Three-dimensional structure of guanylyl cyclase activating protein-2, a calcium-sensitive modulator of photoreceptor guanylyl cyclases

Guanylyl cyclase activating protein-2 (GCAP-2) is a Ca2+-sensitive regulato r of phototransduction in retinal photoreceptor cells. GCAP-2 activates ret inal guanylyl cyclases at low Ca2+ concentration (<100 nM) and inhibits the m at high Ca2+ (>500 nM). The light-induced lowering of the Ca2+ level from similar to 500 nM in the dark to similar to 50 nM following illumination i s known to play a key role in visual recovery and adaptation. We report her e the three-dimensional structure of unmyristoylated GCAP-2 with three boun d Ca2+ ions as determined by nuclear magnetic resonance spectroscopy of rec ombinant, isotopically labeled protein. GCAP-2 contains four EF-hand motifs arranged in a compact tandem array like that seen previously in recoverin. The root mean square deviation of the main chain atoms in the EF-hand regi ons is 2.2 Angstrom in comparing the Ca2+-bound structures of GCAP-2 and re coverin. EF-1, as in recoverin, does not bind calcium because it contains a disabling Cys-Pro sequence. GCAP-2 differs from recoverin in that the calc ium ion binds to EF-4 in addition to EF-2 and EF-3. A prominent exposed pat ch of hydrophobic residues formed by EF-1 and EF-2 (Leu(24), Trp(27), Phe(3 1), Phe(45) Phe(48), Phe(49), Tyr(81), Val(82), Leu(85), and Leu(89)) may s erve as a target-binding site for the transmission of calcium signals to gu anylyl cyclase.