acids suffices, and its fragments may also be used.
As to Cry4Aa1, Cry4Aa2 and Cry4Aa3 (Cry4Aa group), for example, which have in common the identical partial sequence Ile696-Pro851 (SEQ ID NO:6) inside the Cter, the portion starting from Ile696, it has been confirmed that a fusion protein consisting of the peptide having this partial sequence and a heterologous protein forms crystals (cf. Examples). Further, among various other partial sequences included inside this partial sequence, the sequence Ile801-Ser829 (SEQ ID NO:7) and some other partial sequences including this within themselves also provided peptides which gave, in combination with a heterologous protein, such a fusion proteins that formed crystals, whereas partial sequences including no Ile801-Ser829 within themselves failed to form crystals (Examples). Therefore, Ile801-Ser829 (SEQ ID NO:7) is considered to be the very sequence that confers fusion proteins the ability to form crystals. Thus, Cter's partial sequences which include this sequence in themselves can be used to form fusion proteins with heterologous proteins so as to confer the latter the ability to form crystals.
Furthermore, Cry4Ba1, Cry4Ba2 and Cry4Ba5 share a completely identical partial amino acid sequence Val652-Pro807 (SEQ ID NO:8), which corresponds to the above-mentioned Cter fragment Ile696-Pro851 of Cry4Aa1, and, further, all of them include a sequence (Ile757-Ser785, SEQ ID NO:9) within themselves which is completely identical to Ile801-Ser829 (SEQ ID NO:7) of Cry4Aa1. Therefore, as to Cry4Ba1, Cry4Ba2 and Cry4Ba5, either their Cter themselves (the portion starting from Val652) or their Cter's partial sequences which include Ile757-Ser785 within themselves can be used as desired for the purpose of the present invention.
As to Cry4Ba4, a partial sequence Val651-Pro806 (SEQ ID NO:10) within its Cter (the portion stating from Val651), which corresponds to the above-mentioned Cter fragment Ile696-Pro851 of Cry4Aa1, is completely identical to the partial amino acid sequence Val652-Pro807 of Cry4Ba1, Cry4Ba2 and Cry4Ba5, and they differ only in their starting region in their respective Cry proteins. Therefore, again, Cry4Ba4 includes within itself a sequence (Ile756-Ser784, SEQ ID NO:11) that is the same as Ile801-Ser829 (SEQ ID NO:7) of Cry4Aa1. Thus, as to Cry4Ba4 also, either its Cter itself (the portion starting from Val651) or any of its fragments which includes Ile756-Ser784 within itself can be used as desired for the purpose of the present invention.
As to Cry4Ba3, a partial sequence Val652-Pro807 (SEQ ID NO:12) within its Cter (the portion starting from Val652), which corresponds to the above-mentioned Cter fragment le696-Pro851 of Cry4Aa1, also includes within itself a sequence (Ile757-Ser785, SEQ ID NO: 13) which is completely identical to Ile801-Ser829 (SEQ ID NO:7) of Cry4Aa1. Therefore, as to Cry4Ba3, too, either its Cter (the portion starting from Val652) itself or any of its fragments which includes Ile757-Ser785 within itself can be used as desired for the purpose of the present invention.
As for the Cry1Aa group including e.g., Cry1Aa1 as shown in the above table, their Cter's are the portions all starting from Ala622, among which their partial sequences Ala622-Pro777 are completely identical with one another in their amino acids. A fusion protein of the peptide consisting of this identical portion and a heterologous protein was confirmed to form crystals (see Example in which a peptide originating from Cry1Aa3 is utilized). Therefore, as regards the Cry1Aa group, either their Cter's (the portion starting from Ala622) themselves or any of their fragments which includes Ala622-Pro777 within itself can be used as desired in the present invention.
Regarding the CryAb group including e.g., Cry1Ab3 (but except Cry1Ab2) as shown in the above table, their Cter's are the portions all starting from Ala623, among which their partial sequence Ala623-Pro778, which correspond to the above-mentioned Cter fragment Ile696-Pro851 of Cry4Aa1, are completely identical with one another in their amino acids. And this partial sequence, when compared with the partial sequence Ala622-Pro777 of the Cter's of the above Cry1Aa group, differs only in that the Gln664 in the Cry1Aa group is replaced with 665Lys in the CryAb group, and has no other difference except that the amino acid position numbers in the former partial sequence as a whole are greater by 1 than the latter. As the proteins belonging to the CryAb group are those which form a parasporal inclusion bodies and the partial sequence Ala623-Pro778 or their Cter's differ only in one single amino acid compared with the partial sequence Ala622-Pro777 of the Cter of the Cry1Aa group (with which formation of crystals is confirmed in Example), either the Cter's of the CryAb group (the portion starting from Ala623) themselves or any of their fragments which include Ala623-Pro778 within themselves can be used as desired in the present invention.
The Cter of Cry1Ab2 is the portion which starts from Ala624, and an amino acid sequence Ala624-Pro779 included within it differs only in that Ser703 and Asp712 of the Cry1Ab group are replaced in it with Asn704 and His713, respectively, and has no other difference except that its amino acid position numbers as a whole are shifted by 1. For this reason, and since Cry1Ab2 is a protein which forms parasporal inclusion bodies, either the Cter of this (the portion starting from Ala624) itself or any of its fragments which include Ala624-Pro779 within themselves can be used as desired in the present invention.
In the above tables, the group consisting of Cry1Ac1, Cry1Ac4, Cry1Ac7, Cry1Ac8, Cry1Ac9, Cry1Ac10, Cry1Ac11, Cry1Ac16 and Cry1Ac19 (Cry1Ac(I) group) have their Cter's which start from Ala624, and the group consisting of Cry1Ac5, Cry1Ac12, Cry1Ac14, Cry1Ac15, and Cry1Ac20 (Cry1Ac(II) group) have their Cter's which start from Ala623. And the partial sequence Ala624-Pro779 of the former is completely identical in their amino acids to the partial sequence Ala623-Pro778 of the latter. Judging from the fact that a fusion protein of the peptide consisting of the partial sequence Ala624-Pro779 of Cry1Ac1 and a heterologous protein was confirmed to form crystals (see Examples), also with the Cry1Ac(I) and Cry1Ac(II) groups, either their Cter's themselves or any of the fragments of the Cter which include the partial sequence Ala624-Pro779 for the Cry1Ac(I) group or the partial sequence Ala623-Pro778 for the Cry1Ac(II) group, can be used in the present invention.
In the present invention, the DNA coding for the fusion protein consisting a protein of interest and a Cter (or its fragment) is prepared by combining a DNA coding for the Cter or its fragment mentioned above is combined in-frame with a DNA coding for the protein of interest on its 3′ or 5′ end.
In the present invention, by employing as a template the total DNA of a Bacillus thuringiensis strain which has the gene of a particular Cry protein that is to be utilized, a DNA coding for the Cter (or its fragment) of the protein can be prepared by PCR. In this preparation process, the primers employed may be provided with restriction sites by a conventional method for incorporation into an expression vector. In the case where the template total DNA of the Bacillus thuringiensis strain needed is unavailable, the DNA region of interest may be synthesized by first preparing primer DNAs (50-60 basis at longest), several to about 10 in number, that cover the entire region of the Cter (or its fragment) by a conventional method based on publicized/registered (e.g., at DDBJ) information of that gene's nucleotide sequence/amino acid sequences, and then carrying out a recursive PCR using the primer DNAs.
