PROTEOLYTIC DEGRADATION OF THE RGD-BINDING AND NON-RGD-BINDING CONFORMERS OF HUMAN PLATELET INTEGRIN GLYCOPROTEIN IIB IIIA - CLUES FOR IDENTIFICATION OF REGIONS INVOLVED IN THE RECEPTORS ACTIVATION/

The human integrin glycoprotein (GP)IIb/IIIa plays a central role in h aemostasis as an inducible receptor for fibrinogen and other RGD-conta ining adhesive proteins at the platelet plasma membrane. Expression of the fibrinogen receptor on platelet activation involves conformationa l changes in the quaternary structure of GPIIb/IIIa. Little is known, however, about the nature of this conformational transition. Given tha t isolated GPIIb/IIIa contains a mixture of RGD-binding and non-RGD-bi nding heterodimers, we used limited proteolysis as a tool for investig ating the structural differences between the two con formers. Comparis on of their fragmentation patterns shows that, whereas in the non-RGD- binding form of GPIIb/IIIa the N-terminal half of the heavy chain of G PIIb (GPIIbH) and the central region of GPIIIa are cleaved by endoprot einase Arg-C, these domains associate tightly with one another in the RGD-binding GPIIb/IIIa and are thus protected from proteolysis. In add ition, the C-terminal half of GPIIb becomes more susceptible to degrad ation in the non-RGD-binding GPIIb/IIIa conformer. Our interpretation, in the context of available structural and functional data, is that a major relative reorientation of the GPIIbH and GPIIIa extracellular d omains takes place along the subunit interface during the:conformation al transition of the platelet integrin.