Structure analysis of streptococcal protein G Fc binding domain

The gene fragment (191 bp) encoding protein G IgG Fc binding domain was isolated by PCR from group G streptococcus (CMCC32138), and a clone containing this gene fragment was found to give fine reactivity to human IgG when expressed in Escherichia coli. The complete nucleotide sequence of the gene fragment was determined. One base pair differs from previously reported protein G nucleotide sequences, and results in an amino acid change (Ala-Thr), but this variation makes no difference in binding to the IgG Fc part by ELISA. The secondary structure of the protein G IgG Fc binding domain has been estimated by circular dichroism and assigned by computer algorithm. It shows a typical alpha-helix region in this domain. By breaking this alpha-helix region with recombinant DNA techniques, a 44 peptide, which contained the N-terminal 27 amino acid residues of this domain, was expressed in E. coli and showed no reactivity to IgG. The hydropathicity of this domain was also analyzed and compared with that of protein A relevant domain. Some similarity was found. These results suggest that the binding mechanism of protein G to the IgG Fc part depends on hydrophobic action which comes from the alpha-helix in protein G molecule, just as protein A binding to IgG Fc part.