Structural Plasticity\nas a Driver of the Maturation\nof Pro-Interleukin-18

Dynamics are often critical for biomolecular function.\nHerein we\nexplore the role of motion in driving the maturation process of pro-IL-18,\na potent pro-inflammatory cytokine that is cleaved by caspases-1 and\n-4 to generate the mature form of the protein. An NMR dynamics study\nof pro-IL-18, probing time scales over 12 orders of magnitude and\nfocusing on <sup>1</sup>H, <sup>13</sup>C, and <sup>15</sup>N spin\nprobes along the protein backbone and amino-acid side chains, reveals\na plastic structure, with millisecond time scale dynamics occurring\nin a pair of β-strands, β1 and β*, that show large\nstructural variations in a comparison of caspase-free and bound pro-IL-18\nstates. Fits of the relaxation data to a three-site model of exchange\nshowed that the ground state secondary structure is maintained in\nthe excited conformers, with the side chain of I48 that undergoes\na buried-to-exposed conformational change in the caspase-free to -bound\ntransition of pro-IL-18, sampling a more extensive range of torsion\nangles in one of the excited states characterized, suggesting partial\nunpacking in this region. Hydrogen exchange measurements establish\nthe occurrence of an additional process, whereby strands β1\nand β* locally unfold. Our data are consistent with a hierarchy\nof dynamic events that likely prime pro-IL-18 for facile caspase binding.