IMMUNOGLOBULIN-BINDING ARTIFICIAL PROTEIN

Abstract: An immunoglobulin-combining artificial protein (linked protein) prepared by linking a number of units together, said unit comprising at least one of the IgG-combining domains of protein A. The linked protein is superior to natural protein A in the ability to purify IgG and is excellent as a molecular weight marker.

Description: SPECIFICATION Field of the Invention The present invention relates to an immunoglobulin-binding artificial protein, whose immunoglobulin G (IgG)-binding ability is higher than that of protein A, a naturally occurring IgG-binding protein. The artificial IgG-binding protein may be used in a wide range of biotechnological fields. Techniques and Disadvantages of the Prior Art It is already known that protein A is a cell wall protein of Staphylococcus aureus, and can be isolated from culture media of Cowan I strain ("Dictionary of Biotechnology", supervised by Saburo Mitsui, et al. , CMC, Inc., p.958, 1986-10-9). There are two examples of repetitive artificial proteins similar to the protein described in the present invention, which are synthesized by genetic engineering techniques with repeating the gene of one of IgG-binding domain of protein A. One is repetitive artificial proteins of mutant domain Z by Nilsson, et al. (Protein Engineering 1, 107-113, 1987) and the other is repetitive artificial proteins of domain B by Saito, et al. (Protein Engineering 2, 481-487, 1989). In the present invention the relationship between the IgG-binding ability of the repetitive artificial proteins and their repetition number are examined in detail. Means to Solve the Points at Issue In the present invention, the gene of domains A and B of the IgG-binding domains of protein A molecule may be freely amplified using the PCR method (primer-utilizing gene amplification method), making possible for the first time the easy production of repetitive artificial proteins having IgG-binding ability. Among the repetitive artificial proteins obtained in this manner (referred to as-simply "repetitive proteins" hereunder), some proteins exhibited, quite unexpectedly, an ability to bind specifically to IgG which was higher than that of naturally occurring protein A, and it was the result of further research based on this finding that led to completion of the present invention. The gene of domains A and B of the IgG-binding domain of protein A was freely amplified using the PCR method, resulting in success for the first time to produce repetitive proteins having IgG-binding ability by linking these domains. The present invention provides from one aspect an immunoglobulin-binding artificial protein obtainable by a method which comprises using the IgG-binding domains A and B of protein A as a unit and making repetitive links of the A and B domains, wherein the number of repetitions of the A and B domains is four. The invention also provides a method for isolating or removing IgG from a sample containing IgG, which comprises binding the immunoglobulin-binding artificial protein as defined in the past paragraph to IgG and recovering or removing IgG. Brief Description of Figures Fig. 1 shows a homology in the amino acid sequences of the five IgG-binding domains (E, D, A, B, C) of protein A molecule. The amino acids are underlined in the portions below three mathes among the five domains. Fig. 2 is a schematic strategy of construction of an expression plasmid used in the present invention. Fig. 3 shows the nucleotide sequences for the two types of primers (PROTS, PROTAS) necessary for amplification of the gene of the AB domain of protein A using the PCR method, and for the two types of primers (ACCS, ACCAS) necessary for introduction of the initiation codon and the Acc I site into the expression vector. Fig. 4 is an affinity chromatogram of rabbit antibody on a PROT-ABIV- Sepharose column ( ○ : protein, Δ : IgG). Fig. 5 shows the binding capacity of six types of recombinant protein A for IgG from rabbit antiserum. Fig. 6 is an SDS-polyacrylamide gel electrophoretic pattern of rabbit IgG purified using a PROT-ABIV-Sepharose column. (1: molecular weight marker, 2: rabbit antiserum, 3: purified IgG). Fig. 7 is an SDS-polyacrylamide gel electrophoretic pattern of proteins prepared as disclosed hereinafter. The gel was stained with coomassie brilliant blue R-250 (CBBR). The lanes in Fig. 7 represent the following: lane M, molecular weight marker of BRL Co.; 1, PROT-ABI; 2, PROT-ABII; 3, PROT-ABIII; 4, PROT-ABIV; 5, PROT-ABV; 6, PROT-ABVI; 7, a mixture of PROT-ABI to VI. For the CBBR staining, 1.0 µg each of PROT-ABI, II and 0.5 µg each of PROT-ABIII to VI were used. PROT-ABIV is the protein which is in accordance with the claimed invention. As will be made clear from the following description, the repetitive protein of the present invention may be easily prepared by a gene technology, and