A NOVEL MODE OF TARGET RECOGNITION SUGGESTED BY THE 2.0 ANGSTROM STRUCTURE OF HOLO S100B FROM BOVINE BRAIN

Background: S100B, a small acidic calcium-binding protein, is a member of the S100 protein family and is a multifunctional protein capable o f binding several target molecules, such as cytoskeletal proteins and protein kinases, in a calcium-dependent manner, S100B is a homodimer o f S100 beta subunits (beta beta) with a total of four calcium-binding motifs called EF hands, S100B is found abundantly in nervous tissue an d has been implicated in Alzheimer's disease and Down's syndrome, Stru ctural analysis of S100B in the calcium-bound state is required to gai n a better understanding of the conformational changes that occur to S 100B upon calcium binding and to elucidate the mode of recognition bet ween S100B and its target molecules. Results: We have determined the t hree-dimensional structure of hole S100B from bovine brain at 2.0 Angs trom resolution by X-ray diffraction, The dimeric S100B molecule is fo rmed by non-covalent interactions between large hydrophobic surfaces o n both S100 beta subunits, There are two EF-hand motifs per S100 beta subunit, each of which binds one calcium ion, We observe, in the calci um-bound structure, dramatic changes in the conformation of the termin al helices, from the compact structure in the apo form to a more exten ded form upon binding calcium, Following these changes, an exposed hyd rophobic core, surrounded by many negatively charged residues, is reve aled. Cys84 is positioned at an exposed surface of S100B, surrounded b y hydrophobic residues, and could form a disulfide bond to tau protein , one of the known target molecules thought to interact with S100B in this way. Conclusions: The molecular structure of hole S100B suggests a novel mode of target recognition for the S100 family of calcium-bind ing proteins. Upon calcium binding, dramatic changes occur in the term inal helices of S100B, revealing a large hydrophobic surface, not obse rved in the apo form. It is through hydrophobic interactions and possi bly a Cys84-mediated disulfide bond that S100B is thought to bind its target molecules.