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

In this report, we describe the intracellular localization of multimer in in platelets and its biosynthesis by Dami cells, a megakaryocytic c ell line. Immunoelectron microscopy was used to examine frozen thin se ctions of resting and activated platelets. Multimerin was localized wi thin the platelet alpha-granule in an eccentric position. Within activ ated platelets, multimerin was found in the surface-connected open can nalicular system and on the external plasma membrane. Light microscopi c immunocytochemistry demonstrated multimerin in normal megakaryocytes and in Dami cells after stimulation with PMA. Confirmation of multime rin biosynthesis by Dami cells was obtained by metabolic labeling stud ies. Both platelet and Dami cell multimerin demonstrated several subun it sizes on reduced SDS-PAGE. However, peptide mapping confirmed struc tural homology between the different multimerin subunits. Glycosidase digestion demonstrated that multimerin is heavily glycosylated with ma inly complex, N-linked carbohydrate. In contrast to the multimerin iso lated from platelets, cultured Dami cells secreted mainly smaller mult imers of the protein. Biosynthesis of multimerin by a megakaryocytic c ell line supports endogenous biosynthesis by megakaryocytes as the ori gin of this platelet alpha-granule protein.