Analyses of cellular multimerin I receptors:: in vitro evidence of binding mediated by αIIbβ3 and αvβ3

Multimerin I (MMRNI) is a large, soluble, polymeric, factor V binding protein and member of the EMILIN protein family. In vivo, MMRNI is found in platelets, megakaryocytes, endothelium and extracellular matrix fibers, but not in plasma. To address the mechanism of MMRNI binding to activated platelets and endothelial cells,we investigated the identity of the major MMRNI receptors on these cells using wild-type and RGE-forms of recombinant MMRNI. Ligand capture,cell adhesion, ELISA and flow cytometry analyses of platelet-MMRNI binding, indicated that MMRNI binds to integrins alpha llb beta 3 and alpha v beta 3. Endothelial cell binding to MMRNI was predominantly mediated by alpha v beta 3 and did not require the MMRNI RGD site or cellular activation. Like many other alpha c beta 3 ligands, MMRNI had the ability to support adhesion of additional cell types, including stimulated neutrophils. Expression studies, using a cell line capable of endothelial-like MMRNI processing, indicated that MMRNI adhesion to cellular receptors enhanced its extracellular matrix fiber assembly. These studies implicate integrin-mediated binding in MMRNI attachment to cells and indicate that MMRNI is a ligand for alpha llb beta 3 and alpha v beta 3.