Structural properties of the human acidic ribosomal P proteins forming the P1-P2 heterocomplex.

The ribosome has a morphologically distinct structural feature called the stalk, recognized as a vital element for its function. The ribosomal P proteins constitute the main part of the eukaryotic ribosomal stalk, forming a pentameric structure P0-(P1-P2)(2). The group of P1/P2 proteins in eukaryotes is very diverse, and in spite of functional and structural similarities they do not fully complement one another, probably constituting an adaptive feature of the ribosome from a particular species to diverse environmental conditions. The functional differences among the P1/P2 proteins were analysed in vivo several times; however, a thorough molecular characterization was only done for the yeast P1/P2 proteins. Here, we report a biophysical analysis of the human P1 and P2 proteins, applying mass spectrometry, CD and fluorescence spectroscopy, cross-linking and size exclusion chromatography. The human P1/P2 proteins form stable heterodimer, as it is the case for P1/P2 from yeast. However, unlike the yeast complex P1A-P2B, the human P1-P2 dimer showed a three-state transition mechanism, suggesting that an intermediate species may exist in solution.