High‐temperature solution NMR structure of Tm Csp

Cold shock proteins (Csps) are assumed to play a central role in the regulation of gene expression under cold shock conditions. Acting as single‐stranded nucleic acid‐binding proteins, they trigger the translation process and are therefore involved in the compensation of the influence of low temperatures (cold shock) upon the cell metabolism. However, it is unknown so far how Csps are switched on and off as a function of temperature. The aim of the present study is the study of possible structural changes responsible for this switching process. ¹H‐¹⁵N HSQC spectra recorded at different temperatures and chemical‐shift analysis have indicated subtle conformational changes for the cold‐shock protein from the hyperthermophilic bacterium Thermotoga maritima (TmCsp) when the temperature is elevated from 303 K to its physiological temperature (343 K). The three‐dimensional structure of TmCsp was determined by nuclear magnetic resonance (NMR) spectroscopy at 343 K to obtain quantitative information concerning these structural changes. By use of residual dipolar couplings, the loss of NOE information at high temperature could be compensated successfully. Most pronounced conformational changes compared with room‐temperature conditions are observed for amino acid residues closely neighbored to two characteristic β‐bulges and a well‐defined loop region of the protein. Because the residues shown to be responsible for the interaction of TmCsp with single‐stranded nucleic acids can almost exclusively be found within these regions, nucleic acid‐binding activity might be down‐regulated with increasing temperature by the described conformational changes.