Structural and functional characterization of major platelet membrane components derived by limited proteolysis of glycoprotein IIIa

The authors isolated a product of proteolytic degradation of glycoprotein IIIa (GPIIIa) which is formed on the surface of human platelets during incubation with chymotrypsin and which was previously described as the 66 kDa platelet membrane component. This component migrated with an apparent Mr 62,400 in a non-reduced system of sodium dodecyl sulfate polyacrylamide gel electrophoresis. In a reduced system it yielded two major subunits migrating with apparent Mr 14,000-17,000 and 65,000. The low-molecular weight component began with the NH2-terminal sequence of GPIIIa (GPNICTTR...) and the larger component with residue 348 of GPIIIa (GKIRSKKA...) as deduced from a cDNA clone of this glycoprotein. The two subunits appeared to be linked by one or more S-S bridges supporting the contention that GPIIIa is a highly folded molecule on the platelet membrane. In contrast to GPIIIa, the '66 kDa component' did not bind to GRGDSPK-agarose, to fibrinogen-agarose nor to insolubilized monoclonal antibody recognizing the GPIIb/IIIa complex. The exposure of fibrinogen receptors during the course of incubation of platelets with chymotrypsin preceded the formation of the '66 kDa component' characterized in this study. An intermediate product of GPIIIa proteolysis migrating with an apparent Mr 120,000 in a non-reduced system and Mr 80,000 in a reduced system was identified as a precursor of the '66 kDa component'. The '120 kDa component' was not retained on GRGDSPK-agarose or on fibrinogen-agarose but it was retained on insolubilized antibody recognizing the GPIIb/IIIa complex. Incubation of platelets with porcine pancreatic elastase or human granulocytic elastase resulted in the formation of similar proteolytic degradation fragments.