Ca2+-dependent conformational changes in bovine GCAP-2.

GCAP-2, a mammalian photoreceptor-specific protein, is a Ca2+-dependent regulator of the retinal membrane guanylyl cyclases (Ret-GCs). Sensing the fall in intracellular free Ca2+ after photo-excitation, GCAP-2 stimulates the activity of Ret-GC leading to cGMP production. Like other members of the recoverin superfamily, GCAP-2 is a small N-myristoylated protein containing four EF-hand consensus motifs. In this study, we demonstrate that like recoverin and neurocalcin, GCAP-2 alters its conformation in response to Ca2+-binding as measured by a Ca2+-dependent change in its far UV CD spectrum. Differences in the conformation of the Ca2+-bound and Ca2+-free forms of GCAP-2 were also observed by examining their relative susceptibility to V8 protease. In contrast to recoverin, we do not observe proteolytic cleavage of the myristoylated N-terminus of Ca2+-bound GCAP-2. NMR spectra also show that, in contrast to recoverin, the chemical environment of the N-terminus of GCAP-2 is not dramatically altered by Ca2+ binding. Despite the similarity of GCAP-2 and recoverin, the structural consequences of Ca2+-binding for these two proteins are significantly dissimilar.