BINDING OF NATIVE ALPHA(2)-MACROGLOBULIN TO HUMAN GROUP-G STREPTOCOCCI

Binding of human alpha(2)-macroglobulin (alpha(2)M) tb group G strepto cocci and to their immunoglobulin G (IgG)-binding proteins (protein G) was investigated. Native alpha(2)M bound specifically to strain G-148 with an apparent dissociation constant of (2.2 +/- 1.5) x 10(-9) M. P roteinase complexed alpha(2)M did not compete for the binding sites, a nd I-125 labelled proteinase-complexed alpha(2)M did not bind to the b acteria. Binding of native alpha(2)M to the cells was not affected by IgG or protein G consisting of only IgG-binding domains. I-125-labelle d recombinant protein G did not bind to native or proteinase-complexed alpha(2)M. However, a lysate of G-148 cells inhibited binding of alph a(2)M to the bacteria, and immobilized wild-type protein G bound alpha (2)M directly from fresh human plasma. In 13 group G streptococcal iso lates, IgG-binding proteins were immunologically identified as protein G. In 11 isolates, these molecules reacted also with alpha(2)M and hu man serum albumin (HSA). Western blots (immunoblots) of two wild-type protein G variants revealed identical bands reactive with goat IgG, HS A, and native alpha(2)M. Digestion of wild-type protein G with clostri pain destroyed in both variants the binding sites for alpha(2)M but no t for albumin and IgG. N-terminal fragments of protein G (lacking the IgG-binding region) bound both alpha(2)M and HSA, whereas a similar HS A-binding peptide lacking the first 80 amino acids did not react with alpha(2)M. Our findings are consistent with a specific binding site fo r native alpha(2)M in the N-terminal region of protein G and suggest t hat binding of alpha(2)M via IgG-binding proteins may be a general fea ture of human group G streptococci.