Abstract:
A series of protein A mutants having high alkali resistance, and methods of using the protein A mutants are provided. The protein A mutants have a high binding affinity for regions of immunoglobulin proteins other than the complementarity determining regions. The protein A mutants can be coupled to a solid support for immunoglobulin isolation, or conjugated to a label for immunoglobulin detection. This series of protein A mutants has high chemical stability under alkaline conditions of pH 13-14, and can also be used as chromatography ligands for purification procedures that use alkaline solutions under harsh conditions, such as Clean-In-Place (CIP). Also provided are methods of immunoglobulin separation and purification, and alkali regeneration of affinity chromatography medium that uses protein A as a ligand.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Section 371 of International Application No. PCT/CN2013/076445, filed May 30, 2013, which was published in the Chinese language on Sep. 25, 2014 under International Publication No. WO 2014/146350 A1, and the disclosure of which is herein incorporated by reference. 
     
    
     REFERENCE TO SEQUENCE LISTING SUBMITTED ELECTRONICALLY 
       [0002]    This application contains a sequence listing, which is submitted electronically via EFS-Web as an ASCII formatted sequence listing with a file name “688096-79US Sequence Listing”, creation date of Nov. 9, 2015, and having a size of 17 kb. The sequence listing submitted via EFS-Web is part of the specification and is herein incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0003]    The invention belongs to the field of protein engineering, relates to a series of Staphylococcal protein A mutants with high alkali resistance and their application. 
       BACKGROUND OF THE INVENTION 
       [0004]    Biotechnology is one of the fastest growing high-tech fields in the world. As one of the hot spots, antibody drug development continues to improve the health and the life quality of many patients and has achieved remarkable market performance in recent years. Although investments in research and development of new drugs have been increased continuously, the number of innovative drugs that reach the market has dramatically decreased, and many small molecule drugs face the threat of the patent cliff. Therefore, in order to seek new growing points, many pharmaceutical companies are entering the field of antibody drug development. Currently, antibody drugs are widely used in basic biomedical research, diagnosis and treatment of diseases such as cancers, organ transplant rejection and autoimmune diseases. 
         [0005]    In recent years, as a large number of therapeutic antibody drugs have been invented and used in the medical field, the production process draws a lot of attention from people. In general, large-scaled economic production of antibody drugs and diagnostic reagents are produced by cell culture at the intracellular level or secreted into the culture medium. The cell culturing process requires a culture medium supplemented with sugars, amino acids, and certain growth factors. 
         [0006]    Therefore antibody must be isolated from culture medium as well as other cellular components and purified to a certain level before it can be used as a human therapeutic agent. Currently the most widely used method for antibody purification is affinity chromatography which is simple, fast and highly selective. With those advantages, affinity chromatography significantly reduces the subsequent steps of purification to save time and cost with no sacrifice of purity. Cost control is very important in the modern industrial production processes. If chromatography medium could be used repeatedly, it would significantly reduce the production cost of an antibody. However, previously used chromatography medium may retain un-eluted proteins, protein aggregates, or residual substances that could be harmful, such as viruses or endotoxins. So it is very critical that the previously used media must be cleaned before reusing. The most effective way to clean chromatography medium is Clean-In-Place (CIP) using alkaline solutions. The method involves a treatment with a buffer or solution containing 0.5M Sodium Hydroxide (NaOH). Using this rigorous clean method, impurities can be effectively removed from the medium, however, it may also damage the chromatography medium itself. Therefore, Protein A molecules with high alkali resistance which bind immunoglobulin as described in the present invention can be used as effective ligands for the purification of antibodies. Importantly, the chromatography medium can be treated by CIP with alkali solution and regenerated for repetitive use. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    This invention aims to address the disadvantages of existing technology and provide a specific series of mutations to the Protein A molecule which enable it high alkali resistance. 
         [0008]    Another objective of the present invention is to provide applications of this protein A molecule. 
         [0009]    The objective of this invention can be achieved by the following technical solutions: 
         [0010]    A series of Protein A mutants of which the amino acid sequences are defined by SEQ ID NO:1 or SEQ ID NO:2, or defined by the amino acid sequences within immunoglobulin binding region that have more than 99% homology to the sequences defined by SEQ ID NO:1 or SEQ ID NO:2. Here immunoglobulin binding region is defined from the 7th amino acid residue to the 54th. of SEQ ID NO: 1 or SEQ ID NO:2. 
         [0011]    Nucleic Acids Encoding the Protein A. 
