Studies on the nature of the human platelet alloantigen, PlA1: localization to a 17,000-dalton polypeptide.

We have examined the biochemical properties of the human platelet alloantigen system, PlA, using a preparation enriched in plasma membrane glycoproteins (GPs) IIb and IIIa as the starting material. After confirming that GPIIIa contains the PlA epitope, attempts were made to distinguish the two allelic forms of PlA (A1 and A2) using electrophoretic techniques. Whereas no difference could be discerned in the mol. wt of GPIIIa extracted from A1/A1, A1/A2 or A2/A2 platelets by one-dimensional SDS-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional electrophoresis revealed a reproducible difference in the isoelectric point of GPIIIa derived from A2/A2 individuals. Treatment of GPIIIa with a combination of exo- and endoglycosidases resulted in apparently complete deglycosylation of the molecule, as indicated by its co-migration with chemically deglycosylated GPIIIa in SDS-PAGE. The enzymatically deglycosylated protein retained its full ability to react with anti-PlA1 antibodies. Tryptic digestion of GPIIIa resulted in the generation of a number of smaller polypeptides, including one of 17,000 daltons, that contained the PlA1 determinant. These results suggest that the carbohydrate moieties of GPIIIa are unimportant to the expression of the PlA system, and that the charge difference between the two allelic forms of GPIIIa may reflect a subtle amino acid difference(s) within a 17K polypeptide region of the GP.