Recoverin or visinin (VIS)

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Recoverin or visinin (VIS) regulates guanylate cyclase activity in retinal rod outer segments of vertebrates. In the dark, cationic channels are kept open by bound cGMP. Ca2+ enters through these channels; this influx is matched by efflux through a Na+-K+, Ca2+ exchanger. Light activates an enzyme cascade that stimulates cGMP hydrolysis, leading to channel closure, which blocks calcium influx but not its efflux. Free [Ca2+] in the cell drops within 0.5 s of light stimulation. Recoverin prolongs the photo-response by blocking the phosphorylation of photoexcited rhodoposin by rhodopsin kinase. Isoforms, visinin and neurocalcin, are found in nerve cells; frequenin was characterized in Drosophila. Recoverin is N-myristylated. In the crystal structure [21] of nonmyristoylated recoverin a single Sm3+ ion is bound in place of Ca2+ in EF-hand 2. The hydrophobic patches of lobe 1,2 and of lobe 3,4 are exposed and orientated so that their two clefts could not enfold a target helix as seen in CAM, TNC, ELC, RLC and BM40. NMR studies [22] showed that the myristoyl side chain is buried between the two lobes in the apo-form but is exposed following calcium binding to EF-hands 2 and 3.