Detection of Ca2+-dependent cyclic GMP binding protein in frog rod outer segments.

For the identification of the cGMP-sensitive ion channel protein of frog rod outer segments (ROS), we analyzed cGMP binding proteins in the ROS by photoaffinity labeling with [3H]cGMP. We found three cGMP binding polypeptides (66 kDa, 92 kDa and 100 kDa) in the membrane protein fraction of ROS. cGMP binding to the 66 kDa polypeptide required the addition of 2 mM CaCl2. We propose that this polypeptide corresponds to the cGMP-activated channel protein reported by Cook et al. [(1987) Proc. Natl. Acad. Sci. USA 84, 585-589]. The 100 kDa and 92 kDa polypeptides are subunits of the cGMP phosphodiesterase.