GTP-binding proteins in human platelet membranes serving as the specific substrate of islet-activating protein, pertussis toxin

Abstract

Two GTP-binding proteins serving as the specific substrate of islet-activating protein (IAP), pertussis toxin, were purified from human platelet membranes as heterotrimers with an αβγ-subunit structure. The α of the major IAP substrate had a molecular mass of 40 kDa and differed from that of G i 1 or G o previously purified from brain membranes. The partial amino acid sequences of the 40 kDa α completely matched with the sequences which were deduced from the nucleotide sequences of the human G i 2 α gene. On the other hand, the α of the minor IAP substrate purified from human platelets was about 41 kDa and cross-reacted with an antibody raised against α of brain G i 1 (G i 1 α). These results indicate that the major IAP substrate present in human platelet membranes is a product of the G i 2 α gene.