         [0012]    The present invention relates to protein A which is essentially a series of immunoglobulin-binding molecules which can tightly bind with immunoglobulin at regions other than the complementarity determining region, and at the meantime can withstand an alkali condition of pH13-14. Such an immunoglobulin binding protein A is composed of three a helical regions, which fold into a three-helix bundle structure. Protein A binds to the Fc part of an immunoglobulin through the residues on the helix 1 and 2 surface. Also Protein A has lower affinity toward the Fab part of an immunoglobulin, through the residues on the surface of helix 2 and 3. The residues in the center of the three-helix bundle form an hydrophobic core, of which the hydrophobic interactions could enable the high thermal stability of protein A. 
         [0013]    Different from other types of non-alkali resistant Protein A, when the alkali resistant protein A species are placed in an alkaline environment of pH13-14, even though the hydrogen bonds which help to maintain the protein tertiary structure disappeared and all three α helixes would be stretched, the strong hydrophobic interactions among three helixes would still enforce α helix 1,2 3 to stay closely to each other and to ensure their relative positions do not change significantly. Once the pH are brought back to a neutral environment, such as pH7-8, hydrogen bonds among α helix 1,2,3 would be recovered, then the overall conformation or tertiary structure of the tri-helix bundle could be restored to the one before alkali treatment. While other non-alkali resistant proteins are not able to restore their tertiary structures due to the lacking of this kind of hydrophobic core to maintain their structure. After being placed in an alkaline environment, like pH13-14, the structure of non-alkali resistant proteins would be completely destroyed, so when they are put back into a neutral environment like pH 7-8, the overall conformation or tertiary structure of the protein could not be restored to the one before alkali treatment. With this alkali resistance characteristics, monomeric Protein A or multimeric protein A coupled to a solid phase support which can be used for the isolation and purification purpose of an immunoglobulin molecule, or they can be also used for detection of immunoglobulin molecules by conjugating a marker or label. If these alkali resistant protein A are used as ligands for chromatography medium, the chromatography medium can be cleaned and regenerated by Clean-In-Place (CIP) with alkaline solutions. 
         [0014]    As described in the invention, the molecular weight of the protein A monomer is around 6KD, composed of 58 amino acids, which can be chemically synthesized or recombinantly expressed. If recombinantly expressed, cell strains must be constructed firstly to carry a gene that encodes the amino acid sequence of the protein. Therefore, the following steps of procedure are required: first, obtain the protein A gene encoding the amino acid sequence by gene synthesis; second, insert the protein A gene into an expression vector. Here tags for purification or labels for identification can be inserted into the expression vectors as needed. And then transform the vector into a suitable cell line for further expression. By doing this, protein A (including monomeric or polymeric) which have immunoglobulin molecule binding affinity could be obtained from culture medium or cells. It should be noticed that the expression of protein A needs to be tightly controlled. So selecting vectors for the expression of protein A is very important, and those vectors should have the following features: 1) an expression vector should have a promoter or transcription initiation site, 2) an expression vector should have the gene operons for expression switching, 3) an expression vector should have a ribosome binding site, and 4) an expression vector should have transcription and translation termination sites, all of which can improve stability of the transcription and translation products. The expression vectors recommended for protein A expression are pET (Novagen), pQE30 (Qiagen), PGS21a (genscript), pGAPZα A (Invitrogen), etc., in corresponding, the host cells which could be used for protein A expression are genetically engineered  Escherichia coli  ( E. coli ) or  Pichia pastoris  yeast strains. The host cells can express protein A on membranes or extracellularly. 
         [0015]    According to different needs, those Protein A and/or their mutants could be isolated and purified using their physical and/or chemical properties. Usually those isolation and purification methods are protein precipitation (salting out), centrifugation, osmotic pressure shock, sonication, ultrafiltration, gel filtration, adsorption chromatography, ion exchange chromatography, affinity chromatography, high performance liquid chromatography or other liquid chromatography, dialysis etc., and the combination of these methods. Furthermore, target proteins fused with an affinity tag can be isolated and purified by affinity purification. Affinity tags described herein are, e.g., poly-histidine tag and FLAG tag, which could be used for purification of protein A. 
         [0016]    The binding affinity of protein A toward immunoglobulin can be measured by ELISA assay. For example, Protein A was immobilized on a solid phase carrier (here for example: a plate), free Protein A which was not immobilized on the carrier was washed out. And then an enzyme-labeled immunoglobulin was added and incubated for a certain time to allow efficient Protein A binding. By washing the solid phase carrier, free immunoglobulin which did not bind with immobilized protein A and other substances in liquid phase were removed from immunoglobulin-Protein A complexes which were immobilized on the solid phase carrier. Usually, the ratio of the amount of the enzyme-labeled immunoglobulin to the amount of immobilized Protein A in such a complex is 1:1. After adding chromogenic substrate, the substrate was catalyzed by the enzyme which bound to the immunoglobulin into colored product, and then the amount of immunoglobulin which bound to Protein A could be qualified and quantified by analyzing depth of coloration. 
