Quality control of glycogen through direct ubiquitylation by RNF213

Quality control of biomolecules is vital for organismal health. Although DNA repair and protein quality control are well understood, how cells monitor other important biomolecules such as glycogen remains ill defined. Accumulation of aberrant, poorly branched glycogen into insoluble polyglucosan bodies causes severe disease^1,2. Here we identify autophagy of ubiquitylated aberrant glycogen as a quality control mechanism safeguarding the brain from polyglucosan buildup. This mechanism depends on the E3 ubiquitin ligase RNF213. Mice lacking ligase activity in RNF213 accumulate polyglucosan in the cerebellum, pons and hippocampus. Using cells engineered to produce polyglucosan, we show that RNF213 selectively ubiquitylates abnormal glycogen. Cryo-electron microscopy analysis of RNF213 bound to glycogen-derived maltoheptaose showed that its CBM20 domain binds linear oligosaccharides. Disruption of carbohydrate binding results in gain of E3 ligase activity towards physiological glycogen, indicating that the CBM20 domain limits RNF213 activity towards physiological glycogen. Epistasis analysis places RNF213 upstream of LUBAC, suggesting a hierarchical network of multiple E3 ligases surveying glycogen quality. Ubiquitylated polyglucosan recruits autophagy receptors SQSTM1, TAX1BP1 and optineurin, thereby triggering uptake into autophagosomes. These findings identify RNF213 as a quality control factor preventing polyglucosan accumulation in astrocytes through direct ubiquitylation of polyglucosan, demonstrating an essential role for non-protein ubiquitylation in glycogen quality control.