The three-dimensional crystal structure of seryl-transfer RNA synthetase from Escherichia coli, refined at 2.5 A resolution, is described. It has an N-terminal domain that forms an antiparallel alpha helical coiled-coil, stretching 60 A out into the solvent and stabilized by interhelical hydrophobic interactions and an active-site alpha-beta domain based around a seven-stranded antiparallel beta sheet. Unlike the three other known synthetase structures, the enzyme contains no classical nucleotide-binding fold, and is the first representative of a second class of aminoacyl-tRNA synthetase structures.

The coupling between protein dynamics and hydration-water dynamics was assessed by perdeuteration, temperature-dependent neutron scattering, and molecular dynamics simulations. Mean square displacements of water and protein motions both show a broad transition at 220 K and are thus coupled. In particular, the protein dynamical transition appears to be driven by the onset of hydration-water translational motion.