Synthesis of actin by cultured guinea pig megakaryocytes. Complex formation with fibrin.

Guinea pig megakaryocytes were isolated from femoral marrow and cultured in the presence of radioactive amino acids. Radioactivity was incorporated into several proteins including a 42 000 dalton polypeptide identified as actin by DNAase agarose affinity chromatography. Quantitative immunoelectrophoresis of megakaryocyte extract revealed that 3.0% of the total solubilized cellular protein was fibrinogen. Immunoabsorption studies using anti guinea pig fibrinogen beads failed to reveal the presence of newly synthesized radioactive fibrinogen in the cellular extract, however, radioactive actin was detected in the eluates obtained from the immune beads. When guinea pig fibrinogen was clotted with thrombin in the presence of radioactive megakaryocyte extract, a complex formed between a high molecular weight species of fibrin and actin. No actin fibrinogen complex was detected. The results suggest that actin synthesized by megakaryocytes complexes with fibrin formed from a relatively large pool of non-radioactive intracellular fibrinogen.