An ion-counterion interaction between the lysine of the NKXD motif in the GTPase domain and an aspartate in the inserted helical domain of alpha subunits of heterotrimeric G proteins, Lys-278 and Asp-158, respectively, of Gs alpha is shown to be essential for activation by AlF4- and 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)triphosphate. Proximity of the helical domain to the GTPase domain is thus involved in the fundamental inactive-->active transition of the protein in a way that further distinguishes alpha subunits of heterotrimeric G proteins from ras and ras-like GTPases that lack helical domains and are neither activated by AlF4- nor combine with beta gamma dimers.