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 (mcm ⁵ s ² U) and pseuduridine (Ψ) are thought to be required for an efficient decoding process. In Saccharomyces cerevisiae, the simultaneous presence of mcm ⁵ s ² U and Ψ38 in tRNA ^Gln _UUG 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 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 mcm ⁵ s ² U 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.