Journal of Cell Biology最新文献

Micronuclei (MN), a hallmark of chromosome instability, frequently rupture, leading to protumorigenic consequences. MN rupture requires nuclear lamina defects, yet their underlying causes remain unclear. Here, we demonstrate that MN lamina gaps are linked to excessive MN growth resulting from impaired protein export. This export defect arises from reduced levels of the transport protein RCC1 in MN. Overexpressing RCC1 increases protein export and protects MN from rupture. Differences in RCC1 levels linked to chromatin state also explain why high euchromatin content increases the stability of small MN. Additional RCC1 loss in euchromatic MN results in impaired protein import. For these MN, increasing RCC1, directly or through increasing histone methylation, accelerates rupture. Our findings define a new model of MN rupture, where defects in protein export drives continuous MN growth causing nuclear lamina gaps that predispose MN to membrane rupture and where chromatin-specific features can alter rupture of small MN by further impairing nuclear transport.

Hundreds of mitochondrial proteins rely on N-terminal presequences for organellar targeting and import. While generally described as positively charged amphiphilic helices, presequences lack a consensus motif and thus likely promote protein import into mitochondria with variable efficiencies. Indeed, the concept of presequence strength underlies biological models such as stress sensing, yet a quantitative analysis of what dictates strong versus weak presequences is lacking. Furthermore, the extent to which presequence strength affects mitochondrial function and cellular fitness remains unclear. Here, we capitalize on the MitoLuc protein import assay to define multiple aspects of presequence strength. We find that select presequences, including those that regulate the mitochondrial unfolded protein response (UPRmt), impart differential import efficiencies during mitochondrial uncoupling. Surprisingly, we find that presequences beyond those associated with stress signaling promote highly variable import efficiency in vitro, suggesting presequence strength may influence a broader array of processes than currently appreciated. We exploit this variability to demonstrate that only presequences that promote robust in vitro import can fully rescue defects in respiratory growth in complex IV-deficient yeast, suggesting that presequence strength dictates metabolic potential. Collectively, our findings demonstrate that presequence strength can describe numerous metrics, such as total imported protein, maximal import velocity, or sensitivity to uncoupling, suggesting that the annotation of presequences as weak or strong requires more nuanced characterization than typically performed. Importantly, we find that such variability in presequence strength meaningfully affects cellular fitness beyond stress signaling, suggesting that organisms may broadly exploit presequence strength to fine-tune mitochondrial import and thus organellar homeostasis.

BAR domain-containing proteins are key regulators of endocytosis and actin remodeling. Their function in morphogenesis remains to be investigated. We report that the I-BAR domain-containing protein, missing-in-metastasis (MIM) (also called MTSS1), promotes branched actin network formation and endocytosis to drive rapid, cyclical plasma membrane remodeling during syncytial divisions in Drosophila embryos. Actin-rich villous protrusions in the apical caps in interphase are depleted in metaphase, concurrent with furrow extension between adjacent nuclei. MIM depletion results in a loss of furrow extension and in longer, more abundant apical protrusions containing the formin diaphanous. Branched actin networks promoted by MIM are in balance with bundled actin networks induced by RhoGEF2 and diaphanous. Cyclical recruitment of MIM to the cortex promotes localization of active Rac, the WAVE regulatory complex, and the Arp2/3 complex to drive endocytic membrane remodeling. These findings identify MIM as an integrator of actin and endocytic dynamics that enables rapid membrane remodeling during Drosophila syncytial division cycles.

Collective chemotaxis is often viewed through a mesenchymal lens, emphasizing peripheral polarity and force generation. In this issue, Diaz and Mayor (https://doi.org/10.1083/jcb.202507211) reveal that epithelial-like neural crest clusters achieve directed chemotaxis through a junction-centered strategy that redistributes polarity, contractility, and traction forces internally.

Epithelial tissues are populated with accessory cells including pigment-producing melanocytes, which must migrate between tightly adherent epithelial cells, but how cells migrate through confined epithelial spaces without impairing barrier function is poorly understood. Using live imaging of the mouse epidermis, we captured the migration of embryonic melanocytes (melanoblasts) while simultaneously visualizing the basement membrane or epithelial surfaces. We show that melanoblasts migrate through basal and suprabasal layers of the epidermis where they use keratinocyte surfaces, as well as the basement membrane, as substrates for migration. Melanoblasts form atypical and dynamic E-cadherin attachments to keratinocytes that largely lack cytoplasmic catenins known to anchor E-cadherin to F-actin. We show E-cadherin is needed in both melanoblasts and keratinocytes to stabilize migratory protrusions, and that depleting E-cadherin results in reduced melanoblast motility and ventral depigmentation in adult mice. These findings illustrate how migratory cells modify the cell adhesion machinery to invade between connected epithelial cells without interrupting the skin barrier.

Lipid bilayers form the basis of organellar architecture, structure, and compartmentalization in the cell. Decades of biophysical, biochemical, and imaging studies on purified or in vitro-reconstituted liposomes have shown that variations in lipid composition influence the physical properties of membranes, such as thickness and curvature. However, similar studies characterizing these membrane properties within the native cellular context have remained technically challenging. Recent advancements in cellular cryo-electron tomography (cryo-ET) imaging enable high-resolution, three-dimensional views of native organellar membrane architecture preserved in near-native conditions. We previously developed a "Surface Morphometrics" pipeline that generates surface mesh reconstructions to model and quantify cellular membrane ultrastructure from cryo-ET data. Here, we expand this pipeline to measure the distance between the phospholipid head groups of the membrane bilayer as a readout of membrane thickness. Using this approach, we demonstrate thickness variations both within and between distinct organellar membranes. We show that organellar membrane thickness positively correlates with other features, such as membrane curvedness, in cells. Further, we show that subcompartments of the mitochondrial inner membrane exhibit varying membrane thicknesses that are independent of whether the mitochondria are in fragmented or elongated networks. We also demonstrate that our technique, when applied to three-dimensional data, yields results that match existing measurements obtained from two-dimensional data of in vitro samples. Finally, we demonstrate that large membrane-associated macromolecular complexes exhibit distinct density profiles that correlate with local variations in membrane thickness. Overall, our updated Surface Morphometrics pipeline provides a framework for investigating how changes in membrane composition in various cellular and disease contexts affect organelle ultrastructure and function.

Lipid droplets (LDs), originating from the ER, play critical roles in lipid metabolism. ER-LD contacts enable lipid exchange and support essential cellular processes. However, how viruses utilize ER-LD coordination remains elusive. Here, we demonstrate that hepatitis C virus (HCV) infection markedly increases LDs abundance and enhances ER-LD contacts. Through a targeted screen of ER-LD tethering proteins, we identified that the NRZ complex, composed of nonsteroidal anti-inflammatory drug-activated gene (NAG), RAD50 interactor 1 (RINT1) and zeste white 10 (ZW10), is essential for HCV-induced ER-LD association and viral infection. Mechanistically, RINT1 and ZW10 interact with the HCV envelope protein E1. Ectopic E1 expression is sufficient to promote ER-LD contacts, which are abolished upon NRZ depletion. NRZ depletion also impairs Dengue virus (DENV) and Zika virus (ZIKV) infection, suggesting its conserved proviral function. Together, this work uncovers a critical mechanism by which host inter-organelle tethering complexes regulate viral infection, offering new insights into virus-host interactions and potential antiviral targets.