Yeast Pak1 Kinase Associates with and Activates Snf1

Abstract

Members of theThe Snf1 kinase of budding yeast and the mammalian AMPactivated protein kinase (AMPK) play critical roles in signaling nutrient stress (9). When activated, Snf1 and AMPK downregulate metabolic pathways that consume ATP and stimulate pathways that promote ATP synthesis. Nutrient-sensing pathways, in particular, glucose-sensing pathways, are critical signaling pathways that are deregulated in type 2 diabetes. Recent studies of AMPK underscore its importance as a metabolic switch and suggest that activation of AMPK may provide a therapeutic benefit for patients with type 2 diabetes (38). The budding yeast Snf1 kinase and mammalian AMPK are orthologous proteins, and they play similar roles in controlling cellular metabolism. Snf1 kinase is needed for the proper response to nutrient stress conditions, such as growth on alternative carbon sources and sporulation (2). A full understanding of the Snf1/AMPK signaling pathways will first require identification of all of the components of the pathway. While several downstream targets of Snf1 and AMPK are both known and conserved between species (39), much less is known about components acting upstream of Snf1 and AMPK.Biochemical and genetic experiments have shown that members of the Snf1/AMP-activated protein kinase family are regulated by phosphorylation of the conserved threonine residue in the kinase activation loop. This mechanism for controlling protein kinase activity is used both by members of the serine threonine protein kinase family and by members of the tyrosine protein kinase family. Biochemical fractionation of rat liver has shown that threonine 172 of the mammalian AMPK enzyme is phosphorylated by a distinct protein kinase called AMPK kinase (AMPKK) (11). In budding yeast, the analogous threonine is located at position 210. Carlson and coworkers first showed that replacement of this residue with alanine in-

Ubp8 and SAGA Regulate Snf1 AMP Kinase Activity

“…One of the most studied modes of Snf1 regulation is the phosphorylation of threonine 210, a mark which closely follows Snf1 activation (14,24,36). In this study, we found that in addition to being modified by phosphorylation, Snf1 is modified by ubiquitination.…”

“…Here, we report that Ubp8 can deubiquitinate Snf1, a highly conserved AMP-activated serine/threonine protein kinase that serves as an energy sensor in the cell. Our data indicate that hyperubiquitination of Snf1 affects its stability as well as its phosphorylation status, which is known to affect Snf1 kinase activity (14,24,36). Together, our results suggest that SAGA functions as a master regulator of gene expression not only by altering chromatin structures but also by regulating the stability and activity of transcriptional modulators.…”

“…One highly conserved mechanism involves the phosphorylation of the T loop at threonine 210 (T210) (14,19,24,36), which closely parallels the activation of Snf1 and many other AMP kinases. When yeast cells grown in glucose are switched to less-preferred carbon sources, Snf1 becomes phosphorylated and then activates the expression of genes involved in utilization of these carbon sources by regulating the activity of downstream transcriptional modulators (12,20,28,29,33,39).…”

Hexokinase 2 Is an Intracellular Glucose Sensor of Yeast Cells That Maintains the Structure and Activity of Mig1 Protein Repressor Complex

“…Snf1 kinase forms a complex with an activating subunit Snf4 and one of the three Snf1-interacting proteins Sip1, Sip2, or Gal83, which target Snf1 to distinct subcellular locations (32). Regulation of Snf1 activity involves phosphorylation of the Snf1 catalytic subunit at threonine 210 by one of the three kinases, Sak1, Tos3, or Elm1 (33)(34)(35). ADP also appears to play a significant role in Snf1 activation in response to glucose limitation by protecting the enzyme against dephosphorylation by the Glc7-Reg1 phosphatase (36).…”

Heterotrimer-independent regulation of activation-loop phosphorylation of Snf1 protein kinase involves two protein phosphatases

“…Phosphorylation of the activation-loop Thr of the catalytic subunit activates SNF1/AMPK. For SNF1, the protein kinases Sak1, Tos3, and Elm1 phosphorylate Thr-210 on the Snf1/α subunit in response to glucose limitation and other stresses (5)(6)(7)(8), but there is no evidence that these Snf1-activating kinases are regulated (9,10). Less is understood about the protein phosphatases that deactivate SNF1.…”