Specific cGMP binding by retinal rod axoneme and its modulation by calcium ions and calmodulin.

The high-affinity binding of cGMP to the retinal rod axoneme was identified. The axoneme was shown to contain two types of binding sites, with concentrations of 3.6 X 10(-10) and 5.8 X 10(-11) mol mg protein-1. The cGMP concentration for half-saturation of binding was 0.35 microM. The inhibition of cGMP binding by colchicine and vinblastine was 20% of Ca2+ and calmodulin control cGMP binding. The effect of calmodulin is explained by its interaction with specific binding sites which are possibly associated with Ca(2+)-induced depolymerization of axoneme microtubules.