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Release of eIF6 (p27BBP) from the 60S subunit allows 80S ribosome assembly

Abstract

The assembly of 80S ribosomes requires joining of the 40S and 60S subunits, which is triggered by the formation of an initiation complex on the 40S subunit1. This event is rate-limiting for translation2, and depends on external stimuli3 and the status of the cell4. Here we show that 60S subunits are activated by release of eIF6 (also termed p27BBP)5,6. In the cytoplasm, eIF6 is bound to free 60S but not to 80S. Furthermore, eIF6 interacts in the cytoplasm with RACK17, a receptor for activated protein kinase C (PKC). RACK1 is a major component of translating ribosomes, which harbour significant amounts of PKC. Loading 60S subunits with eIF6 caused a dose-dependent translational block and impairment of 80S formation, which were reversed by expression of RACK1 and stimulation of PKC in vivo and in vitro. PKC stimulation led to eIF6 phosphorylation, and mutation of a serine residue in the carboxy terminus of eIF6 impaired RACK1/PKC-mediated translational rescue. We propose that eIF6 release regulates subunit joining, and that RACK1 provides a physical and functional link between PKC signalling and ribosome activation.

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Figure 1: Endogenous RACK1 is found on ribosomes and binds to eIF6 in the cytoplasm.
Figure 2: eIF6 blocks translation and RACK1 stimulates translation in vivo.
Figure 3: The RACK1–PKC complex acts on eIF6.
Figure 4: Reduction of anti-association activity by phosphorylation in vivo.
Figure 5: eIF6 anti-association activity in vitro is modulated by RACK1–PKC βII.

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Acknowledgements

We thank E. Rizzo, S. Tognin, A. Donadini, A. M. Barbieri and L. Castelli for technical advice; for providing salt-washed purified ribosomal subunits, we thank T. Pestova. For sharing reagents, information and critical advice, we thank A. Bachi, T.-A. Sato, F. Loreni, F. Amaldi, P. Linder, J. Traugh, A. Johnson, A. Hinnebusch, A. Burlando, L. Spremulli, E. Villa, J. Verdiere, D. Ron, T. Dever, C. Groft and M. Foiani. This work was supported by AIRC, MURST (P.C.M., S.B.). This study was carried out under the framework of the Italian MUIR Center of Excellence in Physiopathology of Cell Differentiation.

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Correspondence to Stefano Biffo.

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Ceci, M., Gaviraghi, C., Gorrini, C. et al. Release of eIF6 (p27BBP) from the 60S subunit allows 80S ribosome assembly. Nature 426, 579–584 (2003). https://doi.org/10.1038/nature02160

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