RAPID KINETICS OF G-PROTEIN SUBUNIT ASSOCIATION - A RATE-LIMITING CONFORMATIONAL CHANGE

G protein subunit association and dissociation are thought to play an important role in signal transduction. We measured alpha beta gamma he terocomplex formation using resonance energy transfer. Fluorescein-lab elled alpha(F-alpha) emission was quenched similar to 10% on mixing wi th eosin-labelled beta gamma(E-beta gamma). Unlabelled beta gamma did not quench F-alpha fluorescence. Stopped-flow kinetics showed a t(1/2) ranging from 2.5 s to 0.25 s for 50 nM to 1200 nM E-beta gamma. The r ate saturated at high E-beta gamma concentrations consistent with a tw o-step mechanism. We report the first rapid-mix studies of G protein s ubunit association kinetics which suggest that alpha and beta gamma co mbine by a two-step process with a maximal rate of 4.1 +/- 0.4 s(-1).