SpAB protein domain

In molecular biology, the domain B, refers to the immunoglobulin-binding domain found in the Staphylococcus aureus virulence factor protein A. Hence, it is abbreviated to SpA_B.

Function

SpA_B enables theStaphylococcus aureus bacteria to evade the host's immune system through the disruption of opsonization and phagocytosis. It does this though SpA_B binding to the Fc fragment of IgG.

The B domain of SpA consists of three a-helices which are retained upon interaction with the Fc fragment of IgG. Protein A contains five highly homologous immunoglobulin -binding domains in tandem, which share a common structure consisting of three helices in a closed left-handed twist. Protein A can exist in both secreted and membrane-bound forms, and has two distinct Ig-binding activities: each domain can bind Fc-gamma and Fab.
The native state of the B domain, deviates a lot since its inter-helical angles fluctuate. It appears to be relatively thermodynamically more stable than the E domain. The increased stability of the B domain may be due to heightened
mobility, and therefore entropy, in the native state and decreased mobility entropy in the more compact denatured state.