Selective role of G protein γ subunits in receptor interaction

Receptor stimulation of nucleotide exchange in a heterotrimeric G protein (alpha beta gamma) is the primary event-modulating signaling by G proteins, The molecular mechanisms at the basis of this event and the role of the G protein subunits, especially the py complex, in receptor activation are unclear. In a reconstituted system, a purified muscarinic receptor, M2, activates G protein heterotrimers alpha i2 beta1 gamma5 and alpha i2 beta1 gamma7 with equal efficacy. However, when the ct subunit type is substituted with alphao, alphao beta1 gamma7 shows a 100% increase in M2-stimulated GTP hydrolysis compared with alphao beta1 gamma5, Using a sensitive assay based on py complex stimulation of phospholipase C activity, we show that both beta1 gamma5 and beta1 gamma7 form heterotrimers equally well with alphao and alphai, These results indicate that the gamma subunit interaction with a receptor is critical for modulating nucleotide exchange and is influenced by the subunit-type composition of the heterotrimer.