The technique of recursive PCR is well known to those skilled in the art. Synthesis of the DNA by this method for the region of Cter (or its fragment) may be carried out in the following manner, by way of example, 4AaCter(696-851) [Ile696-Pro851 in the Cry4Aa group]: the nucleotide sequence coding for 4AaCter(696-851) is divided into 10 portions and each portion is chemically synthesized as primers (FIG. 1), in such a manner as leaving complementary bases at its end(s) for permitting its hybridization with adjacent portion(s) at their ends, and using these, a recursive PCR is carried out to obtain the DNA coding for the full-length 4AaCter (696-851). In the nucleotide sequences presented in FIG. 1, the bases shown by upper-case letters are those located within the nucleotide sequence coding for 4AaCter (696-851), and the bases shown by lower-case letters are those which were added for convenience in manipulation.
(1) Primer C1-1-f (SEQ ID NO:15)
(2) Primer C1-1&2-r (SEQ ID NO:16)
(3) Primer C1-2&3-f (SEQ ID NO:17)
(4) Primer C1-3&4-r (SEQ ID NO:18)
(5) Primer C1-4&5-f (SEQ ID NO:19)
(6) Primer C1-5&6-r (SEQ ID NO:20)
(7) Primer C1-6&7-f (SEQ ID NO:21)
(8) Primer C1-7&8-r (SEQ ID NO:22)
(9) Primer C1-8&9-f (SEQ ID NO:23)
(10) Primer C1-9-r (SEQ ID NO:24)
A DNA fragment prepared by PCR using a pair of primers C1-1-f and C1-1&2r, which had at one end of them sequences complementary with each other, and a DNA fragment having at both ends respective complementary sequences and prepared by PCR using a series of primer pairs, C1-2&3-f, C1-3&4-r, C1-4&5f, C1-5&6-r, C1-6&7-f and C1-7&8-r, and a DNA fragment prepared by PCR using a pair of primers C1-8&9-f and C1-9r were provided. Using these fragments, the DNA (SEQ ID NO:25) coding for the full-length 4AaCter(696-851) is prepared by PCR (FIG. 2).
It is also possible, in order to introduce a spacer which gives an amino acid sequence that can be cleaved specifically by an enzyme (e.g., an amino acid sequence targeted by a protease) between the heterologous protein of interest and the Cter (or its fragment), to introduce in advance a DNA coding for such a spacer. Insertion of such a DNA can be done by choosing a proper nucleotide sequence for the primers, which is also well known to those skilled in the art.
When the gene prepared as above is introduced into a host bacterium, such as E. coli, and the host is induced to express the fusion protein, the fusion protein accumulates within the host cells in a great amount forming crystals. Crystals thus formed can be readily observed under an optical microscope.
Through fracturing the host cells obtained above which have a large amount of accumulated crystals, and centrifugation which follows, the crystals can be collected as the precipitate. The crystals thus collected can be dissolved by suspending them in an alkaline aqueous solution, e.g., sodium a carbonate buffer with a pH of about 10.5-12 and incubating the mixture at 37° C. for 30 minutes to 2 hours. After dissolution of the crystals, the fusion protein, which retains the biological activity is recovered in the supernatant by centrifugation.
Depending on the intended purpose, the active fusion protein recovered above may be used directly, or subjected to further purification by commonly used purification means, such as ion-exchange chromatography, as needed.
Further, if a site which is cleaved specifically by an enzyme is inserted between the heterologous protein and Cter (or its fragment), treatment of the fusion protein with the enzyme to cleave it at the sequence, followed by removal of the Cter portion (or its fragment), will give the protein of interest.
In order to produce it in the form of a fusion protein in host cells, the molecular weight of a heterologous protein fused with a Cter (or its fragment) of a Cry protein is preferably not more than 50 kDa.
In a convenient way, detection of a fusion protein obtained by the method according to the present invention can be done by detecting the Cter portion of the fusion protein using an anti-Cter antiserum (or an anti-Cter antibody isolated from the antiserum) collected from an animal (in particular, mammal, e.g., rabbit, goat, etc.) which has been immunized with the Cter used in the fusion protein formation according to a conventional method (Example 8). For detection, any method well known to those skilled in the art may be employed, such as Western blotting, ELISA, immunoprecipitation, and the like. A column prepared by immobilizing an antiserum to the Cter or an anti-Cter antibody on a solid phase may also be used for isolation and purification of a fusion protein.
Further, an antiserum reactive to the original protein before fusion can also be obtained by immunizing an animal in a conventional manner with the fusion protein prepared according to the present invention, and collecting the serum (see Example 7). Such an antiserum or an antibody (polyclonal) to the original protein isolated from the antiserum, can be used in the analysis (detection, semiquantitative or quantitative) of a sample (e.g., a biological sample of an animal, esp. of a mammal, among others, human tissues, blood, plasma and serum). Specifically, for example, such an antiserum or antibody is immobilized on a proper carrier, like as latex particles, gelatin particles, colloidal gold, polystyrene beads, or a polystyrene plate. And any of techniques well known to those skilled in the art, such as immunoagglutination, enzyme labeling, chemiluminescence, and the like, can be performed on them for detection of the presence of the original protein contained in a sample, or for semiquantitative or quantitative analysis through comparison with a reference standard. Thus, an antiserum to the original protein (or an antibody to the original protein) obtained from an animal immunized with a fusion protein can be provided as a testing reagent, directly in the form of a solution or a lyophilized preparation, or in such forms in which it is immobilized on a proper carrier, like latex particles, gelatin particles, colloidal gold, polystyrene beads, a polystyrene plate, and the like.
Isolation and purification of the antibody to the original protein from the antiserum to the fusion protein can be done, in a conventional manner, i.e., by preparing an affinity column to which the original protein or the Cter employed in the fusion is bound, and by allowing the antibody to the original protein or the antibody to the Cter, respectively, to be specifically adsorbed.
EXAMPLES
The present invention is described in further detail below. It should be noted, however, that the present invention is not intended to be limited to the examples.
Example 1
Production of Fusion Protein of 4AaCter(696-851) and Glutathione-S-transferase
According to the following procedure, glutathione-S-transferase (GST) originating from Schistosoma japonicum was expressed in a large amount and was let accumulate in E. coli cells, in the form of a fusion protein with 4AaCter (696-851), which is a fragment of the Cter of one of Cry proteins, Cry4Aa2, and corresponds to amino acids Ile 698 ̃Pro 851 of Cry4Aa2.
1. Preparation of a DNA Coding for 4AaCter(696-851) (FIG. 4)
Cry4Aa-S2 gene (formally named, in the priority document, syn4A gene) (which is a gene designed to express a polypeptide consisting of the full-length Cry4Aa's 1180 amino acids) was synthesized by recursive PCR. For this total synthesis, 50 to 55-base synthetic oligonucleotide primers were used which had been designed to cover the full-length nucleotide sequence of interest and at the same time to form base pairs consisting of 10 to 15 bases overlapping between adjacent ones of these primers. The nucleotide sequence of the open reading frame (ORF) of cry4Aa-S2 gene is shown as SEQ ID NO:26. Using this as a template, PCR was carried out to amplify the DNA fragment coding for 4AaCter(696-851). The nucleotide sequences of the primers employed are shown in FIG. 5 and below. The DNA fragment obtained by this has a XhoI site at its each end.
(1) Primer X-Syn4A-C1-f: 5′-GGCTCGAGATCATCAACACCTTCTAC-3′ [nucleotides 3-26 (single-underscored in the upper part of FIG. 5) give an XhoI site, nucleotide 9-26 (double-underscored) a terminal sequence of 4AaCter(696-851)] (SEQ ID NO:27).
(2) Primer X-S-Syn4A-C1-r: 5′-GGCTCGAGCCCGGGCCGGCACATTCATGATT-3′ [nucleotides 3-8 (single-underscored in the lower part of FIG. 5) give an XhoI site, nucleotide 17-31 (double-underscored) a terminal sequence of 4AaCter(696-851)] (SEQ ID NO:28).
<Reaction Solution>















