Ribosomal proteins were extracted from 30 S subunits of Halobacterium marismortui under native conditions. Their separation was based on gel filtration and hydrophobic chromatography, performed at a concentration of 3.2 M KCl to avoid denaturation. A total of nine proteins were isolated, purified and identified by partial amino-terminal sequences and two-dimensional gel electrophoresis. There is a high degree of sequence homology with 30 S proteins from H. cutirubrum, and also some with 30 S proteins of eubacteria. Proton NMR data indicate unfolding of the proteins in low salt. One of the proteins, however, retains its secondary structure at a salt concentration as low as 0.1 M NaCl, and even in 8 M urea. One reason for this outstanding stability could be the high proportion (50%) of β-structure in this protein as determined from circular dichroism measurements. In general, there is a higher β-sheet content than for 30 S proteins from Escherichia coli. Measurements of Stokes radii indicate several of the proteins to have a rather elongated shape. One of these is a complex consisting of L3/L4 and L20, similar to the L8-complex from E. coli. The presence of this 50 S complex in the preparation of the small subunit suggests a location on the interface between the subunits.
