Functional role of acidic ribosomal proteins. Interchangeability of proteins from bacterial and eukaryotic cells

Core particles derived from yeast ribosomes by treatment with 50% ethanol and 0.4 M NH4Cl (P0.4 cores) are derived of the acidic proteins L44/45 functionally equivalent to the bacterial proteins L7 and L12. These bacterial proteins are able to reconstitute the EF-2-dependent GDP binding capacity of the yeast cores but not their GTPase activity. On the other hand, yeast particles prepared in similar conditions but in the presence of 1 M NH4Cl (P1.0 cores) lose proteins L44/45, L15, and S31. These particles are able to reconstitute both activities by the bacterial proteins L7 and L12. Proteins L15 and S31 somehow affect the interaction of bacterial proteins L7 and L12 with the yeast particles. Indeed, in their presence only one dimer of L7 and L12 is bound to the P0.4 cores, while in their absence (P1.0 cores) the amount of bacterial proteins retained by the yeast particles is doubled. Elongation factor EF-2 seems to play an important role in the binding of the bacterial proteins to the yeast cores. Our results suggest that the two dimers of L7 and L12 normally present in the ribosomes might play a different functional role, one of the dimers being related to the binding of the substrate and the other one involved in the GTPase active center.