Phosphodiesterase of Cone Photoreceptors from the Lizard, Anolis carolinensis

Cone and rod photoreceptors utilize cyclic guanosine monophosphate (cGMP) in the light regulation of membrane polarization. The prototype for visual transduction is established for rod photoreceptors, which utilize a cascade of reactions to regulate a cyclic nucleotide phosphodiesterase (PDE) (EC 3.1.4.17) and thereby control the intracellular concentration of cGMP. Although cones appear to utilize a comparable cGMP cascade for their phototransduction, evidence exists that the PDE from cone photoreceptors may be different from that of rods. Dissociated cone photoreceptors, isolated retinas, and cone outer segments from the lizard, Anolis carolinensis, have been used to identify and characterize a PDE enzyme complex that shares several features in common with the rod outer segment (ROS) PDE complex. Immunoadsorption and sodium dodecyl sulfate-polyacrylamide gel electrophoresis have identified a subunit of lizard cone PDE that has an apparent electrophoretic mobility of 84 kDa and a subunit of lizard rod PDE that migrates at ∼90 kDa. The lizard cone PDE complex is similar in size, extraction, activation, and immunological characteristics to the PDE complex of rod photoreceptors from lizard, bovine, and human retinas. The lizard cone PDE complex, and perhaps that from cone photoreceptors in general, differs from that of ROS in its chromatographic properties on anion-exchange resins. The sharing of physical and activation properties of the rod and cone PDE complex is compatible with the phototransduction process occurring by a similar mechanism in both cell types. The differences in light sensitivity and speed of response may be attributable to features of the individual proteins that form the PDE complexes of rods and cones or to other undisclosed features of the respective cascades.