Interactions within the coiled-coil domain of RetGC-1 guanylyl cyclase are optimized for regulation rather than for high affinity

RetGC-1, a member of the membrane guanylyl cyclase family of proteins, is regulated in photoreceptor cells by a Ca2+-binding protein known as GCAP-1, Proper regulation of RetGC-1 is essential in photoreceptor cells for normal light adaptation and recovery to the dark state. In this study we show that cGMP synthesis by RetGC-1 requires dimerization, because critical functions in the catalytic site must be provided by each of the two polypeptide chains of the dimer, We also show that an intact cy-helical coiled-coil structure is required to provide dimerization strength for the catalytic domain of RetGC-1, However, the dimerization strength of this domain must be precisely optimized for proper regulation by GCAP-1, We found that Arg(838) within the dimerization domain establishes the Ca2+ sensitivity of RetGC-1 by determining the strength of the coiled-coil interaction. Arg(838) substitutions dominantly enhance cGMP synthesis even at the highest Ca2+ concentrations that occur in normal dark-adapted photoreceptor cells, Molecular dynamics simulations suggest that Arg(838) substitutions disrupt a small network of salt bridges to allow an abnormal extension of coiled-coil structure. Substitutions at Arg(838) were first identified by linkage to the retinal degenerative disease, autosomal dominant cone rod dystrophy (adCORD). Consistent with the characteristics of this disease, the Arg(898)-substituted RetGC-1 mutants exhibit a dominant biochemical phenotype, We propose that accelerated cGMP synthesis in humans with adCORD is the primary cause of cone-rod degeneration.