title: Protein targeting and degradation are coupled for elimination of mislocalized proteins.
abstract: A substantial proportion of the genome encodes membrane proteins that are delivered to the endoplasmic reticulum by dedicated targeting pathways. Membrane proteins that fail targeting must be rapidly degraded to avoid aggregation and disruption of cytosolic protein homeostasis. The mechanisms of mislocalized protein (MLP) degradation are unknown. Here we reconstitute MLP degradation in vitro to identify factors involved in this pathway. We find that nascent membrane proteins tethered to ribosomes are not substrates for ubiquitination unless they are released into the cytosol. Their inappropriate release results in capture by the Bag6 complex, a recently identified ribosome-associating chaperone. Bag6-complex-mediated capture depends on the presence of unprocessed or non-inserted hydrophobic domains that distinguish MLPs from potential cytosolic proteins. A subset of these Bag6 complex 'clients' are transferred to TRC40 for insertion into the membrane, whereas the remainder are rapidly ubiquitinated. Depletion of the Bag6 complex selectively impairs the efficient ubiquitination of MLPs. Thus, by its presence on ribosomes that are synthesizing nascent membrane proteins, the Bag6 complex links targeting and ubiquitination pathways. We propose that such coupling allows the fast tracking of MLPs for degradation without futile engagement of the cytosolic folding machinery.
authors: Cell Biology and Metabolism Program, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors: Tara
authors: T
authors: Hessa
authors: Ajay
authors: A
authors: Sharma
authors: Malaiyalam
authors: M
authors: Mariappan
authors: Heather D
authors: HD
authors: Eshleman
authors: Erik
authors: E
authors: Gutierrez
authors: Ramanujan S
authors: RS
authors: Hegde
mesh_terms: D000818:Animals; D053501:Arsenite Transporting ATPases / Q000378:metabolism; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D018832:Molecular Chaperones / Q000378:metabolism; D046912:Multiprotein Complexes / Q000378:metabolism; D009478:Neuropeptide Y / Q000737:chemistry; D011328:Prions / Q000737:chemistry / Q000378:metabolism*; D011388:Prolactin / Q000737:chemistry; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D017510:Protein Folding; D011498:Protein Precursors / Q000737:chemistry; D021382:Protein Sorting Signals / Q000502:physiology; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D012270:Ribosomes / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism; D054875:Ubiquitination
chemical_list: C092009:BAG6 protein, human; D018832:Molecular Chaperones; D046912:Multiprotein Complexes; D009478:Neuropeptide Y; D011328:Prions; D011498:Protein Precursors; D021382:Protein Sorting Signals; D025801:Ubiquitin; C017671:preprolactin; D011388:Prolactin; D046988:Proteasome Endopeptidase Complex; D053501:Arsenite Transporting ATPases