Arrestin–GRK1 Competition Affects Rhodopsin Active Time

Phototransduction begins when a photon activates a rhodopsin molecule. Activated rhodopsin activates the G-protein transducin, which activates phosphodiesterase, leading to decreased cGMP concentration and closure of cyclic-nucleotide-gated channels. Termination of this process requires inactivation of rhodopsin and transducin. The former is mediated by arrestin, which separates the bleached chromophore from the opsin. The affinity of arrestin for rhodopsin is increased by triple phosphorylation of the rhodopsin by the G-proteincoupled-receptor kinase GRK1. But arrestin can also bind to unphosphorylated and semiphosphorylated rhodopsin, leading Doan et al. to hypothesize that competition for rhodopsin binding between GRK1 and arrestin influences the rate of rhodopsin inactivation. Analysis of single-photon responses in mice that expressed reduced levels of either GRK1 or arrestin provided support for this hypothesis. In addition, the results suggested that, contrary to previous reports, the active time of rhodopsin was longer than that of transducin. The authors show this depends on recording conditions.