Retina International's Scientific Newsletter - Recovery of cGMP and Cation Concentrations of the Dark State

	Retina International's Scientific Newsletter
	The Protein Pages Recovery of cGMP and Cation Concentrations of the Dark State

Recent update from: 10.01.2000

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While PDE hydrolyses cGMP there must be a mechanism that restores the cGMP concentration after bleaching and in response to changes in Ca²⁺ ions produced by a closure of the CNCG at low cGMP levels.
Guanylate cyclase (RetGC) , that is bound to membranes (particulate), forms cGMP from GTP after illumination, dependent on Ca²⁺ and an activating protein (GCAP) (1), (5), (7). Reduced cGMP concentrations close the CNCG . Closure of the CNCG leads to a decrease of the intracellular Ca²⁺ concentration. GCAP is reported to bind GC after release of Ca²⁺ ions. On binding of GCAP, GC starts to produce cGMP that is decreased on PDE activity. By opening the CNCG again Ca²⁺ concentration increases and GCAP-Ca²⁺ is removed from GC which is then inactive.
Two proteins termed retinal GC (RetGC) and rod outer segment GC (ROSGC) have been reported. They were found to be encoded by the same gene (4) . Additionally, it could be shown that rods and cones contain different forms of RetGC (6).
Concerning GCAP two proteins (GCAP1 and GCAP2-P24) were introduced (1) (3) (5) (2) . Gorczyca et al. (5) reported on the activity of the bovine form of GCAP to be possibly involved in activation of several proteins of the visual cascade. Results on GC showed rod specific GC to be regulated by GCAP1 and GCAP2 while cone GC is regulated by GCAP1.

References

Baehr,W., Gorczyca,W., Subbaraya,I., Ohguro,H., Johnson,R.S., Walsh,K.A., Gray Keller,M.P., Detwiler,P.B., Crabb,J., and Palczewski,K. Isolation, cloning, and functional characterization of bovine retina guanylate cyclase activating protein (GCAP). 1994; Invest.Ophthalmol.Vis.Sci. 35 (Suppl.): 1486
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Dizhoor,A.M., Olshevskaya,E.V., and Hurley,J.B. A novel retinal Ca2+-binding protein, p24, stimualtes photoreceptor guanylyl cyclase and associates with the outer segment membranes in a calcium-sensitive manner. 1995; Invest.Ophthalmol.Vis.Sci. 36: S223
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Dizhoor,A.M., Olshevskaya,E.V., Laura,R., Lowe,D.G., and Hurley,J.B. A membrane guanylyl cyclase RetGC from human photoreceptors is regulated by a specific activator and calcium. 1994; Invest.Ophthalmol.Vis.Sci. 35 (Suppl.): 1486
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Duda,T., Goraczniak,R.M., Sitaramayya,A., and Sharma,R.K. Cloning of a photoreceptor guanylate cyclase. 1994; Invest.Ophthalmol.Vis.Sci. 35 (Suppl.): 1463
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Gorczyca,W.A., Gray Keller,M.P., Detwiler,P.B., and Palczewski,K. Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods. 1994; Proc.Natl.Acad.Sci.U.S.A. 91: 4014-4018.
Link to PubMed
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Liu,M.Y., Li,J., and Yau,K.W. Phosphorylation of the N-terminal domain of human rod cyclic nucleotide-gated channel by protein kinase c and the cAMP- dependent protein kinase. 1994; Invest.Ophthalmol.Vis.Sci. 35 (Suppl.): 1474
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Oliveira,L., Miniou,P., Viegas-Pequignot,E., Rozet,J.M., Dollfus,H., Pittler,S.J., and Viegas Pequignot,E. Human retinal guanylate cyclase (GUC2D) maps to chromosome 17p13.1. 1994; Genomics. 22: 478-481.
Link to PubMed
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Contact the editor	This site is maintained and edited by Dr. rer. medic. Markus Preising, Dipl.Biol. Molecular Genetics Laboratory Department of Paediatric Ophthalmology, Strabismology and Ophthalmogenetics University of Regensburg Head: Prof. Dr. med. Birgit Lorenz