The present invention provides modified G protein .alpha.-subunits which are characterized by constitutive localization to the plasma membrane; enhanced binding to one or more of the normal receptor binding partners for that .alpha.-subunit; and efficient binding to and activation of G protein binding partners. The distribution of these modified .alpha.-subunits, which are "tethered" to the plasma membrane, allows the regulation of receptor-G protein coupling, and thus G-protein signaling, in various biological systems.