Specific recognition and ubiquitination of translating ribosomes by mammalian CCR4-NOT

Translation impacts mRNA stability and, in yeast, this is mediated by the Ccr4-Not deadenylation complex. The details of this process in mammals remain unclear. Here, we use cryoEM and crosslinking mass spectrometry to show that mammalian CCR4-NOT specifically recognizes ribosomes that are stalled during translation elongation in an in vitro reconstituted system. Similar to yeast, CCR4-NOT inserts a helical bundle of its CNOT3 subunit into the empty E site of the ribosome. Our cryoEM structure shows that human CNOT3 also locks the L1 stalk in an open conformation to inhibit further translation. CCR4-NOT is required for stable association of the non-constitutive subunit CNOT4, which ubiquitinates the ribosome, likely to signal stalled translation elongation. Overall, our work shows that human CCR4-NOT not only detects but also enforces ribosomal stalling to couple translation and mRNA decay.