Structural differences between Saccharomyces cerevisiae ribosomal stalk proteins P1 and P2 support their functional diversity

The eukaryotic acidic P1 and P2 proteins modulate the activity of the ribos omal stalk but playing distinct roles. The aim of this work was to analyze the structural features that are behind their different function. A structu ral characterization of Saccharomyces cerevisaie P1 alpha and P2 beta prote ins was performed by circular dichroism, nuclear magnetic resonance, fluore scence spectroscopy, thermal denaturation, and protease sensitivity. The re sults confirm the low structure present in both proteins but reveal clear d ifferences between them. P1 alpha shows a virtually unordered secondary str ucture with a residual helical content that disappears below 30 degrees C a nd a clear tendency to acquire secondary structure at low pH and in the pre sence of trifluoroethanol, In agreement with this higher disorder P1 alpha has a fully solvent-accessible tryptophan residue and, in contrast to P2 be ta, is highly sensitive to protease degradation. An interaction between bot h proteins was observed, which induces an increase in the global secondary structure content of both proteins. Moreover, mixing of both proteins cause s a shift of the Plot tryptophan 40 signal, pointing to an involvement of t his region in the interaction. This evidence directly proves an interaction between P1 alpha and P2 beta before ribosome binding and suggests a functi onal complementation between them. On a whole, the results provide structur al support for the different functional roles played by the proteins of the two groups showing, at the same time, that relatively small structural dif ferences between the two stalk acidic protein types can result in significa nt functional changes.