10 × PCR buffer (for KOD plus)
5.0 μL



2 mM dNTP
5.0 μL



25 mM MgSO4
2.4 μL



Primer X-Syn4A-C1-f (10 μM)
1.5 μL



Primer X-S-Syn4A-C1-r (10 μM)
1.5 μL



Template DNA(25 ng)
1.0 μL



Sterilized water (DDW)
32.6 μL 



DNA polymerase (KOD plus, TOYOBO)
1.0 μL



Total volume
50.0 μL 









<Reaction Conditions>

The above reaction solution was set on a thermal cycler (Gene Amp PCR system 9700, PE Applied Biosystems), and reaction was allowed to proceed under the following condition: 94° C. for 2 min; (94° C. for 15 sec, then 55° C. for 30 sec, then 72° C. for 1 min)×25 cycles; 72° C. for 7 min; 4° C. for an indefinite period.
3. Construction of Expression Vector pGST-4AaCter
The fragment obtained above was treated with XhoI. A commercially available expression vector pGEX-6P-1 (GE Healthcare Bio-Science, FIG. 6) was provided. The nucleotide sequence of the multicloning region of this vector is shown as SEQ ID NO:29, and the amino acid sequence coded for by this region as SEQ IN DO:30. Into the Xhol site within the region was inserted in a conventional manner the above-mentioned 4AaCter(696-851) fragment that had been amplified and then treated with Xhol, and thus expression vector pGST-4AaCter was constructed (FIG. 4). This fragment 4AaCter(696-851) is designed in such a manner that GST and 4AaCter(696-851) are combined in-frame if it is inserted in the correct orientation. The orientation of the inserted 4AaCter(696-851) gene was confirmed based on the restriction enzyme pattern utilizing unique KpnI and NaeI sites within the sequence, and by sequencing.
4. Transformation of Host E. coli Cells by Introduction of the Gene
E. coli BL21 strain cells were transformed by introduction into them of pGST-4AaCter constructed above. Namely, 0.1 mL of overnight culture of E. coli BL21 cells was applied to 5 mL of LB medium, and the mixture was shake cultured at 37° C. until the turbidity of the culture reached 0.5 (for about 2 hours). From 1 mL of this, bacterial cells were collected by centrifugation, suspended in 0.5 mL of ice-cooled 50 mM CaCl2, and were let stand on ice for 30 minutes. To a 0.2-mL suspension taken from this was added pGST-4AaCter, and the mixture was let stand on ice for 30 minutes, then subjected to a heat shock at 42° C. for 30 seconds, and to this was added 0.8 mL of LB medium (to 1 mL in total). After shake cultured at 37° C. for 1 hour, the culture was streaked onto ampicillin-containing LB agar plates, and after an overnight culture at 37° C., an E. coli strain transformed with pGST-4AaCter was obtained.
5. Induction of Expression
The E. coli was precultured. Five mL of the medium (TB) was put in a test tube and cultured overnight. Two mL of this overnight culture was added to 200 mL of TB medium. The mixture was cultured on a shake culture apparatus (New Brunswick Scientific INOVA4230) at 240 rpm for 2-3 hours at 37° C., until the OD600 reached 0.6-0.8. IPTG (isopropyl-β-D-thiogalactopyranoside) was added to the culture fluid to make a final concentration of 0.06 mM. Culture was continued for further 2-4 hours (240 rpm 37° C.) to induce expression. Crystal formation within the cells was observed (FIG. 7, arrowheads).
6. Solubilization of the Crystals
E. coli cells expressing GST-4AaCter were collected and suspended in 25 mL of PBS, and then lysozyme (final concentration: 1 mg/mL) and phenylmethylsulfonyl fluoride (PMSF, final concentration: 1 mg/mL) were added to the suspension. The cells then were fractured by ultrasonication (for 6 min in total, repeating ON (20 sec) and OFF (10 sec)), and insoluble fraction was precipitated by centrifugation at 11000 rpm for 15 minutes. The precipitate, which contained crystals of GST-4AaCter, was washed by centrifugation with a proper volume of PBS, and the precipitate was suspended in 100 mM Na2CO3 (pH 10.5) solution and incubated at room temperature for 1-2 hours to solubilize the GST-4AaCter crystals. After centrifugation, the supernatant, which contained GST-4AaCter, was subjected to analysis by SDS-PAGE. It was confirmed that the fusion protein was localized in the insoluble fraction and solubilized by alkali treatment (FIG. 8).
The GST activity of GST-4AaCter which had been solubilized in a buffer with a pH of 10.5 was assayed by CDNB (1-chloro-2,4-dinitrobenzene) method. Namely, a protein sample containing GST was put in the wells of a 96-well plate, and after addition of 200 μL of the following substrate solution to this, let stand for 1 minute at room temperature. This sample was set on an absorptiometer (Spectra MAX 250, Molecular Devices), and the change in its absorption per unit time was measured at 340 nm.
<Substrate Solution (for 4 Samples)>















100 mM potassium phosphate [pH 7.4]
960 μL 



50 mM GSH*
20 μL



50 mM CDNB
20 μL



Total volume
 1 mL





*GSH: reduced glutathione






The result confirmed that GST-4AaCter exhibits a potent GST activity, though having fallen short of GST as a control (produced from E. coli carrying GST expression vector pGEX-6P-1) (Table 3). Besides, the maximum yield of GST-4AaCter so far reached is 0.6 mg per 3 mL culture.







