The E144 residue of Scherffelia dubia centrin discriminates between the DNA repair protein XPC and the centrosomal protein Sfi1

• We used isothermal titration calorimetry to study how algal centrin binds to its targets.
• Centrin with the mutation F109L reduced binding to both XPC and Sfi1.
• Centrin with the mutation E114A reduced binding to XPC but not to Sfi1.
• The double mutation F109L–E144A further reduced XPC binding compared to the single mutations.

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.