Physical Studies on the Conformation of Ribosomal Protein S4 from Escherichia coli

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Abstract
PMR, circular dichroism and IR spectroscopy were used to investigate the secondary and tertiary structure of the 16-S RNA binding protein S4 from E. coli ribosomes. The PMR spectra of protein S4 in ribosomal reconstitution and low-salt buffers were identical and showed little dipolar broadening of the peaks, suggesting that the protein had an open extended structure. A ring-current-shifted apolar methyl resonance in the high-field region of the spectrum, together with a perturbation of the tyrosine ring proton resonance in the low-field region, indicated the existence of a specific tertiary fold in the polypeptide chain. This structure disappeared on lowering the pH below 5 or on heating above 30.degree. C, both processes being reversible. Circular dichroism measurements on protein S4 showed an .alpha.-helix content of 32% in reconstitution buffer compared with 26% in low-salt buffer. Heating the protein solution in reconstitution buffer above 35.degree. C reversibly disrupted this extra helix. IR studies on both solid films and solutions of protein S4 indicated the presence of little or no .beta.-structure. These results correlate well with the known RNA binding properties of protein S4.