Sharing the load: Mex67–Mtr2 cofunctions with Los1 in primary tRNA nuclear export

Abstract

Eukaryotic transfer RNAs (tRNAs) are exported from the nucleus, their site of synthesis, to the cytoplasm, their site of function for protein synthesis. The evolutionarily conserved β-importin family member Los1 (Exportin-t) has been the only exporter known to execute nuclear export of newly transcribed intron-containing pre-tRNAs. Interestingly, is unessential in all tested organisms. As tRNA nuclear export is essential, we previously interrogated the budding yeast proteome to identify candidates that function in tRNA nuclear export. Here, we provide molecular, genetic, cytological, and biochemical evidence that the Mex67-Mtr2 (TAP-p15) heterodimer, best characterized for its essential role in mRNA nuclear export, cofunctions with Los1 in tRNA nuclear export. Inactivation of Mex67 or Mtr2 leads to rapid accumulation of end-matured unspliced tRNAs in the nucleus. Remarkably, merely fivefold overexpression of Mex67-Mtr2 can substitute for Los1 inΔ cells. Moreover, in vivo coimmunoprecipitation assays with tagged Mex67 document that the Mex67 binds tRNAs. Our data also show that tRNA exporters surprisingly exhibit differential tRNA substrate preferences. The existence of multiple tRNA exporters, each with different tRNA preferences, may indicate that the proteome can be regulated by tRNA nuclear export. Thus, our data show that Mex67-Mtr2 functions in primary nuclear export for a subset of yeast tRNAs.

tRNA dynamics between the nucleus, cytoplasm and mitochondrial surface: Location, location, location

“…The second potential tRNA nuclear exporter uncovered [35] is the Mex67-Mtr2 heterodimer (NXF-NXT or TAP-p15 in metazoans) [37]. This essential heterodimer has a well-studied role in mRNA nuclear export [Review [38]].…”

“…Moreover, mere 5-fold over-expression of Mex67-Mtr2 substitutes for Los1 in los1 Δ cells, thereby confirming its role in tRNA nuclear export. Finally, tRNAs co-purify with Mex67, documenting a direct role for Mex67 in tRNA nuclear export [37]. The essential functions of Mex67-Mtr2 are evolutionarily conserved, as human TAP-p15 suppresses lethality of a yeast mex67 mtr2 double mutant strain [39].…”

“…However, accumulation of capped pre-tRNAs was not observed in mex67-5 or mtr2 ts cells at the non-permissive temperature. Rather, northern hybridization data document that mex67-5 or mtr2 ts cells accumulate pre-tRNAs with processed ends [37]. Perhaps, the lack of accumulation of 5′ extended, capped pre-tRNAs in mex67-5 or mtr2 ts cells is due to increased retention in the nucleus, allowing sufficient time for end-processing.…”

“…Though Mex67-Mtr2 has been recently been characterized as a bona fide tRNA exporter [37], the mechanism of Mex67-Mtr2-mediated tRNA export is not clear. Various biochemical as well as structural studies of Mex67-Mtr2 and their homologs have identified two mechanisms by which the heterodimer exports specific RNA cargoes [Reviews [38,50–52]].…”

“…The data indicate that there is competition between tRNA end processing and tRNA nuclear export by Los1 and thereby may provide an explanation as to why Los1 is present in limiting amounts in the nucleus [60]. Mex67-Mtr2 levels may also be limiting for tRNA export [37], however, the fidelity of such Los1-independent tRNA export pathways has not been investigated.…”