Molecular basis of signaling specificity between GIRK channels and GPCRs

Stimulated muscarinic acetylcholine receptors (M2Rs) release G beta gamma subunits, which slow heart rate by activating a G protein-gated K+ channel (GIRK). Stimulated beta 2 adrenergic receptors (beta 2ARs) also release G beta gamma subunits, but GIRK is not activated. This study addresses the mechanism underlying this specificity of GIRK activation by M2Rs. K+ currents and bioluminescence resonance energy transfer between labelled G proteins and GIRK show that M2Rs catalyze G beta gamma subunit release at higher rates than beta 2ARs, generating higher G beta gamma concentrations that activate GIRK and regulate other targets of G beta gamma. The higher rate of G beta gamma release is attributable to a faster G protein coupled receptor - G protein trimer association rate in M2R compared to beta 2AR. Thus, a rate difference in a single kinetic step accounts for specificity.