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The striatin-interacting phosphatase and kinase (STRIPAK) complex is a large, multisubunit protein phosphatase 2A (PP2A) assembly that integrates diverse cellular signals in the Hippo pathway to regulate cell proliferation and survival. The architecture and assembly mechanism of this critical complex are poorly understood. Using cryo-EM, we determine the structure of the human STRIPAK core comprising PP2AA, PP2AC, STRN3, STRIP1, and MOB4 at 3.2-Å resolution. Unlike the canonical trimeric PP2A holoenzyme, STRIPAK contains four copies of STRN3 and one copy of each the PP2AA–C heterodimer, STRIP1, and MOB4. The STRN3 coiled-coil domains form an elongated homotetrameric scaffold that links the complex together. An inositol hexakisphosphate (IP 6 ) is identified as a structural cofactor of STRIP1. Mutations of key residues at subunit interfaces disrupt the integrity of STRIPAK, causing aberrant Hippo pathway activation. Thus, STRIPAK is established as a noncanonical PP2A complex with four copies of regulatory STRN3 for enhanced signal integration. A cryo-EM structure of the striatin-interacting phosphatase and kinase (STRIPAK) complex reveals the overall architecture of this large, multisubunit assembly that broadly regulates different signaling pathways.

MicroRNAs regulate the expression of many proteins and require specific maturation steps. Primary microRNA transcripts (pri-miRs) are cleaved by Microprocessor, a complex containing the RNase Drosha and its partner protein, DGCR8. Although DGCR8 is known to bind heme, the molecular role of heme in pri-miR processing is unknown. Here we show that heme is critical for Microprocessor to process pri-miRs with high fidelity. Furthermore, the degree of inherent heme dependence varies for different pri-miRs. Heme-dependent pri-miRs fail to properly recruit Drosha, but heme-bound DGCR8 can correct erroneous binding events. Rather than changing the oligomerization state, heme induces a conformational change in DGCR8. Finally, we demonstrate that heme activates DGCR8 to recognize pri-miRs by specifically binding the terminal loop near the 3′ single-stranded segment. Drosha and DGCR8 constitute the core Microprocessor complex, which processes primary microRNAs (pri-miRs) into mature microRNAs. Here the authors show that heme is essential for the proper processing of pri-miRs by Drosha-DGCR8, and the molecular mechanism by which heme enhances processing fidelity.

Defects in plasma membrane repair can lead to muscle and heart diseases in humans. Tripartite motif-containing protein (TRIM)72 (mitsugumin 53; MG53) has been determined to rapidly nucleate vesicles at the site of membrane damage, but the underlying molecular mechanisms remain poorly understood. Here we present the structure of Mus musculus TRIM72, a complete model of a TRIM E3 ubiquitin ligase. We demonstrated that the interaction between TRIM72 and phosphatidylserine-enriched membranes is necessary for its oligomeric assembly and ubiquitination activity. Using cryogenic electron tomography and subtomogram averaging, we elucidated a higher-order model of TRIM72 assembly on the phospholipid bilayer. Combining structural and biochemical techniques, we developed a working molecular model of TRIM72, providing insights into the regulation of RING-type E3 ligases through the cooperation of multiple domains in higher-order assemblies. Our findings establish a fundamental basis for the study of TRIM E3 ligases and have therapeutic implications for diseases associated with membrane repair. The authors present the full-length dimeric TRIM72 E3 ubiquitin ligase and the architecture of its high-order assembly bound to a phosphatidylserine-enriched membrane, providing insights into its role in membrane repair and ubiquitylation.

The spindle checkpoint maintains genomic stability and prevents aneuploidy. Unattached kinetochores convert the latent open conformer of the checkpoint protein Mad2 (O-Mad2) to the active closed conformer (C-Mad2), bound to Cdc20. C-Mad2–Cdc20 is incorporated into the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex/cyclosome (APC/C). The C-Mad2-binding protein p31 comet and the ATPase TRIP13 promote MCC disassembly and checkpoint silencing. Here, using nuclear magnetic resonance (NMR) spectroscopy, we show that TRIP13 and p31 comet catalyze the conversion of C-Mad2 to O-Mad2, without disrupting its stably folded core. We determine the crystal structure of human TRIP13, and identify functional TRIP13 residues that mediate p31 comet –Mad2 binding and couple ATP hydrolysis to local unfolding of Mad2. TRIP13 and p31 comet prevent APC/C inhibition by MCC components, but cannot reactivate APC/C already bound to MCC. Therefore, TRIP13–p31 comet intercepts and disassembles free MCC not bound to APC/C through mediating the local unfolding of the Mad2 C-terminal region. The spindle checkpoint ensures the fidelity of chromosome segregation during mitosis and meiosis. Here the authors use a combination of biochemical and structural biology approaches to show how the TRIP13 ATPase and its adaptor, p31 comet , catalyze the conversion of the checkpoint protein Mad2 between latent and active forms

