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The Nuclear Hexokinase 2 Acts as a Glucose Sensor in Saccharomyces cerevisiae

Open AccessPublished:August 05, 2016DOI:https://doi.org/10.1074/jbc.L116.738237
      This is a response to a letter by Kriegel et al. (
      • Kriegel T.M.
      • Kettner K.
      • Rödel G.
      • Sträter N.
      • et al.
      Regulatory function of hexokinase 2 in glucose signaling in Saccharomyces cerevisiae.
      ).
      In our work (
      • Vega M.
      • Riera A.
      • Fernández-Cid A.
      • Herrero P.
      • Moreno F.
      • et al.
      Hexokinase 2 is an intracellular glucose sensor of yeast cells that maintains the structure and activity of Mig1 protein repressor complex.
      ) we analyze the mechanism by which hexokinase 2 (Hxk2) regulates its incorporation into the repressor complex of the Mig1-dependent gene promoters. Kriegel et al. (
      • Kriegel T.M.
      • Kettner K.
      • Rödel G.
      • Sträter N.
      • et al.
      Regulatory function of hexokinase 2 in glucose signaling in Saccharomyces cerevisiae.
      ) claim that the lack of experimental data on Hxk2 phosphorylation state and conformation questions our conclusions. Nevertheless, it has been described that the shuttling back and forth between the nucleus and the cytoplasm of Hxk2 is regulated by phosphorylation and dephosphorylation of serine 14, mediated by Snf1 kinase and Reg1-dependent Glc7 protein phosphatase, respectively (
      • Fernández-García P.
      • Peláez R.
      • Herrero P.
      • Moreno F.
      • et al.
      Phosphorylation of yeast hexokinase 2 regulates its nucleocytoplasmic shuttling.
      ). In contrast, Kettner et al. (
      • Kettner K.
      • Krause U.
      • Mosler S.
      • Bodenstein C.
      • Kriegel T.M.
      • Rödel G.
      • et al.
      Saccharomyces cerevisiae gene YMR291W/TDA1 mediates the in vivo phosphorylation of hexokinase isoenzyme 2 at serine-15.
      ) suggest that Tda1 protein is the necessary kinase for serine 14 phosphorylation, but in our hands, the Hxk2 nucleocytoplasmic distribution is not affected in Δtda1 mutant cells (
      • Fernández-García P.
      • Peláez R.
      • Herrero P.
      • Moreno F.
      • et al.
      Phosphorylation of yeast hexokinase 2 regulates its nucleocytoplasmic shuttling.
      ). Moreover, in vitro experiments, with purified proteins, demonstrate that Snf1 kinase directly phosphorylates Hxk2, and this phosphorylation regulates its binding capacity to the Xpo1 transporter (
      • Fernández-García P.
      • Peláez R.
      • Herrero P.
      • Moreno F.
      • et al.
      Phosphorylation of yeast hexokinase 2 regulates its nucleocytoplasmic shuttling.
      ).
      However, Hxk2 phosphorylation by the Snf1 kinase is not critical to regulate the Hxk2-SUC2 promoter binding (
      • Vega M.
      • Riera A.
      • Fernández-Cid A.
      • Herrero P.
      • Moreno F.
      • et al.
      Hexokinase 2 is an intracellular glucose sensor of yeast cells that maintains the structure and activity of Mig1 protein repressor complex.
      ). It is well documented that high glucose and low glucose induce a closed and open conformation of S. cerevisiae Hxk2, respectively (
      • Kuser P.R.
      • Krauchenco S.
      • Antunes O.A.
      • Polikarpov I.
      • et al.
      The high resolution crystal structure of yeast hexokinase PII with the correct primary sequence provides new insights into its mechanism of action.
      ,
      • Shoham M.
      • Steitz T.A.
      • et al.
      The 6-hydroxymethyl group of a hexose is essential for the substrate-induced closure of the cleft in hexokinase.
      ). Because in the presence of xylose both in vivo and in vitro Hxk2-Mig1 interaction is abolished, as happens in low glucose conditions, we conclude that xylose induces a Hxk2 conformation similar to that observed in low glucose conditions (
      • Shoham M.
      • Steitz T.A.
      • et al.
      The 6-hydroxymethyl group of a hexose is essential for the substrate-induced closure of the cleft in hexokinase.
      ). Moreover, the conformation of Hxk2 induced by xylose does not require the Hxk2 phosphorylation or an ATP-induced conformation, because it is also observed in the absence of ATP or the non-hydrolyzable analog AMP-PNP (adenosine 5′-(β,γ-imino)triphosphate), respectively (
      • Vega M.
      • Riera A.
      • Fernández-Cid A.
      • Herrero P.
      • Moreno F.
      • et al.
      Hexokinase 2 is an intracellular glucose sensor of yeast cells that maintains the structure and activity of Mig1 protein repressor complex.
      ).

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