Assignment and secondary structure of calcium-bound human S100B.

The NMR assignments of backbone 1H, 13C, and 15N resonances for calcium-bound human S100B were completed via heteronuclear multidimensional NMR spectroscopic techniques. NOE correlations, amide exchange, 3JHNH alpha coupling constants, and CSI analysis were used to identify the secondary structure for Ca-S100B. The protein is comprised of four helices (helix I, Glu2-Arg20; helix II, Glu31-Asn38; helix III, Gln50-Thr59; helix IV, Phe70-Phe87), three loops (loop I, Glu21-His25; loop II, Glu39-Glu49; loop III, Leu60-Gly66), and two beta-strands (strand I, Lys26-Lys28; strand II, Glu67-Asp69) which form a short antiparallel beta-sheet. Helix IV is extended by approximately one turn when compared to the secondary structures of apo-rat [Drohat et al. (1996) Biochemistry, 35, 11577-11588] and bovine S100B [Kilby et al. (1996) Structure, 4, 1041-1052]. In addition, several residues outside the calcium-binding loops in S100B undergo significant backbone chemical shift changes upon binding calcium which are not observed in the related protein calbindin D9k. Together these observations support previous site-directed mutagenesis, absorption spectroscopy, and cysteine chemical reactivity experiments, suggesting that the C-terminus in Ca-S100B is important for interactions with other proteins.