CALEXCITIN - A SIGNALING PROTEIN THAT BINDS CALCIUM AND GTP, INHIBITSPOTASSIUM CHANNELS, AND ENHANCES MEMBRANE EXCITABILITY

A previously uncharacterized 22-kDa Ca2+-binding protein that also bin ds guanosine nucleotides was characterized, cloned, and analyzed by el ectrophysiological techniques, The cloned protein, calexcitin, contain s two EF-hands and also has homology with GTP-binding proteins in the ADP ribosylation factor family. In addition to binding two molecules o f Ca2+, calexcitin bound GTP and possessed GTPase activity, Calexcitin is also a high affinity substrate for protein kinase C. Application o f calexcitin to the inner surface of inside-out patches of human fibro blast membranes, in the presence of Ca2+ and the absence of endogenous Ca2+/calmodulin kinase type II or protein kinase C activity, reduced the mean open time and mean open probability of 115 +/- 6 pS K+ channe ls, Calexcitin thus appears to directly regulate K+ channels, When mic roinjected into molluscan neurons or rabbit cerebellar Purkinje cell d endrites, calexcitin was highly effective in enhancing membrane excita bility. Because calexcitin translocates to the cell membrane after pho sphorylation, calexcitin could serve as a Ca2+-activated signaling mol ecule that increases cellular excitability, which would in turn increa se Ca2+ influx: through the membrane, This is also the first known ins tance of a GTP-binding protein that binds Ca2+.