Molecular properties of human guanylate cyclase–activating protein 2 (GCAP2) and its retinal dystrophy–associated variant G157R

In murine and bovine photoreceptors, guanylate cyclase–activating protein 2 (GCAP2) activates retinal guanylate cyclases (GCs) at low Ca(2+) levels, thus contributing to the Ca(2+)/cGMP negative feedback on the cyclase together with its paralog guanylate cyclase–activating protein 1, which has the same function but different Ca(2+) sensitivity. In humans, a GCAP2 missense mutation (G157R) has been associated with inherited retinal degeneration (IRD) via an unknown molecular mechanism. Here, we characterized the biochemical properties of human GCAP2 and the G157R variant, focusing on its dimerization and the Ca(2+)/Mg(2+)-binding processes in the presence or absence of N-terminal myristoylation. We found that human GCAP2 and its bovine/murine orthologs significantly differ in terms of oligomeric properties, cation binding, and GC regulation. Myristoylated GCAP2 endothermically binds up to 3 Mg(2+) with high affinity and forms a compact dimer that may reversibly dissociate in the presence of Ca(2+). Conversely, nonmyristoylated GCAP2 does not bind Mg(2+) over the physiological range and remains as a monomer in the absence of Ca(2+). Both myristoylated and nonmyristoylated GCAP2 bind Ca(2+) with high affinity. At odds with guanylate cyclase–activating protein 1 and independently of myristoylation, human GCAP2 does not significantly activate retinal GC1 in a Ca(2+)-dependent fashion. The IRD-associated G157R variant is characterized by a partly misfolded, molten globule-like conformation with reduced affinity for cations and prone to form aggregates, likely mediated by hydrophobic interactions. Our findings suggest that GCAP2 might be mostly implicated in processes other than phototransduction in human photoreceptors and suggest a possible molecular mechanism for G157R-associated IRD.