The contribution of the factor Va C1 domain (fVa-C1) to assembly of the prothrombinase complex has not been previously investigated. The homologous fVa-C2 domain contains a binding site for phosphatidylserine (PS) that includes the indole moieties of Trp(2063)/Trp(2064) at the apex of spike-1. In order to investigate the structure and function of fVa-C1 a molecular model was constructed based on the structure of fVa-C2. The aromatic and hydrophobic side chains of Tyr (1956) /Leu (1957) in fVa-C1 are located at the predicted apex of spike-3. Exposed charged and hydrophobic residues in fVa-C1 were changed to alanine in clusters of 1-3 mutations per construct. The resultant 20 mutants were expressed in COS cells and screened for binding to immobilized PS and prothrombinase activity on phospholipid vesicles containing either 25% or 5% PS. Two mutants, (Y1956,L1957)A, and (R2023,R2027)A showed both decreased binding to immobilized PS and a selective decrease in prothrombinase activity on membranes containing 5% PS. The interaction of purified (Y1956,L1957)A with phospholipid vesicles was studied using fluorescence resonance energy transfer and prothrombinase assays. The affinity of (Y1956,L1957)A binding to 25% PS membranes was reduced 12-fold compared to rHFVa. Prothrombin activation in the presence of (Y1956,L1957)A was markedly impaired on phos-pholipid vesicles containing 10% or less PS. We conclude that solvent exposed hydrophobic and aromatic amino acids in both fVa-C1 and fVa-C2 contribute to the interaction of factor V with PS membranes.