G-ALPHA-I-G-ALPHA-S CHIMERAS DEFINE THE FUNCTION OF ALPHA-CHAIN DOMAINS IN CONTROL OF G-PROTEIN ACTIVATION AND BETA-GAMMA-SUBUNIT COMPLEX INTERACTIONS

Gs and G12 are G proteins whose α subunits are 65% homologous. Within the 355 amino acid α12 polypeptide, substitution of residues Ile213-Lys319 with the corresponding αs region (Ile235-Arg356) generated a chimera that activated adenylyl cyclase, indicating that the αs activation domain resides within this 122 amino acid αs sequence. Mutation within αs residues Glu15-Pro144 resulted in an αs polypeptide having an enhanced rate of GDP dissociation. Mutation within two regions of the N-terminus influenced the ability of pertussis toxin to ADP-ribosylate the α subunit polypeptide, a reaction controlled by the βγ subunit complex. The findings define the G protein α subunit N-terminus as a regulatory region controlling βγ subunit interactions and GDP dissociation independent of the GTPase and effector activation domains. © 1990.