A Novel Mechanism for the Control of Translation Initiation by Amino Acids, Mediated by Phosphorylation of Eukaryotic Initiation Factor 2B

Abstract

Eukaryotic initiation factor 2B (eIF2B) plays a key role in controlling the initiation of mRNA translation. eIF2B is heteropentamer whose catalytic (ε) subunit promotes GDP/GTP exchange on eIF2. We show here that depriving human cells of amino acids rapidly results in the inhibition of eIF2B, independently of changes in eIF2 phosphorylation. Although amino acid deprivation also inhibits signaling through the mammalian target of rapamycin complex 1 (mTORC1), the inhibition of eIF2B activity by amino acid starvation is independent of mTORC1. Instead, amino acids repress the phosphorylation of a novel site in eIF2Bε. We identify this site as Ser525, located adjacent to the known phosphoregulatory region in eIF2Bε. Mutation of Ser525 to Ala abolishes the regulation of eIF2B and protein synthesis by amino acids. This indicates that phosphorylation of this site is crucial for the control of eIF2B and protein synthesis by amino acids. These findings identify a new way in which amino acids regulate a key step in translation initiation and indicate that this involves a novel amino acid-sensitive signaling mechanism.

ACKNOWLEDGMENTS

This study was supported by funding from the Wellcome Trust and the Faculty of Medicine, University of British Columbia.

We are very grateful to Ronald Wek (University of Indiana, Indianapolis) for providing Gcn2^+/+ and Gcn2^−/− MEFs and to Rui Liu for creating expression vectors of subunits of eIF2. We thank Nick Morrice (University of Dundee), Bryan Ballif (formerly Harvard University) and Jürgen Kast (University of British Columbia) for help with the mass spectrometric analyses.