Metal-binding properties of calmodulin.

Metal-binding properties of calmodulin have been studied by using trivalent lanthanide ions as analogs of Ca2+. In agreement with a report published as this work was in progress [Kilhoffer, M.-C., Demaille, J. G., and Gerald, D. (1980) FEBS Lett. 116, 269-272] we found that sites I and II are the high-affinity sites, while sites III and IV are the low-affinity sites for Tb3+. Competition experiments suggest the same preference in binding also applies to Ca2+. With calmodulin selectively nitrated at either of the two tyrosine residues we found that, although both tyrosine groups can transfer energy to the bound Tb3+, the fluorescence of only Tyr-138 is sensitive to metal binding. Direct excitation of bound Eu3+ ions using a laser indicates that all four sites possess very similar microenvironments. These studies demonstrate that the binding properties of calmodulin are different from those of the homologous protein troponir C.