Structural and Dynamic Characterization of Intrinsically Disordered Human Securin by NMR Spectroscopy

Understanding the molecular action of securin, the inhibitor of separase in mitosis, is of immense theoretical and biomedical importance. The residue-level structural description of an intrinsically disordered protein of this length (202 amino acids, containing 24 prolines), however, represents a particular challenge. Here we combined H-1-detected and C-13-detected protonless NMR experiments to achieve full assignment of securin's backbone amide resonances. Chemical shifts, N-15 relaxation rates (R-1, R-2, H-1-N-15 NOEs), H-1 exchange rates with the solvent (CLEANEX-PM), and H-1-N-15 residual dipolar couplings were determined along the entire length of the protein. This analysis showed that securin is not entirely disordered, but segregates into a largely disordered N-terminal half and a C-terminal halt with transient segmental order, within which the segment D-150-F-159 has a significant helical tendency and segments E-113-S-127 and W-174-L-178 also show a significant deviation from random-coil behavior. These results, in combination with bioinformatic and biochemical data on the securin/separase interaction, shed light on the inhibitory action of securin on separase.