Structural Differences betweenSaccharomyces cerevisiaeRibosomal Stalk Proteins P1 and P2 Support Their Functional Diversity

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Abstract
The eukaryotic acidic P1 and P2 proteins modulate the activity of the ribosomal stalk but playing distinct roles. The aim of this work was to analyze the structural features that are behind their different function. A structural characterization of Saccharomyces cerevisaie P1α and P2β proteins was performed by circular dichroism, nuclear magnetic resonance, fluorescence spectroscopy, thermal denaturation, and protease sensitivity. The results confirm the low structure present in both proteins but reveal clear differences between them. P1α shows a virtually unordered secondary structure with a residual helical content that disappears below 30 °C and a clear tendency to acquire secondary structure at low pH and in the presence of trifluoroethanol. In agreement with this higher disorder P1α has a fully solvent-accessible tryptophan residue and, in contrast to P2β, is highly sensitive to protease degradation. An interaction between both proteins was observed, which induces an increase in the global secondary structure content of both proteins. Moreover, mixing of both proteins causes a shift of the P1α tryptophan 40 signal, pointing to an involvement of this region in the interaction. This evidence directly proves an interaction between P1α and P2β before ribosome binding and suggests a functional complementation between them. On a whole, the results provide structural support for the different functional roles played by the proteins of the two groups showing, at the same time, that relatively small structural differences between the two stalk acidic protein types can result in significant functional changes.