Abstract

In eukaryotic cells, the translation of mRNA into protein is tightly regulated at multiple levels. In this talk, I will present two complementary studies that explore distinct, yet mechanistically intertwined, aspects of translational control. The first part focuses on how the Hepatitis C virus (HCV) internal ribosomal entry site (IRES) manipulates host translation machinery to drive cap-independent viral protein synthesis. Through a series of high-resolution cryo-EM structures, we reveal how the IRES restructures eukaryotic initiation factor 3 (eIF3), with its core subunits being displaced by tight interaction with the IRES while the non-core subunits remain positioned on the ribosome. Unexpectedly, the N-terminal domain of the eIF3c subunit interacts with the 60S ribosomal subunit during elongation, suggesting that eIF3 plays roles beyond initiation, potentially extending to elongation, termination, and ribosome recycling. In the second part, we uncover a novel function of the RNA helicase DHX29 in regulating mRNA stability based on codon optimality. Using genome-wide CRISPR screening, selective ribosome profiling, and cryo-EM, we demonstrate that DHX29 binds near the A-site entrance of translating ribosomes and senses the decoding efficiency of incoming aminoacyl-tRNAs. This ribosomal interaction allows DHX29 to recruit the GIGYF2 •4EHP complex, thereby linking slow translation caused by non-optimal codons to mRNA decay pathways. Together, these studies highlight how both viral elements and host RNA -binding proteins reshape the translational landscape—either to hijack the host machinery or to maintain transcriptome integrity—offering new structural and mechanistic insights into translational regulation.