The myristoylation of guanylate cyclase-activating protein-2 causes an increase in thermodynamic stability in the presence but not in the absence of Ca2+

Guanylate cyclase activating protein-2 (GCAP-2) is a Ca2+-binding protein of the neuronal calcium sensor (NCS) family. Ca2+-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 Ca2+-dependent conformational switch of GCAP-2 is missing so far, as no atomic resolution structures of the Ca2+-free state have been determined. The role of the myristoyl moiety remains poorly understood. Available functional data is incompatible with a Ca2+-myristoyl switch as observed in the prototype NCS protein, recoverin. For the homologous GCAP-1, a Ca2+-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 Ca2+-bound and Ca2+-free forms, respectively, to gain information on the nature of the Ca2+-dependent conformational switch of the protein and shed some light on the role of its myristoyl group. In the absence of Ca2+, the stability of the myristoylated and non-myristoylated forms was indistinguishable. Ca2+ 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 Ca2+-free GCAP-2, whereas it interacts with a hydrophobic part of the protein's structure in the Ca2+-bound state.