         [0017]    The alkali resistance and the chemical stability of these proteins can be easily confirmed by a person with technical expertise in this field. For example, proteins having been soaked in 0.5M NaOH for at least 60 hours are measured for immunoglobulin binding activity according to the ELISA assay described above. If the proteins having been treated by alkaline solutions can still maintain good binding activity, then these proteins are stable in an alkali environment. Another example is using an amino group or carboxyl group of the proteins to couple the proteins onto a solid phase carrier (here for example: Sepharose 4B) by diepoxide, epichlorohydrin, cyanogen bromide, N-hydroxysuccinimide and other coupling agents, which is packed into a chromatography column. The following steps (1 to 4) are performed as a cycle: 1) The column was loaded with excessive immunoglobulin molecules and then 2) washed with phosphate buffer at pH 8.0 thoroughly. 3) Immunoglobulin was eluted with glycine buffer at pH 3.0. 4) The elution was analyzed to determine the amount of total immunoglobulin that bound onto the column as binding capacity. The binding capacity was recorded after each cycle. In between each cycle, one step of Clean-In-Place (CIP) was done using alkali solution that was consisted of 0.5M NaOH and the column was regenerated and equilibrated with proper buffers. After 100 cycles, if the total binding capacity is not reduced, which means such proteins are very stable in the alkali environment. The proteins are very suitable for affinity purification process of immunoglobulins as affinity ligands. 
         [0018]    Protein a Multimers which Contains Two or More Repetitive Units 
         [0019]    The proteins discussed above are monomer. However, these proteins can be linked to form multimeric proteins, such as dimer, trimer, tetramer and others alike. Therefore, monomeric Protein A proteins described above are belong to the present invention and multimeric Protein A proteins which are formed by one kind protein of itself or the combinations of all other kinds Protein A proteins described above are also belong to the present invention. The multimeric protein A of the invention contain a linker unit which is composed of 4 to 10 optimized amino acids. This linker unit does not change the conformation or tertiary structure of Protein A, which makes multimeric Protein A sufficiently stable in the same alkaline environment as monomeric Protein A does. So the linker will not jeopardize the alkali resistance characteristics of the Protein A. In the present implementation of application, polymer is the dimer of Protein A, which is connected by a linker containing 4 amino acids (ADGK; SEQ ID NO: 56). 
         [0020]    The present invention includes nucleic acid sequences encoding the above proteins and the sequences of the above Protein A multimers. The nucleic acid sequences were codon-optimized to avoid rare codons and the formation of secondary structures of mRNAs, the genes were synthesized using overlapping primers. 
         [0021]    This invention includes such Protein A or Protein A multimer derivatives of which the N-terminal, C-terminal or side chain groups that have been chemically modified, such as acylation at the N-terminal amino group or esterification at the C-terminal carboxyl group, and those derivatives can tightly bind immunoglobulin. Modifications on a protein will play important roll on regulating protein&#39;s pI, stability, solubility, reactivity and biological activity. If providing a cysteine residue to the C-terminal of Protein A, then the protein could be coupled to a solid phase carrier with a thioether bond through this provided cysteine. This coupling method is easy to be applied with standard techniques and equipments. 
         [0022]    Applications using Protein A, Protein A multimers or their derivatives as described above in isolation, purification or detection procedures of immunoglobulin. 
         [0023]    Applications using genes of protein A, protein A multimers or their derivatives as described above in isolation, purification or detection procedures of immunoglobulin. 
         [0024]    Commercial applications using the proteins described above include purification and detection of immunoglobulin. Applications for the immunoglobulin purification include affinity chromatographic separation methods, wherein at least one of the target molecules is separated from the matrix by binding to protein A or protein A multimers as described above. Specific steps involved are: 1) sample solution containing immunoglobulin is loaded through the protein A chromatography medium. In this step, components in the sample solution other than immunoglobulin will flow through unhindered, but immunoglobulin will be adsorbed onto the chromatographic medium. 2) Wash the medium with proper buffer (e.g., phosphate buffer) to remove non-specifically bound residual impurities. As discussed above, an alkaline solution could be used here if necessary. 3) Elute immunoglobulin from the medium with elution buffer. Usually immunoglobulin can be eluted by changing the elution pH, ionic strength of the buffer or by adding the competitive binding substances against protein A. The applications for detection of immunoglobulin include 1) Label or mark Protein A, Protein A multimers or their derivatives with an enzyme, chemiluminescent or isotope reagent, 2) Incubate labeled Protein A with the samples needed to be detected, 3) By analyzing protein A to detect immunoglobulin, which makes detection of immunoglobulin visualized. 
