N-MYRISTOYLATION OF RECOVERIN ENHANCES ITS EFFICIENCY AS AN INHIBITOROF RHODOPSIN KINASE

Recoverin, a recently identified member of the EF-hand superfamily of Ca2+-binding proteins, is capable to inhibit rhodopsin phosphorylation by rhodopsin kinase at high but not at low free [Ca2+]. The N-termina l glycine residue of retinal recoverin is heterogeneously acylated wit h myristoyl or related N-acyl group. To clarify the role of the IV-ter minal acylation of recoverin in its inhibitory action upon rhodopsin p hosphorylation, we compared the efficiency of myristoylated and non-my ristoylated forms of recombinant recoverin as inhibitors of rhodopsin kinase activity, We have found that rhodopsin phosphorylation by purif ied rhodopsin kinase, which does not depend on free [Ca2+] in the abse nce of recoverin, is regulated by Ca2+ in the presence of both forms o f the recombinant protein, EC(50), values for Ca2+ are the same (2 mu M) for the myristoylated and non-myristoylated forms; the Hill coeffic ients of 1.7 and 0.9, respectively, indicate that the effect is cooper ative with respect to Ca2+ only for myristoylated recoverin, In the pr esence of Ca2+, both forms of recoverin taken at saturated concentrati ons cause an almost equal inhibition of rhodopsin phosphorylation, How ever, the inhibitory action of the myristoylated form occurs at much l ower its concentrations than that of the non-myristoylated form (EC(50 ) are 0.9 and 6.5 mu M, respectively).