Light-Mediated Activation of Rac-1 in Photoreceptor Outer Segments

• Published in: Current biology 2003-08, Vol.13 (15), p.1306-1310
• Main Authors: Balasubramanian, Nagaraj, Slepak, Vladlen Z.
• Format: Article
• Language: English
• Subjects: Animals; Blotting, Western; Cattle; Electrophoresis, Polyacrylamide Gel; Fluorescent Antibody Technique; Guanosine 5'-O-(3-Thiotriphosphate) - metabolism; Humans; Photic Stimulation; Precipitin Tests; rac GTP-Binding Proteins - metabolism; Rhodopsin - metabolism; Rod Cell Outer Segment - metabolism; Signal Transduction
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/S0960-9822(03)00511-6

Small GTP binding proteins regulate diverse biological processes including gene expression, cytoskeleton reorganization, and protein and vesicular transport [1–4]. While small GTPases have been investigated in a wide variety of cells, few studies have addressed their role in photoreceptors. In vertebrate retinal rods, the light stimulus is transmitted from rhodopsin via the pathway mediated by the heterotrimeric G protein transducin [5–7]. To increase their sensitivity to light, photoreceptors accumulate remarkably high concentrations of rhodopsin and transducin in specialized cellular compartments, the outer segments (OS). Transport of these proteins from the inner segments is regulated by the small GTPases Rab6 and Rab8, which do not enter OS [8–10]. Here, we asked if small G proteins have other functions in photoreceptors. We show that OS contain the small GTPase Rac-1, a member of the Rho family. In contrast to other cells, Rac-1 in OS is exclusively associated with the membranes and resides in lipid rafts. Most importantly, Rac-1 is activated by light. This activation is specifically blocked by a synthetic peptide corresponding to the Asn-Pro-X-X-Tyr motif found in rhodopsin, and Rac-1 coprecipitates with rhodopsin on Concanavalin A Sepharose. These data provide the first direct evidence for the existence of a novel pathway activated by rhodopsin.