Summary

Factor V is an essential coagulation cofactor that circulates inplasma and platelet a -granules where it is stored complexed tomultimerin 1 (MMRN1).To gain insights into the origin and processingof human platelet factor V, and factor V-MMRN1 complexes,we studied factor V in cultured megakaryocytes. FactorV mRNA was detected in all megakaryocyte cultures. However,like albumin, IgG and fibrinogen, factorV protein was detectableonly in megakaryocytes cultured with exogenous protein. Theamount of factor V associated with megakaryocytes was influencedby the exogenous factorV concentration.Similar to platelet factor V, megakaryocyte factor V was proteolyzed andcomplexed with megakaryocyte-synthesized MMRN1. Withsecretagogues, megakaryocytes released factorV, IgG, fibrinogenand MMRN1.Immunofluorescent and electron microscopy confirmedfactorV uptake by endocytosis and its trafficking to megakaryocytea -granules. These data provide direct evidence thathuman megakaryocytes process plasma-derived factor V intoa -granules and generate factor V-MMRN1 complexes from endogenouslyand exogenously synthesized proteins.