Multimerin is a large soluble protein, with an uncertain function, found in platelets, megakaryocytes, endothelium and extracellular matrix fibers but not in plasma. The observation that multimerin contains structural features of an adhesive protein, including an Arg-Gly-Asp (RGD) sequence, led us to investigate its ability to support adhesion of platelets, megakaryocytes, endothelial cells and other cell types. Multimerin had the ability to support the adhesion of both platelets and megakaryocytes and this required cellular activation and the multimerin RGD site. Studies of normal and Glanzmann platelets indicated that multimerin interacted with the major platelet integrin receptor, αIIbβ3 and radioimmunoprecipitation analyses confirmed that multimerin bound to αIIbβ3. Multimerin also supported adhesion of endothelial cells, neutrophils and other cells including smooth muscle cells, fibroblast cells, human embryonic kidney (HEK293) and epithelial cells. Unlike platelets, these cells do not express αIIbβ3; this indicated that other integrin or non-integrin receptors could be involved in cellular adhesion to multimerin. Comparisons of cell adhesion to wild-type and RGE-multimerin indicated that unlike platelets and megakaryocytes, some other cell types (e.g. endothelial cells, smooth muscle cells and neutrophils) were capable of adhering to RGE-multimerin. This suggested that cellular adhesion to multimerin occurs by both RGD and non-RGD dependent mechanisms. Finally, unlike platelets, megakaryocytes and neutrophils, adhesion of other cell types to multimerin did not require cellular activation. In conclusion, our data indicate multimerin has fairly broad proadhesive properties, involving RGD and non-RGD dependent mechanisms, and that cellular receptors including αIIbβ3 interact with multimerin to mediate its binding to activated platelets, endothelial cells and potentially other cell types.

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