Structure of calcium-bound human S100A13 at pH 7.5 at 1.8 Å resolution

Abstract

S100A13 is a member of the S100 family of EF-hand-containing calcium-binding proteins. S100A13 plays an important role in the secretion of fibroblast growth factor-1 and interleukin 1α, two pro-angiogenic factors released by the nonclassical endoplasmic reticulum/Golgi-independent secretory pathway. The X-ray crystal structure of human S100A13 at pH 7.5 was determined at 1.8 Å resolution. The structure was solved by molecular replacement and was refined to a final R factor of 19.0%. The structure revealed that human S100A13 exists as a homodimer with two calcium ions bound to each protomer. The protomer is composed of four α-helices (α1- α4), which form a pair of EF-hand motifs. Dimerization occurs by hydrophobic interactions between helices α1 and α4 and by intermolecular hydrogen bonds between residues from helix α1 and the residues between α2 and α3 of both chains. Despite the high similarity of the backbone conformation in each protomer, the crystal structures of human S100A13 at pH 7.5 (this study) and at pH 6.0 [Li et al. (2007), Biochem. Biophys. Res. Commun. 356, 616-621] exhibit recognizable differences in the relative orientation (∼2.5°) of the protomers within the dimer and also remarkable differences in the side-chain conformations of several amino-acid residues. © International Union of Crystallography 2008.