Diverse and dynamically regulated mRNP composition regulating translation

ABSTRACT Cells tightly regulate translation initiation in order to control which proteins they synthesize and how much of each protein they produce. This regulation of protein synthesis matches translation levels with the cell's translational capacity and physiological needs. Translation initiation, in particular, is a key point for both global and transcript-specific regulation. In the canonical pathway for translation initiation, an mRNA is first activated by the formation of a closed-loop complex bridging between the 5'-methylguanosine cap and the 3'-polyadenylate tail. A small ribosomal subunit, accompanied by a variety of other initiation factors, is recruited to the mRNA and scans in order to begin translation at the first AUG. Recent evidence suggests that translation initiation does not proceed down such a uniform pathway. Individual translation factors are subject to regulation downstream of major signaling pathways, including MAP kinase cascades, mTOR kinase signaling, and the integrated stress response. Activation or inhibition of core translation initiation factors can produce transcript- specific changes in translation, leading to broad translational reprogramming. Translation of developmentally regulated genes also depends on cryptic initiation factors such as eIF2A, eIF2D, and DENR/MCTS-1. Our motivating hypothesis is that this heterogeneous landscape of translation initiation complexes underlies dynamic, mRNA-specific control of protein synthesis. Here, we propose to use proximity labeling of protein and RNA in order to survey the composition of translation initiation complexes that assemble in vivo and understanding how this changes in response to physiological and environmental signals. We will couple this with an analysis of translational across the transcriptome. Together, these results will reveal the full diversity of pathways for translation initiation in vivo and show how these different pathways mediate translational expression programs.