Multimerin 1 is a massive, soluble, disulfide-linked homopolymeric protein that is expressed in megakaryocytes, platelets and endothelial cells. Normally, multimerin 1 undergoes efficient sorting to secretion granules, and it is not detectable in plasma. Recently, multimerin 1 was designated as a member of the EMILIN protein family, a group of structurally similar, disulfide-linked multimeric proteins. Multimerin 1 has the structural features of an adhesive protein and it supports the adhesion of many different cell types in vitro, including activated platelets, neutrophils, and endothelial cells. Multimerin 1 also has the ability to self associate and form large, branching matrix fibers. In platelet alpha-granules, multimerin 1 functions as the binding protein for coagulation factor V, a key regulator of coagulation. This review summarizes the current knowledge on multimerin 1 including its orthologous genes, restricted pattern of expression, structure, biosynthesis and functions.

Multimerin 1 (MMRN1) is a large, soluble, polymeric, factor V binding protein and member of the EMILIN protein family. In vivo, MMRN1 is found in platelets, megakaryocytes, endothelium and extracellular matrix fibers, but not in plasma. To address the mechanism of MMRN1 binding to activated platelets and endothelial cells, we investigated the identity of the major MMRN1 receptors on these cells using wild-type and RGE-forms of recombinant MMRN1. Ligand capture, cell adhesion, ELISA and flow cytometry analyses of platelet-MMRN1 binding, indicated that MMRN1 binds to integrins alphaIIbbeta3 and alphavbeta3. Endothelial cell binding to MMRN1 was predominantly mediated by alphavbeta3 and did not require the MMRN1 RGD site or cellular activation. Like many other alphavbeta3 ligands, MMRN1 had the ability to support adhesion of additional cell types, including stimulated neutrophils. Expression studies, using a cell line capable of endothelial-like MMRN1 processing, indicated that MMRN1 adhesion to cellular receptors enhanced its extracellular matrix fiber assembly. These studies implicate integrin-mediated binding in MMRN1 attachment to cells and indicate that MMRN1 is a ligand for alphaIIbbeta3 and alphavbeta3.

Multimerin 1 (MMRN1) is a polymeric, factor V (FV) binding protein that is stored in platelet and endothelial cell secretion granules but is undetectable in normal plasma. In human platelet alpha-granules, FV is stored complexed to MMRN1, predominantly by noncovalent binding interactions. The FV binding site for MMRN1 is located in the light chain, where it overlaps the C1 and C2 domain membrane binding sites essential for activated FV (FVa) procoagulant function. Surface plasmon resonance (SPR), circular dichroism (CD) and thrombin generation assays were used to study the binding of FV and FVa to MMRN1, and the functional consequences. FV and FVa bound MMRN1 with high affinities (K(D): 2 and 7 nM, respectively). FV dissociated more slowly from MMRN1 than FVa in SPR experiments, and CD analyses suggested greater conformational changes in mixtures of FV and MMRN1 than in mixtures of FVa and MMRN1. SPR analyses indicated that soluble phosphatidylserine (1,2-Dicaproylsn-glycero-3-phospho-L-serine) competitively inhibited both FV-MMRN1 and FVa-MMRN1 binding. Furthermore, exogenous MMRN1 delayed and reduced thrombin generation by plasma and platelets, and it reduced thrombin generation by preformed FVa. Exogenous MMRN1 also delayed FV activation, triggered by adding tissue factor to plasma, or by adding purified thrombin or factor Xa to purified FV. The high affinity binding of FV to MMRN1 may facilitate the costorage of the two proteins in platelet alpha-granules. As a consequence, MMRN1 release during platelet activation may limit platelet dependent thrombin generation in vivo.

In platelets, coagulation cofactor V is stored in complex with multimerin 1 in ␣-granules for activation-induced release during clot formation. The molecular nature of multimerin 1 factor V binding has not been determined, although multimerin 1 is known to interact with the factor V light chain. We investigated the region in factor V important for multimerin 1 binding using modified enzyme-linked immunoassays and recombinant factor V constructs. Factor V constructs lacking the C2 region or entire light chain had impaired and absent multimerin 1 binding, respectively, whereas the B domain deleted construct had modestly reduced binding. Analyses of point mutated constructs indicated that the multimerin 1 binding site in the C2 domain of factor V partially overlaps the phosphatidylserine binding site and that the factor V B domain enhances multimerin 1 binding. Multimerin 1 did not inhibit factor V phosphatidylserine binding, and it bound to phosphatidylserine independently of factor V. There was a reduction in factor V in complex with multimerin 1 after activation, and thrombin cleavage significantly reduced factor V binding to multimerin 1. In molar excess, multimerin 1 minimally reduced factor V procoagulant activity in prothrombinase assays and only if it was added before factor V activation. The dissociation of factor V-multimerin 1 complexes following factor V activation suggests a role for multimerin 1 in delivering and localizing factor V onto platelets prior to prothrombinase assembly.Activated coagulation factor V is a key non-enzymatic cofactor that is an essential component of the prothrombinase complex (1, 2). The active form of factor V, factor Va, is generated through consecutive cleavages by thrombin or factor Xa at residues Arg-709, Arg-1018, and Arg-1545, which produce factor Va heterodimers containing a heavy chain (A1 and A2 domains) and light chain (A3, C1, and C2 domains). The B domain of factor V, which facilitates thrombin cleavage and enhances factor V anticoagulant activity, is released on activation (3-7). The incorporation of factor Va into the prothrombinase complex provides binding sites for factor Xa, resulting in a 300,000-fold increase in the V max of prothrombin activation (8, 9).In blood, much of the procoagulant factor V is stored in platelets as a complex with the ␣-granule protein multimerin 1 (MMRN1) for activation-induced release during clot formation (10, 11). Several differences have been noted in the functional properties of platelet and plasma factor V (reviewed in Ref. 12), which are encoded by the same gene (1), but the contributions of MMRN1 1 to these differences are unknown. MMRN1 is recognized to bind factor V, factor Va, and the isolated light chain of factor Va (11). Studies of activated platelets show that factor V-MMRN1 complexes dissociate after exposure to thrombin (11), similar to the way factor VIII-von Willebrand factor (VWF) 1 complexes dissociate upon exposure to thrombin to liberate factor VIIIa for tenase complex assembly during coagulation (13). This i...