Structural Preordering in the N-Terminal Region of Ribosomal Protein S4 Revealed by Heteronuclear NMR Spectroscopy

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Abstract
Protein S4, a component of the 30S subunit of the prokaryotic ribosome, is one of the first proteins to interact with rRNA in the process of ribosome assembly and is known to be involved in the regulation of this process. While the structure of the C-terminal 158 residues of Bacillus stearothermophilus S4 has been solved by both X-ray crystallography and NMR, that of the N-terminal 41 residues is unknown. Evidence suggests that the N-terminus is necessary both for the assembly of functional ribosomes and for full binding to 16S RNA, and so we present NMR data collected on the full-length protein (200 aa). Our data indicate that the addition of the N-terminal residues does not significantly change the structure of the C-terminal 158 residues. The data further indicate that the N-terminus is highly flexible in solution, without discernible secondary structure. Nevertheless, structure calculations based on nuclear Overhauser effect spectroscopic data combined with 15N relaxation data revealed that two short segments in the N-terminus, S12RRL15 and P30YPP33, adopt transiently ordered states in solution. The major conformation of S12RRL15 appears to orient the arginine side chains outward toward the solvent in a parallel fashion, while that of P30YPP33 forms a nascent turn of a polyproline II helix. These segments contain residues that are highly conserved across many prokaryotic species, and thus they are reasonable candidates respectively for sites of interaction with RNA and other ribosomal proteins within the intact ribosome.