Streptococcal protein G. Gene structure and protein binding properties.
Protein G was solubilized from 31 human group C and G streptococcal strains with the muralytic enzyme mutanolysin. As judged by the mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the binding patterns of the solubilized protein G molecules in Western blot experiments, the strains could be divided into three groups, represented by the group G streptococcal strains G148 and G43 and the group C streptococcal strain C40. The 65-kDa G148 protein G and the 58-kDa C40 protein G showed affinity for both immunoglobulin G (IgG) and human serum albumin (HSA), whereas the 40-kDa G43 protein G bound only IgG. Despite the different molecular patterns, the three protein G species had identical NH2-terminal amino acid sequences. Apart from the 65-kDa peptide, digestion of G148 streptococci with mutanolysin also produced a 52-kDa IgG- and HSA-binding peptide and a 14-kDa HSA-binding peptide. It was demonstrated that these peptides resulted from cleavage of 65-kDa protein G by proteolytic components in the mutanolysin preparation. The protein G genes of the C40 and G43 strains were cloned and sequenced, and their structure was compared to the previously published sequence of the G148 protein G gene. As compared to G148, both the C40 and G43 genes lacked a 210-base pair fragment in the IgG-binding region, accounting for the 10-fold lower affinity of these proteins for IgG. The G43 gene also lacked a 450-base pair fragment in the 5'-end of the gene, explaining why the G43 protein G did not bind HSA. The differences in protein G structure did not correlate with the clinical origin of the strains used in this study. The IgG-binding region of protein G was further mapped. Thus, a peptide corresponding to a single IgG-binding unit was obtained by the cloning and expression of a 303-base pair polymerase chain reaction-generated DNA fragment. The affinity of this 11.5-kDa peptide for human IgG was 8.0 x 10(7) M-1, as determined by Scatchard plots. Finally, a 55-amino acid-long synthetic peptide, corresponding to one of the three repeated domains in the COOH-terminal half of strain G148 protein G, effectively blocked binding of protein G to IgG.
Sjöbring
U
U
Department of Medical Microbiology, University of Lund, Sweden.
Björck
L
L
Kastern
W
W
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D001483:Base Sequence; D005798:Genes, Bacterial; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes; D010455:Peptides / Q000138:chemical synthesis; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D012709:Serum Albumin / Q000378:metabolism; D013291:Streptococcus / Q000235:genetics* / Q000254:growth & development
D001426:Bacterial Proteins; C034284:IgG Fc-binding protein, Streptococcus; D007074:Immunoglobulin G; D015345:Oligonucleotide Probes; D010455:Peptides; D012709:Serum Albumin