During cytokinesis of animal and fungal cells, a contractile actomyosin ring (CAR) assembles at target locations and constricts to drive cell separation. In animal cells, the position of the CAR is determined by the mitotic spindle so that the cleavage plane is perpendicular to the mitotic axis. However, in budding yeasts, the location of CAR assembly is specified by a cortical septin cytoskeleton that recruits CAR components to the neck. In the polymorphic fungus Aureobasidium pullulans, we show that septins assemble at mother-bud necks and predict the sites of CAR assembly. Cells lacking septins stochastically failed to assemble CARs at a subset of bud necks. However, even cells lacking all four core septins were able to assemble CARs at 75% of bud necks. Our findings suggest the existence of a novel pathway that enables CAR positioning, assembly, and constriction with no requirement for septin scaffolds or nuclear/spindle cues.

Centriolar satellites (CS) are dynamic and heterogeneous granular assemblies that concentrate around centrosomes and contribute to ciliogenesis. In this issue, Begar et al. (https://doi.org/10.1083/jcb.202509238) examine the CS scaffold protein PCM1 to dissect CS assembly and structure during the cell cycle and ciliogenesis.

Unattached kinetochores promote microtubule capture while preventing cell cycle progression during mitosis. The Mps1 kinase controls these events by mediating kinetochore assembly of the fibrous corona in animal cells and by triggering the spindle checkpoint. In budding yeast, which does not assemble a fibrous corona, the Stu1 and Slk19 spindle proteins promote microtubule capture by clustering unattached kinetochores, but the underlying mechanism is unclear. Here, we show that Mps1 controls this pathway by phosphorylating two conserved MELT motifs in Stu1 to recruit Slk19 and mediate kinetochore clustering. Structural analysis of the Stu1:Slk19 complex reveals long, string-like filaments and offers mechanistic insight into how kinetochores might cluster. Our findings suggest the regulation of kinetochore capture is a conserved Mps1 function across eukaryotes.

Nucleotide excision repair is essential for genome stability; however, enzymatic scissors that remove damage must be tightly regulated to prevent erroneous DNA cleavage. In this issue, Muniesa-Vargas et al. (https://doi.org/10.1083/jcb.202602121) used live-cell imaging to uncover how cutting DNA by XPG endonuclease is controlled during repair.

Fatty acids (FAs) are transported from lipid droplets (LDs) to mitochondria for β-oxidation during cell starvation. Starvation also triggers engulfment of LDs by autophagosomes and their subsequent degradation by lysosomes (lipophagy). The mechanisms coordinating these pathways remain unclear. Here, we demonstrate that PISD-LD, an LD-localized isoform of phosphatidylserine decarboxylase, facilitates FA transfer while inhibiting lipophagy. PISD-LD mediates LD-mitochondrion (LD-mito) contacts via interaction with mitochondrial PISD. In PISD-LD KD cells, LDs are larger, and FA trafficking and mitochondrial FA β-oxidation are suppressed. The lipid transfer proteins ATG2A/B are recruited by PISDs to mediate FA transfer from LDs to mitochondria. Disruption of PISD-LD-mediated LD-mito contacts activates lipophagy, aiding LD degradation. PISD-LD binds the lipophagy receptor Spartin and inhibits lipophagy by impeding Spartin-LC3 interaction. PISD-LD also regulates LD-mito contacts and lipid metabolism in mouse liver. Thus, PISD-LD serves as a switch between LD-to-mitochondrion FA transfer and lipophagy, ensuring efficient energy production.

The intestinal epithelium, predominantly composed of enterocytes (ECs), integrates dietary cues to regulate intestinal stem cell (ISC) activity and maintain intestinal homeostasis. However, the underlying mechanisms remain incompletely understood. Here, we demonstrate the functions of essential nutrient-sensing mediators Rag GTPases and mTORC1 on Drosophila ISC activity. Inhibition of Rag GTPases in ECs triggers Mitf activation, which subsequently induces Upd3 expression to non-cell autonomously increase the ISC proliferation and differentiation through stimulating the JAK-STAT pathway. Inactivation of mTORC1 in ECs also activates the Upd3-JAK-STAT axis and increases ISC activity, but through a Mitf-independent mechanism. Moreover, in contrast to mTORC1 inactivation blocking ISC proliferation and differentiation, depletion of Rag GTPases in ISCs shows no obvious cell autonomous effect on ISC activity. Consistent with inhibition of the nutrient-responsive pathway in ECs promoting ISC proliferation, long-term nutrient starvation enhances ISC activity and Upd3 expression. Together, our findings reveal a nutrient-sensing regulatory network that controls ISC proliferation involving the Rag GTPases-Mitf axis and mTORC1 activity.

