• Structural and functional studies on S100A1

      Wright, Nathan Thompson; Weber, David J., Ph.D. (2008)
      S100 proteins are small, dimeric calcium binding proteins that contain two EF-hands per subunit. While these proteins are expressed plentifully in a tissue-specific manner, the cellular functions of most S100 proteins remain understudied. This dissertation describes efforts to elucidate both the structure and the function of one member of this family, S100A1, in various biologically relevant states. Multidimensional NMR spectroscopy techniques were used to solve the solution structure of S100A1 in its Ca 2+-bound form. The protein folds as a symmetric homodimer, with an X-type helix bundle comprising the dimer interface. A large conformational change, involving the reorientation of helix 3, accompanies S100A1 calcium binding. This change exposes a previously hidden hydrophobic pocket which is the general target protein binding site and specifically the binding site for the calmodulin binding domain of the ryanodine receptor. The solution structure of S100A1 bound to a peptide from this region, along with whole-cell calcium transient measurements in S100A1 knockout mice, provide evidence that S100A1 directly increases RyR1-mediated calcium release by binding to a discrete area on the cytoplasmic face of RyR1. This binding event is driven by both hydrophobic and ionic interactions between S100A1 and the RyR peptide. A competition between S100A1 and calmodulin for the same region of RyR1 likely represents a cellular mechanism of modulating SR Ca2+ release in skeletal muscle fibers.;Studies detailing the internal backbone dynamics of the Ca2+-loaded form of another S100 protein, S100B, are also presented in this manuscript. These experiments were conducted to further our understanding of the molecular mechanisms driving S100 protein structure and function. Here, S100B is shown to be a stable protein in the Ca2+-bound form, with significant motions in the hydrophobic pocket. These motions may assist in peptide binding, and support a previously suggested coupling between Ca2+ and target peptide binding in S100 proteins.