Differential regulation of G protein-gated inwardly rectifying K+ channel kinetics by distinct domains of RGS8

1. The contribution of endogenous regulators of G protein signalling (RGS) proteins to G protein modulated inwardly rectifying K+ channel (GIRK) activ ation/deactivation was examined by expressing mutants of G alpha (oA) insen sitive to both pertussis toxin (PTX) and RGS proteins in rat sympathetic ne urons. 2. GIRK channel modulation was reconstituted in PTX-treated rat sympathetic neurons following heterologous expression of G protein subunits. Under the se conditions, noradrenaline-evoked GIRK channel currents displayed: (1) a prominent lag phase preceding, activation, (2) retarded activation and deac tivation kinetics, and (3) a lack of acute desensitization. 3. Unexpectedly, heterologous expression of RGS8 in neurons expressing PTX- i-RGS-insensitive G alpha (oA) shortened the lag phase and restored rapid a ctivation, but retarded the deactivation phase further. These effects were found to arise from the N-terminus, but not the core domain, of RGS8 thus s uggesting actions on channel modulation independently of GTPase acceleratio n. 4. These findings indicate that different domains of RGS8 make distinct con tributions to the temporal regulation of GIRK channels. The RGS8 core domai n accelerates termination of the G-protein cycle presumably by increasing G alpha GTPase activity. In contrast, the N-terminal domain of RGS8 appears to promote entry into the G protein cycle, possibly by enhancing coupling o f receptors to the G protein heterotrimer. Together, these opposing effects should allow for an increase in temporal fidelity without a dramatic decre ase in signal strength.