Proteolytic dissection of eIF4G reveals the closed-loop mRNP as an architecture for translation repression.

Abstract

Formation of a "closed-loop" mRNP, in which the 5' cap and 3' poly(A) tail are bridged by eIF4E-eIF4G-PABP interactions, has long been proposed to drive efficient translation initiation. Direct tests of this model in mammalian cells have remained elusive. Using auxin-inducible degron (AID) technology to acutely deplete eIF4G1, we find that global translation is only partially reduced and recovers without restoration of eIF4G1 levels. We identify eIF4G3 as an underappreciated contributor to basal translation that buffers translational output upon eIF4G1 loss without increased protein expression, explaining the modest defects observed in prior RNAi-based studies. Systematic replacement of eIF4G1 with defined cleavage products and interaction mutants reveals that PABP binding by eIF4G1 is dispensable for bulk translation initiation: the central caspase-3 cleavage fragment of eIF4G1 (casp3-cpM), which lacks the PABP-interaction domain, fully rescues global protein synthesis, and acute depletion of both major cytoplasmic PABP paralogs primarily destabilizes mRNAs rather than impairing initiation. In contrast, the N-terminal enteroviral 2A cleavage product (2A-cpN) is a potent, dominant translational repressor that requires simultaneous eIF4E and PABP engagement to form a dead-end closed-loop mRNP that sequesters initiation factors without enabling 43S recruitment. These findings reveal that the eIF4G-PABP closed-loop architecture is not required for productive initiation but can be actively co-opted for translational silencing. This explains why viral eIF4G cleavage, but not factor depletion, produces near-complete translational shutoff. The modular architecture of eIF4G enables diametrically opposing translational outcomes through selective proteolytic processing.