Phosphorylation stabilizes the active conformation of rhodopsin

Deactivation of many G protein coupled receptors (GPCRs) is now known to require phosphorylation of the activated receptor. The first such GPCR so analyzed was rhodopsin, which upon light activation forms an intramolecular equilibrium between the two conformers, metarhodopsin I and II (MI and MII). In this study, we find surprisingly that rhodopsin phosphorylation increases rather than diminishes the formation of MII, the conformation that activates G protein. The MI-MII equilibrium constant was progressively shifted toward MII as the experimental phosphorylation stoichiometry was increased from 0 to 6.4 phosphates Fer rhodopsin. Increasing phosphorylation both increased MII's formation rate (k(1)) and decreased its rate of loss (k(-1)). The direct effect of cytoplasmic surface phosphorylation on intramolecular conformer eqililibria observed here may be important to functional state modulation of other membrane proteins.