Characterization of retinal guanylate cyclase‐activating protein 3 (GCAP3) from zebrafish to man

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Abstract
Calmodulin‐like neuronal Ca2+‐binding proteins (NCBPs) are expressed primarily in neurons and contain a combination of four functional and nonfunctional EF‐hand Ca2+‐binding motifs. The guanylate cyclase‐activating proteins 1–3 (GCAP1–3), the best characterized subgroup of NCBPs, function in the regulation of transmembrane guanylate cyclases 1–2 (GC1–2). The pairing of GCAPs and GCs in vivo depends on cell expression. Therefore, we investigated the expression of these genes in retina using in situ hybridization and immunocytochemistry. Our results demonstrate that GCAP1, GCAP2, GC1 and GC2 are expressed in human rod and cone photoreceptors, while GCAP3 is expressed exclusively in cones. As a consequence of extensive modification, the GCAP3 gene is not expressed in mouse retina. However, this lack of evolutionary conservation appears to be restricted to only some species as we cloned all three GCAPs from teleost (zebrafish) retina and localized them to rod cells, short single cones (GCAP1–2), and all subtypes of cones (GCAP3). Furthermore, sequence comparisons and evolutionary trace analysis coupled with functional testing of the different GCAPs allowed us to identify the key conserved residues that are critical for GCAP structure and function, and to define class‐specific residues for the NCBP subfamilies.