         [0025]    Beneficial Effects (Benefits) 
         [0026]    The present invention provides protein A, multimeric protein A and their derivatives which can be firmly bonded to regions of an immunoglobulin molecule other than the complementarity determining region, and also can be coupled to a solid phase carrier as ligands for isolation of immunoglobulin. This kind of protein A can maintain its chemical stability in an alkaline environment with pH 13-14, which enable the Protein A much more tolerant to the harsh condition that is used in Cleaning-In-Place (CIP) procedure. Therefore, the invention of protein A, its multimers and derivatives can be used in the purification and/or detection of immunoglobulin. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0028]    In the drawings: 
           [0029]      FIG. 1  shows expression of protein A in  Escherichia coli  detected by SDS-PAGE, wherein lane M is a Protein Marker (Genscript 94KD, 66 kD, 36KD, 25KD, 14KD), lanes 1 to 4 are expression of protein A in different  Escherichia coli  colonies; 
           [0030]      FIG. 2  shows expression of protein A dimer in  Escherichia coli  detected by SDS-PAGE, wherein lane M is a Protein Marker (Genscript 94KD, 66 kD, 36KD, 25KD, 14KD), lanes 1 to 4 are expression of the protein A dimer in different  Escherichia coli  colonies; 
           [0031]      FIG. 3  is an image of SDS-PAGE; Protein A was purified by Ni column, wherein lane 1 is Protein Marker (Genscript 94KD, 66 kD, 36KD, 25KD, 14KD), lane 2 is protein A elution; 
           [0032]      FIG. 4  is an image of SDS-PAGE; Protein A dimer was purified by Ni column, wherein lane 1 is Protein Marker (Genscript 94KD, 66 kD, 36KD, 25KD, 14KD), lane 2 is cell lysate supernatant, lane 3 is flow-through, lane 4 is protein A dimer eluted by the equilibration buffer, lane 5 is protein A dimer elution; 
           [0033]      FIG. 5  is an image of SDS-PAGE; Protein A as an affinity ligand to purify immunoglobulin from human serum, wherein lane 1 is Protein Marker (Genscript 94KD, 66 kD, 36KD, 25KD, 14KD), lane 2 is human serum, lane 3 is purified human immunoglobulin using protein A as ligand for affinity chromatography; 
           [0034]      FIG. 6  is an image of SDS-PAGE; Protein A dimer as an affinity ligand to purify immunoglobulin from human serum, where lane 1 is Protein Marker (Genscript Corporation) 220 KD, 150 KD, 100 KD, 75 KD, 50 KD, 35 kD, 25 kD, 15 KD), Lane 2 is human serum, lane 3 is proteins eluted by washing buffer (phosphate buffer), lane 4 is purified human immunoglobulin using protein A dimer as ligand for affinity chromatography; and 
           [0035]      FIG. 7  shows alkali resistance tests for Protein A dimer as an affinity ligand. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example 1 
     Construction of Vector Containing a Gene which is Encoding Protein a Fused with 6 Histidine Residues at its N-Terminus 
       [0036]    According to the codon preference of  E. coli  and to avoid secondary structure formation of mRNA, gene sequences of protein A fused with 6 histidine residues at its N-terminus were designed and optimized, which are shown here as SEQ ID NO:4 and SEQ ID NO:6 respectively; the corresponding amino acid sequences are shown as SEQ ID NO:3 and SEQ ID NO:5 respectively, of which the former is called protein A1 and the latter is called protein A2 in the following description. Using gene design software, multiple small gene fragments with certain length of overlapping sequences and similar annealing temperature were designed. All the small genes pieced together would cover the whole length of protein A1/A2 genes. The primer pairs were designed and synthesized according to those small gene sequences, of which the sequences were shown as here: Primers 4-1 to 4-8 (SEQ ID NOs: 16-23, respectively) are for the Protein A1 and Primers 6-1 to 6-8 (SEQ ID NOs: 24-31, respectively) are for the Protein A2. Two rounds of PCR reactions were done to synthesize the genes. For the 1st round PCR, all the primers 4-1 to 4-8 or primers 6-1 to 6-8 were mixed together as primers and templates, PBO polymerase (Genscript Corporation) was used for PCR reaction. All PCR reactions were done with 2720 thermal cycler (Applied Biosysytems) using the following method: the reaction cycles were 95° C. for 20 s, 55° C. for 20 s, 72° C. for 20 s, after a total of 25 cycles, one extension step was taken at 72° C. for 3 minutes, the PCR solutions were cooled down to 4° C. and stored as the 1st PCR products. These 1st PCR products were used as templates for the 2nd round. Respectively, forwarding primers (primer 4-1/primer 6-1) and reversing primers (primer 4-8/primer 6-8) were used in the 2nd PCR reaction to amplify whole genes of Protein A1/A2 with the same PCR method as described above. The PCR products from the 2nd round PCR reactions were loading to an agarose gel containing 1% ethidium bromide and purified by electrophoresis. The DNA bands were visualized under UV light and cut out from the gel. The amplified DNA fragments were purified using Quick Gel Extraction Kit (Genscript Corporation) and the protocol provided by the manufacturer. The purified DNA fragments were sequenced using ABI PRISM Bigdye Terminator Cycle Sequencing Ready Reaction Kit and 3730×196-capillary DNA analyzer from Applied Biosystems Inc. 
         [0000]    
       
