Abstract
tRNAs are unique among various RNAs in that they shuttle between the nucleus and the cytoplasm, and their localization is regulated by nutrient conditions. Although nuclear export of tRNAs has been well documented, the import machinery is poorly understood. Here, we identified Ssa2p, a major cytoplasmic Hsp70 in Saccharomyces cerevisiae, as a tRNA-binding protein whose deletion compromises nuclear accumulation of tRNAs upon nutrient starvation. Ssa2p recognizes several structural features of tRNAs through its nucleotide-binding domain, but prefers loosely-folded tRNAs, suggesting that Ssa2p has a chaperone-like activity for RNAs. Ssa2p also binds Nup116, one of the yeast nucleoporins. Sis1p and Ydj1p, cytoplasmic co-chaperones for Ssa proteins, were also found to contribute to the tRNA import. These results unveil a novel function of the Ssa2p system as a tRNA carrier for nuclear import by a novel mode of substrate recognition. Such Ssa2p-mediated tRNA import likely contributes to quality control of cytosolic tRNAs.
Highlights
Most cytoplasmic RNAs are exported unidirectionally from the nucleus across the nuclear envelope (NE) after their birth and appropriate processing in the nucleus (Lei and Silver, 2002; Grunwald et al, 2011). tRNAs, are unique among major classes of cytoplasmic RNAs in that they shuttle between the nucleus and the cytoplasm (Takano et al, 2005; Shaheen and Hopper, 2005; for review; Yoshihisa, 2006; Phizicky and Hopper, 2010; Hopper, 2013)
The following lines of evidence support the idea that Ssa2p has a novel function in intracellular dynamics of tRNAs as their nuclear import carrier
We noticed that mutations in the conserved residues in the nuclear binding domain (NBD) of the Ssa proteins affect the tRNA binding ability in vitro differently between Ssa1p and Ssa2p (Figure 6C)
Summary
Most cytoplasmic RNAs are exported unidirectionally from the nucleus across the nuclear envelope (NE) after their birth and appropriate processing in the nucleus (Lei and Silver, 2002; Grunwald et al, 2011). tRNAs, are unique among major classes of cytoplasmic RNAs in that they shuttle between the nucleus and the cytoplasm (Takano et al, 2005; Shaheen and Hopper, 2005; for review; Yoshihisa, 2006; Phizicky and Hopper, 2010; Hopper, 2013). We and others demonstrated that mature tRNAs move from the cytoplasm back into the nucleus (Shaheen and Hopper, 2005; Takano et al, 2005). Los1p/exportin-t is considered as a primary export carrier of mature tRNAs in yeast, plants, and vertebrates (Arts et al, 1998; Hellmuth et al, 1998; Kutay et al, 1998; Sarkar and Hopper, 1998; Li and Chen, 2003; Park et al, 2005; Cook et al, 2009). Yeast Los1p exports pre-tRNAs for cytoplasmic splicing (Sarkar and Hopper, 1998; Yoshihisa et al, 2003).
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