amazingly excels in its ability to bind IgG specifically. Therefore, the repetitive protein of the present invention is able to selectively bind IgG in an IgG-containing sample, and thus can be used to purify and separate IgG. Likewise, it may be used in the opposite manner to selectively eliminate IgG from said sample. In addition, though the repetitive protein has an excellent ability to bind IgG, the bound IgG can be easily eluted with an appropriate eluent. Either a heterogeneous or a homogeneous system may be used to purify IgG using the repetitive protein prepared according to the present invention, and any suitable conventional method may be employed in either case. For example, if a heterogeneous system is used, then the repetitive protein prepared according to the present invention may be immobilized on a container, the wall of a container or a carrier using any conventional method, and an IgG-containing sample may be contacted with the protein using either a batch method or a continuous method. The method for immobilization may be a method suitable for immobilizing microorganisms and enzymes, preferably one of the various types of carrier-binding methods such as the covalent bond method, the ionic bond method or the physical adsorption method. Also, the material chosen for the carrier or the wall of the container may be that which is most suitable for the binding method used, with a wide range of choices available, for example, synthesized or natural organic substances such as synthesized resins, polysaccharides, etc.; inorganic substances such as glass, metal, porous clay, etc.; biological substances such as blood cells, cells, microorganic cells, etc. The carrier which thus immobilizes the repetitive protein prepared according to the present invention is, for example, filled into a column, after which an IgG-containing sample is put into the column to cause only the IgG to selectively bind to the immobilizing carrier. Then, when an elution is drawn therefrom the IgG is eluted out, and thus purified IgG may be obtained in a continuous manner. Moreover, this procedure may be repeated in order to achieve a higher degree of purity, and as the need arises retreatment may be done using a newly prepared column filled with said protein A immobilizing carrier. Likewise, when using a batch method, purification may be done by binding of IgG to the carrier in the same manner, separation of solid and liquid, and finally elution of IgG. Example 1 (1) Construction of plasmids pTRP-PROT-ABI to VI The AB domain of protein A was selected as a unit part of the repetitive proteins, and its gene was synthesized by the PCR method using commercially available protein A-fusion protein-expression vector pRIT2T as the template (Fig. 2). Here, a sequence recognized by restriction enzyme Acc I was introduced to both termini of the gene, to cause the gene to be linked in a head-to-tail manner. The sequence recognized by AccI is non-palindromic, and thus the DNA fragment which contains such a sequence on both termini is characteristically only linked in a fixed direction. There are some other such restriction enzymes, for example, AfL III, Ava I, Ban I, Ban II, Hgi AI, etc. A so-called ATG vector (pTRPACC) with a trp promoter was prepared as the expression vector, which carries an AccI site as the cloning site, and initiation codon ATG followed by lysine codon AAA in order to achieve a highly efficient expression in E. coli. The above mentioned gene fragment of the AB domain of the protein A was polymerized one to six times and inserted into the expression vector. In the PCR reaction, a pRIT2T cut off with Hin cII was used as the template, while 5'-GGTAGACGCTGATAACAATTTCAACAAA-3' (Fig. 3, PROTS) was used as a sense primer and 5'-GGTCTACTTTTGGTGCTTGAGCATCATTTA-3' (Fig. 3, PROTAS) was used as an antisense primer. Here, the gene for the target AB domain was amplified with Taq polymerase (2.5 units); the condition of PCR was 30 cycles of 940C for 1 minute, 50°C for 1 minute and 72°C for 5 minutes. The expression vector was also synthesized by a PCR method in the same manner using the sense primer (ACCS) and antisense primer (ACCAS) shown in Fig. 3 to introduce an initiation and lysine codons, and AccI site for insertion of the polymerized gene fragments. The ligation reaction was conducted at 14°C for 16 hours, using T4DNA ligase (350 units). The product of this reaction was then used for transformation of E. coli JM109 (recAl, endAl, gyrA96, thi, hsdR17, supE44, relAl, λ-, Δ(lac-proAB), [F', traD36, proAB, lacIa, lacZΔ M15]), and the cells were then mixed with an agar culture medium (composition: 10 g of trypton, 5 g of yeast extracts and 10 g of sodium chloride per liter; pH 7.4) containing 0.7% agar and 100 µg/ml of ampicillin. These cells were then laid on the plate of an L-agar culture medium containing 1.5% agar and 100 µg/ml