Illuminated Rhodopsin Is Required for Strong Activation of Retinal Guanylate Cyclase by Guanylate Cyclase-Activating Proteins

摘要

We have recently shown that activation of retinal guanylate cyclase (retGC) by GC-activating proteins (GCAPs) is much stronger than that previously reported and that preincubation of photoreceptor outer segment homogenates with ATP or its analogue,adenylyl imidodiphosphate (AMP-PNP),is required for the strong activation [Yamazaki,A.,Yu,H.,Yamazaki,M.,Honkawa,H.,Matsuura,I.,Usukura,J.,and Yamazaki,R.K.(2003) J.Biol Chem.278,33150-33160].Here we show that illuminated rhodopsin is essential for development of the AMP-PNP incubation effect.This was demonstrated by illumination of dark homogenates and treatments of illuminated homogenates with 11 -cis-retinal and hydroxylamine prior to the AMP-PNP incubation and by measurement of the GCAP2 concentration required for 50% activation.We also found that the AMP-PNP incubation effect was not altered by addition of guanosine 5'-O-(3-thiotriphosphate),indicating that transducin activation is not required.It is concluded that illuminated rhodopsin is involved in retGC activation in two ways: to initiate the ATP incubation effect for preparation of retGC activation as shown here and to reduce the Ca~(2+) concentrations through cGMP phosphodiesterase activation as already known.These two signal pathways may be activated in a parallel and perhaps proportional manner and finally converge for strong activation of retGC by Ca~(2+)-free GCAPs.