TABLE 3







GST Activity of Fusion Protein











GST activity



Protein
(μmol/min/nmol GST)






GST (control, 27 kDa)
242



GST-4AaCter (fusion protein, 44 kDa)
181









Example 2
Production of Fusion Protein of Various Fragments of Cry4A and Glutathione-S-Transferase
Study was made to identify where the indispensable region resides in 4AaCter for a fusion protein to form crystals. As shown in FIG. 9, various parts of the amino acid sequence Cry4Aa were amplified, and their fusion proteins with GST were produced according to a procedure similar to that followed in Example 1. Each of them was examined for their formation of crystals in the same manner as in Example 1. The fusion protein with 4AaCter(852-1180) was not found to form clear crystals, nor did the fusion protein with GST-4AaCter(696-799) exhibit formation of crystals. On the other hand, crystal formation was confirmed with GST-4AaCter(696-851) [amino acid sequence of the Cter(696-851) portion: SEQ ID NO:6] and with those fusion proteins prepared using gradually shortened peptide chains, i.e., GST-4AaCter(801-851), GST-4AaCter(801-834) [amino acid sequence of the 4AaCter(801-834) portion: SEQ ID NO:31], and GST-4AaCter(801-829) [amino acid sequence of the 4AaCter(801-829) portion: SEQ ID NO:7]. This indicates that in 4AaCter, the sequence essential for crystal formation is a polypeptide chain portion consisting of 29 amino acids, 801-829, equally included in these latter Cter portions.
Example 3
Production of Fusion Protein of 4AaCter(696-851) with MM29 kD
MM29 kD, a cytotoxic protein derived from B. thuringiensis, exhibits a potent toxicity to mammalian cells (esp. to leukemia and cancer cells). MM29 kD is produced and accumulated in B. thuringiensis cells in the form of a precursor consisting of 304 amino acids (whose nucleotide and amino acid sequences shown as SEQ ID NO:32 and SEQ ID NO:33, respectively), and converted to the active form through removal of 28 amino acids from the N-terminus and some from the C-terminus by proteinase K. Though it is not known as yet how many amino acids are removed from its C-terminus in nature, removal of 23 amino acids gives an active protein. And based on studies of its molecular size, it is considered that about 23 amino acids are removed also in nature. The nucleotide sequence of the active MM29 kD, which is formed by removal of N-terminal 28 amino acids and C-terminal 23 amino acids, is presented as SEQ ID NO:34, and the corresponding amino acid sequence as SEQ ID NO:35.
While it has so far been prepared through its expression in the form of a fusion protein with GST in E. coli cells, it has been difficult to obtain MM29 kD in high yield. Namely, MM29 kD is a protein which cannot be expressed easily. This is thought to be due to the adverse effects of MM29 kD, which accumulates in the soluble fraction, on the viability and reproduction of the E. coli cells. An attempt was made to let this protein accumulate in the form of crystals (i.e., in a manner that the soluble fraction is kept free of this protein) and thus obtain high yield while averting the adverse effects. For this purpose, fusion proteins were constructed in which 4AaCter was added in-frame to either the N- or C-terminus of MM29 kD, respectively, i.e., 4AaCter-MM29 kD and MM29 kD-4AaCter (FIG. 10).
1. Construction of Expression Vector for Production of Fusion Protein 4AaCter-MM29 kD (FIG. 11)
Using the total DNA from B. thuringiensis MM50G2 strain as a template and the following set of primers, PCR was performed to prepare the MM29 kD gene DNA having a BamHI site on the upstream end and an EcoRI site on the downstream end (FIG. 10).









(1) Primer Cytox-N-f-Bam:



(SEQ ID NO: 36)



GTGGATCCGTTATTCAAGAATACCTTACGTTTAATG



 



(2) Primer Cytox-r-999-Eco:



(SEQ ID NO: 37)



