Platelet membrane glycoprotein IIb heavy chain forms a complex with glycoprotein IIIa that binds Arg-Gly-Asp peptides

Platelet membrane GPIIb is comprised of a disulfide-linked heavy chain (GPIIb(H)) and light chain (GPIIb(L)). We have examined the role of the two chains of GPIIb in the maintenance of the GPIIb-IIIa heterodimer and Arg-Gly-Asp (RGD) peptide-binding function. Lysates of surface radioiodinated platelets were treated with 1% 2-mercaptoethanol for 18 hours at 4 degrees C. Reduction of the interchain disulfide in GPIIb was followed by immunoprecipitation with antipeptide antibodies specific for GPIIb(H) or GPIIb(L). In addition to the GPIIb-IIIa complex, a polypeptide of 120 Kd was precipitated by anti-GPIIb(H) and a polypeptide of 23 Kd was precipitated by anti-GPIIb(L) from reduced platelet lysates. To determine whether GPIIb(H) or GPIIb(L) remained complexed with GPIIIa, reduced platelet lysates were immunoprecipitated with AP3, a monoclonal anti-GPIIIa antibody, resulting in the coimmunoprecipitation of GPIIb(H) but not GPIIb(L). Conversely, the monoclonal anti-GPIIb(H) antibody PMI-1 immunoprecipitated GPIIIa with GPIIb(H). Thus GPIIb(H) maintains its association with GPIIIa. Furthermore, the GPIIb(H)-IIIa complex retains its reactivity with AP2, a monoclonal antibody (MoAb) specific for the nondissociated GPIIb-IIIa complex. Affinity chromatography of reduced platelet lysates on immobilized KYGRGDS resulted in binding and specific elution of the GPIIb(H)-IIIa complex. These findings indicate that GPIIb(H) contains sufficient information for maintenance of a complex with GPIIIa and support of the binding of the heterodimer to RGD peptides.