Jacobs, S. E. | bioRxiv Science

bioRxiv2024

Engineering a Cell-Based Orthogonal Ubiquitin Transfer Cascade for Profiling the Substrates of RBR E3 Parkin

The E3 ubiquitin (UB) ligase Parkin utilizes a Ring-Between-Ring (RBR) domain to mediate the transfer of UB to its substrates to regulate diverse cellular functions, including mitochondrial quality control, cell cycle progression, metabolism programming, and the establishment of synaptic functions. Mutations affecting the E3 ligase activity of Parkin are associated with cancer and Parkinsons disease (PD). An essential role of Parkin is to synthesize UB chains on the surface of damaged mitochondria to initiate mitophagy. Still, it is not clear how Parkin carries out other biological functions through the ubiquitination of its downstream targets in the cell. We hypothesized that a comprehensive substrate profile of Parkin would facilitate the discovery of ubiquitination pathways underpinning its multifaceted roles in cell regulation and reveal mechanistic linkages between Parkin malfunction and disease development. Here, we used phage display to assemble an orthogonal ubiquitin transfer (OUT) cascade of Parkin that can exclusively deliver an engineered UB mutant (xUB) to Parkin and its substrates in living cells. We then generated a substrate profile of Parkin by purifying xUB-conjugated proteins from cells and identifying them by proteomics. The OUT screen identified Parkin substrates involved in DNA replication, protein translation, intracellular protein transport, and rhythmic regulation. Based on previous literature implicating alterations in membrane vesicle trafficking in PD, we verified Parkin-catalyzed ubiquitination of Rab GTPases (Rab1a, Rab5a, Rab5c, Rab7a, Rab8a, Rab10, an Rab13) as well as CDK5, with reconstituted ubiquitination reactions in vitro and in cells. We also found chemical-induced stimulation of mitophagy enhanced Parkin-mediated ubiquitination of Rab proteins. These findings demonstrate that the OUT cascade of Parkin can serve as an empowering tool for identifying Parkin substrates to elucidate its cellular functions.

biochemistry↗

bioRxiv2023

The E3 ubiquitin ligase RNF216 contains a linear ubiquitin chain-determining-like domain that functions to regulate dendritic arborization and dendritic spine type in hippocampal neurons

Of the hundreds of E3 ligases found in the human genome, the RING-between RING (RBR) E3 ligase in the LUBAC (linear ubiquitin chain assembly complex) complex HOIP (HOIL-1-interacting protein or RNF31), contains a unique domain called LDD (linear ubiquitin chain determining domain). HOIP is the only E3 ligase known to form linear ubiquitin chains, which regulate inflammatory responses and cell death via activation of the NF-{kappa}B pathway. We identified an amino acid sequence within the RNF216 E3 ligase that shares homology to the LDD domain found in HOIP (R2-C). Here, we show that the R2-C domain of RNF216 promotes self-assembly of all ubiquitin chains, with a dominance for those assembled via K63-linkages. Deletion of the R2-C domain altered RNF216 localization, reduced dendritic complexity and changed the distribution of apical dendritic spine morphology types in primary hippocampal neurons. These changes were independent of the RNF216 RBR catalytic activity as expression of a catalytic inactive version of RNF216 had no effect. These data show that the R2-C domain of RNF216 diverges in ubiquitin assembly function from the LDD of HOIP and and functions independently of RNF216 catalytic activity to regulate dendrite development in neurons.

neuroscience↗