Vmsl and ANKZF1 peptidyl-tRNA hydrolases release nascent chains from stalled ribosomes

Ribosomal surveillance pathways scan for ribosomes that are transiently paused or terminally stalled owing to structural elements in mRNAs or nascent chain sequenced(1,2). Some stalls in budding yeast are sensed by the GTPase Hbsl, which loads Dom34, a catalytically inactive member of the archaeo-eukaryotic release factor 1 superfamily. Hbsl-Dom34 and the ATPase Rlil dissociate stalled ribosomes into 40S and 60S subunits. However, the 60S subunits retain the peptidyl-tRNA nascent chains, which recruit the ribosome quality control complex that consists of Rqcl-Rqc2-Ltn1-Cdc48-Ufd1-Np14. Nascent chains ubiquitylated by the E3 ubiquitin ligase Ltn1 are extracted from the 60S subunit by the ATPase Cdc48-Ufd1-Np14 and presented to the 26S proteasome for degradation(3-9). Failure to degrade the nascent chains leads to protein aggregation and proteotoxic stress in yeast and neurodegeneration in mice(10-14). Despite intensive investigations on the ribosome quality control pathway, it is not known how the tRNA is hydrolysed from the ubiquitylated nascent chain before its degradation. Here we show that the Cdc48 adaptor Vmsl is a peptidyl-tRNA hydrolase. Similar to classical eukaryotic release factor 1, Vmsl activity is dependent on a conserved catalytic glutamine. Evolutionary analysis indicates that yeast Vms1 is the founding member of a Glade of eukaryotic release factor 1 homologues that we designate the Vmsl-like release factor 1 clade.