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.