HighlightsCK2 phosphorylates human centrin 1 at T138.CK2 phosphorylates human centrin 2 at T138 and S158.CK2 phosphorylation of centrin 1 abolishes its binding to transducin β.CK2 phosphorylation regulates the binding of human centrins to their targets.The binding of phosphorylated centrin to its target was measured by ITC.

Changes in Ca 2+ concentrations act as a second messenger to regulate biological processes. Ca 2+ sensor proteins transduce these Ca 2+ signals upon binding to protein targets that are involved in the cellular process that is being regulated. Ca 2+ sensor proteins such as calmodulin and centrin bind Ca 2+ through acidic residues that compose the EFhand motifs and further bind targets via surfaces that recognize specific motifs on targets. However, in some cases, the binding of a Ca 2+ sensor protein to a target can occur in the absence of Ca 2+. Upon Ca 2+ binding, Ca 2+-sensor proteins undergo conformational changes, which lead to the exposure of surface that interacts with target. Moreover, Ca 2+ binding and the binding of Ca 2+ sensors to targets induce conformational changes that drive regulation. Comparisons of the Ca 2+-sensor proteins calmodulin and centrin provide information on regulatory processes.

Centrins are members of the EF-hand family of calcium-binding proteins, which are highly conserved among eukaryotes. Centrins bind to several cellular targets, through a hydrophobic triad. However, the W1xxL4xxxL8 triad in XPC (Xeroderma Pigmentosum Group C protein) is found in the reverse orientation, as in the L8xxxL4xxW1 triad in Sfi1 (Suppressor of Fermentation-Induced loss of stress resistance protein 1). As shown by previous NMR studies of human centrin 2 in complex with XPC or Sfi1, the E148 residue of human centrin 2 is in contact with XPC but is pushed away from the triad of Sfi1. We corroborated these findings using site-directed mutagenesis to generate mutations in Scherffelia dubia centrin (SdCen) and by using isothermal titration calorimetry to analyze the binding affinity of these mutants to XPC and Sfi1. We mutated the F109 residue, which is the main residue involved in target binding regardless of triad orientation, and the E144 residue, which was thought to be involved only in XPC binding. The F109L mutation reduced the binding of SdCen to XPC and Sfi1 and the negative effect was greater upon temperature increase. By contrast, the E144A mutation reduced the binding to XPC but had no effect on Sfi1 binding. The F109L-E144A mutation enhanced the negative effect of the two single mutations on XPC binding. Sfi1 proteins from Ostreococcus lucimarinus and Ostreococcus tauri, which belong to the same clade as S. dubia, were also investigated. A comparative analysis shows that the triad residues are more conserved than those in human Sfi1.

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