Isolation and biochemical characterization of the α- and β-subunits of glycoprotein IIb of human platelet plasma membrane

Abstract

The α- and β-subunits of glycoprotein IIb (GPIIb) of human platelet plasma membrane were isolated in fully reduced, partially reduced and alkylated, and fully alkylated forms, by size-exclusion chromatography after reduction of pure GPIIb. The sugar moiety of GPIIbα accounts for 16.4% of its total weight, whereas that of GPIIbβ accounts for only 10.2%. The molar percentages (per 100 mol of total amino acids) of neuraminic acid and galactose in the α-subunit more than double those in the β-subunit, whereas galactosamine is present only in GPIIbα. From the amino acid and sugar compositions the acidic nature of both subunits was confirmed. The M(r) values obtained, 114,000 for GPIIbα and 22,200 for GPIIbβ, are in very good agreement with those obtained by physical methods. We found by stepwise reduction of pure GPIIb with dithioerythritol and GPIIbα and GPIIbβ are joined by a single interchain disulphide bridge, while the remaining half-cystine residues participate in intrachain bonds, six in GPIIbα and one in GPIIbβ, the intersubunit disulphide bond being that reduced first. Neither of the two subunits is liberated from isolated plasma membranes when this GPIIb interchain bond is reduced in isolated membranes.