Factor Va (FVa) is a cofactor for the serine protease factor Xa that activates prothrombin to thrombin in the presence of Ca2+ and a membrane surface. FVa is a heterodimer composed of one heavy chain (A1 and A2 domains) and one light chain (A3, C1, and C2 domains). We use fluorescence, circular dichroism, and equilibrium dialysis to demonstrate that (1) the FV C2 domain expressed in Sf9 cells binds one molecule of C6PS with a kd of ∼2 μM, (2) stabilizing changes occur in the FV C2 domain upon C6PS binding, (3) the C6PS binding site in the FV C2 domain is located near residue Cys2113, which reacts with DTNB, and (4) binding to a PS-containing membrane is an order of magnitude tighter than that to soluble C6PS. Coupled with a recently published crystal structure of the C2 domain, these results support a model for the mechanism of C2-membrane interaction.
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