eIF3-mediated ribosome recruitment by histone H4 mRNA during translation initiation

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Hayek, Hassan | Schaeffer, Laure | Janvier, Aurélie | Martin, Franck | Eriani, Gilbert | Allmang, Christine

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International audience. Eukaryotic mRNA translation initiation is a coordinated process that requires numerous initiation factors (eIFs) to trigger mRNA attachment to the small 40S ribosomal subunit, mRNA scanning and start codon selection as well as ribosomal assembly. Recent studies have unveiled a remarkable diversity of ribosome recruitment processes during the initiation of eukaryotic mRNA translation.Our team has shown that the initiation of translation of histone H4 mRNA combined canonical (cap) and viral (no-scanning) initiation strategies (Martin et al., Mol Cell 2011, Nature Com 2016). The ribosomes are recruited by the initiation factors bound to specific structures in the histone mRNA coding region and deposited directly on the initiator AUG codon. This tethering mechanism leads to direct initiation on the AUG without scanning of the 5' untranslated region. Recent work has shown that some subunits of the eIF3 translation initiation factor ensure selective recruitment of cellular mRNAs to control their expression. The histone H4 mRNA is one of the mRNAs controlled by eIF3 (Lee et al., 2015, Villa et al., 2013). In higher eukaryotes eIF3 is composed of 13 subunits (a to m). Among them eIF3d was shown to exhibit a cap binding activity. We deciphered the eIF3-H4 mRNA interaction and analyzed his role during the translation of histone H4. Combining cross- linking and ribonucleoprotein immunoprecipitation (RNP IP) in vivo and in vitro we found that eIF3 interacted with H4 but also H1, H2A, H2B and H3 histone mRNAs. Furthermore, we have demonstrated a direct interaction between the H4 mRNA and the eIF3c, d, e and g subunits. The functional role of eIF3 subunits has been investigated in vivo by siRNA interference and specific analysis of histone neosynthesis after [35S] pulse labelling. Results suggest that eIF3 plays a positive role on H4 mRNA translation. Combined with our previous cryo-EM results on the H4 mRNA/80S complex these data suggest that histone H4 mRNA may interact with eIF3 at both ends of the mRNA channel where the eIF3b/g/i subcomplex and the eIF3d subunits should be located. eIF3-mediated ribosome recruitment is therefore likely to play a critical role in the H4 unconventional translation initiation mechanism.

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