The eukaryotic stalk, which is responsible for the recruitment of translation factors, is a pentamer containing two P1–P2 dimers with unclear modes of action. In Saccharomyces cerevisiae, P1/P2 proteins (individual P1 and P2 proteins) are organized into two distinct dimers, P1A–P2B and P1B–P2A. To investigate the functional contribution of each dimer on the ribosome, RTA (ricin A chain), which binds to the stalk to depurinate the SRL (sarcin/ricin loop), was used as a molecular probe in yeast mutants in which the binding site for one or the other dimer on P0 was deleted. Ribosome depurination and toxicity of RTA were greatly reduced in mutants containing only P1A–P2B on the ribosome, whereas those with only P1B–P2A were reduced less in depurination and were unaffected in toxicity. Ribosomes bearing P1B–P2A were depurinated by RTA at a similar level as wild-type, but ribosomes bearing P1A–P2B were depurinated at a much lower level in vitro. The latter ribosomes showed the lowest association and almost no dissociation with RTA by surface plasmon resonance. These results indicate that the P1B–P2A dimer is more critical for facilitating the access of RTA to the SRL, providing the first in vivo evidence for functional divergence between the two stalk dimers on the ribosome.

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