Function of CNG-modulin, a novel regulator of the cone cGMP-gated channel

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DESCRIPTION (provided by applicant): The objective of this research project is to uncover molecular mechanisms responsible for light adaptation in cones. In any given species, rod responses to light are slower in time course and more sensitive to light than cone responses. But the most dramatic difference between rods and cones is in their ability to adapt to ambient illumination. Cones can adapt to over 9 orders of magnitude of the background light intensity, while rods have a limited ability to adapt. Our hypothesis states that light adaptation in cones arises, in part, from Ca2+dependent changes in the ligand sensitivity of the cGMP-gated ion channels. In our previous work, we characterized cone modulation and observed a significantly larger modulation than rods, which is mediated by an unknown soluble modulator protein. Recently, we have identified a novel protein that we named CNG-modulin, which is expressed in cones, but not in rods, and binds to the cone cGMP-gated channel while co-expressed in yeast and in cultured cell lines. In pilot experiments, this protein modulates the sensitivity of the channel to cGMP in a Ca2+dependent manner in excised membrane patches from striped bass cones. These cumulative observations strongly suggest that CNG-modulin is the authentic cone channel modulator, but further functional studies are required for an unambiguous assignment of this function. We will test this hypothesis with experiments designed (1) to compare the quantitative features of the modulation using recombinant CNG-modulin in cone membrane patches with those of the endogenous modulation, and (2) through functional studies of genetically modified cone photoreceptors altered to suppress the expression of CNG-modulin. Successful completion of this project will verify, for the first time, the identification of the cone channel modulator and will provide us with the understanding of the role of this novel molecular mechanism in the cone function. It will also provide another target protein in the molecular analysis of photoreceptor-related genetic diseases. PUBLIC HEALTH RELEVANCE: This research project aims to characterize the function of a newly discovered cone- specific protein, a modulator of the cone cyclic nucleotide gated channel. It will further our understanding of the molecular mechanism of light adaptation and will provide a basis for exploring its possible role in photoreceptor-related genetic diseases.