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 tRNAGlnUUG was shown to mediate efficient synthesis of the Q/N rich [PIN+] prion forming protein Rnq1. 1 In the absence of these two tRNA modifications, higher than normal levels of hypomodified tRNAGlnUUG, but not its isoacceptor tRNAGlnCUG 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.