A Ca2+-Binding Protein that Regulates the Lifetime of the Transducin Active State

Abstract

Abstract—It has been previously shown in the study of the kinetic behavior of cGMP-specific phosphodiesterase in preparations of the outer segments of bovine retinal rods, which was temporarily activated by 2 μM GTP at high and low concentrations of free ions of Ca²⁺ (100 μM and <10 nm, respectively), that the lifetime of cGMP-specific phosphodiesterase in the active state significantly decreased (by ≈2 times) when low concentrations of free calcium ions were used. This suggested that there is a Ca²⁺-binding protein in the outer segments of bovine retinal rods that can interact with the so-called free transducin and regulate the rate of hydrolysis of GTP, which is bound in its active site, depending on the concentration of free Ca²⁺ ions. This paper is devoted to a discussion of the known Ca²⁺-binding proteins that occur in the outer segments of bovine retinal rods that could play the role of a potential regulator.