Platelets and endothelial cells store the polymeric factor V(a) binding protein, multimerin 1 (MMRN1), for release upon agonist stimulation. In human megakaryocytes, factor V binding to MMRN1 follows plasma factor V endocytosis, resulting in stored complexes of MMRN1 and factor V in platelet α-granules. The C2 domain of the factor V light chain contains a MMRN1 binding site; however, the affinity and stoichiometry of factor V-MMRN1 binding have not been determined, direct comparisons of factor V and Va binding to MMRN1 have not been done, and potential homologous roles of C1 and C2 domain structures in MMRN1 binding have not been studied. To further explore the mechanism of factor V and Va binding to MMRN1, and the roles of B domain release and C1 domain residues in MMRN1 binding, we used surface plasmon resonance and solid-phase binding studies. Functional consequences of factor V-MMRN1 binding were tested in competitive binding assays with the soluble phospholipid 1,2-Dicaproyl-sn-glycero-3-phospho-L-serine (C6PS), and calibrated automated thrombinography (CAT). Factor V bound to MMRN1 with a higher affinity than factor Va (approximately 2 nM versus 12 nM), and a stoichiometry consistent with binding to MMRN1 trimers. The higher affinity of factor V for MMRN1 was mainly due to differences in rates of formation of a more stable, secondary complex with MMRN1. Factor V activation by thrombin dissociated bound factor V from MMRN1, consistent with the reduced affinity of factor Va for MMRN1. A panel of point mutated, B domain deleted factor V constructs were used to identify MMRN1 binding residues in the C1 domain of factor V and Va. On a three dimensional model of factor Va, these residues mapped to a large, predominantly contiguous region between the C1 and C2 domains, that overlapped residues critical for factor Va phospholipid binding and procoagulant function. Consistent with the lowered affinity of factor Va for MMRN1, C6PS significantly inhibited factor Va-MMRN1, but not factor V-MMRN1 binding (p<0.05). Overlap between the MMRN1 and phospholipid binding sites was verified by CAT assays, as MMRN1 caused dose-dependent, significant reductions in plasma thrombin generation in these assays, by increasing lag time (p<0.01), and reducing peak (p<0.01) and total thrombin generation (p<0.01). Taken together, these data indicate that the functional homologies between the C domains of factor V extend to their MMRN1 binding sites. Moreover, thrombin has modulating effects on factor V-MMRN1 binding that mimic its effects on factor VIII-von Willebrand factor binding. The affinity of factor V-MMRN1 binding could be important to promote the association of MMRN1 with factor V in platelets, until factor V release and activation for prothrombinase assembly.