Zinc causes an apparent increase in rhodopsin phosphorylation

Purpose. Rhodopsin is a zinc-binding protein. We investigated the effect of low concentrations of zinc on the initial phosphorylation of rhodopsin. Methods. Dark-adapted bovine rod outer segments (ROS) were incubated with (gamma(32)P)ATP and 5 mM magnesium in the presence and absence of micromolar amounts of zinc. The ROS were exposed to light to initiate phosphorylation under conditions which allow only limited initial phosphorylation. Results. We found that zinc enhanced the rhodopsin phosphorylation apparent on autoradiographies by several fold. Phosphorylation reactions conducted in the presence of potent phospho-opsin phosphatase inhibitors show a comparable zinc-enhanced phosphorylation of rhodopsin. Under our reaction conditions, ROS membranes also appear more red upon initial exposure to light when zinc is present. Conclusions. Zinc can increase initial rhodopsin phosphorylation, apparently acting at the substrate rhodopsin and not at relevant phosphatases or rhodopsin kinase. How zinc binding to rhodopsin might increase its ability to serve as a substrate for phosphorylation is under investigation.