The kinase Isr1 negatively regulates hexosamine biosynthesis in S. cerevisiae

PLoS Genet. 2020 Jun 24;16(6):e1008840. doi: 10.1371/journal.pgen.1008840. eCollection 2020 Jun.


The S. cerevisiae ISR1 gene encodes a putative kinase with no ascribed function. Here, we show that Isr1 acts as a negative regulator of the highly-conserved hexosamine biosynthesis pathway (HBP), which converts glucose into uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), the carbohydrate precursor to protein glycosylation, GPI-anchor formation, and chitin biosynthesis. Overexpression of ISR1 is lethal and, at lower levels, causes sensitivity to tunicamycin and resistance to calcofluor white, implying impaired protein glycosylation and reduced chitin deposition. Gfa1 is the first enzyme in the HBP and is conserved from bacteria and yeast to humans. The lethality caused by ISR1 overexpression is rescued by co-overexpression of GFA1 or exogenous glucosamine, which bypasses GFA1's essential function. Gfa1 is phosphorylated in an Isr1-dependent fashion and mutation of Isr1-dependent sites ameliorates the lethality associated with ISR1 overexpression. Isr1 contains a phosphodegron that is phosphorylated by Pho85 and subsequently ubiquitinated by the SCF-Cdc4 complex, largely confining Isr1 protein levels to the time of bud emergence. Mutation of this phosphodegron stabilizes Isr1 and recapitulates the overexpression phenotypes. As Pho85 is a cell cycle and nutrient responsive kinase, this tight regulation of Isr1 may serve to dynamically regulate flux through the HBP and modulate how the cell's energy resources are converted into structural carbohydrates in response to changing cellular needs.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Energy Metabolism
  • Glucose / metabolism
  • Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing) / genetics
  • Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing) / metabolism*
  • Hexosamines / biosynthesis*
  • Mutation
  • Phosphorylation
  • Protein Kinases / genetics
  • Protein Kinases / metabolism*
  • Protein Processing, Post-Translational
  • Protein Stability
  • Saccharomyces cerevisiae / enzymology*
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism*
  • Uridine Diphosphate N-Acetylglucosamine / metabolism


  • Hexosamines
  • Saccharomyces cerevisiae Proteins
  • Uridine Diphosphate N-Acetylglucosamine
  • GFA1 protein, S cerevisiae
  • Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing)
  • Protein Kinases
  • Glucose