of ampicillin, and cultured overnight at 37°C. The plasmid DNA of the transformant was then isolated using the alkali-SDS method, after which agarose gel electrophoresis was conducted and the clone with the target DNA fragment inserted therein was selected. The obtained plasmid DNA was partially digested by AccI to determine how many units of the gene of the AB domain of protein A had been inserted, and the plasmids containing fragments of one unit to six units were named pTRP-PROT-ABI to pTRP-PROT-ABVI, respectively. The pTRP-PROT-ABIV-transformed E. coli is deposited at the Fermentation Research Institute, Agency of Industrial Science and Technology in Japan as FERM BP-3930, under the identification of Escherichia coli JM109/pTRP-PROT-AB4. (2) Culturing of transformed E. coli JM109/pTRP-PROT-ABI to VI E. coli which were transformed with plasmids pTRP-PROT-ABI to VI were cultured in an LB culture medium containing 100 µg/ml of ampicillin, at 37°C for 24 hours. The cells thus obtained were stored at -20°C. (3) Disruption of the cells and purification of each protein The frozen cells were thawed at room temperature, after which they were suspended in a 50 mM phosphate buffer solution (pH 7.5) containing 0.1 mM EDTA and 0.15 M NaCl, the volume of which was 3 times the cells, and the cells were disrupted with a Dyno-Mill. To this was added 1/50 volume of 10% Lubrol PX and stirring was done at 0°C for 15 minutes followed by centrifugation at 15,000 rpm for 20 minutes at 40C. The supernatant thereof was 30% saturated with ammonium sulfate, and was subjected to centrifugation. This supernatant was then 60% saturated with ammonium sulfate and then subjected again to centrifugation to obtain 30-60% saturated ammonium sulfate precipitation fractions. They were then dissolved in a small amount of buffer solution A (a 50 mM phosphate buffer solution containing 0.1 mM EDTA, pH 7.5), and after desalting with a Sephadex G-25 column it was applied to a DEAE-Sepharose column which had been equilibrated with buffer solution A. The object protein adsorbed on the DEAE-Sepharose column under these conditions. The column was then adequately washed with buffer solution A, after which elution was done using buffer solution A with a linear concentration gradient of 0 - 1 M NaCl. The object protein was detected by ELISA using HRP-conjugated goat anti-rabbit IgG antibody. The object protein fractions were then applied to an IgG-Sepharose column which had been equilibrated with a 50 mM Tris-HCl buffer solution, pH 7.4, containing 0.15 M NaCl and 0.05% Tween 20. After adequately washing the column with the same buffer solution, elution was done using a 0.5 M ammonium acetate buffer solution (pH 3.4). The eluted fractions were then precipitated with 80% saturated ammonium sulfate, and the precipitate was dissolved in a small amount of buffer solution A, after which the resulting solution was subjected to high-performance molecular sieve chromatography on a TSKgel G3000SW column which had been equilibrated with a 0.1 M phosphate buffer solution, pH 7.0, containing 0.2 M NaCl. Thus obtained fractions produced a single band in SDS-polyacrylamide gel electrophoresis. Table 1 below lists the number of repeat of the AB domain of each repetitive protein along with its molecular weight. One AB domain is composed of 116 amino acids, and its molecular weight is 13,205.24. There are four amino acids, MetLysValAsp, attached to the N-terminus of each repetitive protein, and eight amino acids, ThrGlyArgArgPheThrThrSer, attached to the C-terminus thereof. In addition, each AB domain is linked with two amino acids, ValAsp. Number of links of the AB domain of each protein and its molecular weight. Protein AB domain No. Molecular weight PROT-ABI 1 14,586 PROT-ABII 2 27,987 PROT-ABIII 3 41,389 PROT-ABIV 4 54,790 PROT-ABV 5 68,192 PROT-ABVI 6 81,593 Structure of protein: NH 2 -MKVD-(AB domain-VD) n TGRRFTTS-COOH n=number of repeats of AB domain Protein PROT-ABIV is the subject of the present invention; PROT-ABI-III and PROT-ABV/VI are outside the scope of the claimed invention. Example 2 (1) Preparation of immobilizing carrier In a column, 0.3 g of a carrier (cyanogen bromide activated Separose 4B: Pharmacia Co.) was swelled with 1 mM HCl for 15 minutes. The swollen gel was then washed with 1 mM HCl (200 ml per 1 g of dried gel). It was further washed with a coupling buffer ( 0.1 M of NaHCO 3 (pH 8.3) - 0.5 M NaCl) (5 ml per 1 g of dried gel). Separately, a ligand protein (the repetitive proteins prepared in Example 1: one to six times repeated AB domains (i.e., PROT AB1-VI) were dissolved in a coupling buffer. Commercially available native protein A was also dissolved in the same buffer for control ligand solution. The solutions were each mixed with the above-mentioned gel at a proportion of 2 ml of ligand solution to 1 ml of gel, and the suspensions were shaken