THE EXCHANGEABLE YEAST RIBOSOMAL ACIDIC PROTEIN YP2-BETA SHOWS CHARACTERISTICS OF A PARTLY FOLDED STATE UNDER PHYSIOLOGICAL CONDITIONS

The eukaryotic acidic ribosomal P proteins, contrary to the standard r -proteins which are rapidly degraded in the cytoplasm, are found formi ng a large cytoplasmic pool that exchanges with the ribosome-bound pro teins during translation. The native structure of the P proteins in so lution is therefore an essential determinant of the protein-protein in teractions that take place in the exchange process, In this work, the structure of the ribosomal acidic protein YP2 beta from Saccharomyces cerevisiae has been investigated by fluorescence spectroscopy, circula r dichroism (CD), nuclear magnetic resonance (NMR): and sedimentation equilibrium techniques, We have established the fact that YP2 beta bea rs a 22% alpha-helical secondary structure and a noncompact tertiary s tructure under physiological conditions (pH 7.0 and 25 degrees C); the hydrophobic core of the protein appears to be solvent-exposed, and ve ry low cooperativity is observed for heat- or urea-induced denaturatio n. Moreover, the H-1-NMR spectra show a small signal dispersion, and v irtually all the amide protons exchange with the solvent on a very sho rt time scale, which is characteristic of an open structure. At low pH , YP2 beta maintains its secondary structure content, but there is no evidence for tertiary structure. 2,2,2-Trifluoroethanol (TFE) induces a higher amount of alpha-helical structure but also disrupts any trace of the remaining tertiary fold. These results indicate that YP2 beta may have a flexible structure in the cytoplasmic pool, with some of th e characteristics of a ''molten globule'', and also point out the phys iological relevance of such flexible protein states in processes other than protein folding.