15N relaxation studies of apo-Mts1:: A dynamic S100 protein

Mts1 is a member of the S 100 family of EF-hand calcium-binding proteins. Like most S100 proteins, Mts1 exists as a dimer in solution and contains one canonical and one pseudo-EF-hand motif per monomer, each of which consists of two alpha helices connected by a loop capable of coordinating a calcium ion. The backbone dynamics of murine apo-Mts1 homodimer have been examined by nuclear magnetic resonance spectroscopy. Longitudinal and transverse relaxation data and steady-state H-1-N-15 nuclear Overhauser effects were analyzed using model-free formalism. The extracted global correlation time is 9.94 ns. Results indicate that the protein backbone is most rigid at the dimer interface, made up of helices I and 4 from each monomer with mean S-2 (S-avg(2)) values similar to 0.9, flanked by helices 2 and 3 with lower S-avg(2) values of 0.84 and 0.77, respectively. Each calcium-binding site along with the hinge joining the two EF-hands and the N- and C-termini are considerably more flexible than the dimer interface on a range of time scales and more flexible than the corresponding regions of other S100 proteins studied to date. As the hinge and the C-terminal tail are believed to interact with target proteins, these dynamic characteristics may have implications for Mts1 activity.