Academic journal article Genetics

General Amino Acid Control and 14-3-3 Proteins Bmh1/2 Are Required for Nitrogen Catabolite Repression-Sensitive Regulation of Gln3 and Gat1 Localization

Academic journal article Genetics

General Amino Acid Control and 14-3-3 Proteins Bmh1/2 Are Required for Nitrogen Catabolite Repression-Sensitive Regulation of Gln3 and Gat1 Localization

Article excerpt

WHEN simultaneously provided with both a good and poor nitrogen source, Saccharomyces cerevisiae cells will exhaust the good source from the medium before using the poor one (Watson 1977). However, there is no particular structural requirement for a compound to function as a good nitrogen source beyond being present in sufficient amounts, and transported and catabolized rapidly. These requirements result in the quality of a nitrogen source being somewhat dependent on the strain background analyzed. The regulatory system mediating this selectivity is designated nitrogen catabolite repression (NCR) (Cooper 1982, 2004; Broach 2012; Ljungdahl and Daignan-Fornier 2012; Conrad et al. 2014). The functional heart of NCR is the GATA factors, Gln3 and Gat1, which activate the transcription of genes needed to transport and catabolize poorly used nitrogen sources. In nitrogen-replete conditions, Gln3- and Gat1mediated transcription is repressed, whereas it is derepressed when nitrogen supplies are meager.

Mechanism of TorCI-mediated Gln3 regulation

Once GATA factors were identified and the transcriptional wiring of NCR-sensitive transcriptional control elucidated (Hofman-Bang 1999; Cooper 2002, 2004; Magasanik and Kaiser 2002), the pressing question became how the response to such a breadth of compounds could be integrated and directed to regulate Gln3 and Gat1. The answer began to emerge by connecting the dots between Gln3 and its negative regulator, Ure2, with mechanistic Tor protein kinase complex 1 (mTorC1) (Blinder et al. 1996; Beck and Hall 1999; Cardenas et al. 1999; Hardwick et al. 1999; Bertram et al. 2000; Kulkarni et al. 2001; Carvalho and Zheng 2003). This global regulator integrates and generates responses to a diverse spectrum of nutrient- and stress-related inputs (Crespo and Hall 2002; Kim and Guan 2011; Loewith and Hall 2011; Laplante and Sabatini 2012; Lamming and Sabatini 2013; Bar-Peled and Sabatini 2014; Swinnen et al. 2014; Shimobayashi and Hall 2016; Stauffer and Powers 2016). When activated, mTorC1 upregulates processes associated with increased cell division, e.g., protein or ribosomal protein synthesis; and conversely downregulates those associated with nutrient limitation, e.g., NCR-sensitive transcription and autophagy. In nutrient-poor conditions the regulatory outcomes are reversed.

Models conceptualizing this and observations from subsequent investigations posit that excess nitrogen or glutamine activates mTorC1 via the intermediate action of a leucyl transfer RNA (tRNA)-Ego-Gtr complex (figure 1 in Rai et al. 2016) (Binda et al 2009, 2010; Bonfils et al 2012; Crespo et al 2002). Thus activated, mTorC1 phosphorylates Gln3 and Tor-associated protein, Tap42 (Di Como and Arndt 1996; Beck and Hall 1999; Bertram et al. 2000). Phosphorylation of Gln3 facilitates its interaction with Ure2, sequestering it in the cytoplasm (Bertram et al. 2000). Tap42 phosphorylation facilitates its interaction with the Sit4 and PP2A phosphatases (Di Como and Arndt 1996; Jiang and Broach 1999). The Tap42-phosphatase complexes, bound to mTorC1, result in their inactivation; further maintaining Gln3 in its phosphorylated, Ure2-bound form (Beck and Hall 1999; Bertram et al. 2000; Wang et al. 2003; Yan et al. 2006). In limiting nitrogen or after rapamycin inhibition of mTorC1, the Tap42Sit4 complex is released and thus activated, and it dephosphorylates Gln3; permitting it to dissociate from Ure2, enter the nucleus, and activate NCR-sensitive transcription (Beck and Hall 1999; Bertram et al. 2000; Wang et al. 2003; Yan et al. 2006) .

Need to enlarge the mTorCI control model

As elegant and engaging as this model is, it has become increasingly clear that nitrogen-responsive mTorC1 regulation was insufficient to explain NCR (Cox et al. 2004a,b; Tate et al. 2006, 2009, 2010; Georis et al. 2008, 2011a,b; FayyadKazan et al. 2016). Most significantly, the five methods interchangeably used to downregulate mTorC1 activity in fact represented five physiologically distinct conditions when a single reporter, Gln3, was employed (Tate and Cooper 2013): Rap-elicited nuclear Gln3 localization required both Sit4 and PP2A phosphatases. …

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