Membrane binding controls ordered self-assembly of animal septins

The structural organisation of pentraxin-3 and its interactions with heavy chains of inter-α-inhibitor regulate crosslinking of the hyaluronan matrix

Pentraxin-3 (PTX3) is an octameric protein, comprised of eight identical protomers, that has diverse functions in reproductive biology, innate immunity and cancer. PTX3 interacts with the large polysaccharide hyaluronan (HA) to which heavy chains (HCs) of the inter-α-inhibitor (IαI) family of proteoglycans are covalently attached, playing a key role in the (non-covalent) crosslinking of HC•HA complexes. These interactions stabilise the cumulus matrix, essential for ovulation and fertilisation in mammals, and are also implicated in the formation of pathogenic matrices in the context of viral lung infections. To better understand the physiological and pathological roles of PTX3 we have analysed how its quaternary structure underpins HA crosslinking via its interactions with HCs. A combination of X-ray crystallography, cryo-electron microscopy (cryo-EM) and AlphaFold predictive modelling revealed that the C-terminal pentraxin domains of the PTX3 octamer are arranged in a central cube, with two long extensions on either side, each formed from four protomers assembled into tetrameric coiled-coil regions, essentially as described by (Noone et al., 2022; doi:10.1073/pnas.2208144119). From crystallography and cryo-EM data, we identified a network of inter-protomer salt bridges that facilitate the assembly of the octamer. Small angle X-ray scattering (SAXS) validated our model for the octameric protein, including the analysis of two PTX3 constructs: a tetrameric 'Half-PTX3' and a construct missing the 24 N-terminal residues (Δ1-24_PTX3). SAXS determined a length of ∼520 Å for PTX3 and, combined with 3D variability analysis of cryo-EM data, defined the flexibility of the N-terminal extensions. Biophysical analyses revealed that the prototypical heavy chain HC1 does not interact with PTX3 at pH 7.4, consistent with our previous studies showing that, at this pH, PTX3 only associates with HC•HA complexes if they are formed in its presence. However, PTX3 binds to HC1 at acidic pH, and can also be incorporated into pre-formed HC•HA complexes under these conditions. This provides a novel mechanism for the regulation of PTX3-mediated HA crosslinking (e.g., during inflammation), likely mediated by a pH-dependent conformational change in HC1. The PTX3 octamer was found to associate simultaneously with up to eight HC1 molecules and, thus, has the potential to form a major crosslinking node within HC•HA matrices, i.e., where the physical and biochemical properties of resulting matrices could be tuned by the HC/PTX3 composition.

Septin assemblies promote the lipid organization of membranes

Cytoskeletal-mediated membrane compartmentalization is essential to support cellular functions, from signaling to cell division, migration, or phagocytosis. Septins are cytoskeletal proteins that directly interact with membranes, acting as scaffolds to recruit proteins to cellular locations and as structural diffusion barriers. How septins interact with and remodel the lipid organization of membranes is unclear. Here, we combined minimal reconstituted systems and yeast cell imaging to study septin-mediated membrane organization. Our results show that at low concentrations membrane-diffusive septins self-assemble into sub-micrometric patches that co-exist with the septin collar at the division site. We found that patches are made of short septin filaments and that are able to modulate the lipid organization of membranes. Furthermore, we show that the polybasic domain of Cdc11 influences the membrane-organizing and curvature-sensing properties of septins. Collectively, our work provides understanding of the molecular mechanisms by which septins can support cellular functions intimately linked to membranes.

Cooperativity in septin polymerization is tunable by ionic strength and membrane adsorption

Cells employ cytoskeletal polymers to move, divide, and pass information inside and outside of the cell. Previous work on eukaryotic cytoskeletal elements such as actin, microtubules, and intermediate filaments investigating the mechanisms of polymerization have been critical to understand how cells control the assembly of the cytoskeleton. Most biophysical analyses have considered cooperative versus isodesmic modes of polymerization; this framework is useful for specifying functions of regulatory proteins that control nucleation and understanding how cells regulate elongation in time and space. The septins are considered a fourth component of the eukaryotic cytoskeleton, but they are poorly understood in many ways despite their conserved roles in membrane dynamics, cytokinesis, and cell shape, and in their links to a myriad of human diseases. Because septin function is intimately linked to their assembled state, we set out to investigate the mechanisms by which septin polymers elongate under different conditions. We used simulations, in vitro reconstitution of purified septin complexes, and quantitative microscopy to directly interrogate septin polymerization behaviors in solution and on synthetic lipid bilayers of different geometries. We first used reactive Brownian dynamics simulations to determine if the presence of a membrane induces cooperativity to septin polymerization. We then used fluorescence correlation spectroscopy (FCS) to assess septins' ability to form filaments in solution at different salt conditions. Finally, we investigated septin membrane adsorption and polymerization on planar and curved supported lipid bilayers. Septins clearly show signs of salt-dependent cooperative assembly in solution, but cooperativity is limited by binding a membrane. Thus, septin assembly is dramatically influenced by extrinsic conditions and substrate properties and can show properties of both isodesmic and cooperative polymers. This versatility in assembly modes may explain the extensive array of assembly types, functions, and subcellular locations in which septins act.

