A Role for Cytoskeletal Elements in the Light-Driven Translocation of Proteins in Rod Photoreceptors

PURPOSE.—Light-driven protein translocation is responsible for the dramatic redistribution of some proteins in vertebrate rod photoreceptors. In this study, the involvement of microtubules and microfilaments in the light-driven translocation of arrestin and transducin was investigated. METHODS.—Pharmacologic reagents were applied to native and transgenic Xenopus tadpoles, to disrupt the microtubules (thiabendazole) and microfilaments (cytochalasin D and latrunculin B) of the rod photoreceptors. Quantitative confocal imaging was used to assess the impact of these treatments on arrestin and transducin translocation. A series of transgenic tadpoles expressing arrestin truncations were also created to identify portions of arrestin that enable arrestin to translocate. RESULTS.—Application of cytochalasin D or latrunculin B to disrupt the microfilament organization selectively slowed only transducin movement from the inner to the outer segments. Perturbation of the microtubule cytoskeleton with thiabendazole slowed the translocation of both arrestin and transducin, but only in moving from the outer to the inner segments. Transgenic Xenopus expressing fusions of green fluorescent protein (GFP) with portions of arrestin implicates the C terminus of arrestin as an important portion of the molecule for promoting translocation. This C-terminal region can be used independently to promote translocation of GFP in response to light. CONCLUSIONS.—The results show that disruption of the cytoskeletal network in rod photoreceptors has specific effects on the translocation of arrestin and transducin. These effects suggest that the light-driven translocation of visual proteins at least partially relies on an active motordriven mechanism for complete movement of arrestin and transducin. Transport of molecules is critical to the proper functioning of cells, but even more so for polarized cells. In neurons, perhaps the epitome of polarized cells, molecular transport uses both fast and slow components to renew membrane lipids and protein elements in the membrane and cytosol. Rod photoreceptors are arguably one of the most highly polarized cells with regard to both structure and function. Structurally, rods are divided into two segments, the rod outer segment (ROS) and the rod inner segment (RIS), joined by a narrow connecting cilium. The ROS contains a highly elaborate system of stacked, disc-shaped membranes that are densely packed with the visual pigment rhodopsin, whereas the RIS is more typical of the