Calcium binding, structural stability and guanylate cyclase activation in GCAP1 variants associated with human cone dystrophy.

Guanylate cyclase activating protein 1 (GCAP1) is a neuronal Ca(2+) sensor (NCS) that regulates the activation of rod outer segment guanylate cyclases (ROS-GCs) in photoreceptors. In this study, we investigated the Ca(2+)-induced effects on the conformation and the thermal stability of four GCAP1 variants associated with hereditary human cone dystrophies. Ca(2+) binding stabilized the conformation of all the GCAP1 variants independent of myristoylation. The myristoylated wild-type GCAP1 was found to have the highest Ca(2+) affinity and thermal stability, whereas all the mutants showed decreased Ca(2+) affinity and significantly lower thermal stability in both apo and Ca(2+)-loaded forms. No apparent cooperativity of Ca(2+) binding was detected for any variant. Finally, the non-myristoylated mutants were still capable of activating ROS-GC1, but the measured cyclase activity was shifted toward high, nonphysiological Ca(2+) concentrations. Thus, we conclude that distorted Ca(2+)-sensor properties could lead to cone dysfunction.