Calcium and cyclic GMP regulation of light-sensitive protein phosphorylation in frog photoreceptor membranes.
In frog photoreceptor membranes, light induces a dephosphorylation of two small proteins and a phosphorylation of rhodopsin. The level of phosphorylation of the two small proteins is influenced by cyclic GMP. Measurement of their phosphorylation as a function of cyclic GMP concentration shows fivefold stimulation as cyclic GMP is increased from 10(-5) to 10(-3) M. This includes the concentration range over which light activation of a cyclic GMP phosphodiesterase causes cyclic GMP levels to fall in vivo. Cyclic AMP does not affect the phosphorylations. Calcium ions inhibit the phosphorylation reactions. Calcium inhibits the cyclic GMP-stimulated phosphorylation of the small proteins as its concentration is increased from 10(-6) to 10(-3) M, with maximal inhibition of 70% being observed. Rhodopsin phosphorylation is not stimulated by cyclic nucleotides, but is inhibited by calcium, with 50% inhibition being observed as the Ca++ concentration is increased from 10(-9) to 10(-3) M. A nucleotide binding site appears to regulate rhodopsin phosphorylation. Several properties of the rhodopsin phosphorylation suggest that it does not play a role in a rapid ATP-dependent regulation of the cyclic GMP pathway. Calcium inhibition of protein phosphorylation is a distinctive feature of this system, and it is suggested that Ca++ regulation of protein phosphorylation plays a role in the visual adaptation process. Furthermore, the data provide support for the idea that calcium and cyclic GMP pathways interact in regulating the light-sensitive conductance.
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002625:Chemistry, Organic; D006152:Cyclic GMP / Q000502:physiology*; D005136:Eye Proteins / Q000378:metabolism*; D008027:Light; D008566:Membranes / Q000378:metabolism; D055601:Organic Chemistry Phenomena; D010766:Phosphorylation; D010786:Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D011892:Rana catesbeiana; D011898:Ranidae; D012243:Rhodopsin / Q000378:metabolism; D013997:Time Factors
D005136:Eye Proteins; D012243:Rhodopsin; D006152:Cyclic GMP; D002118:Calcium