Recoverin or visinin (VIS)

entry list
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.