Binding of native α2-macroglobulin to human group G streptococci

Abstract

Binding of human α2-macroglobulin (α2M) to group G streptococci and to their immunoglobulin G (IgG)binding proteins (protein G) was investigated. Native α2M bound specifically to strain G-148 with an apparent dissociation constant of (2.2 ± 1.5) x 10-9 M. Proteinase-complexed α2M did not compete for the binding sites, and 125I-labelled proteinase-complexed α2M did not bind to the bacteria. Binding of native α2M to the cells was not affected by IgG or protein G consisting of only IgG-binding domains. 125I-labelled recombinant protein G did not bind to native or proteinase- complexed α2M. However, a lysate of G-148 cells inhibited binding of α2M to the bacteria, and immobilized wild-type protein G bound α2M directly from fresh human plasma. In 13 group G streptococcal isolates, IgG-binding proteins were immunologically identified as protein G. In 11 isolates, these molecules reacted also with α2M and human serum albumin (HSA). Western blots (immunoblots) of two wild-type protein G variants revealed identical bands reactive with goat IgG, HSA, and native α2M. Digestion of wild-type protein G with clostripain destroyed in both variants the binding sites for α2M but not for albumin and IgG. N-terminal fragments of protein G (lacking the IgG-binding region) bound both α2M and HSA, whereas a similar HSA- binding peptide lacking the first 80 amino acids did not react with α2M. Our findings are consistent with a specific binding site for native α2M in the N-terminal region of protein G and suggest that binding of α2M via IgG- binding proteins may be a general feature of human group G streptococci.