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 Ca(2+)-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 alpha-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 Ca(2+) sensitivity of RetGC-1 by determining the strength of the coiled-coil interaction. Arg(838) substitutions dominantly enhance cGMP synthesis even at the highest Ca(2+) 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(838)-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.
Ramamurthy
V
V
Howard Hughes Medical Institute and Department of Biochemistry, University of Washington, Seattle, Washington 98195, USA.
Tucker
C
C
Wilkie
S E
SE
Daggett
V
V
Hunt
D M
DM
Hurley
J B
JB
D002118:Calcium / Q000378:metabolism; D006162:Guanylate Cyclase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D011956:Receptors, Cell Surface; D012374:Rod Cell Outer Segment / Q000378:metabolism
D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C110715:guanylate cyclase 1; D006162:Guanylate Cyclase; D002118:Calcium