The backbone structure of the thermophilic Thermoanaerobacter tengcongensis ribose binding protein is essentially identical to its mesophilic E. coli homolog

We report the X-ray crystal structure of a Thermoanaerobacter tengcongensis ribose binding protein (tteRBP) determined to 1.9 ？ resolution. We find that tteRBP is significantly more stable (appTm value ~102°C) than the mesophilic Escherichia coli ribose binding protein (ecRBP) (appTm value ~56°C). The tteRBP has essentially the identical backbone conformation (0.41 ？ RMSD of 235/271 Cα positions and 0.65 ？ RMSD of 270/271 Cα positions) as ecRBP. Classification of the amino acid substitutions as a function of structure therefore allows the identification of amino acids which potentially contribute to the observed thermal stability of tteRBP in the absence of large structural heterogeneities.The near identity of backbone structures of this pair of proteins entails that the significant differences in their thermal stabilities are encoded exclusively by the identity of the amino acid side-chains. Furthermore, the degree of sequence divergence is strongly correlated with structure; with a high degree of conservation in the core progressing to increased diversity in the boundary and surface regions. Different factors that may possibly contribute to thermal stability appear to be differentially encoded in each of these regions of the protein. The tteRBP/ecRBP pair therefore offers an opportunity to dissect contributions to thermal stability by side-chains alone in the absence of large structural differences.The mechanisms that contribute to protein thermal stability are varied, subtle, and complex [1-5]. Various contributing factors to thermal stability have been proposed by comparative analysis of thermophilic and mesophilic proteins [4,6]. Proposed mechanisms can be categorized [5] generally as contributions by the main-chain structure (new folds [7], loop shortening [8]), or by side-chain interactions (increased packing in core [9] or surface [10], alteration of amino acid composition [11-13]), post-translational modifications [14] or co-factor binding [4,15]). Usually