Binding of protein S4 to 16S rRNA is critical for the subsequent binding of other ribosomal proteins. In addition S4 regulates its own translation and that of three other ribosomal proteins. Following our description of the solution structure of S4's mRNA binding domain (S4delta41, 159 residues), we showed that the structure of the C-terminal 158 residues of the intact protein is the same that of S4delta41. In contrast, the 45 N-terminal residues are highly flexible, but contain two conserved regions, S12RRL15 and P30YPP33, that adopt transiently ordered structures in solution. These transiently ordered conformations, predicted by NMR, were similar to those observed in the independently determined crystal structure of the 30S ribosomal unit from T. thermophilus. During the course of the structural work on the S4 N-terminus, severe overlap problems were encountered in the NMR spectra resulting from chemical shift degeneracy and the high concentration of proline residues. To overcome these problems, we developed two novel experiments that used the carbonyl chemical shifts to remove signal degeneracy. These experiments enabled us to make complete signal assignments of the N-terminal domain of intact S4. 15N relaxation data (T1, T2, and {H}15N NOE) have been collected on both S4 and S4delta41,. These data were analyzed using an anisotropic Model-free approach. The analysis yielded average overall correlation times and anisotropies of rotational diffusion of 8.39 plus/minus .02 ns and 1.53 plus/minus .04 for S4delta41, and 11.40 plus/minus .10 ns and 1.41 plus/minus .06 for S4. In addition transverse relaxation dispersion measurement indicated that there were no motions on the millesecond-to-microsecond timescale within the C-terminal 159 residues of the protein. Taken together, the relaxation data indicate the C-terminal 159 residues of S4 reorient as an axially symmetric rigid body. This observation suggests that this portion of S4 and acts as a folding template upon which ribosomal RNA can organize.
| Sayers, E W; Gerstner, R B; Draper, D E et al. (2000) Structural preordering in the N-terminal region of ribosomal protein S4 revealed by heteronuclear NMR spectroscopy. Biochemistry 39:13602-13 |
| Markus, M A; Gerstner, R B; Draper, D E et al. (1999) Refining the overall structure and subdomain orientation of ribosomal protein S4 delta41 with dipolar couplings measured by NMR in uniaxial liquid crystalline phases. J Mol Biol 292:375-87 |
