Purpose: To determine the temporal kinetics of the simultaneous translocation of arrestin and rod alpha-transducin in mice exposed to different lighting environments and to compare the subcellular compartmentation of cone alpha-transducin with arrestin.
Methods: Double labeling immunofluorescence microscopy and image analysis are used to visualize and quantify the concentrations of rod arrestin and alpha-transducin in the subcellular compartments of the rod outer segments, the rod inner segments and the synaptic terminals.
Results: The magnitude of the effects of the translocation are clearly contrasted in images of the retinas of animals that have been maximally light adapted verses retinas that have been maximally dark adapted. The onset of light results in a rapid, simultaneous, translocation of arrestin and alpha-transducin from their respective compartments (alpha-transducin in the rod outer segment and arrestin in the rod inner segment) to the opposite compartment. Almost all of alpha-transducin has translocated in less than two min whereas the translocation of the majority of arrestin requires at least five to six min. Translocation in the opposite direction, from light to dark, occurs more slowly for both proteins with arrestin requiring almost 30 min and alpha-T needing more than 200 min to complete its journey. Under the same lighting conditions, cone arrestin translocation is incomplete. Cone alpha-transducin does not translocate under any the lighting conditions tested. Unlike the frog, continuous exposure of mice to light does not result in arrestin translocating back to the rod inner segment.
Conclusions: These data suggest that there are four mechanisms involved in the translocation of these two proteins. They also support the conclusion that the more important cellular function of the translocation process is to terminate phototransduction in rod and cone photoreceptors, which could provide protection against light damage. The secondary function of translocation is to maximize rod sensitivity to light during dark adaptation. The restricted localization of cone alpha-transducin to the cone outer segment is consistent with the function of cones in bright light, just as the concentration of rod alpha-transducin in dark adapted rod outer segment is consistent with their functioning in dim light.