Probing the structure of the Caulobacter crescentus ribosome with chemical labeling and mass spectrometry

The ribosomal proteins of Caulobacter crescentus were amidinated before and after disassembly of the organelle and the results analyzed by mass spectrometry. Comparison with structural information from previous X-ray crystal studies of other bacterial ribosomes provides insight about the C. crescentus ribosome. In total, 47 of the 54 proteins present in the ribosome of C. crescentus were detected after labeling. The extent of derivatization for each protein is strongly dependent on the solvent accessibility of its target residues. Proteins of the ribosome stalk, which are known to be largely solvent-accessible, were labeled quite extensively. In striking contrast, other proteins that are known to be highly shielded in their subunits were labeled at very few of their potential sites. Furthermore, evidence that protein L12 binds to the ribosome via its N-terminal domain is consistent with previous findings.