Human platelet antigen 3 (HPA-3): localization of the determinant of the alloantibody Lek(a) (HPA-3a) to the C-terminus of platelet glycoprotein IIb heavy chain and contribution of O-linked carbohydrates.

The human platelet antigen (HPA) 3 system is expressed on GPIIb, one subunit of GPIIb-IIIa, the platelet fibrinogen receptor. It was recently shown that HPA-3 was associated with an Ile843/Ser polymorphism. To investigate further HPA-3 determinant structure, we localized an HPA-3a determinant, recognized by the alloantiserum Lek(a), within the last 29 amino acids of GPIIb alpha. This region encompasses the polymorphic Ile843, which, as expected, is substituted into Ser in Lek(a)-negative individuals, as shown by DNA sequence after polymerase chain reaction on platelet RNA. In addition, contribution of glycosylation to the determinant structure was demonstrated since the Lek(a) antigenicity was strongly decreased after specifically removing non-terminal O-linked sugars, but not terminal sialic acids. We have thus refined the localization of an HPA-3a determinant within the last 29 amino acids, including Ile843, of GPIIb heavy chain, and shown that the Lek(a) HPA-3a determinant is dependent, in part, upon the serine-linked carbohydrates adjacent to Ile/Ser843.