Arrestin facilitates rhodopsin dephosphorylation in vivo

Deactivation of G protein-coupled receptors (GPCRs) involves multiple phosphorylations followed by arrestin binding, which uncouples the GPCR from G protein activation. Some GPCRs, such as rhodopsin, are reused many times. Arrestin dissociation and GPCR dephosphorylation are key steps in the recycling process. In vitro evidence suggests that visual arrestin (Arr1) binding to light-activated, phosphorylated rhodopsin hinders dephosphorylation. Whether Arr1 binding also affects rhodopsin dephosphorylation in vivo is not known. We investigated this using both male and female mice lacking Arr1. Mice were exposed to bright light and placed in darkness for different periods of time, and differently phosphorylated species of rhodopsin were assayed by isoelectric focusing. For wildtype mice, rhodopsin dephosphorylation was nearly complete by one hour in darkness. Surprisingly, we observed that in the ARR1 knockout rods, rhodopsin remained phosphorylated even after three hours. Delayed dephosphorylation in ARR1 knockout rods cannot be explained by cell stress induced by persistent signaling, since it is not prevented by the removal of transducin, the visual G protein, nor can it be explained by downregulation of protein phosphatase 2A, the putative rhodopsin phosphatase. We further show that cone arrestin (Arr4), which binds light-activated, phosphorylated rhodopsin poorly, had little effect in enhancing rhodopsin dephosphorylation, whereas mice expressing binding-competent mutant Arr1-3A showed a similar time course of rhodopsin dephosphorylation as wildtype. Together, these results reveal a novel role of Arr1 in facilitating rhodopsin dephosphorylation in vivo.

Automated summary

Arr1 plays a key role in facilitating the dephosphorylation of rhodopsin in vivo. Mice lacking Arr1 show delayed dephosphorylation of rhodopsin, which cannot be explained by cell stress or downregulation of protein phosphatase 2A.