RIBOSOMAL-PROTEIN L9 - A STRUCTURE DETERMINATION BY THE COMBINED USE OF X-RAY CRYSTALLOGRAPHY AND NMR-SPECTROSCOPY

The structure of protein L9 from the Bacillus stearothermophilus ribos ome has been determined at 2.5 Angstrom resolution by refinement again st single crystal X-ray diffraction data with additional constraints p rovided by NMR data. This highly elongated protein consists of two dom ains separated by a nine-turn connecting helix. Conserved aromatic and positively charged amino acid residues on the surface of each domain are likely to be directly involved in binding 23 S ribosomal RNA. The shape of the protein, with its two widely spaced RNA-binding sites, su ggests that it may serve as a ''molecular strut'', most likely playing a role in ribosome assembly and/or maintaining the catalytically acti ve conformation of the ribosomal RNA. The combined use of X-ray and NM R data in the refinement procedure was essential in defining the N-ter minal domain of the protein, which was relatively poorly determined by the X-ray data alone. In addition to resolving the ambiguities in def ining the hydrophobic core and side-chain conformations with the N-ter minal domain, this combined NMR-X-ray analysis provides the first deta iled and accurate view of the N-terminal RNA-binding site. NMR data al so showed that the N-terminal domain is stable in solution, indicated by amide protons that are protected from solvent exchange. The lack of definition of the N-terminal domain in the X-ray structure is therefo re likely due to packing disorder within the crystal rather than struc tural instability. This combined NMR-X-ray analysis provides a useful model as to how X-ray and NMR data can be practically and logically co mbined in the determination of the structure of a single protein molec ule. (C) 1996 Academic Press Limited