Nuclear export of ubiquitinated proteins via the UBIN-POST system.

Although mechanisms for protein homeostasis in the cytosol have been studied extensively, those in the nucleus remain largely unknown. Here, we identified that a protein complex mediates export of polyubiquitinated proteins from the nucleus to the cytosol. UBIN, a ubiquitin-associated (UBA) domain-containing protein, shuttled between the nucleus and the cytosol in a CRM1-dependent manner, despite the lack of intrinsic nuclear export signal (NES). Instead, the UBIN binding protein polyubiquitinated substrate transporter (POST) harboring an NES shuttled UBIN through nuclear pores. UBIN bound to polyubiquitin chain through its UBA domain, and the UBIN-POST complex exported them from the nucleus to the cytosol. Ubiquitinated proteins accumulated in the cytosol in response to proteasome inhibition, whereas cotreatment with CRM1 inhibitor led to their accumulation in the nucleus. Our results suggest that ubiquitinated proteins are exported from the nucleus to the cytosol in the UBIN-POST complex-dependent manner for the maintenance of nuclear protein homeostasis.

Proteotoxic stress and ageing triggers the loss of redox homeostasis across cellular compartments.

The cellular proteostasis network integrates the protein folding and clearance machineries in multiple sub-cellular compartments of the eukaryotic cell. The endoplasmic reticulum (ER) is the site of synthesis and folding of membrane and secretory proteins. A distinctive feature of the ER is its tightly controlled redox homeostasis necessary for the formation of inter- and intra-molecular disulphide bonds. Employing genetically encoded in vivo sensors reporting on the redox state in an organelle-specific manner, we show in the nematode Caenorhabditis elegans that the redox state of the ER is subject to profound changes during worm lifetime. In young animals, the ER is oxidizing and this shifts towards reducing conditions during ageing, whereas in the cytosol the redox state becomes more oxidizing with age. Likewise, the redox state in the cytosol and the ER change in an opposing manner in response to proteotoxic challenges in C. elegans and in HeLa cells revealing conservation of redox homeostasis. Moreover, we show that organelle redox homeostasis is regulated across tissues within C. elegans providing a new measure for organismal fitness.

ERdj8 governs the size of autophagosomes during the formation process.

In macroautophagy, membrane structures called autophagosomes engulf substrates and deliver them for lysosomal degradation. Autophagosomes enwrap a variety of targets with diverse sizes, from portions of cytosol to larger organelles. However, the mechanism by which autophagosome size is controlled remains elusive. We characterized a novel ER membrane protein, ERdj8, in mammalian cells. ERdj8 localizes to a meshwork-like ER subdomain along with phosphatidylinositol synthase (PIS) and autophagy-related (Atg) proteins. ERdj8 overexpression extended the size of the autophagosome through its DnaJ and TRX domains. ERdj8 ablation resulted in a defect in engulfing larger targets. C. elegans, in which the ERdj8 orthologue dnj-8 was knocked down, could perform autophagy on smaller mitochondria derived from the paternal lineage but not the somatic mitochondria. Thus, ERdj8 may play a critical role in autophagosome formation by providing the capacity to target substrates of diverse sizes for degradation.

The AAA+ ATPase/ubiquitin ligase mysterin stabilizes cytoplasmic lipid droplets.

Mysterin, also known as RNF213, is an intracellular protein that forms large toroidal oligomers. Mysterin was originally identified in genetic studies of moyamoya disease (MMD), a rare cerebrovascular disorder of unknown etiology. While mysterin is known to exert ubiquitin ligase and putative mechanical ATPase activities with a RING finger domain and two adjacent AAA+ modules, its biological role is poorly understood. Here, we report that mysterin is targeted to lipid droplets (LDs), ubiquitous organelles specialized for neutral lipid storage, and markedly increases their abundance in cells. This effect was exerted primarily through specific elimination of adipose triglyceride lipase (ATGL) from LDs. The ubiquitin ligase and ATPase activities of mysterin were both important for its proper LD targeting. Notably, MMD-related mutations in the ubiquitin ligase domain of mysterin significantly impaired its fat-stabilizing activity. Our findings identify a unique new regulator of cytoplasmic LDs and suggest a potential link between the pathogenesis of MMD and fat metabolism.

Alternative exon skipping biases substrate preference of the deubiquitylase USP15 for mysterin/RNF213, the moyamoya disease susceptibility factor.

The deubiquitylating enzyme USP15 plays significant roles in multiple cellular pathways including TGF-ß signaling, RNA splicing, and innate immunity. Evolutionarily conserved skipping of exon 7 occurs during transcription of the mRNAs encoding USP15 and its paralogue USP4, yielding two major isoforms for each gene. Exon 7 of USP15 encodes a serine-rich stretch of 29 amino acid residues located in the inter-region linker that connects the N-terminal putative regulatory region and the C-terminal enzymatic region. Previous findings suggested that the variation in the linker region leads to functional differences between the isoforms of the two deubiquitylating enzymes, but to date no direct evidence regarding such functional divergence has been published. We found that the long isoform of USP15 predominantly recognizes and deubiquitylates mysterin, a large ubiquitin ligase associated with the onset of moyamoya disease. This observation represents the first experimental evidence that the conserved exon skipping alters the substrate specificity of this class of deubiquitylating enzymes. In addition, we found that the interactomes of the short and long isoforms of USP15 only partially overlapped. Thus, USP15, a key gene in multiple cellular processes, generates two functionally different isoforms via evolutionarily conserved exon skipping.