The secondary structure of salt-extracted ribosomal proteins from Escherichia coli as studied by circular dichroic...

Ribosomal proteins from Escherichia coli MRE600 have been obtained by a new, mild purification procedure. This involves extraction of the subunits with salt followed by chromatographic fractionation in the presence of salt. The use of urea or other denaturing agents and conditions is avoided. A survey of the secondary structure of the 30 S and 50 S proteins, as observed by circular dichroic spectroscopy, is presented. The spectra have been analysed by a new procedure which uses a library of 16 circular dichroic spectra of proteins with a known three-dimensional structure. This method provides a more reliable analysis, especially of the contribution from beta-sheet. The results show that most of the 30 S proteins have a high alpha-helix content, whereas the 50 S proteins are more diverse. The latter group shows a larger contribution from beta-sheet. The data presented here are compared with those already published for a number of proteins which were, with one exception, prepared in the presence of urea. In most cases we find higher alpha-helix and beta-sheet values for the salt-extracted proteins than for the corresponding urea-treated proteins. In those cases, however, where special care was taken to renature the urea-treated proteins agreement is found to within the expected experimental error. The results show that salt-extracted ribosomal proteins have a well-defined secondary structure with a relatively small contribution from unordered structure.