The In Vitro Synthesis of Polypeptides for the Platelet Membrane Glycoproteins IIb and IIIa

The platelet membrane glycoproteins lIb (Gpllb) and GpIIIa form calcium-dependent heterodimers containing binding sites for fibrinogen, von Willebrand factor, and fibronectin. Although Gpllb and GplIIa are distinct proteins, both Gpllb and Gpllla are deficient in platelets from individuals with the recessive disorder Glanzmann’s thrombasthenia. To gain a better understanding of the genetic basis for Gpllb and Gpllla synthesis, we studied their synthesis by two human leukemia cell lines, HEL and K562. HEL cells contained complexes of Gpllb and Gpllla, and K562 cells expressed Gpllla, but not Gpllb, when stimulated with phorbol-12-myristate-13-acetate (PMA). RNA from HEL cells directed the in vitro synthesis of a 110,000-Mrprecursor for Gpllb and a 92,000-Mrprecursor for Gpllla, which indicates that the synthesis of Gpllb and Gpllla by HEL cells is directed by separate mRNAs. In contrast, RNA from PMA-stimulated K562 cells only directed the synthesis of a 92,000-Mrprecursor for Gpllla. The dissociation of Gpllb and Gpllla synthesis in K562 cells suggests that Gpllb and Gpllla may be the products of separate genes.