Calcium-activated opsin phosphatase activity in retinal rod outer segments

Abstract

We describe the presence in bovine retinal rod outer segments of a phosphatase which dephosphorylates phosphoopsin with an efficiency similar to that of PP2A, and which is stimulated by submicromolar levels of Ca2+ (half-maximal activation, 0.4-0.5 μM). This enzyme is designated Ca2+- activated opsin phosphatase (CAOP). CAOP has a molecular mass of 70-75 kDa as determined by gel filtration on Superose 12 and exhibits reversible Ca2+- dependent oligomerization. An unidentified protein of approximately 25 kDa is necessary for full activity of CAOP and for cooperative binding of Ca2+ (h>2). CAOP does not require Mg2+ and is inhibited by okadaic acid (median inhibitory concentration >25 μM), which suggests that it is related to the PP1/2A/2B class of protein phosphatases. Like PP2B, CAOP is inhibited by trifluoperazine (median inhibitory concentration 40 μM), but calmodulin has no effect on CAOP activity, and CAOP is inhibited by mastoparan at much higher concentrations than PP2B. This combination of properties suggests that CAOP is not identical to any characterized protein phosphatase. Since the cytoplasmic concentration of Ca2+ in retinal rod outer segments is reduced upon excitation by light, the existence of a Ca2+-sensitive opsin phosphatase activity suggests that light-dependent Ca2+ levels may control rhodopsin dephosphorylation.