Identification of PlAl alloantigen domain on a 66 kDa protein derived from glycoprotein IIIa of human platelets.

Incubation of platelets with chymotryptin leads to the exposure of fibrinogen receptors and to the appearance of a 66 kDa membrane component on the surface of platelets. Both glycoprotein IIIa (GP IIIa) and a 66 kDa component were precipitated from detergent extracts of solubilized, surface radiolabeled chymotrypsin-treated platelets by human anti-PlAl antisera. Moreover, the presence of the P1A1 antigen was identified on GP IIIa (but not on GP IIb) and on a 66 kDa protein by means of immunoblot procedures using platelet Triton X-114 extracts and these purified proteins. Anti-PlAl antiserum did not recognize GP IIIa on the surface of intact (untreated) platelets nor the 66 kDa protein on the surface of chymotrypsin-treated platelets of PlAl-negative individuals. The present data demonstrate directly that the 66 kDa protein is derived from GP IIIa and contains the PlAl alloantigen.