Calcium and membrane binding properties of bovine neurocalcin delta expressed in Escherichia coli.

Neurocalcins are brain-specific proteins that belong to a new subclass of the EF-hand superfamily of calcium binding proteins, defined by the photoreceptor cell-specific protein, recoverin. Recoverin, which regulates the desensitization of photo-excited rhodopsin, is myristoylated and exhibits a calcium-myristoyl switch. Like recoverin, neurocalcins have a signal for N-myristoylation and possess four EF-hands, although the first one lacks some residues critical for calcium binding. In this work, I have examined the calcium and membrane binding properties of recombinant myristoylated and unmyristoylated neurocalcin delta. I show that neurocalcin, like recoverin, binds to biological membranes in a calcium- and myristoyl-dependent manner. Both myristoylated and unmyristoylated proteins bind three calcium ions. However, the unmyristoylated form exhibits a higher affinity for calcium than the myristoylated protein but shows a lower cooperativity in binding calcium. These data support the model for the calcium-myristoyl switch mechanism proposed for recoverin (Zozulya, S., and Stryer, L. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11569-11573; Dizhoor, A. M., Chen, C. K., Olshevskaya, E., Sinelnikova, V. V., and Hurley, J. B. (1993) Science 259, 829-832). Using point mutations, I have investigated the relative importance of each of the three functional EF hands (EF2, EF3, and EF4) in the calcium and membrane binding properties of neurocalcin. Calcium and membrane binding properties of the mutant-myristoylated proteins suggest that binding of calcium to EF2 is critical in triggering the binding of the protein to membranes.

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