title: Large protein complexes retained in the ER are dislocated by non-COPII vesicles and degraded by selective autophagy.
abstract: Multisubunit protein complexes are assembled in the endoplasmic reticulum (ER). Existing pools of single subunits and assembly intermediates ensure the efficient and rapid formation of complete complexes. While being kinetically beneficial, surplus components must be eliminated to prevent potentially harmful accumulation in the ER. Surplus single chains are cleared by the ubiquitin-proteasome system. However, the fate of not secreted assembly intermediates of multisubunit proteins remains elusive. Here we show by high-resolution double-label confocal immunofluorescence and immunogold electron microscopy that naturally occurring surplus fibrinogen Aα-γ assembly intermediates in HepG2 cells are dislocated together with EDEM1 from the ER to the cytoplasm in ER-derived vesicles not corresponding to COPII-coated vesicles originating from the transitional ER. This route corresponds to the novel ER exit path we have previously identified for EDEM1 (Zuber et al. Proc Natl Acad Sci USA 104:4407-4412, 2007). In the cytoplasm, detergent-insoluble aggregates of fibrinogen Aα-γ dimers develop that are targeted by the selective autophagy cargo receptors p62/SQSTM1 and NBR1. These aggregates are degraded by selective autophagy as directly demonstrated by high-resolution microscopy as well as biochemical analysis and inhibition of autophagy by siRNA and kinase inhibitors. Our findings demonstrate that different pathways exist in parallel for ER-to-cytoplasm dislocation and subsequent proteolytic degradation of large luminal protein complexes and of surplus luminal single-chain proteins. This implies that ER-associated protein degradation (ERAD) has a broader function in ER proteostasis and is not limited to the elimination of misfolded glycoproteins.
authors: Division of Cell and Molecular Pathology, Department of Pathology, University of Zurich, 8091 Zurich, Switzerland.
authors: Valerie
authors: V
authors: Le Fourn
authors: Sujin
authors: S
authors: Park
authors: Insook
authors: I
authors: Jang
authors: Katarina
authors: K
authors: Gaplovska-Kysela
authors: Bruno
authors: B
authors: Guhl
authors: Yangsin
authors: Y
authors: Lee
authors: Jin Won
authors: JW
authors: Cho
authors: Christian
authors: C
authors: Zuber
authors: Jürgen
authors: J
authors: Roth
mesh_terms: D001343:Autophagy; D022162:Cytoplasmic Vesicles / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D060746:Endoplasmic Reticulum-Associated Degradation / Q000502:physiology*; D005340:Fibrinogen / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism / Q000648:ultrastructure; D056945:Hep G2 Cells; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology; D017510:Protein Folding; D021381:Protein Transport
chemical_list: C486654:EDEM1 protein, human; D006023:Glycoproteins; D008565:Membrane Proteins; C464794:fibrinogen Aalpha; D005340:Fibrinogen