Effects of protein S and protein S-C4BP on APC-mediated proteolysis of factor Va at R³⁰⁶ and R⁵⁰⁶. Both protein S and protein S-C4BP are cofactors for APC that stimulate APC-mediated proteolysis at R³⁰⁶ in factor Va 18-fold and 11-fold, respectively. While protein S has no effect on APC-mediated inactivation at R⁵⁰⁶ in factor Va, the protein S-C4BP complex specifically inhibits APC-mediated cleavage at R⁵⁰⁶ 3- to 4-fold.