The Transducin Cycle in the Phototransduction Cascade

Abstract

Among the numerous heterotrimeric G-proteins that have now been characterized, transducin, the G-protein that conveys the visual transduction signal in vertebrate retinal rods, was often considered as a special case: It is enormously abondant in the highly specialised rod outer segment (10 to 20% of total protein content of the retinal rod outer segment) and it is soluble: its extraction from ROS membranes does not require any detergent. Indeed, in the physiological cycle, the TαGTP subunit is released from the membrane after the rhodopsin-catalysed exchange of GDP for GTP, and it takes a cytoplasmic route to reach its membrane bound effector. This solubility might be uniquely related to the very peculiar morphology of the ROS that are filled up with tightly stacked discs. The cytoplasm is divided in extremely thin layers, (150 Å thickness) in which transducin remains confined at a high concentration (500 μM). Thus a soluble TαGTP cannot drift very far while shuttling between rhodopsin and PDE molecules present on the two membrane surfaces that limit the cytoplasmic layer. Other G-proteins, in more usual cellular environments, would dilute in the cytoplasm if they were released from the cell membrane. However transducin does not seem to differ drastically from the other G-proteins, none of which is an intrinsic membrane protein.