Combined tRNA modification defects impair protein homeostasis and synthesis of the yeast prion protein Rnq1

Modified nucleosides in tRNA anticodon loops such as 5-methoxy-carbonyl-methyl-2-thiouridine (mcm⁵s²U) and pseuduridine (Ψ) are thought to be required for an efficient decoding process. In Saccharomyces cerevisiae, the simultaneous presence of mcm⁵s²U and Ψ38 in tRNA^Gln_UUG was shown to mediate efficient synthesis of the Q/N rich [PIN⁺] prion forming protein Rnq1.¹ Klassen R, Ciftci A, Johanna Funk J, Bruch A, Butter F, Schaffrath R. tRNA anticodon loop modifications ensure protein homeostasis and cell morphogenesis in yeast. Nucleic Acids Res 2016; 44(22):10946-959. pii: gkw705; PMID:27496282; http://dx.doi.org/10.1093/nar/gkw705[Crossref], [PubMed], [Web of Science ®] [Google Scholar] In the absence of these two tRNA modifications, higher than normal levels of hypomodified tRNA^Gln_UUG, but not its isoacceptor tRNA^Gln_CUG can restore Rnq1 synthesis. Moroever, tRNA overexpression rescues pleiotropic phenotypes that associate with loss of mcm⁵s²U and Ψ38 formation. Notably, combined absence of different tRNA modifications are shown to induce the formation of protein aggregates which likely mediate severe cytological abnormalities, including cytokinesis and nuclear segregation defects. In support of this, overexpression of the aggregating polyQ protein Htt103Q, but not its non-aggregating variant Htt25Q phenocopies these cytological abnormalities, most pronouncedly in deg1 single mutants lacking Ψ38 alone. It is concluded that slow decoding of particular codons induces defects in protein homeostasis that interfere with key steps in cytokinesis and nuclear segregation.