Fam20C, the first protein kinase identified in the secretory pathway, governs the majority phosphorylation of the secretome. Although Fam20C has a high propensity for secretion, its kinase activity occurs intracellularly. How Fam20C clients are phosphorylated inside cells remains elusive. Here, we demonstrate that the pseudokinase Fam20A forms a heterocomplex with Fam20C on the Golgi membrane, anchoring the cleaved, mature form of Fam20C within the Golgi. Their Golgi localization is promoted by a set of cargo receptors ERGIC2 and ERGIC3, which drive the ER-to-Golgi transport of Fam20A-Fam20C. Importantly, both ERGIC2 and ERGIC3 are upregulated in the mammary gland during lactation, and Ergic2 or Ergic3 knockout mice display global changes in secretome phosphorylation, less phosphorylated β-casein in milk, and deficiency in offspring growth. Our findings uncover a previously unrecognized mechanism for the spatiotemporal regulation of Fam20 kinases, crucial for efficient phosphorylation of secretory proteins during lactation.

Coenzyme Q (CoQ or ubiquinone) is an essential cofactor for mitochondrial energy production and a vital radical-trapping antioxidant that maintains membrane integrity. Additionally, CoQ shares an early biosynthetic pathway with cholesterol biosynthesis. In this issue, Ndoci et al. (https://doi.org/10.1083/jcb.202507174) reveal a regulatory system that preserves mitochondrial CoQ levels when the mevalonate pathway is impaired, though this prioritization leaves cells vulnerable to oxidative stress.

Actin filaments often appear as branches, nucleated by the Arp2/3 complex. Arp2 and Arp3 are ATPases, which adopt different nucleotide-dependent conformations. We investigated how the nucleotide state of mammalian Arp2/3 complexes affects branch stability, by applying mechanical load. Branch junctions are 30-fold more stable when Arp2/3 is in the ADP-inorganic phosphate (Pi) rather than the ADP state. Pi is in rapid equilibrium with the ADP-Arp2/3 complex at the branch junction (release rate 0.2 s-1). Upon branch dissociation, Arp2/3 complexes remaining attached to the mother filament in the ADP-Pi state are 100-fold more stable, release their phosphate slowly (0.05 s-1), and can regrow branches without reloading ATP. Glia maturation factor (GMF) accelerates the dissociation of surviving ADP-Arp2/3 complexes, but does not prevent branch regrowth at physiological ATP concentration. Cortactin stabilizes branches and enhances renucleation. Neither GMF nor cortactin affects branch stability and renucleation of ADP-Pi-Arp2/3. Overall, these results identify Pi in the Arp2/3 complex as a critical regulator of branched actin network stability.

Epidermal stem cell maintenance depends on complex niche signalling. In this issue Gaeta, Du et al. (https://doi.org/10.1083/jcb.202507165) show that substantial dermal fibroblast depletion has surprisingly minimal effects on epidermal proliferation and skin function during development and adult homeostasis.

Rab5-family GTPases cycle between active and inactive forms to regulate endosomal membrane identity and protein trafficking. VPS9-family guanine nucleotide exchange factors (GEFs) promote Rab activation at endosomes, whereas GTPase-activating proteins (GAPs) oppose Rab signaling. Here, we identify an unexpected role for the yeast VPS9-family GEF complex VINE in promoting the inactivation of the Rab5 homolog Vps21. Through genome-wide proximity screening, predictive modeling, targeted mutagenesis, and in vivo assays, we show that VINE recruits the protein phosphatase Glc7 through the ankyrin repeat-containing domain of its GEF subunit Vrl1. Our results suggest this directs the dephosphorylation of Kxd1, a subunit of the GAP adaptor BLOC-1, which in turn enhances its interaction with the Vps21-specific GAP Msb3 and accelerates GAP-mediated Vps21 inactivation. Thus, VINE is a VPS9-family GEF complex that selectively limits endosomal Rab signaling. These findings reveal a novel mechanism integrating positive and negative Rab regulation, providing insight into how Rab5 signaling is fine-tuned during endosomal trafficking and maturation.

