Protein Kinase A Is Part of a Mechanism That Regulates Nuclear Reimport of the Nuclear tRNA Export Receptors Los1p and Msn5p

Abstract

The two main signal transduction mechanisms that allow eukaryotes to sense and respond to changes in glucose availability in the environment are the cyclic AMP (cAMP)/protein kinase A (PKA) and AMP-activated protein kinase (AMPK)/Snf1 kinase-dependent pathways. Previous studies have shown that the nuclear tRNA export process is inhibited in Saccharomyces cerevisiae deprived of glucose. However, the signal transduction pathway involved and the mechanism by which glucose availability regulates nuclear-cytoplasmic tRNA trafficking are not understood. Here, we show that inhibition of nuclear tRNA export is caused by a block in nuclear reimport of the tRNA export receptors during glucose deprivation. Cytoplasmic accumulation of the tRNA export receptors during glucose deprivation is not caused by activation of Snf1p. Evidence obtained suggests that PKA is part of the mechanism that regulates nuclear reimport of the tRNA export receptors in response to glucose availability. This mechanism does not appear to involve phosphorylation of the nuclear tRNA export receptors by PKA. The block in nuclear reimport of the tRNA export receptors appears to be caused by activation of an unidentified mechanism when PKA is turned off during glucose deprivation. Taken together, the data suggest that PKA facilitates return of the tRNA export receptors to the nucleus by inhibiting an unidentified activity that facilitates cytoplasmic accumulation of the tRNA export receptors when glucose in the environment is limiting. A PKA-independent mechanism was also found to regulate nuclear tRNA export in response to glucose availability. This mechanism, however, does not regulate nuclear reimport of the tRNA export receptors.

A Comprehensive Analysis of Replicative Lifespan in 4,698 Single-Gene Deletion Strains Uncovers Conserved Mechanisms of Aging

“…It is currently unclear whether translation rates are the cause for these differences, or if signal transduction downstream of DR causes post-translational changes of Los1 and/or Tor1. Glucose deprivation may selectively act on Los1 localization via PKA inactivation rather than Snf1/AMPK inactivation (Pierce et al, 2014), which may partially explain the phenotypic differences between these partial DR mimetic mutants.…”

Nup100 regulates Saccharomyces cerevisiae replicative life span by mediating the nuclear export of specific tRNAs

“…It is therefore possible that a nuclear transport or other type of defect in nup100Δ cells activates a signaling response that inhibits the export of specific tRNAs. We predict such a potential Nup100-dependent stress response is dissimilar to amino acid or glucose depletion responses since Los1 and Msn5 are mislocalized under either condition (Huang and Hopper 2014;Pierce et al 2014b), a phenotype we did not observe in nup100Δ cells (Fig. 3A).…”

“…Another potential explanation for nuclear tRNA accumulation in nup100Δ cells is that this retention is caused by an indirect effect of a stress response that inhibits re-export of some tRNAs. Complete removal of amino acids or glucose from media causes significant accumulation of mature, aminoacylated tRNAs in wild-type yeast (Whitney et al 2007;Huang and Hopper 2014;Pierce et al 2014b). It is therefore possible that a nuclear transport or other type of defect in nup100Δ cells activates a signaling response that inhibits the export of specific tRNAs.…”

“…Interestingly, mature cytoplasmic tRNAs appear to be continuously imported back into the nucleus in S. cerevisiae (Takano et al 2005); this retrograde import of tRNAs indirectly requires the NTR Mtr10 (Murthi et al 2010;Huang and Hopper 2015) and may be important for tRNA quality control (Hopper 2013) as well as certain tRNA modifications (Ohira and Suzuki 2011). Environmental conditions play a significant role in regulating tRNA re-export, as mature tRNAs accumulate in the nuclei of wild-type yeast that are amino acid-or glucose-starved (Whitney et al 2007;Huang and Hopper 2014;Pierce et al 2014b). Mutation or inhibition of CEX1, SOL1/2, UTP8, or UTP9 also causes nuclear tRNA accumulation (Steiner-Mosonyi et al 2003;Stanford et al 2004;McGuire and Mangroo 2007;Eswara et al 2009).…”

Author response: Cytosolic Hsp70 and co-chaperones constitute a novel system for tRNA import into the nucleus

“…Since the ssa2Δ phenotype is only obvious under starvation conditions, Ssa2p may provide a secondary pathway to accommodate high demand for the tRNA import under these conditions. In other words, the Ssa2p pathway may confer a regulatory capability to the tRNA nuclear import, responding to growth conditions like the case of tRNA export (Murthi et al, 2010;Huang and Hopper, 2014;Pierce et al, 2014). We speculate that Ssa2p also has some contribution to basal nuclear import of tRNAs under normal conditions by co-operating with the Mtr10p pathway, and that quantitative regulation of the Ssa2p pathway, but not complete on-and-off, enables enhancement of nuclear import under starvation conditions.…”