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).