         
               
             
               
             
               
               
             
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 PCR Primers for synthesizing Protein A1 and Protein A2 genes. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PCR Primers for synthesizing Protein A1 gene  
               
               
                 (SEQ ID NOs: 16-23) 
               
             
          
           
               
                 Primer 4-1 
                 CCATGGGCTCACATCATCATCATCATCACGGCTCGGGTGCGGACGGTAA 
               
               
                   
               
               
                 Primer 4-2 
                 ACGCATTCTGCTGTTCTTTTTCAAATTTACCGTCCGCACCC 
               
               
                   
               
               
                 Primer 4-3 
                 AGAACAGCAGAATGCGTTCTACGAAATTCTGCATCTGCCGA 
               
               
                   
               
               
                 Primer 4-4 
                 CATTACGCTGTTCTTCGGTCAGGTTCGGCAGATGCAGAATT 
               
               
                   
               
               
                 Primer 4-5 
                 CCGAAGAACAGCGTAATGCATTTATCCAGTCTCTGAAAGATGATCCGAGC 
               
               
                   
               
               
                 Primer 4-6 
                 ACCCAGCACGTTCGTAGACTGGCTCGGATCATCTTTCAGA 
               
               
                   
               
               
                 Primer 4-7 
                 TACGAACGTGCTGGGTGAAGCGAAAAAACTGAACGATGCG 
               
               
                   
               
               
                 Primer 4-8 
                 CATATGTCATTTCGGGGCCTGCGCATCGTTCAGTTTTTTC 
               
               
                   
               
             
          
           
               
                 PCR Primers for synthesizing Protein A2 gene  
               
               
                 (SEQ ID NO: 24-31) 
               
             
          
           
               
                 Primer 6-1 
                 CCATGGGCTCGCACCACCACCACCACCACGGCTCGGGCGCAGATGGCAAG 
               
               
                   
               
               
                 Primer 6-2 
                 ATGCGTTCTGTTGTTCTTTTTCAAACTTGCCATCTGCGCC 
               
               
                   
               
               
                 Primer 6-3 
                 AAAGAACAACAGAACGCATTCTACGAAATCCTGCATCTGCCGA 
               
               
                   
               
               
                 Primer 6-4 
                 TGCGTTACGCTGTTCTTCGGTCAGGTTCGGCAGATGCAGGA 
               
               
                   
               
               
                 Primer 6-5 
                 AGAACAGCGTAACGCATTCATCAAGTCTATCCGCGATGATCCG 
               
               
                   
               
               
                 Primer 6-6 
                 CCCAGCACGTTCGTAGACTGGCTCGGATCATCGCGGATAG 
               
               
                   
               
               
                 Primer 6-7 
                 CTACGAACGTGCTGGGCGAAGCGAAAAAACTGAATGATGC 
               
               
                   
               
               
                 Primer 6-8 
                 CATATGTCATTTCGGGGCCTGCGCATCATTCAGTTTTTTCGC 
               
               
                   
               
             
          
         
       
     
         [0037]    DNA fragments which the sequences had been verified were used as templates for protein A1/A2 subcloning. Primer 7 (SEQ ID NO:7) with primer 8 (SEQ ID NO:8) or primers 7 (SEQ ID NO:7) with primer 9 (SEQ ID NO:9) were used amplify cDNA fragments of the protein A1 or A2 respectively. The PCR products were purified by agarose gel electrophoresis. Using CloneEZ cloning kit (Genscript Corporation) and following the kit instruction, gene fragments encoding protein A1/A2 were subcloned into pET15b vectors. Vectors containing protein A1/A2 genes were verified by DNA sequencing and named by the following abbreviation: vector containing gene encoding protein A1 with the amino acid sequence shown as SEQ ID NO:3 was named PET15b-ProteinA1; vector containing gene encoding protein A2 with the amino acid sequence shown as SEQ ID NO:5 was named PET15b-ProteinA2. 
       Example 2 
     Construction of Vector Containing a Gene which is Encoding Protein a Dimer Fused with 6 Histidine Residues at its N-Terminus 
       [0038]    According to the codon preference of  E. coli  and to avoid secondary structure formation of mRNA, gene sequences of protein A dimer fused with 6 histidine residues at its N-terminus were designed and optimized, which are shown here as SEQ ID NO:11 and SEQ ID NO:13 respectively; the corresponding amino acid sequences are shown as SEQ ID NO:10 and SEQ ID NO:12 respectively, of which the former is called protein AA1 and the latter is called protein AA2 for the following description. Using gene design software, multiple small gene fragments with certain length of overlapping sequences and similar annealing temperature were designed. All the small genes pieced together would cover the whole length of protein AA1/AA2 genes. The primer pairs were designed and synthesized according to those small gene sequences, of which the sequences were shown as here: Primers 11-1 to 11-12 (SEQ ID NOs: 32-43) are for the Protein AA1 and Primers 13-1 to 13-12 (SEQ ID NOs: 44-55) are for the Protein AA2. Two rounds of PCR reactions were done to synthesize the genes. For the 1st round PCR, all the primers of 11-1 to 11-12 or primers 13-1 to 13-12 were mixed together as primers and templates, PBO polymerase (Genscript Corporation) was used for PCR reaction. All PCR reactions were done with 2720 thermal cycler (Applied Biosysytems) using the following method: the reaction cycles were 95° C. for 20 s, 55° C. for 20 s, 72° C. for 20 s, after a total of 25 cycles, one extension step was taken at 72° C. for 3 minutes, the PCR solutions was cooled down to 4° C. and stored as the 1st PCR products. These 1st PCR products were used as templates for the 2nd round. Respectively, forwarding primers (primer 11-1/primer 13-1) and reversing primers (primer 11-12/primer 13-12) were used in the 2nd PCR reaction to amplify whole genes of Protein AA1/AA2 with the same PCR method as described above. The PCR products from the 2nd round PCR reactions were loading to an agarose gel containing 1% ethidium bromide and purified by electrophoresis. The DNA bands were visualized under UV light and cut out from the gel. The amplified DNA fragments were purified using Quick Gel Extraction Kit (Genscript Corporation) and the protocol provided by the manufacture. The purified DNA fragments were sequenced using ABI PRISM Bigdye Terminator Cycle Sequencing Ready Reaction Kit and 3730×196-capillary DNA analyzer from Applied Biosystems Inc. 
         [0000]    
       
