New roles for the de-ubiquitylating enzyme OTUD4 in an RNA-protein network and RNA granules.

Mechanisms that regulate the formation of membrane-less cellular organelles, such as neuronal RNA granules and stress granules, have gained increasing attention over the past years. These granules consist of RNA and a plethora of RNA-binding proteins. Mutations in RNA-binding proteins have been found in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). By performing pulldown experiments and subsequent mass spectrometry on mouse brain lysates, we discovered that the de-ubiquitylating enzyme OTU domain-containing protein 4 (OTUD4) unexpectedly is part of a complex network of multiple RNA-binding proteins, including core stress granule factors, such as FMRP (also known as FMR1), SMN1, G3BP1 and TIA1. We show that OTUD4 binds RNA, and that several of its interactions with RNA-binding proteins are RNA dependent. OTUD4 is part of neuronal RNA transport granules in rat hippocampal neurons under physiological conditions, whereas upon cellular stress, OTUD4 is recruited to cytoplasmic stress granules. Knockdown of OTUD4 in HeLa cells resulted in defects in stress granule formation and led to apoptotic cell death. Together, we characterize OTUD4 as a new RNA-binding protein with a suggested function in regulation of translation.

TRIAD3/RNF216 E3 ligase specifically synthesises K63-linked ubiquitin chains and is inactivated by mutations associated with Gordon Holmes syndrome.

TRIAD3/RNF216 is a ubiquitin ligase of the RING-in-between-RING family. Recent publications identified TRIAD3 mutations in patients with neurological diseases, including Gordon Holmes syndrome and Huntington-like disorder. To understand the functional relevance of these disease-associated mutations, we have tested the ubiquitin ligase activity of mutated TRIAD3 in vitro. Several of these point mutations completely abrogated TRIAD3's catalytic activity. Using mass spectrometry, we identified new TRIAD3-interacting proteins/substrates from mouse brain lysate, which provide a new link between TRIAD3 and processes involving clathrin-mediated endocytosis. Strikingly, we found that TRIAD3 synthesises specifically lysine-63 (K63)-linked poly-ubiquitin chains in vitro, a chain type that usually plays a role in mediating signalling events rather than triggering proteasomal degradation. Therefore, this finding is of great importance to further understand TRIAD3's cellular role and loss-of-function in disease.