Crystal-Structure and Biochemical Characterization of Recombinant Human Calcyphosine Delineates a Novel EF-Hand-Containing Protein Family

Calcyphosine is an EF-hand protein involved in both Ca2 +-phosphatidylinositol and cyclic AMP signal cascades, as well as in other cellular functions. The crystal structure of Ca2 +-loaded calcyphosine was determined up to 2.65 Å resolution and reveals a protein containing two pairs of Ca2 +-binding EF-hand motifs. Calcyphosine shares a highly similar overall topology with calmodulin. However, there are striking differences between EF-hand 4, both N-terminal and C-terminal regions, and interdomain linkers. The C-terminal domain of calcyphosine possesses a large hydrophobic pocket in the presence of calcium ions that might be implicated in ligand binding, while its N-terminal hydrophobic pocket is almost shielded by an additional terminal helix. Calcyphosine is largely monomeric, regardless of the presence of Ca2 +. Differences in structure, oligomeric state in the presence and in the absence of Ca2 +, a highly conserved sequence with low similarity to other proteins, and phylogeny define a new EF-hand-containing family of calcyphosine proteins that extends from arthropods to humans.