PLATELET ALPHA-GRANULE FIBRINOGEN, ALBUMIN, AND IMMUNOGLOBULIN-G ARE NOT SYNTHESIZED BY RAT AND MOUSE MEGAKARYOCYTES

It has been assumed that endogenous synthesis by the platelet precursor cell, the bone marrow megakaryocyte, is the major source of platelet α-granule protein. To test this hypothesis, we used mRNA phenotyping to detect in megakaryocytes the presence of mRNA transcripts specific for various proteins. Our results indicate that megakaryocytes synthesize platelet factor 4, a protein relatively specific for platelets, but do not express mRNA transcripts for the fibrinogen, albumin, or IgG found in α-granules. We have previously shown that megakaryocytes endocytose circulating proteins, including fibrinogen, albumin, and IgG, and incorporate them into α-granules. Thus, platelets appear to contain a unique type of secretory granule whose contents originate by both endogenous synthesis and endocytosis from plasma. Under basal conditions, the source of α-granule fibrinogen is plasma.