title: Ubiquitin initiates sorting of Golgi and plasma membrane proteins into the vacuolar degradation pathway.
abstract: BACKGROUND
  In yeast and mammals, many plasma membrane (PM) proteins destined for degradation are tagged with ubiquitin. These ubiquitinated proteins are internalized into clathrin-coated vesicles and are transported to early endosomal compartments. There, ubiquitinated proteins are sorted by the endosomal sorting complex required for transport (ESCRT) machinery into the intraluminal vesicles of multivesicular endosomes. Degradation of these proteins occurs after endosomes fuse with lysosomes/lytic vacuoles to release their content into the lumen. In plants, some PM proteins, which cycle between the PM and endosomal compartments, have been found to be ubiquitinated, but it is unclear whether ubiquitin is sufficient to mediate internalization and thus acts as a primary sorting signal for the endocytic pathway. To test whether plants use ubiquitin as a signal for the degradation of membrane proteins, we have translationally fused ubiquitin to different fluorescent reporters for the plasma membrane and analyzed their transport.
  
  
  RESULTS
  Ubiquitin-tagged PM reporters localized to endosomes and to the lumen of the lytic vacuole in tobacco mesophyll protoplasts and in tobacco epidermal cells. The internalization of these reporters was significantly reduced if clathrin-mediated endocytosis was inhibited by the coexpression of a mutant of the clathrin heavy chain, the clathrin hub. Surprisingly, a ubiquitin-tagged reporter for the Golgi was also transported into the lumen of the vacuole. Vacuolar delivery of the reporters was abolished upon inhibition of the ESCRT machinery, indicating that the vacuolar delivery of these reporters occurs via the endocytic transport route.
  
  
  CONCLUSIONS
  Ubiquitin acts as a sorting signal at different compartments in the endomembrane system to target membrane proteins into the vacuolar degradation pathway: If displayed at the PM, ubiquitin triggers internalization of PM reporters into the endocytic transport route, but it also mediates vacuolar delivery if displayed at the Golgi. In both cases, ubiquitin-tagged proteins travel via early endosomes and multivesicular bodies to the lytic vacuole. This suggests that vacuolar degradation of ubiquitinated proteins is not restricted to PM proteins but might also facilitate the turnover of membrane proteins in the early secretory pathway.
authors.affiliation: Department of Developmental Genetics, Center for Plant Molecular Biology, University of Tübingen, Tübingen 72076, Germany. peter.pimpl@zmbp.uni-tuebingen.de
authors.forename: David
authors.initials: D
authors.lastname: Scheuring
authors.forename: Fabian
authors.initials: F
authors.lastname: Künzl
authors.forename: Corrado
authors.initials: C
authors.lastname: Viotti
authors.forename: Melody San Wan
authors.initials: MS
authors.lastname: Yan
authors.forename: Liwen
authors.initials: L
authors.lastname: Jiang
authors.forename: Swen
authors.initials: S
authors.lastname: Schellmann
authors.forename: David G
authors.initials: DG
authors.lastname: Robinson
authors.forename: Peter
authors.initials: P
authors.lastname: Pimpl
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism*; D004705:Endocytosis; D056827:Endosomal Sorting Complexes Required for Transport; D006056:Golgi Apparatus / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D008954:Models, Biological; D057146:Multivesicular Bodies / Q000378:metabolism; D021381:Protein Transport; D059748:Proteolysis; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014026:Nicotiana / Q000166:cytology / Q000378:metabolism; D025801:Ubiquitin / Q000737:chemistry / Q000378:metabolism*; D014617:Vacuoles / Q000378:metabolism*
chemical_list: D056827:Endosomal Sorting Complexes Required for Transport; D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D025801:Ubiquitin; D049452:Green Fluorescent Proteins