This entry includes a group of EF-hand calcium-binding domain-containing proteins, including EFCAB3 and SPT21 (spermatogenesis-associated protein 21). The function of EFCAB3 is not clear. SPT21 is involved in the differentiation of haploid spermatids [].Many calcium-binding proteins belong to the same evolutionary family and share a type of calcium-binding domain known as the EF-hand. This type of domain consists of a twelve residue loop flanked on both sides by a twelve residue α-helical domain. In an EF-hand loop the calcium ion is coordinated in a pentagonal bipyramidal configuration. The six residues involved in the binding are in positions 1, 3, 5, 7, 9 and 12; these residues are denoted by X, Y, Z, -Y, -X and -Z. The invariant Glu or Asp at position 12 provides two oxygens for liganding Ca (bidentate ligand). Ca2 binding induces a conformational change in the EF-hand motif, leading to the activation or inactivation of target proteins. EF-hands tend to occur in pairs or higher copy numbers [, , , , ].
