NMR-driven secondary and tertiary structure model of Ca2+-loaded calexcitin

Calexcitin (CE) is a Ca2+-binding protein which is expressed in neuronal cells and is a member of the sarcoplasmic Ca2+-binding protein subfamily. The peptide backbone of Ca2+-CE has been assigned by NMR and it shows that CE is composed of nine α-helices—forming four EF-hands and an additional helix near the C-terminus. A structural model of CE suggests the presence of a putative recessed hydrophobic pocket that may be involved in Ca2+-mediated protein–ligand interactions. This feature is unique to CE and is absent in other SCPs, such as those from Branchiostoma and Nereis, and from calerythrin.