         
               
             
               
             
               
               
             
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 PCR Primers for synthesizing Protein AA1 dimer and Protein 
               
               
                 AA2 dimer 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PCR Primers for synthesizing Protein AA1 dimer gene 
               
               
                 (SEQ ID NOs: 32-43) 
               
             
          
           
               
                 Primer 11-1 
                 CCATGGGCTCACATCATCATCATCATCACGGCTCGGGTGCGGACGGTAA 
               
               
                   
               
               
                 Primer 11-2 
                 ACGCATTCTGCTGTTCTTTTTCAAATTTACCGTCCGCACCC 
               
               
                   
               
               
                 Primer 11-3 
                 AGAACAGCAGAATGCGTTCTACGAAATTCTGCATCTGCCGAACCTGACC 
               
               
                   
               
               
                 Primer 11-4 
                 TCAGAGACTGGATAAATGCATTACGCTGTTCTTCGGTCAGGTTCGGCAG 
               
               
                   
               
               
                 Primer 11-5 
                 TGCATTTATCCAGTCTCTGAAAGATGATCCGAGCCAGTCTACGAACGTGC 
               
               
                   
               
               
                 Primer 11-6 
                 CCTGCGCATCGTTCAGTTTTTTCGCTTCACCCAGCACGTTCGTAGACTGG 
               
               
                   
               
               
                 Primer 11-7 
                 TGAACGATGCGCAGGCCCCGAAAGCGGATGGCAAATTCGAAAAAG 
               
               
                   
               
               
                 Primer 11-8 
                 GCAGAATTTCATAGAAGGCGTTCTGCTGTTCTTTTTCGAATTTGCCATCC 
               
               
                   
               
               
                 Primer 11-9 
                 CGCCTTCTATGAAATTCTGCACCTGCCGAATCTGACGGAAGAACAGCGCA 
               
               
                   
               
               
                 Primer 11-10 
                 GCTCGGATCGTCTTTCAGGCTCTGGATGAACGCATTGCGCTGTTCTTCCG 
               
               
                   
               
               
                 Primer 11-11 
                 TGAAAGACGATCCGAGCCAGTCCACGAATGTTCTGGGCGAAGCGAAAAA 
               
               
                   
               
               
                 Primer 11-12 
                 CATATGTCATTTCGGTGCTTGTGCGTCATTCAGTTTTTTCGCTTCGCCCA 
               
               
                   
               
             
          
           
               
                 PCR Primers for synthesizing Protein AA 2 dimer gene 
               
               
                 (SEQ ID NOs: 44-55) 
               
             
          
           
               
                 Primer 13-1 
                 CCATGGGCTCGCACCACCACCACCACCACGGCTCGGGCGCAGATGGCAAG 
               
               
                   
               
               
                 Primer 13-2 
                 TCGTAGAATGCGTTCTGTTGTTCTTTTTCAAACTTGCCATCTGCGCC 
               
               
                   
               
               
                 Primer 13-3 
                 ACAGAACGCATTCTACGAAATCCTGCATCTGCCGAACCTGACCGA 
               
               
                   
               
               
                 Primer 13-4 
                 CGCGGATAGACTTGATGAATGCGTTACGCTGTTCTTCGGTCAGGTTCGGC 
               
               
                   