Mitochondrial protein import is critical for organelle biogenesis, maintenance, and regeneration-essential for cellular homeostasis. Import dysfunction compromises cellular energy supplies, which is damaging to cells, particularly those with high energetic demands like neurons. Previously, we have shown that import failure is rescued by intercellular mitochondrial transfer (IMT) via tunnelling nanotubes (TNTs) however, the fate of the transferred mitochondria and the mechanistic basis for rescue were unresolved. Here, we show that bidirectional mitochondrial trafficking between cells harboring import-defective and import-competent mitochondria is distinct in terms of their regulation and ensuing consequences. Transferred import-defective mitochondria are highly fragmented and destined for canonical lysosomal degradation. In contrast, reactive oxygen species (ROS)-producing mitochondria at the periphery of cells with import-competent mitochondria are transferred into neighboring cells undergoing import failure. These new arrivals then accumulate within previously uncharacterized "mitochondrial degradation bodies" (MDBs). We speculate that the cooperation of these distinct cases of TNT-mediated conventional and noncanonical "trans-mitophagy" instigates mitochondrial regeneration, and thereby rescues mitochondrial function.

Proper control of stem cell behavior is crucial for maintaining tissue homeostasis. In the Drosophila midgut, smooth septate junction (sSJ)-associated proteins play a crucial role in regulating intestinal stem cell (ISC) proliferation via atypical PKC (aPKC) activity. Here, we investigate the underlying mechanism. We found that aPKC in the apical cortex diminishes during enteroblast differentiation, in accordance with the apical accumulation of sSJ proteins. In sSJ protein-deficient conditions, aPKC remained in the apical cortex in enteroblasts, accompanied by ISC overproliferation. Manipulation of Kibra, an activator of the Hippo pathway with aPKC-binding properties, affected sSJ proteins depletion-induced ISC overproliferation in an aPKC-binding domain-dependent manner, revealing a link between aPKC and the Hippo- and/or Misshapen-Yorkie pathway in ISC proliferation. This study provides novel insights into the mechanism underlying stem cell regulation, in which an antagonistic interaction between aPKC and junctional proteins in differentiating progenitors is involved.

Unorthodox AQP12-type channels are poorly understood intracellular aquaporins localized in the endoplasmic reticulum and zymogen granules (ZGs) of pancreatic acinar cells. Despite connections to major diseases, their biophysical properties and intracellular trafficking regulation remain largely unknown. Here, we show that heterologously expressed plant and metazoan AQP12-related channels specifically localize to the intracellular yolk platelet (YP) membrane of frog oocytes, a feature that is recapitulated in vivo for invertebrate and vertebrate orthologs. Using native YP membranes, we show that the vertebrate channels are mercury-sensitive polytransporters with intracellular trafficking regulated by Ca2+ and cAMP signaling pathways. We identify a novel pan-vertebrate C-terminal YP-targeting domain (YPD) in AQP12, which also drives orthodox aquaporin chimeras and truncated channels to YPs and ZGs. In cultured pancreatic cells, the YPD and Ca2+-induced AQP12 N-terminal phosphorylation coregulate secretagogue-triggered channel transport to the ZGs for enzyme secretion. These findings uncover conserved signaling mechanisms for AQP12 trafficking to intracellular protein storage vesicles, and open unexpected avenues for targeted delivery systems.

Actin's transition from monomers (G-actin) to polymers (F-actin) and then into bundled and branched networks underlies many cellular and system functions. Yet, how these networks are dynamically assembled and disassembled is incompletely understood-including why F-actin is often simultaneously and redundantly bundled by different proteins. Here, we focus on fascin and espin, two bundlers that often coexist and robustly bundle F-actin. We find that they synergistically bundle F-actin compared to equal amounts of each one alone. However, we show that bundles containing these two proteins are robustly destabilized by a synergism between Mical and cofilin, indicating mechanisms of how complex bundles are disassembled and remodeled. Yet, our results also reveal that together fascin and espin protect F-actin from this disassembly more effectively than each one alone-including to regulate F-actin disassembly and cellular remodeling in vivo. These findings reveal mechanisms for assembling and disassembling complex networks of bundled F-actin, including a synergism between different bundlers and disassemblers in these processes.