MULTIMERIN IS FOUND IN THE ALPHA-GRANULES OF RESTING PLATELETS AND IS SYNTHESIZED BY A MEGAKARYOCYTIC CELL-LINE

In this report, we describe the intracellular localization of multimerin in platelets and its biosynthesis by Dami cells, a megakaryocytic cell line. Immunoelectron microscopy was used to examine frozen thin sections of resting and activated platelets. Multimerin was localized within the platelet alpha-granule in an eccentric position. Within activated platelets, multimerin was found in the surface-connected open cannalicular system and on the external plasma membrane. Light microscopic immunocytochemistry demonstrated multimerin in normal megakaryocytes and in Dami cells after stimulation with PMA. Confirmation of multimerin biosynthesis by Dami cells was obtained by metabolic labeling studies. Both platelet and Dami cell multimerin demonstrated several subunit sizes on reduced SDS-PAGE. However, peptide mapping confirmed structural homology between the different multimerin subunits. Glycosidase digestion demonstrated that multimerin is heavily glycosylated with mainly complex, N-linked carbohydrate. In contrast to the multimerin isolated from platelets, cultured Dami cells secreted mainly smaller multimers of the protein. Biosynthesis of multimerin by a megakaryocytic cell line supports endogenous biosynthesis by megakaryocytes as the origin of this platelet alpha-granule protein.