Protein A is a 42
kDa surface
protein originally found in the cell wall of the
bacterium Staphylococcus aureus. It is encoded by the
spa gene and its regulation is controlled by DNA topology, cellular osmolarity, and a
two-component system called ArlS-ArlR. It has found use in biochemical research because of its ability to bind
immunoglobulins. It is composed of five homologous Ig-binding domains that fold into a three-helix bundle. Each domain is able to bind proteins from many mammalian species, most notably
IgGs. It binds the heavy chain within the
Fc region of most immunoglobulins and also within the
Fab region in the case of the human VH3 family. Through these interactions in serum, where IgG molecules are bound in the wrong orientation (in relation to normal
antibody function), the bacteria disrupts
opsonization and
phagocytosis.