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.