Stripping of ribosomal proteins from Escherichia coli ribosomes

A number of investigators, employing different techniques, have shown that high concentrations of salt cause the detachment of roughly half of the protein of E. coli ribosomes [l-4] . The detached proteins represent different molecular species than those remaining bound to the ribosomal RNA [5--g]. These findings raise the question of whether the two groups of proteins are bound differently in the intact ribosome or whether the selective detachment may be due to the structure of the ribosome. We have therefore been interested in the question of whether the configuration of the ribosome is a significant factor in determining how much ribosomal protein can be removed by salt. It is known that magnesium plays an important role in determining the shape of the ribosome [lo] . Magnesium ions can be removed with a sequestering agent such as EDTA [ 1 l] , or displaced by monovalent cations when the latter are in large excess [12,13]. Such magnesium-depleted ribosomes maintain a compact form if they are kept in 0.5 M NH,Cl or if magnesium is restored to them. If exposed to low ionic strength in the absence of magnesium, however, they undergo a drastic change in configuration, marked by a drop in sedimentation constant from 30s and 50s to about 10s or less. Protein does not become detached during this unfolding process [11,14]. Accordingly, we have begun a study of the effect of magnesium depletion and the concomitant shape changes on the detachment of ribosomal proteins by salt. The results presented below show that the degree of protein stripping is a function of both ionic strength and magnesium concentration, and that once magnesium has been removed, large amounts of protein are