Component of the Ribosome Quality Control (RQC) complex; RQC is a ribosome-bound complex required for degradation of polypeptides arising from stalled translation; recruits ala- and thr-charged tRNA to the A site and directs CAT (carboxyl-terminal alanine and threonine) tailing, the noncanonical elongation of nascent chains independent of mRNA or 40S subunits, promoting LTN1-mediated ubiquitination and peptide release from stalled ribosomes; monitors translation stress and signals this to Hsf1p