Aurora A kinase is a conserved regulator of mitotic spindle assembly whose activity must be precisely balanced to ensure genome stability. Its activation at centrosomes and spindle microtubules is promoted by CEP192, TACC3 and TPX2, and counteracted by protein phosphatase 6 (PP6), which dephosphorylates the Aurora A T-loop. PP6 forms three holoenzymes defined by regulatory subunits PPP6R1–R3, yet how these confer substrate recognition and regulation is poorly understood. Here, we identify a phenylalanine-rich motif (A-box), present in PPP6R2 and PPP6R3 but absent from PPP6R1, that binds a surface on Aurora A overlapping those used by CEP192, TPX2 and TACC3. Mutation of A-box phenylalanine residues disrupts Aurora A binding and T-loop dephosphorylation, elevating spindle-associated Aurora A activity, and enlarging mitotic spindles. PPP6R2 additionally contains a unique PLK1-docking site downstream of its A-box, explaining how PLK1 competes with Aurora A for PPP6R2 and thereby suppresses PP6 activity towards Aurora A. Together, these findings show that PP6 regulatory subunits mimic Aurora A activators to direct dephosphorylation, and reveal how competing interactions regulate PP6 in mitosis. Aurora A activity must be tightly controlled during mitosis to ensure accurate spindle assembly and chromosome segregation. This work defines how PP6 selectively dephosphorylates Aurora A, revealing an unexpected mechanism in which phosphatase regulatory subunits mimic Aurora A activators and are themselves differentially regulated by PLK1. PP6 selectively dephosphorylates Aurora A via an unexpected mechanism, whereby phosphatase regulatory subunits mimic Aurora A activators and are themselves differentially regulated by PLK1.
Synaptic plasticity requires the coordinated regulation of membrane remodelling, energy production, and protein turnover. However, how these processes are integrated at the molecular level remains unclear. Here, we propose that activity-dependent lipid flux represents a central mechanism linking these processes. Saturated free fatty acids (sFFAs) released from membrane phospholipids, triglycerides, or from extracellular uptake can be activated to acyl-CoA and directed to mitochondrial β-oxidation and synaptic protein lipidation. These represent two emerging pathways adding to the complexity of roles played by sFFAs, including membrane fluidity and receptor activation. Recent work identifies DDHD2 as a mechanistic entry point into this broader synaptic lipid network. We position DDHD2-dependent lipid mobilisation in the wider context of glia-neuron lipid transfer and ApoE/sortilin-dependent fatty acid uptake. Key unresolved questions include the source and fate of fatty acids, the relative contribution of β-oxidation versus glycolysis to ATP production, and whether these pathways are altered in neurodegenerative disease. This Perspective discusses the coordinated regulation of membrane remodelling, energy production, and protein turnover in synaptic plasticity.
RNA-binding E3 ubiquitin ligases have the ability to link RNA regulation to the protein ubiquitination machinery, thereby influencing diverse biological processes. Here, we identify the RNA-binding E3 ubiquitin ligase DZIP3 as an innate immune sensor that couples viral RNA recognition to downstream immune signaling. Upon viral infection, binding of viral RNA to the DZIP3 RNA-binding motif induces conformational changes that promote K63-linked auto-ubiquitination, oligomerization, and stabilization of DZIP3. Activated DZIP3 associates with RIG-I and MAVS at the mitochondrial membrane, promoting K63-linked ubiquitination of RIG-I and robust activation of the type-I interferon response. Consequently, DZIP3 depletion impairs host antiviral defense, whereas DZIP3 restricts infection by a broad range of RNA viruses. Our findings establish DZIP3 as an innate immune co-sensor that integrates viral RNA detection with ubiquitin-mediated signaling, revealing a regulatory mechanism of RIG-I-MAVS-mediated antiviral immunity. RNA-binding E3 ubiquitin ligases are multifunctional regulators that can link RNA sensing to the protein ubiquitination machinery. This study identifies DZIP3 as an innate immune co-sensor that couples viral RNA detection to RIG-I-MAVS activation, revealing a novel regulatory layer in the host antiviral immune response. Binding to viral RNA activates DZIP3, which promotes RIG-I ubiquitination.
Nuclear speckles (NS) enhance the expression of NS-associated genes, possibly by elevating local levels of factors involved in multiple steps of gene expression. While dozens of large NS are distributed throughout interchromatin regions, the extent to which NS components dynamically redistribute between NS to adjust to local physiological demands remains unknown. Here, we used live-cell imaging of endogenous NS proteins to identify an interchromatin network of connections that functionally link NS throughout the nucleus. Over timescales ranging from tens of seconds to minutes, NS material is transferred through these curvilinear connections, rather than through isotropic exchange across the NS/nucleoplasm interface followed by diffusion in all directions throughout the nucleoplasm as predicted by current models. Multiphase NS-connecting structures form through the dynamic juxtapositions of multiple NS component phases. Each phase exhibits distinct yet recurrent dynamic patterns, but together, they integrate into a more stable, multiphase, NS-connecting structure in an ATP- and transcription-dependent manner. Our findings reveal the existence of a cellular mechanism that facilitates coordinated inter-NS protein trafficking through multiphase connections. Nuclear speckles (NS) are nuclear compartments enriched in factors involved in gene expression and can enhance the expression of NS-associated genes, but whether their components dynamically redistribute between speckles to meet local physiological demands remains unknown. This study shows that dynamic multiphase connections link NS and enable coordinated, targeted transport of macromolecules within the nucleus. Dynamic multiphase interchromatin connections enable coordinated, non-random transfer of nuclear speckle components through ATP- and transcription-dependent pathways, providing a mechanism for redistributing gene-regulatory factors within the nucleus.
Selective autophagy of the Golgi apparatus (Golgiphagy) is essential for Golgi quality control, yet its regulation and physiological functions remain poorly understood. Here, we identify the deubiquitinase USP28 as an upstream regulator of the Golgiphagy receptor TM9SF3. USP28 removes degradative K48-linked ubiquitin chains from TM9SF3, preventing its proteasomal degradation and sustaining Golgiphagy under nutrient and Golgi stresses. CRM1-dependent nuclear export enables the stress-induced translocation of USP28 to the Golgi, where it stabilizes TM9SF3, promoting Golgi fragmentation and autophagic clearance. Loss of USP28 or TM9SF3 impairs Golgiphagy, disrupts glycosylation fidelity, and inhibits breast cancer cell proliferation. Proteomic and functional analyses identify the glycosyltransferase MGAT5 as a selective TM9SF3-dependent Golgiphagy cargo, revealing how this pathway maintains glycosylation homeostasis. In syngeneic tumor models, USP28 knockout or pharmacological inhibition reduces TM9SF3 abundance, compromises Golgiphagy, perturbs glycosylation, and markedly suppresses tumor growth—effects restored by wild-type but not LC3-binding-deficient TM9SF3. Together, these findings reveal a USP28-TM9SF3-Golgiphagy axis that couples ubiquitin signaling to Golgi glycosylation control and represents a therapeutically targetable pathway for breast cancer. TM9SF3 is a Golgiphagy receptor that controls Golgi quality control, but its regulation and function remain poorly understood. This study identifies a deubiquitinase (USP28) that stabilizes TM9SF3 under cellular stress, maintaining glycosylation and promoting breast cancer progression. Under stress, TM9SF3 is stabilized by USP28, sustaining Golgiphagy and preserving glycosylation patterns that support breast tumor growth.
