Decoding of the ubiquitin code for clearance of colliding ribosomes by the RQT complex

Abstract

The collision sensor Hel2 specifically recognizes colliding ribosomes and ubiquitinates the ribosomal protein uS10, leading to noncanonical subunit dissociation by the ribosome-associated quality control trigger (RQT) complex. Although uS10 ubiquitination is essential for rescuing stalled ribosomes, its function and recognition steps are not fully understood. Here, we showed that the RQT complex components Cue3 and Rqt4 interacted with the K63-linked ubiquitin chain and accelerated the recruitment of the RQT complex to the ubiquitinated colliding ribosome. The CUE domain of Cue3 and the N-terminal domain of Rqt4 bound independently to the K63-linked ubiquitin chain. Their deletion abolished ribosomal dissociation mediated by the RQT complex. The intrinsically disordered regions of Rqt4 enabled the expansion of the searchable area for interaction with the ubiquitin chain. These findings provide mechanistic insight into the decoding of the ubiquitin code for clearance of colliding ribosomes by the RQT complex.

Proteostasis regulation through ribosome quality control and no‐go‐decay

“…Additionally, double deletion (ΔCUE/ΔN) completely abolished the RQT assembly, indicating that Rqt4 compensates Cue3 for ubiquitin-binding defects. Overall, Rqt4 acts as a second arm of RQT in recognizing the K63-linked polyubiquitin to accelerate the RQT assembly and collided ribosome disassembly (Matsuo et al, 2022). Recent cryo-EM structures propose an initial model for the RQT-mediated subunit splitting mechanism by which the RQT complex exerts a pulling force on the 40S subunit of the leading stalled ribosome together with the colliding ribosome, which acts as a wedge to mediate the separation between 40S and 60S subunits (Best et al, 2023).…”

“…The ubiquitinated 40S subunit is released, and the peptidyl tRNA in the P-site remains bound to the liberated 60S subunit (Matsuo et al, 2020;Young & Guydosh, 2022). The RQT mechanism is conserved in multicellular organisms (Matsuo et al, 2022;Narita et al, 2022). In humans, the hRQT (ASCC) dissociates the leading ribosomes and liberates the nascent chain from the 60S.…”