RAB6 IS ASSOCIATED WITH A COMPARTMENT THAT TRANSPORTS RHODOPSIN FROM THE TRANS-GOLGI TO THE SITE OF ROD OUTER SEGMENT DISK FORMATION IN FROG RETINAL PHOTORECEPTORS

The biogenesis of light sensitive membranes in retinal rod photorecept ors involves polarized sorting and targeting of newly synthesized rhod opsin to a specialized domain, the rod outer segment (ROS). We have is olated and characterized the population of post-Golgi membranes that m ediate intracellular transport of rhodopsin. In the present study we h ave examined the association of small (20-25 kDa) GTP-binding (G) prot eins with these membranes. We found that one of the small G proteins, rab6, behaves like an integral membrane protein of the post-Golgi vesi cles, although approximately 30% of rab6 is soluble. The distribution of the membrane-associated and the soluble forms is highly polarized. By confocal and EM immunocytochemistry it can be seen that most of rab 6 is associated with the photoreceptor trans-Golgi cisternae, trans-Go lgi network (TGN) and post-Golgi vesicles. The photoreceptor axon and synaptic terminal are unlabeled, but dendrites of deeper retinal layer s are labeled. The distribution of rab6 across sucrose density gradien t fractions parallels the distribution of sialyltransferase (a TGN mar ker) activity. About 9% of membrane-bound rab6 is associated, however, with the rhodopsin-bearing sialyltransferase-free post-Golgi vesicles , which represent a very small fraction (< 1 %) of the total retinal m embranes. Rab6 is absent from the mature ROS disk membranes but it is present at the sites of new ROS disk formation and in the ROS cytoplas m. This suggests that rab6 becomes soluble upon disk membrane formatio n. Therefore, rab6 may function not only as a component of the sorting machinery of photoreceptors that delivers rhodopsin to its appropriat e subcellular domain but may also participate in some aspects of ROS d isk morphogenesis.