In the N-end rule pathway, the N-terminal residue of a protein is recognized by specific E3 ligases that promote its ubiquitination and proteasomal degradation. Now the structural basis for the recognition of N-terminal basic residues by the UBR box from yeast Ubr1 is solved. Together with functional analysis, the work reveals that the residue at position 2 of the substrate may influence the binding. The N-end rule pathway is a regulated proteolytic system that targets proteins containing destabilizing N-terminal residues (N-degrons) for ubiquitination and proteasomal degradation in eukaryotes. The N-degrons of type 1 substrates contain an N-terminal basic residue that is recognized by the UBR box domain of the E3 ubiquitin ligase UBR1. We describe structures of the UBR box of Saccharomyces cerevisiae UBR1 alone and in complex with N-degron peptides, including that of the cohesin subunit Scc1, which is cleaved and targeted for degradation at the metaphase-anaphase transition. The structures reveal a previously unknown protein fold that is stabilized by a novel binuclear zinc center. N-terminal arginine, lysine or histidine side chains of the N-degron are coordinated in a multispecific binding pocket. Unexpectedly, the structures together with our in vitro biochemical and in vivo pulse-chase analyses reveal a previously unknown modulation of binding specificity by the residue at position 2 of the N-degron.

The nitrilase superfamily, including 13 branches, plays various biological functions in signaling molecule synthesis, vitamin metabolism, small-molecule detoxification, and posttranslational modifications. Most of the mammals and yeasts have Nit1 and Nit2 proteins, which belong to the nitrilase-like (Nit) branch of the nitrilase superfamily. Recent studies have suggested that Nit1 is a metabolite repair enzyme, whereas Nit2 shows ω-amidase activity. In addition, Nit1 and Nit2 are suggested as putative tumor suppressors through different ways in mammals. Yeast Nit2 (yNit2) is a homolog of mouse Nit1 based on similarity in sequence. To understand its specific structural features, we determined the crystal structure of Nit2 from Kluyveromyces lactis (KlNit2) at 2.2 Å resolution and compared it with the structure of yeast-, worm-, and mouse-derived Nit2 proteins. Based on our structural analysis, we identified five distinguishable structural features from 28 structural homologs. This study might potentially provide insights into the structural relationships of a broad spectrum of nitrilases.

Plant thioredoxins (Trxs) participate in two redox systems found in different cellular compartments: the NADP-Trx system (NTS) in the cytosol and mitochondria and the ferredoxin-Trx system (FTS) in the chloroplast, where they function as redox regulators by regulating the activity of various target enzymes. The identities of the master regulators that maintain cellular homeostasis and modulate timed development through redox regulating systems have remained completely unknown. Here, we show that proteins consisting of a single cystathionine β-synthase (CBS) domain pair stabilize cellular redox homeostasis and modulate plant development via regulation of Trx systems by sensing changes in adenosine-containing ligands. We identified two CBS domain–containing proteins in Arabidopsis thaliana, CBSX1 and CBSX2, which are localized to the chloroplast, where they activate all four Trxs in the FTS. CBSX3 was found to regulate mitochondrial Trx members in the NTS. CBSX1 directly regulates Trxs and thereby controls H2O2 levels and regulates lignin polymerization in the anther endothecium. It also affects plant growth by regulating Calvin cycle enzymes, such as malate dehydrogenase, via homeostatic regulation of Trxs. Based on our findings, we suggest that the CBSX proteins (or a CBS pair) are ubiquitous redox regulators that regulate Trxs in the FTS and NTS to modulate development and maintain homeostasis under conditions that are threatening to the cell.

Mitsugumin 53 (MG53) negatively regulates skeletal myogenesis by targeting insulin receptor substrate 1 (IRS-1). Here, we show that MG53 is an ubiquitin E3 ligase that induces IRS-1 ubiquitination with the help of an E2-conjugating enzyme, UBE2H. Molecular manipulations that disrupt the E3-ligase function of MG53 abolish IRS-1 ubiquitination and enhance skeletal myogenesis. Skeletal muscles derived from the MG53 −/− mice show an elevated IRS-1 level with enhanced insulin signalling, which protects the MG53 −/− mice from developing insulin resistance when challenged with a high-fat/high-sucrose diet. Muscle samples derived from human diabetic patients and mice with insulin resistance show normal expression of MG53, indicating that altered MG53 expression does not serve as a causative factor for the development of metabolic disorders. Thus, therapeutic interventions that target the interaction between MG53 and IRS-1 may be a novel approach for the treatment of metabolic diseases that are associated with insulin resistance. The protein MG53 is known to inhibit myogenesis. Here, Ko et al . show that MG53 is an E3 ubiquitin ligase that mediates degradation of insulin receptor substrate 1 in skeletal muscle, thereby regulating myogenesis and insulin sensitivity in vitro and in vivo .

The original version of this Article contained an error in Fig. 1. In panel d, the model on the right of the panel was incorrectly labeled ‘+Heme’, and should have read ‘− Heme’. This has now been corrected in both the PDF and HTML versions of the Article.