Talk abstracts
Talk on Friday 05:00-06:00pm submitted by Alan Hinnebusch
Decoding Translational Reprogramming: Direct and Indirect Molecular Programs Shaping Eukaryotic Gene Expression
Dr. Alan Hinnebusch (Division of Molecular and Cellular Biology, Eunice K. Shriver National Institute of Child Health and Human Development, National Institutes of Health)
Abstract:
Translational control is an important mode of regulating eukaryotic gene expression, yet its molecular mechanisms remain incompletely understood. Here, we present in vivo and in vitro yeast experiments revealing how translation initiation is modulated by direct mRNA interactions and unexpected, indirect transcript-level effects. First, we show that the DEAD-box RNA helicase Ded1 stimulates initiation primarily by resolving secondary structures in 5-untranslated regions (UTRs) to promote ribosomal scanning to the main open-reading-frame (mORF). Contrary to recent proposals, Ded1 rarely acts by suppressing inhibitory upstream ORFs (uORFs). Second, we challenge the prevailing "closed-loop" model regarding the poly(A) tail-binding protein (Pab1). We find that Pab1 enhances initiation only indirectly by blocking mRNA decapping and degradation. By preserving steady-state transcript levels, Pab1 maintains the proper ratio of mRNAs to the translational machinery, while the physical closed-loop mRNP intermediate plays little role in stimulating translation under normal conditions. Third, the decapping enzyme Dcp2 similarly exerts indirect control by suppressing transcript levels and maintaining robust ribosome biogenesis, thereby preserving the high mRNA-to-ribosome ratios required to translate inefficient "weak" mRNAs. Preferential translation of these weak mRNAs also requires robust ternary complex (TC) assembly. During starvation, phosphorylation of eIF2 by the kinase Gcn2 suppresses assembly of the ternary complex (TC) comprised of eIF2-GTP and methionyl initiator tRNAneeded to produce the 43S preinitiation complex. This reorients translation to favor "strong" mRNAs over "weak" ones, in the manner predicted by the "ribosome concentration model." This indirect mechanism complements the classic, uORF-mediated upregulation of the starvation-activated transcription factor GCN4 and its stress-induced target genes in the Integrated Stress Response. Finally, we successfully reconstituted both Ded1-mediated control and ratio-driven translational reprogramming in a fully defined, cell-free yeast system. These findings demonstrate that the core translation machinery alone is sufficient to execute these complex regulatory programs.
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