               
               
                 Primer 13-5 
                 TCATCAAGTCTATCCGCGATGATCCGAGCCAGTCTACGAACGTGCTGGG 
               
               
                   
               
               
                 Primer 13-6 
                 GGCCTGCGCATCATTCAGTTTTTTCGCTTCGCCCAGCACGTTCGTAG 
               
               
                   
               
               
                 Primer 13-7 
                 AATGATGCGCAGGCCCCGAAAGCGGATGGTAAATTTGAAAAAGAACAGCA 
               
               
                   
               
               
                 Primer 13-8 
                 AGGTGCAGAATTTCATAGAAGGCGTTCTGCTGTTCTTTTTCAAATTTACC 
               
               
                   
               
               
                 Primer 13-9 
                 CTTCTATGAAATTCTGCACCTGCCGAATCTGACGGAAGAACAGCGTAATG 
               
               
                   
               
               
                 Primer 13-10 
                 GCTCGGATCGTCACGAATGCTTTTAATGAACGCATTACGCTGTTCTTCCG 
               
               
                   
               
               
                 Primer 13-11 
                 CGTGACGATCCGAGCCAGAGCACGAATGTCCTGGGCGAAGCCAAAAA 
               
               
                   
               
               
                 Primer 13-12 
                 CATATGTCATTTCGGTGCTTGTGCGTCGTTCAGTTTTTTGGCTTCGCCCA 
               
               
                   
               
             
          
         
       
     
