REQUIREMENT FOR INTRAMOLECULAR DOMAIN INTERACTION IN ACTIVATION OF G-PROTEIN ALPHA-SUBUNIT BY ALUMINUM FLUORIDE AND GDP BUT NOT BY GTP-GAMMA-S

An ion-counterion interaction between the lysine of the NKXD motif in the GTPase domain and an aspartate in the inserted helical domain of c u subunits of heterotrimeric G proteins, Lys-278 and Asp-158, respecti vely, of G(s) alpha is shown to be essential for activation by AlF4- a nd partially so for interaction with beta gamma dimers and activation by GTP and receptor. However, this domain interaction is not required for activation by the non-hydrolyzable analog guanosine 5'-3-O-(thio)t riphosphate. Proximity of the helical domain to the GTPase domain is t hus involved in the fundamental inactive --> active transition of the protein in a way that further distinguishes alpha subunits of heterotr imeric G proteins from ras and ras-like GTPases that lack helical doma ins and are neither activated by AlF4- nor combine with beta gamma dim ers.