The route of the visual receptor, rhodopsin, along the cilium

Abstract

The photoreceptor outer segment is the most elaborate primary cilium, containing large amounts of rhodopsin (RHO) in disk membranes that grow from a connecting cilium. The movement of RHO along the connecting cilium precedes formation of the disk membranes. However, the route that RHO takes has not been clearly determined; some reports suggest that it follows an intracellular, vesicular route along the axoneme, possibly as an adaptation for the high load of delivery or the morphogenesis of the disk endomembranes. We addressed this question by studying RHO in cilia of IMCD3 cells and mouse rod photoreceptors. In IMCD3 cilia, fluorescence recovery after photobleaching (FRAP) experiments with fluorescently-tagged RHO supported RHO motility in the ciliary plasma membrane and was inconsistent with RHO motility within the lumen of the cilium. In rod photoreceptors, FRAP of RHO-EGFP was altered by externally-applied lectin, supporting plasmalemmal RHO dynamics. Quantitative immunoEM corroborated our live cell conclusions, detecting RHO distributed along the plasma membrane of the connecting cilium, with negligible labeling within the axoneme. Together, the present findings demonstrate RHO trafficking entirely via the ciliary plasma membrane.