The novel RNA-binding E3 ubiquitin ligase MEX-3C targets the mRNA of MHC-I gene HLA-A2 for degradation in a RING-dependent manner. Depletion of MEX-3C in NK cells leads to reduced killing of target cells.

Misfolded MHC class I heavy chains (MHC I HCs) are targeted for endoplasmic reticulum (ER)-associated degradation (ERAD) by the ubiquitin E3 ligase HRD1, and E2 ubiquitin conjugating enzyme UBE2J1, and represent one of the few known endogenous ERAD substrates. The mechanism by which misfolded proteins are dislocated across the ER membrane into the cytosol is unclear. Here, we investigate the requirements for MHC I ubiquitination and degradation and show that endogenous misfolded MHC I HCs are recognized in the ER lumen by EDEM1 in a glycan-dependent manner and targeted to the core SEL1L/HRD1/UBE2J1 complex. A soluble MHC I HC lacking its transmembrane domain and cytosolic tail uses the same ERAD components and is degraded as efficiently as wildtype MHC I. Unexpectedly, HRD1-dependent polyubiquitination is preferentially targeted to the ER luminal domain of full-length MHC I HCs, despite the presence of an exposed cytosolic C-terminal tail. MHC I luminal domain ubiquitination occurs before p97 ATPase-mediated extraction from the ER membrane and can be targeted to nonlysine, as well as lysine, residues. A subset of integral membrane proteins, therefore, requires an early dislocation event to expose part of their luminal domain to the cytosol, before HRD1-mediated polyubiquitination and dislocation.T he assembly and regulated expression of plasma membrane and secreted proteins is fundamentally reliant on effective endoplasmic reticulum quality control (ERQC). Endoplasmic reticulum (ER)-associated degradation (ERAD) is central to ERQC, selectively disposing of misfolded or surplus proteins to maintain ER homeostasis. ERAD involves recognition, ubiquitination, and retrotranslocation of substrates from the ER to the cytosol for proteasome-mediated degradation (1). Polyubiquitination of the substrate is critical, both for facilitating ER membrane extraction by the p97 ATPase-ubiquitin fusion degradation 1 (Ufd1)-nuclear protein localization 4 (Npl4) complex (2-4) and for delivery to the proteasome. However, the precise role of ubiquitin in the actual dislocation event is unclear.The cellular ERAD machinery consists of multiprotein complexes, typically comprising a membrane-embedded ubiquitin E3 ligase, which engages substrates directly or via ER luminal adaptors (1). Different substrates degraded by the same E3 may use distinct ERAD cofactors that facilitate substrate delivery to the ligase and E2-conjugating enzyme for ubiquitination.Attempts to delineate distinct degradation pathways according to the site of the defect in a misfolded protein have been made in yeast. Proteins with cytosolic defects (ERAD-C) require Doa10p, whereas proteins with transmembrane (ERAD-M) or luminal (ERAD-L) lesions use Hrd1p (5, 6). The expanded repertoire of ERAD E3s in mammalian cells makes developing analogous rules more challenging. 3-hydroxy-3-methylglutaryl-CoA reductase degradation protein 1 (HRD1) and its homolog gp78/autocrine motility factor receptor (AMFR) are the best-characterized mammalian ERAD ligases. The association of HR...

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Helen Bye and Bryony Gilder set up an in‐house CPD club that has blossomed into into a nationwide learning opportunity used by hundreds of vet nurses.

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