AGGAATTCAAGCTTCTTGCTGTTCAGC






A commercially available protein expression vector pGEX-4T-3 was provided (FIG. 12). The nucleotide sequence of the multicloning region of this expression vector is presented as SEQ ID NO:38, and the amino acid sequence coded for by this as SEQ ID NO:39. One day mutagenesis using PCR was applied in a conventional manner to remove from the vector the entire open reading frame (ORF) of GST leaving behind its start codon “ATG” alone to prepare pΔGST-4T-3 (FIG. 11). The nucleotide sequence of pΔGST-4T-3, part of which is shown in FIG. 13, is presented as SEQ ID NO:40. And the forward primer and the reverse primer shown in FIG. 13 employed in this process are presented as SEQ ID NO:41 and SEQ ID NO:42, respectively. Using these primers, BamHI was inserted immediately after that start codon “ATG” left in pΔGST-4T-3.
Then, the above DNA fragment (which has a BamHI site on the upstream end and an EcoRI site on the downstream end) was inserted in a conventional manner into the BamHI/EcoRI site of pΔGST-4T-3 to form vector pΔGST-MM29 kD. A region of nucleotide sequence near the start codon “ATG” of GST and 5′-end of the above-mentioned DNA coding for MM29 kD is given in FIG. 14 and as SEQ ID NO:43.
The DNA fragment (BamHI)-4AaCter-(BamHI) was inserted into the BamHI site of pΔGST-MM29 kD prepared above to construct vector pΔGST-4AaCter-(MM29 kD).
Then, using this as a template, one day mutagenesis was performed to replace the nucleotide sequence “GCGGATCC”, which connected between 4AaCter and MM29 kD, with “GCG”. 4AaCter and MM29 kD thus were connected in-frame and gave pΔGST-4AaCter-MM29 kD (FIG. 11).
2. Construction of Expression Vector for Production of Fusion Protein MM29 kD-4AaCter (FIG. 15)
Using PCR, DNA fragment (BamHI)-MM29 kD-(HindIII) was prepared by adding a nucleotide sequence “GGATCC” to the 5′ end of the DNA sequence coding for MM29 kD (active form) and a nucleotide “TGAATTC” to the 3′ end, respectively. This process added to the 3′ end of MM29 kD a stop codon “TGA”, and provided a EcoRI site simultaneously. At the same time, a HindIII site was freshly created immediately upstream of this EcoRI site.
A PCR was performed using the DNA coding for 4AaCter. For this process, nucleotide sequences of the primers had been designed so that HindIII restriction sites might be added at either end of this DNA, and that in-frame connection to the terminal HindIII site might become possible (namely, so that “AAGCTTTA” might be added to the 5′ end of the DNA coding for 4AaCter, and “AAGCTT” at the 3′ end).
A recombinant vector pBSII-MM29 kD was prepared by inserting the above DNA fragment, (BamHI)-MM29 kD-(HindIII), into the BamHI/HindIII site of a commercially available cloning vector pBluescript II SK+ (pBSII, Stratagene, USA).
Then, by inserting (HindIII)-4AaCter-(HindIII) fragment into the HindIII site of the recombinant vector pBSII-MM29 kD (thus MM29 kD and 4AaCter linked in-frame), pBSII-MM29 kD-4AaCter was constructed.
By treating pBSII-MM29 kD-4AaCter with a BamHI/XhoI restriction enzyme, a BamHI/XhoI DNA fragment including the (BamHI)-MM29 kD-4AaCter-(HindIII) region was cut out, which then was inserted into the BamHI/XhoI site of pGEX-4T-3 to form pGST-MM29 kD-4AaCter.
One-day mutagenesis using PCR was applied using pGST-MM29 kD-4AaCter DNA as a template, the ORF of GST was removed up to the BamHI recognition site sequence leaving the first “ATG” (start codon) alone, and the tac-promoter upstream of GST and the start codon “ATG” was directly linked in-frame to the MM29 kD gene, to form pΔGST-MM29 kD-4AaCter (FIG. 15).
3. Transformation of Host E. coli Cells by Introduction of a Gene and Collection of Crystals
E. coli BL21 cells were transformed by introduction either of the pΔGST-4AaCter-MM29 kD or of the pΔGST-MM29 kD-4AaCter, which had been constructed above 1 and 2, respectively. Five mL of TB medium was inoculated with either of the transformant BL21 strain cells, and after addition of 10 μL of 50 mg/mL Amp, shake cultured at 37° C. for 12 hours (preculture). Fifty-mL TB medium was inoculated with 500 μL of the preculture, and 100 μL of 50 mg/mL Amp was added to this. Shake culture was done at 37° C. for 3 hours (OD600=about 0.8). To this was added 50 μL of 100 mM IPTG, and culture was continued at 37° C. for 3 hours. Formation of crystals was confirmed in the E. coli cells transformed either with 4AaCter-MM29 kD or with MM29 kD-4AaCter (FIG. 16, arrowheads). The bacterial cells were collected and suspended in 20 mL of PBS [pH7.5] and centrifuged (10000 rpm, 4° C., 10 min: RS-18 IV/TOMY). The precipitate separated was suspended in ice-cooled sterilized water, then fractured by ultrasonication (for 5 min in total, repeating ON (10 sec) and OFF (10 sec)), and centrifuged (10000 rpm, 4° C., 10 min). The precipitate, after washed by centrifugation (not less than 3 times) with ice-cooled sterilized water, was suspended in 2 mL of 50 mM Tris-HCl [pH 7.4]. This suspension was subjected to sucrose density-gradient centrifugation, and the layer which contained crystals (white band) was collected and suspended in 20 mL of PBS [pH7.5]. This was centrifuged (12000 rpm, 4° C., 10 min) and the precipitate was further centrifuged in ice-cooled sterilized water (at least 3 times). The supernatant was fully removed, and the precipitate was suspended in 20 mL of ice-cooled sterilized water. After additional centrifugation (12000 rpm, 4° C., 10 min), precipitate was suspended in 2 mL of ice-cooled sterilized water, then distributed into sample tubes, and stored at −80° C.
4. Solubilization of Crystals
The precipitate was suspended in a solubilization buffer (100 mM Na2CO3 [pH 10.5], 10 mM DTTz) to solubilize the crystals (37° C., 30 min) (crystals can be easily solubilized and collected at pH 10-12). After centrifugation (14000 rpm, 4° C., 10 min), the supernatant (4AaCter-MM29 kD of MM29 kD-4AaCter) was collected. This was separately purified by anionic column chromatography (HiTrap Q XL, GE Healthcare Bioscience). The proteins adsorbed by the column and then was eluted using 100 mM, 200 mM, and 300 mM NaCl stepwise. 4AaCter-MM29 kD and MM29 kD-4AaCter were found to be eluted with 300 mM NaCl. Examination using SDS-PAGE revealed that these standard samples had been purified to a single band (FIG. 17).
5. Release and Collection of MM29 kD from Fusion Protein
Then, to the supernatant containing 4AaCter-MM29 kD or MM29 kD-4AaCter was added proteinase K in a proper amount ( 1/10 of the amount of the protein in the supernatant) to remove the 4AaCter sequence upstream or downstream of MM29 kD (37° C., 1 hour). MM29 kD, which is rather resistant to decomposition with proteinase K, survives this treatment as the core polypeptide. The reaction was terminated by 0.1 M PMSF (phenylmethylsulfonium fluoride) which was added to a final concentration of 1 mM. The protein was purified again by anionic ion-exchanger chromatography. MM29 kD is not adsorbed by the column and flows through. Analysis of the purified MM29 kD by SDS-PAGE confirmed that either protein was purified to a single band.
6. Assessment of Cytotoxicity
Using the proteins, 4AaCter-MM29 kD, MM29 kD-4AaCter, and free MM29 kD, purified above, assessment was made for cytotoxicity (lethal activity) of the purified fusion proteins by MTT assay employing Jurkat cells, which originate from a leukemia cell, as a target. Briefly, the number of the Jurkat cells (purchased from Institute of Physical and Chemical Research) were counted using Burker Turk Deep ( 1/10 mm). The cells were diluted to the density of 5.0×105 cells/mL with a medium for measurement for assay (RPMI 1640 free of phenol red, Nissui). The cell culture was placed, 90 μL each, in a necessary number of wells of a 96-well plate. Ten μL each of the samples which had been diluted to proper concentrations with PBS [pH7.5] was added to each well, and incubation was made (37° C., 3 hrs). Five mg/mL MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide, Sigma, dissolved in the medium for measurement) then was added to each well, and incubation was made (37° C., 3 hrs). 100 μL of acidic isopropanol was added to each well, and after sufficient pipetting, OD (570 nm) was measured by a spectrophotometer to calculate survival rate of the cells. As a result, 4AaCter-MM29 kD exhibited an activity comparable to GST-MM29 kD, which had been used as the standard sample in the studies of MM29 kD. Compared to these, the activity of MM29 kD-4AaCter was found to be several times higher (Table 4). Free MM29 kD, prepared either from 4AaCter-MM29 kD or MM29 kD-4AaCter, exhibited its EC50 at 0.4-0.5 ng/mL (Table 4), which suggests that it is not that MM29 kD is irreversibly inactivated when it is joined by 4AaCter at its N-terminus, but probably is lowered in its apparent biological activity due to some steric hindrance. Besides, the yield of 4AaCter-MM29 kD and MM29 kD-4AaCter obtained above was 0.3-0.5 mg per 50 mL of culture, which was at least 6 times the yield of GST-MM29 kD (usually, about 50 μg per 50 mL of culture), which had so far been utilized to produce MM29 kD in E. coli as a host.







