Tyr51: Key Determinant of the Low Thermostability of the <i>Colwellia psychrerythraea</i> Cold-Shock Protein

Cold-shock proteins (Csps) are expressed at lower-than-optimum temperatures, and they function as RNA chaperones; however, no structural studies on psychrophilic Csps have been reported. Here, we aimed to investigate the structure and dynamics of the Csp of psychrophile <i>Colwellia psychrerythraea</i> 34H, (<i>Cp</i>-Csp). Although <i>Cp</i>-Csp shares sequence homology, common folding patterns, and motifs, including a five β-stranded barrel, with its thermophilic counterparts, its thermostability (37 °C) was markedly lower than those of other Csps. <i>Cp</i>-Csp binds heptathymidine with an affinity of 10^–7 M, thereby increasing its thermostability to 50 °C. Nuclear magnetic resonance spectroscopic analysis of the <i>Cp</i>-Csp structure and backbone dynamics revealed a flexible structure with only one salt bridge and 10 residues in the hydrophobic cavity. Notably, <i>Cp</i>-Csp contains Tyr51 instead of the conserved Phe in the hydrophobic core, and its phenolic hydroxyl group projects toward the surface. The Y51F mutation increased the stability of hydrophobic packing and may have allowed for the formation of a K3–E21 salt bridge, thereby increasing its thermostability to 43 °C. <i>Cp</i>-Csp exhibited conformational exchanges in its ribonucleo­protein motifs 1 and 2 (754 and 642 s^–1), and heptathymidine binding markedly decreased these motions. <i>Cp-</i>Csp lacks salt bridges and has longer flexible loops and a less compact hydrophobic cavity resulting from Tyr51 compared to mesophilic and thermophilic Csps. These might explain the low thermostability of <i>Cp</i>-Csp. The conformational flexibility of <i>Cp-</i>Csp facilitates its accommodation of nucleic acids at low temperatures in polar oceans and its function as an RNA chaperone for cold adaptation