Internal dynamics of the calcium-signaling protein S100B, revealed by NMR relaxation studies

Abstract

The internal dynamics of the calcium-signaling protein S100B have been studied by nuclear magnetic resonance (NMR) spectroscopy. Rates of longitudinal and transverse autorelaxation were measured for backbone nitrogen atoms and side-chain methyl deuterium, using isotopically enriched proteins. Additionally, {lcub}1H)-15N cross-relaxation rates (nuclear Overhauser enhancement; NOE) were measured at multiple fields. Magnetic fields of 9.4 and 14.1 Tesla (T) were used for the 15N relaxation measurements, and 14.1 and 17.6 T for the side-chain experiments. Model-free order parameters for backbone amide 1H-15N internuclear vectors and side-chain methyl axes were calculated for the calcium-free (apo) protein, and in some cases the time-scales of motion were also estimated. The 15N relaxation experiments were repeated in the presence of calcium, both with and without target peptides. In contrast to apo-S100B, model-free analysis was unsuccessful for these data; however, significant insight into the backbone dynamics of S100B in the presence of calcium and ligands was gained by direct examination of the NMR relaxation data, and reduced spectral density mapping. The solution structure of S100B in the presence of the target peptide TRTK-12 was solved by NMR and is also presented. S100B is a highly ordered protein along its backbone, although relatively low order is observed in the empty calcium-binding regions of apo-S100B, and in the hinge region connecting the two EF hands, when calcium is present. Hydrophobic side-chains show a diversity of order parameters, apparently inversely related to the stability of the core region in which they are involved. Methyl axis order parameters at the subnanomolar dieter interface show relatively low order, while those in the subunit cores show relatively high order in the apo-protein. Work remains to determine what changes might occur in side-chain order parameters on binding calcium and target peptide.