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.