Dissociation of Kar2p/BiP from an ER sensory molecule, Ire1p, triggers the unfolded protein response in yeast

Biochem Biophys Res Commun. 2000 Dec 20;279(2):445-50. doi: 10.1006/bbrc.2000.3987.


The unfolded protein response (UPR) is a signal transduction pathway induced by a variety of endoplasmic reticulum (ER) stresses and functions to maintain homeostasis of the cellular membrane in eukaryotes. Various ER stresses result in the accumulation of unfolded proteins in the ER, which is sensed by the transmembrane protein kinase/ribonuclease Ire1p that transmits a signal from the ER to the nucleus in Saccharomyces cerevisiae. Here we report that the yeast ER chaperone Kar2p/BiP, a member of the HSP70 family found in the ER, directly regulates the UPR by the interaction with Ire1p. In the absence of ER stress, Kar2p binds the lumenal domain of Ire1p and keeps Ire1p in an inactive unphosphorylated state. Upon exposure of cells to ER stresses, Kar2p is released from Ire1p, resulting in activation of Ire1p and signal transduction to the nucleus. Subsequently, KAR2 mRNA is induced and Kar2p accumulates in the ER in a time-dependent manner, restoring the system to the basal state. This negative autoregulation is similar to the regulation of mammalian cytosolic chaperone Hsp70 via its interaction with heat shock factor 1.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cloning, Molecular
  • Endoplasmic Reticulum / physiology*
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism*
  • HSP70 Heat-Shock Proteins / genetics
  • HSP70 Heat-Shock Proteins / metabolism*
  • Membrane Glycoproteins / genetics
  • Membrane Glycoproteins / metabolism*
  • Models, Biological
  • Molecular Chaperones / metabolism*
  • Phosphorylation
  • Protein Folding*
  • Protein Serine-Threonine Kinases*
  • Recombinant Proteins / metabolism
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / physiology*
  • Saccharomyces cerevisiae Proteins*
  • Signal Transduction / physiology


  • Fungal Proteins
  • HSP70 Heat-Shock Proteins
  • KAR2 protein, yeast
  • Membrane Glycoproteins
  • Molecular Chaperones
  • Recombinant Proteins
  • Saccharomyces cerevisiae Proteins
  • IRE1 protein, S cerevisiae
  • Protein Serine-Threonine Kinases