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Dissociation of Kar2p/BiP from an ER Sensory Molecule, Ire1p, Triggers the Unfolded Protein Response in Yeast,☆☆

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Abstract

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.

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    This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (#11153216 to K. K.) from the Ministry of Education, Science, Sports, and Culture of Japan, and supported in part by Sapporo Bioscience Foundation.

    ☆☆

    Abbreviations used: ER, endoplasmic reticulum; UPR, unfolded protein response; BiP, immunoglobulin heavy chain binding protein; HA, hemagglutinin; DTT, dithiothreitol; IP, immunoprecipitation; HSF, heat shock factor.

    2

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