TABLE 4







Cytotoxicity of Fusion Protein to Jurkat Cells










Protein
Cytotoxicity (EC50, ng/ml)













GST-MM29kD
23.4



4AaCter-MM29kD
24.6



MM29kD-4AaCter
4.7



Free MM29kD
0.4-0.5









Example 4
Production of Fusion Protein of Cter of Cry1Aa and Glutathione-S-Transferase
As it was confirmed as shown in Examples 1-3 that GST and MM29 kD can be produced in a large amount using E. coli in the form of their fusion proteins with various fragments of 4AaCter, an attempt then was made to produce fusion proteins with the Cter of other Cry proteins. Using 1AaCter, the Cter of Cry1Aa (insecticidal toxin specific to lepidopteran insects (butterflies and moths)), which exhibits an insecticidal spectrum utterly different from that of Cry4Aa (insecticidal toxin specific to dipteran insects (mosquitoes)), a large-scale expression of GST was tried as described below.
1. Preparation of a Gene Coding for GST-1AaCter(622-777) and Construction of an Expression Vector pGST-1AaCter
Based on the result of alignment [using a software ClustalW] with the amino acid sequence of 4AaCter(696-851), a corresponding peptide, 1AaCter(622-777), was selected. To prepare a fusion protein with this peptide chain, a gene fragment containing the coding region for 1AaCter(622-777) (amino acid sequence set forth as SEQ ID NO:1) was amplified by PCR. This PCR was performed using DNAs extracted from B. thuringiensis subsp. sotto T84A1 strain as a template, together with the following primers, which are specific to 1AaCter:









(1) Primer 1Aa3-C1-f:



(SEQ ID NO: 44)



GGATCCGCGGTGAATGAGCTG



 



(2) Primer 1Aa3-C1-r:



(SEQ ID NO: 45)



CTCGAGACCCACATTTACTGT






The gene fragment (nucleotide sequence set forth as SEQ ID NO:46) which was thus amplified is provided with a BamHI site at its upstream end and a XhoI site at its downstream end, respectively. This fragment was inserted in-frame into the BamHI-XhoI site within the multicloning site which is downstream of the GST gene of the expression vector pGEX-6P-1, giving an expression vector, pGST-1AaCter (FIG. 18). pGST-1AaCter is designed to express the fusion protein of GST and 1AaCter (GST-1AaCter).
2. Transformation of E. coli Host by Introduction of the Gene
E. coli BL21 cells were used as the host. The cells were transformed by introduction of the above expression vector pGST-1AaCter in the same manner as in Examples 1 and 2, and following induction of expression, GST-1AaCter was expressed. Thus, formation of crystals was observed in the cells, as was the case with GST-4AaCter (FIG. 19, arrowheads). SDS-PAGE analysis of the supernatant (soluble protein fraction) and the precipitate (insoluble protein fraction) separated by centrifugation showed that GST-1AaCter, which was estimated to be about 45 kDa, was located mainly in the insoluble protein fraction (FIG. 20). The amount of expressed GST-1AcCter was estimated to be about 5 μg per 10 μg of E. coli total proteins, according to computer image analysis. While the GST-1AaCter crystals were scarcely soluble in alkaline buffer solutions with a pH of 9-11, they were solubilized in a buffer with a pH of 12 (FIG. 21). Measurement of GST activity was made by the CDNB assay and revealed their high activity comparable to that of the GST purified standard [purified from pGEX-6P-1-introduced E. coli cells through Glutathione Sepharose 4B (GE Healthcare)] (Table 5).







TABLE 5







GST activity of GST-1AaCter











GST activity



Protein
(μmol/min/nmol GST)













GST purified standard
107.3



GST-1AaCter
99.8









Example 5
Preparation of Fusion Protein with Cter from Cry1Ac and Glutathione-S-Transferase
Cry1Ac is an insecticidal toxin specific to lepidopteran insects (butterflies and moths) like Cry1Aa employed in Example 4. Cry1Ac, however, exhibits different characteristics from those of Cry1Aa, such as its N-acetylgalactosamine (GalNAc) recognizing lectin activity. An examination was carried out in the following manner to find whether a protein (GST) linked to the Cter (1AcCter) from this Cry1Ac protein would form crystals and accumulate in E. coli cells.
1. Preparation of Gene Coding for GST-1AcCter(624-779) and Construction of Expression Vector pGST-1AcCter
Based on the result of alignment [using a software ClustalW] with the amino acid sequence of 4AaCter(696-851), a corresponding part, 1AcCter(624-779), was selected from the sequence of Cry1Ac1. In order to prepare a fusion protein with this, a gene fragment containing the coding region for 1AcCter(624-779) (amino acid sequence set forth as SEQ ID NO:4) was amplified by PCR. This PCR was performed using DNAs extracted from B. thuringiensis subsp. kurstaki HD73 strain as a template, together with the following primers, which are specific to 1AcCter(624-779):









(1) Primer 1Ac1-C1-f:



(SEQ ID NO: 47)



GGATCCGCGGTGAATGCGCTG



 



(2) Primer 1Ac1-C1-r:



(SEQ ID NO: 48)



