Complete localization of the intrachain disulphide bonds and the N-glycosylation points in the alpha-subunit of human platelet glycoprotein IIb.

Glycoprotein IIb (GPIIb), one of the two molecular components of the inducible receptor for fibrinogen on the platelet surface, is formed from two subunits, GPIIb alpha (114 kDa) and GPIIb beta (22.5 kDa), joined by a single disulphide bond. CNBr cleavage of GPIIb, together with tryptic or endoproteinase Lys-C digestion of some of the isolated CNBr peptides, followed by amino acid and N-terminal sequence analysis of the isolated fragments, allowed us to locate unambiguously all the unknown disulphide bonds and the N-glycosylation points in platelet GPIIb. It could be established that each cysteine residue in GPIIb, beginning at alpha-Cys-56, is disulphide-bonded to its nearest neighbour in the amino acid sequence. Given the extensive structural similarity among the two-chain alpha-subunits of Arg-Gly-Asp adhesion receptors and the conservative positions of cysteine residues in their amino acid sequences, the intrachain and interchain disulphide-bond pattern found here in GPIIb will most probably be conserved in all two-chain alpha-subunits of these receptors. The N-linked glycosylation points found here in platelet GPIIb are the same as the five N-glycosylated asparagine residues suggested after cDNA sequencing of human erythroleukaemic-cell GPIIb [Poncz, Eisman, Heindenreich, Silver, Vilaire, Surrey, Schwartz & Bennett (1987) J. Biol. Chem. 262, 8476-8482]. Some of the general features of the structure of GPIIb, such as the existence of distinct domains in the alpha- and beta-subunits, as well as the identification of well-defined points in its external topography, are discussed.