This entry represents immunoglobulin and albumin-binding (GA module) domains from various bacterial proteins, which share a common fold consisting of a left-handed three-helical bundle (mirror topology to spectrin-like fold).
The Staphylococcus aureus virulence factor protein A (SpA) contains five highly homologous Ig-binding domains in tandem (designated domains E, D, A, B and C) that share this common structure. Protein A can exist in both secreted and membrane-bound forms, and has two distinct Ig-binding activities: each domain can bind Fc-gamma (the constant region of IgG involved in effector functions) and Fab (the Ig fragment responsible for antigen recognition) .
Protein G-related albumin-binding (GA) modules occur on the surface of numerous Gram-positive bacterial pathogens. Protein G of group C and G Streptococci interacts with the constant region of IgG and with human serum albumin. The GA module is found in a range of bacterial cell surface proteins [15269208, 9086265]. GA modules may promote bacterial growth and virulence in mammalian hosts by scavenging albumin-bound nutrients and camouflaging the bacteria. Variations in sequence give rise to differences in structure and function between GA modules in different proteins, which could alter pathogenesis and host specificity due to their varied affinities for different species of albumin . Proteins containing a GA module include PAB from Peptostreptococcus magnus .