Rqc2p and 60S ribosomal subunits mediate mRNA-independent elongation of nascent chains

Tagging truncated proteins with CAT tails During the translation of a messenger RNA (mRNA) into protein, ribosomes can sometimes stall. Truncated proteins thus formed can be toxic to the cell and must be destroyed. Shen et al. show that the proteins Ltn1p and Rqc2p, subunits of the ribosome quality control complex, bind to the stalled and partially disassembled ribosome. Ltn1p, a ubiquitin ligase, binds near the nascent polypeptide exit tunnel on the ribosome, well placed to tag the truncated protein for destruction. The Rqc2p protein interacts with the transfer RNA binding sites on the partial ribosome and recruits alanine- and threonine-bearing tRNAs. Rqc2p then catalyzes the addition of these amino acids onto the unfinished protein, in the absence of both the fully assembled ribosome and mRNA. These so-called CAT tails may promote the heat shock response, which helps buffer against malformed proteins. Science, this issue p. 75 Stalled protein translation results in 80S ribosome– and messenger RNA–free amino acid addition to truncated proteins. In Eukarya, stalled translation induces 40S dissociation and recruitment of the ribosome quality control complex (RQC) to the 60S subunit, which mediates nascent chain degradation. Here we report cryo–electron microscopy structures revealing that the RQC components Rqc2p (YPL009C/Tae2) and Ltn1p (YMR247C/Rkr1) bind to the 60S subunit at sites exposed after 40S dissociation, placing the Ltn1p RING (Really Interesting New Gene) domain near the exit channel and Rqc2p over the P-site transfer RNA (tRNA). We further demonstrate that Rqc2p recruits alanine- and threonine-charged tRNA to the A site and directs the elongation of nascent chains independently of mRNA or 40S subunits. Our work uncovers an unexpected mechanism of protein synthesis, in which a protein—not an mRNA—determines tRNA recruitment and the tagging of nascent chains with carboxy-terminal Ala and Thr extensions (“CAT tails”).