- Light adaptation of photoreceptor cells is mediated by Ca2+-dependent mechanisms. In darkness, Ca2+ influx through cGMP-gated channels into the outer segment of photoreceptors is balanced by Ca2+ extrusion via Na+/Ca2+, K+ exchangers (NCKXs). Light activates a G protein signaling cascade, which closes cGMP-gated channels and decreases Ca2+ levels in photoreceptor outer segment because of continuing Ca2+ extrusion by NCKXs. Guanylate cyclase–activating proteins (GCAPs) then up-regulate cGMP synthesis by activating retinal membrane guanylate cyclases (RetGCs) in low Ca2+.
- GCAP1, a member of the neuronal calcium sensor subclass of the calmodulin superfamily, confers Ca2+-sensitive activation of retinal guanylyl cyclase 1 (RetGC1). We present NMR resonance assignments, residual dipolar coupling data, functional analysis, and a structural model of GCAP1 mutant (GCAP1V77E) in the Ca2+-free/Mg2+-bound state. NMR chemical shifts and residual dipolar coupling data reveal Ca2+-dependent differences for residues 170–174. An NMR-derived model of GCAP1V77E contains Mg2+ bound at EF2 and looks similar to Ca2+ saturated GCAP1 (root mean square deviations = 2.0 Å).