Tuning the Affinity for Lanthanides of Calcium Binding Proteins

The possibility of selectively substituting one or more lanthanides into the four canonical calcium binding sites of calcium-loaded vertebrate calmodulin (CaM) was investigated by monitoring changes in the 1H-15N HSQC NMR spectra of the 15N-enriched protein upon titration with Yb3+. The affinity of lanthanides for both N-terminal sites I and II is only moderately higher than that of calcium, and comparable with that of calcium for the two C-terminal sites. This situation induces binding of lanthanides to other noncanonical sites located at the interdomain linker, the N- and C-terminal ends, and at the inter-EF-hand linkers. Therefore, mutants were designed to alter the metal binding properties of calcium sites I (D22N, D24E), II (D58N, N60D, D58N-N60D), III (N97D), II-III (N60D-N97D), and IV (D129N), as well as of the inter-EF-hand linker of the N-terminal domain (N42K, T44K). The most striking effects were obtained for the N60D mutant at site II, as selective lanthanide binding is achieved even in the presence of excess calcium, and little or no population of the noncanonical sites occurs. Similar although less pronounced effects were observed for the N97D mutant. These findings allow us to better define some of the determinants of the relative affinities of calcium and lanthanides in CaM and, by extension, in other calcium binding proteins. Finally, by using CaM samples containing only three of the four calcium ions, it was possible to prepare well-defined Ca3Ln-CaM derivatives (Ln = Tb, Dy, Tm, and Yb), with interesting properties as NMR probes.