Within a single generation time a growing yeast cell imports ~14 million ribosomal proteins (r-proteins) into the nucleus for ribosome production. After import, it is unclear how these intrinsically unstable and aggregation-prone proteins are targeted to the ribosome assembly site in the nucleolus. Here, we report the discovery of a conserved nuclear carrier Tsr2 that coordinates transfer of the r-protein eS26 to the earliest assembling pre-ribosome, the 90S. In vitro studies revealed that Tsr2 efficiently dissociates importin:eS26 complexes via an atypical RanGTP-independent mechanism that terminates the import process. Subsequently, Tsr2 binds the released eS26, shields it from proteolysis, and ensures its safe delivery to the 90S pre-ribosome. We anticipate similar carriers - termed here escortins - to securely connect the nuclear import machinery with pathways that deposit r-proteins onto developing pre-ribosomal particles.
- Ramanujan S Hegde, MRC Laboratory of Molecular Biology, United Kingdom
© 2014, Schütz et al.
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