A POSSIBLE ROLE OF RGS9 IN PHOTOTRANSDUCTION - A BRIDGE BETWEEN THE CGMP-PHOSPHODIESTERASE SYSTEM AND THE GUANYLYL CYCLASE SYSTEM

In the current concept of phototransduction, the concentration of cGMP in retinal rod outer segments is controlled by the balance of two enz yme activities: cGMP phosphodiesterase (PDE) and guanylyl cyclase (GC) . However, no protein directly mediates these two enzyme systems. Here we show that RGS9, which is suggested to control PDE activity through regulation of transducin GTPase activity (He, W., Cowan, C. W., and W ensel, T, G. (1998) Neuron 20, 95-102), directly interacts with GC, Wh en proteins in the Triton X-100-insoluble fraction of bovine rod outer segments were isolated by two-dimensional gel electrophoresis and bin ding of GC to these proteins was examined using a GC-specific antibody , proteins (55 and 32 kDa) were found to interact with GC. However, th e activity of GC bound to the 55-kDa protein was not detected. This ob servation was elucidated by the finding that the 55-kDa protein inhibi ted GC activity in a dose-dependent manner. Amino acid sequence showed that five peptides derived from the 55-kDa protein were identical to corresponding peptides of RGS9. Together with other biochemical charac terization of the 55-kDa protein, these observations indicate that the 55-kDa protein is RGS9 and that RGS9 inhibits GC. RGS9 may serve as a mediator between the PDE and GC systems.