Characterization of the Platelet Fibrinogen Receptor

The stmcture of GPIIb and GPIIIa has been elucidated by biochemical and molecular methods (Fig. 1). Both glycoproteins are integral platelet membrane constituents. Together they account for approximately 18% of the platelet membrane protein mass (26-28). 

GPIIb has a molecular weight of 136,000 daltons. It is composed of a larger alpha (125,000 daltons) and a smaller beta (23,000 daltons) subunit, linked by a single disulfide bond (37). The beta subunit serves to anchor GPIIb in the platelet membrane. The disulfide linked alpha subunit is entirely extracellular. 

The intracellular assembly and membrane expression of the GPIIb-IIIa complex has been 

Recent studies indicate that a limited region of GPIIIa (V275GSDNH280) (58) expressed in an extended loop of the metal ion-dependent adhesion site appears to involved in heterodimer assembly. In addition, the expression of isolated residues 1-233 of GPIIb and 111 -318 of GPIIIa leads to the formation of soluble peptide heterodimers that recognize the RGD sequence (58). 

In 1978, Mustard and co-workers (72) described the first interaction of radio-labeled fibrinogen with washed human and rabbit platelets during ADP-induced aggregation. This observation was confirmed and rapidly extended by numerous other laboratories (summarized in 73). 

---