An HRD / DER - independent ER quality control mechanism involves Rsp 5 p - dependent ubiquitination and ER - Golgi transport Cole

e 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 hrd1 strain is restored to a wild-type rate when CPY* is overexpressed. W Substrates of HIP require vesicular trafficking between the ER and Golgi apparatus before degradation by the ubiquitinproteasome 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.