Abstract
Guanylate cyclase activating protein‐2 (GCAP‐2) is a Ca^2+^‐binding protein of the neuronal calcium sensor (NCS) family. Ca^2+^‐free GCAP‐2 activates the retinal rod outer segment guanylate cyclases ROS‐GC1 and 2. Native GCAP‐2 is N‐terminally myristoylated. Detailed structural information on the Ca^2+^‐dependent conformational switch of GCAP‐2 is missing so far, as no atomic resolution structures of the Ca^2+^‐free state have been determined. The role of the myristoyl moiety remains poorly understood. Available functional data is incompatible with a Ca^2+^‐myristoyl switch as observed in the prototype NCS protein, recoverin. For the homologous GCAP‐1, a Ca^2+^‐independent sequestration of the myristoyl moiety inside the proteins structure has been proposed. In this article, we compare the thermodynamic stabilities of myristoylated and non‐myristoylated GCAP‐2 in their Ca^2+^‐bound and Ca^2+^‐free forms, respectively, to gain information on the nature of the Ca^2+^‐dependent conformational switch of the protein and shed some light on the role of its myristoyl group. In the absence of Ca^2+^, the stability of the myristoylated and non‐myristoylated forms was indistinguishable. Ca^2+^ exerted a stabilizing effect on both forms of the protein, which was significantly stronger for myr GCAP‐2. The stability data were corroborated by dye binding experiments performed to probe the solvent‐accessible hydrophobic surface of the protein. Our results strongly suggest that the myristoyl moiety is permanently solvent‐exposed in Ca^2+^‐free GCAP‐2, whereas it interacts with a hydrophobic part of the protein's structure in the Ca^2+^‐bound state.