Glycoproteins IIb (GPIIb) and IIIa (GPIIIa) form the Ca2+ -dependent GPIIb/IIIa complex, which acts as the fibrinogen receptor on activated platelets. GPIIb and GPIIIa are synthesized as single peptide chains. The GPIIb precursor is processed proteolytically to yield two disulphide-bonded chains, GPIIba and GPIIbß. The GPIIb/IIIa complex has two membrane attachment sites located at the C-tennini ofGPIIß and GPIIIa. The short cytoplasmic tails of GPIIbß and/or GPIIIa become most likely associated to the cytoskeleton of activated platelets. In the present work the C-tenninal amino acid residues of platelet GPIIbß and GPIIIa have been analyzed by protein-chemical methods and compared with those predicted from cDNA analysis. We were able to confirm the positions of the C-tennini in both glycoproteins and the identity of the C-tenninus predicted for GPIIIa, i.e. threonine. However, glutamine, not glutamic acid as predicted for GPIIbß from the human erythroleukemic cell line and megakaryocyte cells, was found to be the C-terminal amino acid of GPIIß. This indicates that the glutamic arid in the GPIIb precursor is posttranslationally modified to glutamine.