         [0039]    DNA fragments which the sequences had been verified were used as templates for protein AA1/AA2 subcloning. Primer 7 (SEQ ID NO:7) with primer 14 (SEQ ID NO:14) or primers 7 (SEQ ID NO:7) with primer 15 (SEQ ID NO:15) were used amplify cDNA fragments of the protein AA1 or AA2 respectively. The PCR products were purified by agarose gel electrophoresis. Using CloneEZ cloning kit (Genscript Corporation) and following the kit instruction, gene fragments encoding protein AA1/AA2 were subcloned into pET15b vectors. Vectors containing protein AA1/AA2 genes were verified by DNA sequencing and named by the following abbreviation: vector containing gene encoding protein AA1 with the amino acid sequence shown as SEQ ID NO:10 was named PET15b-ProteinAA1; vector containing gene encoding protein AA2 with the amino acid sequence shown as SEQ ID NO:12 was named PET 15b-ProteinA2. 
       Example 3 
     Expression of Protein a with N-Terminal Fused Six Histidine Residues 
       [0040]    The plasmid PET15b-ProteinA1 was transformed into competent cells of  Escherichia coli  strain BL21.  Escherichia coli  BL21 containing the plasmid PET15b-ProteinA1 was inoculated into the culture broth (1 g/L peptone, 5 g/L yeast extract, 5 g/L NaCl, and 100 mg/L Ampicillin) and cultured at 37° C. When cells reached the logarithmic growth curve, 0.5 mM IPTG was added into the broth to induce protein expression for 4 hours, cell pellets were collected by centrifugation. A small amount of the cell was heated at high temperature (95° C.) and the whole cell lysate was loaded onto 4˜20% gradient SDS-PAGE gel for analysis. As shown in  FIG. 1 , a clear band was detected around 7˜8 KD which was protein A. 
       Example 4 
     Expression of Protein a Dimer with N-Terminal Fused Six Histidine Residues 
       [0041]    The plasmid PET15b-ProteinAA1 was transformed into competent cells of  Escherichia coli  BL21.  Escherichia coli  BL21 containing the plasmid PET15b-Protein AA1 was inoculated into the culture broth (1 g/L peptone, 5 g/L yeast extract, 5 g/L NaCl, and 100 mg/L Ampicillin) and cultured at 37° C., when cells reached their logarithmic growth curve 0.5 mM IPTG was added to induce protein expression for 4 hours. Cell pellets were collected by centrifugation. A small amount of bacteria was heated at high temperature (95° C.) and the whole cell lysate was loaded onto 4˜20% gradient SDS-PAGE gel for analysis. As shown in  FIG. 2 , a clear band was detected around 14˜15 KD which was protein A dimer. 
       Example 5 
     Purification of Protein a with the N-Terminal Fused Six Histidine Residues 
       [0042]    The constant flow pump was rinsed with distilled water, and then the empty glass chromatographic column. About 200 ml Ni-IDA resin (Genscript Corporate) was packed into the column. Using the constant flow pump, the column was equilibrated with equilibration buffer (20 mM Tris 300 mM NaCl) for about 3 L (20 CV) at the flow rate of 5 ml/min. 10 g cell pellets expressing protein A1 were suspended with 200 ml equilibration buffer (20 mMTris 300 NaCl) and sonicated (Ningbo Xinzhi bio technology limited company JY98 IIIDH). Cell lysate was centrifuged and the supernatant was loaded onto the Ni-IDA column at the flow rate of 2 ml/min. Then the column was excessively washed with equilibration buffer at the flow rate of 5 ml/min to remove unbounded proteins and contaminants until the UV being stable. Then the target protein was eluted with elution buffer (20 mM Tris, 300 mM NaCl, 250 mM Iminazole) at the flow rate of 5 ml/min and collected. The eluted protein was loaded onto 4˜20% gradient SDS-PAGE for analysis. As shown in  FIG. 3 , the purity of protein A after Ni-IDA column was above 90%. 
       Example 6 
     Purification of Protein a Dimer with the N-Terminal Fused Six Histidine Residues 
       [0043]    The constant flow pump was rinsed with distilled water, and then the empty glass chromatographic column. About 200 ml Ni-IDA resin (Genscript Corporate) was packed into the column. Using the constant flow pump, the column was equilibrated with equilibration buffer (20 mM Tris 300 mM NaCl) for about 3 L (20 CV) at the flow rate of 5 ml/min. 10 g cell pellets expressing protein A1 were suspended with 200 ml equilibration buffer (20 mMTris 300 NaCl) and sonicated (Ningbo Xinzhi bio technology limited company JY98 IIIDH). Cell lysate was centrifuged and the supernatant was loaded onto the Ni-IDA column at the flow rate of 2 ml/min. Then the column was excessively washed with equilibration buffer at the flow rate of 5 ml/min to remove unbounded proteins and contaminants until the UV being stable. Then the target protein was eluted with elution buffer (20 mM Tris, 300 mM NaCl, 250 mM Imidazole) at the flow rate of 5 ml/min and collected. The eluted protein was loaded onto 4˜20% gradient SDS-PAGE for analysis. As shown in  FIG. 4 , the purity of protein A after Ni-IDA column was above 90%. 
       Example 7 
     Protein a or Protein a Dimer with the N-Terminal Fused Six Histidine Residues Used as an Ligand for Affinity Chromatographic Medium to Purify Immunoglobulin 
       [0044]    Using the amino group, protein A or protein A dimer with the N-terminal fused six histidine residues were coupled onto the epoxy-based surface of agarose medium to make affinity chromatographic resin. Here 10 mg dimeric protein A or protein A was coupled to an epoxy-based surface of 1 ml Sepharose 4B (GE Healthcare) agarose beads to make Protein A affinity resin, of which 0.5 ml resin was used for immunoglobulin purification test. Firstly, the constant flow pump was cleaned with 20 ml double distilled water, then the empty chromatographic column was cleaned. 0.5 ml resin was packed into the column. Using constant flow pump the column was equilibrated with 10 ml phosphate buffer (containing 0.15M NaCl, 30 mM Na2HPO4, 10 mM NaH2PO4 pH 7) at the flow rate of 1 ml/min. 
         [0045]    15 ml of human serum at the concentration of 5 mg/ml was used as testing sample and loaded to the packed column at the flow rate of 0.5-1 ml/min to the saturate the binding of immunoglobulin to the protein A ligands, and then washed the column with 20 ml (40 CV) phosphate buffer (containing 0.15M of NaCl, 30 mM Na2HPO4, 10 mMNaH2PO4 pH 7.0) to remove unbound proteins and contaminants. Human immunoglobulin was finally eluted with 0.1M pH 3.0 glycine buffer and collected with UV detection. 20 uL of the eluted fraction was loaded onto 4-20% gradient SDS-PAGE gel for analysis. As shown in  FIGS. 5 and 6 , protein A or protein A dimer can isolate immunoglobulin with high purity from the serum. 
       Example 8 
     Alkali Resistance Tests of Protein a or Protein a Dimer with the N-Terminal Fused Six Histidine Residues as Ligands of Affinity Chromatographic Medium for Purifying Immunoglobulin 
       [0046]    0.5 ml of Sepharose beads conjugated protein A dimer with six N-terminal histidine residues was used for Clean-In-Place (CIP) test using alkaline solutions. First, the procedure of Example 7 was performed, and the total amount of eluted immunoglobulin was calculated as column capacity. After elution, CIP was performed using 15 ml 0.5M NaOH solution at a flow rate of 1 ml/min to clean the resin thoroughly. And then followed with 10 ml of phosphate buffer (containing 0.15M of NaCl, 30 mMNa2HPO4, 10 mM NaH2PO4 adjusted to pH 7.0) at the same flow rate to wash out NaOH and re-equilibrate the column. Another procedure of Example 7 was performed to determine the binding capacity of the column, which could be used as binding ability of protein A ligand to immunoglobulin. As shown in  FIG. 7 , after 100 cycles of CIP with alkaline solution, the protein A dimer used as an affinity chromatographic ligand still remained good binding ability to immunoglobulin. 
         [0047]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.