An <i>HRD</i>/<i>DER</i>-independent ER quality control mechanism involves Rsp5p-dependent ubiquitination and ER-Golgi transport

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  • Cole M. Haynes
    University of Missouri-Kansas City, Division of Cell Biology and Biophysics, School of Biological Sciences, Kansas City, MO 64110
  • Sabrina Caldwell
    University of Missouri-Kansas City, Division of Cell Biology and Biophysics, School of Biological Sciences, Kansas City, MO 64110
  • Antony A. Cooper
    University of Missouri-Kansas City, Division of Cell Biology and Biophysics, School of Biological Sciences, Kansas City, MO 64110

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<jats:p>We have identified a new pathway of ER-associated degradation in Saccharomyces cerevisiae that functions separately from the HRD/DER pathway comprised of Hrd1p, Hrd3p, Der1p, and Ubc7p. This pathway, termed Hrd1p independent-proteolysis (HIP), is capable of recognizing and degrading both lumenal (CPY* and PrA*), and integral membrane proteins (Sec61–2p) that misfold in the ER. CPY* overexpression likely saturates the HRD/DER pathway and activates the HIP pathway, so the slowed degradation kinetics of CPY* in a hrd1Δ strain is restored to a wild-type rate when CPY* is overexpressed. Substrates of HIP require vesicular trafficking between the ER and Golgi apparatus before degradation by the ubiquitin-proteasome system. Ubiquitination of HIP substrates does not involve the HRD/DER pathway ubiquitin ligase Hrd1p, but instead uses another ubiquitin ligase, Rsp5p. HIP is regulated by the unfolded protein response as Ire1p is necessary for the degradation of CPY* when overexpressed, but not when CPY* is expressed at normal levels. Both the HIP and HRD/DER pathways contribute to the degradation of CPY*, and only by eliminating both is CPY* degradation completely blocked.</jats:p>

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