C-terminal half of human centrin 2 behaves like a regulatory EF-hand domain

Human centrin 2 (HsCen2) is an EF-hand protein that plays a critical role in the centrosome duplication and separation during cell division. We studied the structural and Ca2+-binding properties of two C-termiml fragments of this protein: SC-HsCen2 (T94-YI72), covering two EF-hands, and LCHsCen2 (M84-Y172), having 10 additional residues. Both fragments are highly disordered in the apo state but become better structured (although not conformationally homogeneous) in the presence of Ca2+ and depending on the nature of the cations (K+ or Na+) in the buffer. Only the longer C-terminal domain, in the Ca2+-saturated state and in the presence of Na+ ions, was amenable to structure determination by nuclear magnetic resonance. The solution structure of LC-HsCen2 reveals an open two EF-hand structure, similar to the conformation of related Ca2+-saturated regulatory domains. Unexpectedly, the N-terminal helix segment (F86-T94) lies over the exposed hydrophobic cavity. This unusual intramolecular interaction increases considerably the Ca2+ affinity and constitutes a useful model for the target binding.