CTCGAGTGGCACATTTACTGT 






The gene fragment (nucleotide sequence set forth as SEQ ID NO:49) which was thus amplified was provided with a BamHI site at its upstream end and a XhoI site at its downstream end, respectively. This fragment was inserted in-frame into the BamHI-XhoI site downstream of the GST gene of the expression vector pGEX-6P-1, which gave pGST-1AcCter an expression vector for a fusion protein of GST and 1AcCter (GST-1AcCter).
2. Transformation of E. coli host by Introduction of the Gene
Using E. coli BL21 cells in the same manner as in Example 1, CST-1AcCter was expressed. As a result, formation of crystals was observed in the cells, as were the cases with GST-4AaCter and GST-1AaCter (FIG. 22, arrowheads). SDS-PAGE analysis of the supernatant (soluble protein fraction) and the precipitate (insoluble protein fraction) separated by centrifugation showed that GST-1AcCter, which was estimated to be of about 45 kDa, was found localized mainly in the insoluble protein fraction (FIG. 23). The amount of expressed GST-1AcCter was about 5 μg per 10 μg of the E. coli total proteins, as estimated by computer image analysis. While the GST-1AcCter crystals were scarcely soluble in alkaline solutions with a pH of 9-11, they were solubilized in a buffer with a pH 12 (FIG. 24). Measurement of the activity of GST-1AcCter thus solubilized was made by the CDNB assay and revealed that its activity was comparable to that of the purified standard (Table 6).







TABLE 6







GST activity of GST-1AcCter











GST activity



Protein
(μmol/min/nmol GST)






GST purified standard
107



GST-1AcCter
105









Example 6
Preparation of Fusion Protein with Cter from Cry8Ca
An attempt then was made to prepare a fusion protein with a Cter from Cry8Ca1. Based on the result of alignment [using a software ClustalW] with the amino acid sequence of 4AaCter(696-851), a corresponding sequence, 8CaCter(672-829), was selected. In order to prepare a fusion protein with this, a vector having a DNA in which a gene fragment coding for 8CaCter(Lys672-Pro829) (amino acid sequence set forth as SEQ ID NO:14) had been inserted in-frame downstream of a gene fragment coding for GST was introduced to E. coli BL21 strain cells to transform these, and the expression of the fusion protein was induced. As a result, formation of crystals was observed in the E. coli cells.
Example 7
Preparation of C-Reactive Protein (CRP) Utilizing 4AaCter
C-reactive protein (CRP), which occurs in the blood in response to an inflammation, is an inflammation marker, and its measurements in the blood: which is performed using an antibody reactive specifically to CRP (anti-CRP antibody), can be used as an index to the activity and severity, in follow-up observations and prognosis of an inflammatory diseases. There is a high demand for anti-CRP antibody, and so is the demand for CRP itself, which is the necessary immunogen in order to produce the antibody. However, as CRP collected and purified from the body contains a substantial amount of contaminants originating from its sources, an anti-CRP antibody prepared using it as the immunogen could react with other compounds occurring in the body, thereby affecting the measurement. For this reason, as well as for steady supply of the immunogen CRP, it has been desired that production of anti-CRP antibody is made using, as the immunogen, a recombinant CRP produced by microorganisms. Thus, an attempt was made to produce CRP utilizing 4AaCter.
<Construction of Expression Vector for Production of Fusion Protein>
Human CRP gene which had been prepared by gene synthesis in the following manner was inserted in-frame between the BamHI and XhoI sites of the above mentioned pΔGST-4T-3 to construct pΔGST-CRP.
Namely, the gene segment encoding human CRP was synthesized by recursive PCR performed with reference to a database (GenBank NM—00567). PCR was performed using a DNA fragment prepared by PCR using a pair of primers each of which had at an end a sequence which was complementary to the sequence at an end of the other: i.e., CRP—1f (SEQ ID NO:97)(6 nucleotides at the 5′-end of this form a BamHI site) and CRP—2r (SEQ ID NO:98); and fragments prepared by PCR using a series of primers each having at its ends sequences complementary to its flanking primers: i.e., CRP—3f (SEQ ID NO:99), CRP—4r (SEQ ID NO:100), CRP—5f (SEQ ID NO:101), CRP—6f (SEQ ID NO:102), CRP—7f (SEQ ID NO:103), CRP—8r (SEQ ID NO:104), CRP—9f (SEQ ID NO:105), CRP—10r (SEQ ID NO:106), CRP—11f (SEQ ID NO:107), CRP—12r (SEQ ID NO:108)(6 nucleotides at the 5′-end of this form a XhoI site), and thus a DNA coding for the full length CRP (SEQ ID NO:109)(respective 6 nucleotides at 5′- and 3′-ends form restriction sites required in subcloning) was prepared. In the amino acid sequence (SEQ ID NO:110) coded for by the DNA, the parts consisting of two amino acids at the N- and C-termini, respectively, are linker sequences which have been brought in as part of the restriction sites. The DNA (SEQ ID NO:109) coding for the full length human CRP was inserted in-frame into pΔGST-4T-3 which had been digested with BamHI and XhoI, to construct pΔGST-CRP.
Then a fragment coding for 4AaCter was inserted in-frame into the BamHI site of pΔGST-CRP to construct pΔGST-4AaCter-CRP. Namely, PCR was performed using the open reading frame of the above-mentioned cry4Aa-S2 gene as a template, and using primers B-Syn4A-C1-f (SEQ ID NO:111)(6 nucleotides at its 5′-end form a BamHI site) and B-Syn4A-C1-rn (SEQ ID NO:112)(6 nucleotides at its 5′-end form a BamHI site) to amplify the DNA fragment coding for 4AaCter(696-851). The fragment thus obtained is provided with BamHI sites at it both ends. The reaction solution and reaction conditions for this PCR were as follows.
<Reaction Solution>















