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

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.