Calexcitin B Is a New Member of the Sarcoplasmic Calcium-binding Protein Family

Abstract

Calexcitin (CE) is a calcium sensor protein that has been implicated in associative learning. The CE gene was previously cloned from the long-finned squid, Loligo pealei, and the gene product was shown to bind GTP and modulate K+ channels and ryanodine receptors in a Ca2+-dependent manner. We cloned a new gene from L. pealei, which encodes a CE-like protein, here named calexcitin B (CEB). CEB has 95% amino acid identity to the original form. Our sequence analyses indicate that CEs are homologous to the sarcoplasmic calcium-binding protein subfamily of the EF-hand superfamily. Far and near UV circular dichroism and nuclear magnetic resonance studies demonstrate that CEB binds Ca2+ and undergoes a conformational change. CEB is phosphorylated by protein kinase C, but not by casein kinase II. CEB does not bind GTP. Western blot experiments using polyclonal antibodies generated against CEB showed that CEB is expressed in the L. pealei optic lobe. Taken together, the neuronal protein CE represents the first example of a Ca2+ sensor in the sarcoplasmic calcium-binding protein family.