SOLUTION STRUCTURE OF RAT APO-S100B(BETA-BETA) AS DETERMINED BY NMR-SPECTROSCOPY

S100B(beta beta), a member of the S100 protein family, is a Ca2+-bindi ng protein with noncovalent interactions at its dimer interface. Each apo-S100 beta subunit (91 residues) has four alpha-helices and a small antiparallel beta-sheet, consistent with two predicted helix-loop-hel ix Ca2+-binding domains known as EF-hands [Amburgey et al. (1995) J. B iomol. NMR 6, 171-179]. The three-dimensional solution structure of ap o-S100B(beta beta) from rat has been determined using 2672 distance (1 4.7 per residue) and 88 dihedral angle restraints derived from multidi mensional nuclear magnetic resonance spectroscopy. Apo-S 100B(PP) is f ound to be globular and compact with an extensive hydrophobic core and a highly charged surface, consistent with its high solubility. At the symmetric dimer interface, 172 intermolecular nuclear Overhauser effe ct correlations (NOEs) define the antiparallel alignment of helix I wi th I' and of helix IV with IV'. The perpendicular association of these pairs of antiparallel helices forms an X-type four-helical bundle at the dimer interface. Whereas, the four helices within each apo-S100 be ta subunit adopt a unicornate-type four-helix bundle, with helix I pro truding from the parallel bundle of helices II, III, and IV. According ly, the orientation of helix III relative to helices I, II, and IV in each subunit differs significantly from that known for other Ca2+-bind ing proteins. Indeed, the interhelical angle (Omega) observed in the C -terminal EF-hand of apo-S100 beta is -142 degrees, whereas Omega rang es from 118 degrees to 145 degrees in the apo state and from 84 degree s to 128 degrees in the Ca2+-bound state for the EF-hands of calbindin D-9k, calcyclin, and calmodulin. Thus, a significant conformational c hange in the C-terminal EF-hand would be required for it to adopt a st ructure typical of the Ca2+-bound state, which could readily explain t he dramatic spectral effects observed upon the addition of Ca2+ to apo -S100B(beta beta).