Phototransduction is a series of enzymatic events within retinal photoreceptor cells that converts light to chemical and electrical energy. Light activates the visual photopigment rhodopsin, leading to activation of phosphodiesterase (PDE) which hydrolyzes cyclic GMP (cGMP) to GMP. The decline in cGMP levels closes cGMP-gated sodium/calcium channels resulting in photoreceptor hyperpolarization. A new enzyme was recently discovered in the bovine retina. This enzyme, recovering, is a 26 kilodalton calcium-binding protein that is thought to reverse the effects of light on cGMP levels by activating guanylate cyclase. Guanylate cyclase produces cGMP to overcome the cGMP- hydrolyzing effect of PDE, and reopens the sodium/calcium channels in photoreceptor outer segments. A recovering-like protein may also be present in some bipolar cells. We have recently cloned and sequenced two closely related forms of recovering in human retina. The goal of the proposed research is to utilize specific nucleotide and immunochemical probes to identify the cellular sites of recovering synthesis in the human retina, and to characterize the regulation of recovering gene expression. We also propose to characterize and further develop polymorphic sequences at the recovering locus. These markers can eventually be used to determine if the recovering gene is genetically-linked to loci responsible for inherited diseases of vision. Many investigators suspect that the various forms of retinitis pigmentosa may be due to abnormalities of proteins involved in phototransduction. The important role of recovering in visual transduction makes it plausible that mutations in the gene encoding recovering could be responsible for a form of retinal degeneration.
