Direct visualization of degradation microcompartments at the ER membrane

Proc Natl Acad Sci U S A. 2020 Jan 14;117(2):1069-1080. doi: 10.1073/pnas.1905641117. Epub 2019 Dec 27.


To promote the biochemical reactions of life, cells can compartmentalize molecular interaction partners together within separated non-membrane-bound regions. It is unknown whether this strategy is used to facilitate protein degradation at specific locations within the cell. Leveraging in situ cryo-electron tomography to image the native molecular landscape of the unicellular alga Chlamydomonas reinhardtii, we discovered that the cytosolic protein degradation machinery is concentrated within ∼200-nm foci that contact specialized patches of endoplasmic reticulum (ER) membrane away from the ER-Golgi interface. These non-membrane-bound microcompartments exclude ribosomes and consist of a core of densely clustered 26S proteasomes surrounded by a loose cloud of Cdc48. Active proteasomes in the microcompartments directly engage with putative substrate at the ER membrane, a function canonically assigned to Cdc48. Live-cell fluorescence microscopy revealed that the proteasome clusters are dynamic, with frequent assembly and fusion events. We propose that the microcompartments perform ER-associated degradation, colocalizing the degradation machinery at specific ER hot spots to enable efficient protein quality control.

Keywords: ERAD; cdc48; cryo-electron tomography; phase separation; proteasome.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Chlamydomonas reinhardtii / metabolism
  • Chlamydomonas reinhardtii / ultrastructure
  • Cryoelectron Microscopy
  • Cytosol / metabolism
  • Endopeptidases
  • Endoplasmic Reticulum / metabolism*
  • Endoplasmic Reticulum / ultrastructure*
  • Endoplasmic Reticulum-Associated Degradation / physiology*
  • Optical Imaging
  • Proteasome Endopeptidase Complex / metabolism
  • Proteolysis*
  • Ribosomes / metabolism
  • Ribosomes / ultrastructure
  • Valosin Containing Protein / metabolism


  • Endopeptidases
  • Proteasome Endopeptidase Complex
  • Valosin Containing Protein