A protein G-related cell surface protein in Streptococcus zooepidemicus

This work describes the cloning and sequencing of a gene encoding a plasma protein receptor from Streptococcus zooepidemicus. This receptor, termed protein ZAG, is a 45-kDa protein that binds alpha 2-macroglobulin (alpha 2M), serum albumin, and immunoglobulin G (IgG). The IgG-binding 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 receptors. The IgG-binding profile of protein ZAG is similar to that previously reported for S. zooepidemicus. Binding to serum albumin is mediated by a short amino acid sequence in the middle of the molecule. This domain shows homology to previously described albumin-binding proteins from streptococci, and the albumin-binding profile of protein ZAG is similar to that of streptococcal protein G. The N-terminal part of protein ZAG, which mediates binding to the plasma proteinase inhibitor alpha 2M, is composed of a unique stretch of amino acids. Protein ZAG competes for the same, or nearby, binding site(s) in alpha 2M as do two recently described Streptococcus dysgalactiae receptors, although the sequences of the alpha 2M-binding domains in these three receptors show only minor sequence similarities.