Focus on molecules: guanylate cyclase-activating proteins (GCAPs).

GCAPs are neuronal Ca2þ-binding proteins of the calmodulin (CaM) superfamily expressed in photoreceptors (Palczewski et al., 1994). Whereas mammals express up to three GCAPs (GCAP1-3), as many as eight distinct GCAPs can be identified in the genomes of lower vertebrates. These proteins exhibit about 50% sequence similarity and identical domain structures; all are N-myristoylated or modified by other fatty acid groups. GCAPs are composed of four EF-hand motifs, two each in the N-terminal and C-terminal domains. Crystallization of chicken myrGCAP1 with Ca2þ bound reveals a compact structure (Fig. 1) (Stephen et al., 2007). In contrast to other neuronal Ca2þ-binding proteins, the myristoyl group in GCAP1 is completely buried between the Nand Cterminal helices (green and blue in Fig. 1), and remains buried even after Ca2þ dissociation from the protein. Hallmarks of GCAP structure are high affinity Ca2þ-binding sites termed EF-hand motifs consisting of helix-loop-helix secondary elements. In the canonical EF-hand motif, the loop consists of a 12 amino acid-long sequence rich in acidic residues that provide oxygen groups for Ca2þ coordination. Residues adjacent to this loop are generally hydrophobic (I, L, Y, and W), and the entire loop is flanked by a-helical structures. In GCAP1, EF-hand 1 (residues 17–42) and EF-hand 2 (residues 50–82) form the N-terminal domain, whereas EF-hand 3 (residues 87–118) and EF-hand 4 (residues 130–160) are contained in the C-terminal domain. EF