Selective role of G protein γ subunits in receptor interaction

Receptor stimulation of nucleotide exchange in a heterotrimeric G protein (αβγ) 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 βγ complex, in receptor activation are unclear. In a reconstituted system, a purified muscarinic receptor, M2, activates G protein heterotrimers αi2β1γ5 and αi2β1γ7 with equal efficacy. However, when the α subunit type is substituted with αo, αoβ1γ7 shows a 100% increase in M2-stimulated GTP hydrolysis compared with αoβ1γ5. Using a sensitive assay based on βγ complex stimulation of phospholipase C activity, we show that both β1γ5 and β1γ7 form heterotrimers equally well with αo and αi. These results indicate that the γ subunit interaction with a receptor is critical for modulating nucleotide exchange and is influenced by the subunit-type composition of the heterotrimer.