Cex1p is a novel cytoplasmic component of the Saccharomyces cerevisiae nuclear tRNA export machinery

Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada.
The EMBO Journal (Impact Factor: 10.43). 02/2007; 26(2):288-300. DOI: 10.1038/sj.emboj.7601493
Source: PubMed


The Saccharomyces cerevisiae Yor112wp, which we named Cex1p, was identified using a yeast tRNA three-hybrid interaction approach and an in vivo nuclear tRNA export assay as a cytoplasmic component of the nuclear tRNA export machinery. Cex1p binds tRNA saturably, and associates with the nuclear pore complex by interacting directly with Nup116p. Cex1p co-purifies with the nuclear tRNA export receptors Los1p and Msn5p, the eukaryotic elongation factor eEF-1A, which delivers aminoacylated tRNAs to the ribosome, and the RanGTPase Gsp1p, but not with Cca1p, a tRNA maturation enzyme that facilitates translocation of non-aminoacylated tRNAs across the nuclear pore complex. Depletion of Cex1p and eEF-1A or Los1p significantly reduced the efficiency of nuclear tRNA export. Cex1p interacts with Los1p but not with eEF-1A in vitro. These findings suggest that Cex1p is a component of the nuclear aminoacylation-dependent tRNA export pathway in S. cerevisiae. They also suggest that Cex1p collects aminoacyl-tRNAs from the nuclear export receptors at the cytoplasmic side of the nuclear pore complex, and transfers them to eEF-1A using a channelling mechanism.

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    • "For example, Utp8 is an essential protein that has been reported to chaperone nuclear tRNAs and to facilitate the formation of RanGTP-NTR-tRNA complexes [37] [38]. In the cytoplasm , Cex1 was suggested to target aminoacylated tRNAs from the cytoplasmic face of the NPC to the eukaryotic translation elongation factor eEF1A [39], which has also been suggested to be involved in one of the tRNA export pathways [40]. An intriguing development in the field of tRNA biogenesis arose approximately 15 years ago when it was observed in yeast that mature tRNAs derived from intron-containing pre-tRNAs accumulate in the nucleoplasm upon depletion of factors required for tRNA export [35] [41]. "
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    • "One proposal is that it would cooperate with another protein able to distinguish mature tRNAs from intron-containing pretRNAs , and aminoacylated from uncharged tRNAs. The translation elongation factor Tef1/2 (vertebrate eEF1-a) is a candidate for such a protein because it interacts only with mature aminoacylated tRNA, has been implicated in tRNA nuclear export (Grosshans et al. 2000; McGuire and Mangroo 2007), and is able to move into and out of the yeast nucleus (Murthi et al. 2010). Other yeast proteins including Utp9 (Eswara et al. 2009), Utp8 (McGuire et al. 2009), and Sol1/2 (Shen et al. 1996; Stanford et al. 2004) have also been implicated in the nuclear–cytoplasmic dynamics of mature tRNAs, but further studies are required to learn the roles of these proteins in tRNA re-export. "
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    • "We established biochemically that tRNA is a poor substrate for Msn5p in the absence of Gsp1p-GTP (Figure 14A and C), but like other nuclear export receptors including Los1p, Msn5p binding to tRNA requires Gsp1p-GTP (Figure 14C), suggesting that Msn5p also functions as a nuclear tRNA export receptor. This notion is consistent with in vitro and in vivo studies showing that Msn5p interacts with Utp8p and Cex1p (McGuire and Mangroo, 2007; Strub et al., 2007). Msn5p may export both aminoacylated and non-aminoacylated tRNA, since it can bind both forms of tRNA in a Gsp1p-GTP-dependent manner (Figure 14C). "
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