Protein A is a 42 kDa surface protein originally found in the cell wall of the bacterium Staphylococcus aureus. The protein is composed of five homologous Ig-binding domains that fold into a three-helix bundle. Each domain can bind proteins from many mammalian species, most notably IgGs (immunoglobulins). Protein A has the remarkable ability to bind to the Fc region of most immunoglobulins. 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.
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