overnight at 4°C. After the ligand solution was eliminated, a blocking reagent (0.2 M glycine, pH 8.0) was then added thereto and the suspensions were kept at room temperature for 2 hours. The resulting gels were washed with the coupling buffer, 0.1 M acetate buffer (pH 4.0) - 0.5 M NaCl, and again with the coupling buffer in this order. Seven kinds of columns were prepared filled with the carrier immobilizing each repetitive proteins and protein A. Excellent binding efficiency to carriers was observed, and the binding efficiency of PROT-ABIV (the subject of the claimed invention) with its carrier was about 95%. (2) Isolation and purification of IgG Two ml of rabbit serum was diluted by adding 4 ml of buffer B [0.1 M sodium phosphate (Na-Pi), pH 7.0], and filtered. It was then applied on each of the columns prepared as described above. Each column was washed with 10 ml of buffer B. Bound IgG was eluted with buffer C (0.3 M KCl-HCl, pH 2.3). Eluate was collected at 1 ml each of fractions. The protein and IgG amounts were measured with absorbance at 280 nm (A 280 ) and by the ELISA method with absorbance at 492 nm (A 492 ) respectively. From these results, the amount of IgG which adsorbed to 1 ml of each immobilizing carrier and eluted out was calculated, thereby providing-the value as the binding capacity. The result of affinity chromatography for PROT-ABIV is shown in Fig. 4, which clearly indicates that IgG is eluted mainly in fractions Nos. 19 and 20. The results of measurement of the binding capacity of each repetitive protein and commercially available native protein A for IgG are shown in Fig. 5. As these results make clear, the binding capacity of the repetitive proteins for IgG increased in proportion with the increase in the number of repeat of the IgG-binding domain; for example, the binding capacity of PROT-ABIV was about 35% greater than that of protein A. Also, when 4 µg of the rabbit IgG which was purified with the PROT-ABIV-Sepharose column was applied to SDS polyacrylamide gel electrophoresis, the pattern shown in Fig. 6 was obtained. In this figure, lane 1 represents a commercially available molecular weight marker, lane 2 represents rabbit serum (8 µg), and lane 3 represents purified IgG (4 µg). The results shown in Fig. 6 clearly substantiate the fact that the object IgG was concentrated and purified to a very high degree. Example 3 The six types of proteins produced in Example 1 were subjected to SDS-polyacrylamide gel electrophoresis, and the gel was stained with coomassie brilliant blue R-250. The result is shown in Fig. 7. In this figure, lane M is for a molecular weight marker of BRL Co., lanes 1-6 are for PROT-ABI to VI, and lane 7 is for a mixture of PROT-ABI to VI. Each protein clearly showed that it had a specific molecular weight. Effects of the Invention The present invention is directed to an immunoglobulin-binding artificial protein with an excellent ability to purify IgG using genetic engineering techniques. It is possible to industrially produce the protein having exact molecular weight with ease, which exhibits an ability to separate and purify IgG in a more excellent manner than native protein A. Thus, using the repetitive protein of the present invention it is possible to efficiently separate and purify IgG, which plays an important role in all fields of biotechnology, and an efficient method is therefore provided for producing it in a highly pure form. Further, using the repetitive protein of the present invention, IgG may be selectively isolated and removed from a sample, and thus the repetitive protein may be used for exact and convenient bioassays of IgG in a sample. References to the microorganism deposited under Rule 13.2 1. Escherichia coli JM109/pTRP-PROT-AB4 (a) Name and Address of the depository with which said microorganism is deposited. Name: Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry Address: 1-3, Higashi 1 chome, Tsukuba-shi, Ibarakiken 305, Japan (b) Date deposited with the depository in (a): July 15, 1992 (c) Deposit Receipt No. (Accession No.) assigned to the deposit with the depository in (a): FERM BP-3930

Claims: An immunoglobulin-binding artificial protein obtainable by a method which comprises using the IgG-binding domains A and B of protein A as a unit and making repetitive links of the A and B domains, wherein the number of repetitions of the A and B domains is four. A method for isolating or removing IgG from a sample containing IgG, which comprises binding the immunoglobulin-binding artificial protein according to claim 1 to IgG and recovering or removing IgG.

Inventors: AIBA SACHIKO, KIHIRA YASUNORI, AIBA, SACHIKO, KIHIRA, YASUNORI, AIBA, SACHIKO, 1041, YASHIMA AZAI-CHO, KIHIRA, YASUNORI, 5-44, SUEHIRO-CHO
Applicants: ORIENTAL YEAST CO LTD, ORIENTAL YEAST CO., LTD.
Application year: 1992
Main classification: C12N15
IPC codes: C07K14, C12N15