Light-Dependent Translocation of Arrestin in Rod Photoreceptors is Signaled through a Phospholipase C Cascade and Requires ATP

Light adaptation of rod photoreceptors induces translocation of arrestin from inner segments (IS) to outer segments (OS). Our study suggests that components of the G-protein linked phosphoinositide pathway play a role in signaling the initiating events of arrestin translocation. We show that arrestin translocation can be stimulated by activators of phospholipase C (PLC) and protein kinase C (PKC) in the absence of light. Conversely, arrestin translocation to the OS is significantly slowed by inhibitors of PLC and PKC.In the second part of this study, we investigated the mechanism by which arrestin translocates in response to light. Other investigators have suggested that arrestin translocation between the OS and IS is a passive process, resulting from arrestin's binding affinity for light-activated, phosphorylated rhodopsin in the outer segments and affinity for microtubules in the inner segments. The central tenet of this model is that arrestin's translocation is an energy independent process. In our investigation of this process, we found that treatment of Xenopus retinas with potassium cyanide inhibits arrestin translocation to the OS in response to light, but that translocation can be restored by the removal of cyanide and addition of ATP. These results were confirmed in the mouse retina and clearly suggest that at least one step in arrestin translocation requires ATP. We also found that an arrestin with scrambled C-terminal 30 amino acids retained its binding for both activated rhodopsin and microtubules, but yet was unable to translocate in response to light. The results obtained from both investigating the signaling cascade and the mechanisms of arrestin translocation indicate that arrestin translocation between the IS and OS is more complex than previously proposed, and likely involves both diffusion and motor-assisted processes.