10 × PCR buffer (for KOL plus)
5.0 μL



2 mM dNTP
5.0 μL



25 mM MgSO4
2.4 μL



Primer B-Syn4A-C1-f (10 μM)
1.5 μL



Primer B-Syn4A-C1-rn (10 μM)
1.5 μL



Template DNA (25 ng)
1.0 μL



Purified water (DDW)
32.6 μL 



DNA polymerase (KOD plus, TOYOBO)
1.0 μL



Total volume
50.0 μL 









<Reaction Condition>

The above reaction solution was set in a thermal cycler (Gene Amp PCR system 9700, PE Applied Biosystems) and reaction was allowed to proceed under the following condition: 94° C. for 2 min; (94° C. for 15 sec, then 55° C. for 30 sec, then 72° C. for 1 min)×25 cycles; 72° C. for 7 min; 4° C. for an indefinite period.
4AaCter(696-851) thus prepared was inserted into a plasmid obtained by digestion of pΔGST-CRP with BamHI to create pΔGST-4AaCter-CRP. pΔGST-4AaCter-CRP will express a fusion protein of 4AaCter and CRP (4AaCter-CRP).
<Expression of Fusion Protein>
Thus created pΔGST-4AaCter-CRP was introduced into E. coli BL21 strain cells. The procedures of introduction of this expression vector and induction of expression of E. coli cells were the same as those described above with regard to the introduction of pGST-4AaCter into E. coli cells and induction of its expression. Expression of 4AaCter-CRP was confirmed as follows: the cells were collected and suspended in 10 mL PBS, and after subjected to sonication (ON for 20 sec, OFF for 10 sec), the buffer containing the fractured cells was run in SDS-PAGE together with human CRP (which had been obtained by inserting human CRP gene into a BamHI and XhoI-digested plasmid pΔGST-4T-3, and introducing the plasmid thus obtained into E. coli cells and inducing its expression). As a result, while a band of interest was detected at about 48 kDa with CRP fused to 4AaCter in comparison with the run of a sample taken before induction of expression, no band of interest was observed at about 29 kDa with CRP which had been expressed without fusion with 4AaCter (FIG. 25).
Western blotting of this and human native CRP biological sample using anti-4AaCter-CRP goat antiserum confirmed that this antiserum was reactive to both 4AaCter-CRP and native CRP (FIG. 26). The above results thus indicate that expression of CRP is now available by fusing 4AaCter with CRP, and that an antibody reactive to native CRP can be obtained using 4AaCter-CRP as the immunogen.
<Method for Preparation of Anti-4AaCter-CRP Goat Antiserum)
A goat which had been kept for at least one week for habituation was immunized with 2 mg of 4AaCter-CRP mixed with Freund's complete adjuvant, 5 times at 2-week intervals, and blood was taken from the jugular vein. The blood thus obtained was kept at 37° C. for one hour and let stand at 4° C. for a day and a night. The supernatant obtained was centrifuged at 3000 rpm for 5 minutes, and the supernatant thus obtained was used as 4AaCter-CRP antiserum.
Example 8
Preparation of Anti-4AaCter Antiserum
Immunoassays using an antibody are often performed following expression processes of recombinant proteins, as a method to confirm whether the proteins expressed are the intended ones. However, it is time consuming and costly to provide antigens which are specific to the proteins of interest. For this reason, a method is employed in which a protein of interest is expressed in the form a fusion protein with some other known protein and the expression of the protein of interest is confirmed using an antibody specific to the known protein. Thus, an examination was carried out to ascertain whether protein expression can be confirmed using an anti-4AaCter antibody in the case of a 4AaCter fusion protein.
pΔGST-4AaCter was created by inserting in-frame a gene fragment coding for 4AaCter into the BamHI site of pΔGST, which had been made by removing the GST gene from pGEX4T-3. This vector, pΔGST-4AaCter, expresses 4AaCter in E. coli cells.
4AaCter was let express in E. coli BL21 cells, and after the cells were fractured, the precipitate that was fractionated by centrifugation was solubilized in an alkaline buffer with a pH of 12. Rabbits were immunized in a conventional manner with the solubilized 4AaCter, and anti-4AaCter antiserum was obtained. Namely, rabbits (New Zealand White) which had been kept for one week for habituation were immunized with 0.5 mg of 4AaCter mixed with Freund's complete adjuvant 5 times at two-week intervals, and blood was taken from the jugular vein. The blood thus obtained was kept at 37° C. for one hour and let stand at 4° C. for a day and a night. The supernatant thus obtained was centrifuged at 3000 rpm for 5 minutes, and the supernatant thus obtained was used as 4AaCter antiserum.
Western blotting of 4AaCter and 4AaCter-CRP using the 4AaCter antiserum obtained above confirmed that this antiserum was reactive to both 4AaCter and 4AaCter-CRP (FIG. 27). This result indicates that it is possible to generate an antiserum reactive to 4AaCter by using 4AaCter as the immunogen, and that the 4AaCter antiserum is also reactive to the 4AaCter fusion protein.
INDUSTRIAL APPLICABILITY
The present invention enables to produce a heterologous protein as a fusion protein with a Cter, in bacterial cells, such as E. coli cells, in a great amount in the form of insoluble crystals retaining the protein's activity, which crystals can be solubilized and recovered as an active protein. Thus, the present invention is utilized for production of heterologous proteins using bacteria, such as E. coli, as a host. Further, an antiserum which is created using a fusion protein of the present invention can be utilized in analysis of the original protein, the protein before fusion with a Cter, e.g., as an testing reagent.
[Sequence Listing]
GP124-PCT.ST25

## Claims

1. A method for production of a protein (A) in the form of a fusion protein, comprising the steps of
(a) preparing a DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and a second peptide chain (B), wherein
the N-terminus of protein (A) is fused to the C-terminus of peptide (B) either directly or via a spacer; or
the C-terminus of protein (A) is fused to the N-terminus of peptide (B) either directly or via a spacer, wherein the peptide chain (B) consists of a part of the amino acid sequence that starts with Ile696 and ends with Glu1180 of SEQ ID NO: 89, and
the part comprises the sequence set forth in SEQ ID NO: 6,
(b) introducing the DNA into a host bacterium, and
(c) expressing the fusion protein in the host bacterium.

2. The method of claim 1, wherein in step (a), a DNA which codes for an amino acid sequence comprising a spacer that can be specifically cleaved by a proteolytic enzyme is interposed between the DNA coding for the protein (A) and the DNA coding for the peptide chain (B).

3. The method of claim 1 comprising a further step of fracturing the host bacterium to collect the fusion protein.

4. The method of claim 3 comprising a further step of purifying the collected fusion protein through solubilization of the fusion protein in an alkaline aqueous solution.

5. A method for production of a protein (A) comprising the steps of
(a) preparing a DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and a second peptide chain (B), wherein
the N-terminus of protein (A) is fused to the C-terminus of peptide (B) either directly or via a spacer; or
the C-terminus of protein (A) is fused to the N-terminus of peptide (B) either directly or via a spacer, wherein the peptide chain (B) consists of a part of the amino acid sequence that starts with Ile696 and ends with Glu1180 of SEQ ID NO: 89, and
the part includes the sequence set forth in SEQ ID NO: 6,
(b) introducing the DNA into a host bacterium,
(c) expressing the fusion protein in the host bacterium,
(d) fracturing the host bacterium to collect the fusion protein,
(e) purifying the collected fusion protein through solubilization of the fusion protein in an alkaline aqueous solution, and
(f) removing the peptide chain (B) from the fusion protein.

6. The method of claim 5, wherein the peptide chain (B) is removed by treating the fusion protein with a proteolytic enzyme which acts on a specific cleavage site located in a spacer situated between the peptide chain forming the protein (A) and peptide chain (B).

7. A fusion protein produced by the method of claim 3.

8. A fusion protein produced by the method of according to claim 1.

9. A fusion protein produced by the method of according to claim 5.