HRD1-mediated ERAD tuning of ER-bound E2 is conserved between plants and mammals

Abstract

When membrane proteins and secretory proteins are misfolded or incompletely folded, they are retained in the endoplasmic reticulum (ER) for further folding or degradation. The HMG-COA reductase degradation 1 (HRD1) and degradation of alpha2 10 (DOA10) complexes are two major components involved in the ER-associated protein degradation (ERAD) system in eukaryotic organisms(1-4). However, the relationship between these two complexes is largely unknown, especially in higher eukaryotes. Here, we report that the plant ubiquitin-conjugating enzyme 32 (UBC32), an ER-bound E2 working in the DOA10 complex, is maintained at low levels under standard conditions by proteasome-dependent degradation mediated by the HRD1 complex, the other E3 complex involved in ERAD. Loss of this negative regulation under ER stress increases capacity for degradation of misfolded proteins retained in the ER. Consistently, UBE2J1, the homologue of UBC32 in mammals, was also identified to be targeted by HRD1 for degradation. Taken together, these results suggest that the regulation of UBC32 (or UBE2J1) by the HRD1 complex is conserved between plants and mammals.

EMR, a cytosolic‐abundant ring finger E3 ligase, mediates ER‐associated protein degradation in Arabidopsis

“…Usually, mutations of plant ERAD‐related genes restore the dwarf phenotype of bri1‐5 or bri1‐9 . Interestingly, UBC32 degrades the misfolded MLO like MLO12, whereas Hrd1a and Hrd1b stabilize it (Chen et al ., ). Similar to other ERAD‐related genes, EMR restored the dwarf phenotype of bri1‐5 , while it led to degradation of MLO12, like UBC32.…”

“…The existence of plant ERAD systems was confirmed by a recent study in Arabidopsis as a model plant (Su et al ., ; Doblas et al ., ). UBC32 is an E2 component involved in the ERAD system in Arabidopsis (Cui et al ., ; Chen et al ., ). Arabidopsis homologs of yeast/mammalian ERAD components, such as AtHrd3/Sel1L (Su et al ., ) and AtHrd1a (Liu et al ., ), are involved in plant stress responses.…”

Plant Virus Infection and the Ubiquitin Proteasome Machinery: Arms Race along the Endoplasmic Reticulum

“…In yeast, the ER bound, HRD1 functions with two E2 Ub-conjugating enzymes identified as Ubc6p and Ubc7p [18,19]. In mammals, there are four ER associated E2 Ub-conjugating enzymes known as UBE2g1, UBE2g2, UBE2j1 and UBE2j2 [20,21].…”

“…Ubc6p and its homologues are ER-integral enzymes, whereas Ubc7p homologs are anchored to the cytoplasmic face of the ER by the integral membrane protein Cue1p [22]. The mammalian UBE2g2 coordinates with gp78 but not HRD1 for ubiquitination of some substrates, suggesting that there are special conditions governing how the E2 and E3 enzymes partner with each other [18,20]. Another conserved component of the retrotranslocon is the p97/cdc48A ATPase that acts on the cytoplasmic face of the ER and helps to pull the ubiquitinated protein from the retrotranslocon toward the proteasome (Figure 1B) [12,23].…”

“…The yeast Doa10p Ub-ligase has a RING-finger domain and typically acts with Ubc6p and Ubc7p to degrade targets [18]. In yeast, Ssa1p and Ydj1p are cytoplasmic chaperones of the heat shock protein 70 (Hsp70) or Hsp40 family, which aid identification of malformed proteins for ubiquitination and degradation via Doa10p [24].…”