A PROTEIN G-RELATED CELL-SURFACE PROTEIN IN STREPTOCOCCUS-ZOOEPIDEMICUS

This work describes the cloning and sequencing of a gene encoding a pl asma protein receptor from Streptococcus zooepidemicus. This receptor, termed protein ZAG, is a 45-kDa protein that binds alpha(2)-macroglob ulin (alpha(2)M), serum albumin, and immunoglobulin G (IgG). The IgG-b inding activity is located in the C-terminal part of the molecule and is mediated by two repeated domains highly homologous to each other as well as to the corresponding domains in streptococcal type III Fc rec eptors. The IgG-binding profile of protein ZAG is similar to that prev iously reported for S. zooepidemicus. Binding to serum albumin is medi ated by a short amino acid sequence in the middle of the molecule. Thi s domain shows homology to previously described albumin-binding protei ns from streptococci, and the albumin-binding profile of protein ZAG i s similar to that of streptococcal protein G. The N-terminal part of p rotein ZAG, which mediates binding to the plasma proteinase inhibitor alpha(2)M, is composed of a unique stretch of amino acids. Protein ZAG competes for the same, or nearby, binding site(s) in alpha(2)M as do two recently described Streptococcus dysgalactiae receptors, although the sequences of the alpha(2)M-binding domains in these three receptor s show only minor sequence similarities.