Lipid packing and local geometry influence septin curvature sensing

Septins assemble into scaffolds that direct cell growth and morphology that are often localized to the plasma membrane. While septins preferentially bind convex membranes via amphipathic helices, their assembly on varied geometries in cells suggests additional localization cues. We tested the hypothesis that lipid composition directs septin assembly through lipid packing properties. Lipid mixtures varying in lipid packing were designed by molecular dynamics simulations and incorporated onto supported lipid bilayers to measure septin adsorption in vitro. Septins strongly favor loosely-packed, disordered lipid bilayers but additional geometry cues act in conjunction with this membrane property. Introducing tighter lipid packing in cells disrupted septin structures in a curvature dependent manner, specifically limiting septin assembly and retention along flat regions of the plasma membrane. This work demonstrates that packing defects and geometry jointly regulate septin localization and highlights how multiple membrane properties are integrated to control organization of the septin cytoskeleton.

Summary

Localization of the septin cytoskeleton is controlled by regulatory factors, membrane curvature, and charge. In this study, changes to lipid composition that modulate lipid packing defects are found to impact septin assemblies in vitro and in cells.

Optimization of Protoplast Preparation Conditions in Lyophyllum decastes and Transcriptomic Analysis Throughout the Process

Protoplasts are essential tools for genetic manipulation and functional genomics research in fungi. This study systematically optimized protoplast preparation conditions and examined transcriptional changes throughout the preparation and regeneration processes to elucidate the molecular mechanisms underlying the formation and regeneration of protoplasts in Lyophyllum decastes. The results indicated an optimal protoplast yield of 5.475 × 106 cells/mL under conditions of fungal age at 10 days, digestion time of 2.25 h, enzyme concentration of 2%, and digestion temperature of 28 °C. The Z5 medium supplemented with L. decastes mycelial extract achieved a high regeneration rate of 2.86. RNA-seq analysis revealed 2432 differentially expressed genes (DEGs) during protoplast formation and 5825 DEGs during regeneration. Casein kinase I, cytochrome P450 (CYP52), and redox-regulated input receptor (PEX5) were significantly upregulated during the protoplast stage, while β-1,3-glucan synthase (SKN1), chitin synthase (CHS2), hydrophobin-1, and hydrophobin-2 showed significant upregulation during the protoplast regeneration phase. These findings provide a reference for the efficient preparation and regeneration of protoplasts and offer new insights into the molecular mechanisms of protoplast formation and cell wall regeneration in fungi.

Drivers of Morphogenesis: Curvature Sensor Self-Assembly at the Membrane

This review examines the relationships between membrane chemistry, curvature-sensing proteins, and cellular morphogenesis. Curvature-sensing proteins are often orders of magnitude smaller than the membrane curvatures they localize to. How are nanometer-scale proteins used to sense micrometer-scale membrane features? Here, we trace the journey of curvature-sensing proteins as they engage with lipid membranes through a combination of electrostatic and hydrophobic interactions. We discuss how curvature sensing hinges on membrane features like lipid charge, packing, and the directionality of membrane curvature. Once bound to the membrane, many curvature sensors undergo self-assembly (i.e., they oligomerize or form higher-order assemblies that are key for initiating and regulating cell shape transformations). Central to these discussions are the micrometer-scale curvature-sensing proteins' septins. By discussing recent literature surrounding septin membrane association, assembly, and their many functions in morphogenesis with support from other well-studied curvature sensors, we aim to synthesize possible mechanisms underlining cell shape sensing.

Septin dynamics and organization in mammalian cells

Septins are involved in many important cellular processes, and septin dysfunction has been implicated in various pathologies, such as cancer. Like other components of the cytoskeleton -F-actin, microtubules, and intermediate filaments-septins can self-assemble into filaments and higher-order structures. These non-polar filaments are assembled from complex and variable multimeric building blocks. Septins exhibit a distinct preference for interacting with actin and microtubule structures, particularly at the interface with cellular membrane. Although they are crucial for many vital cellular functions and are frequently observed at prominent cellular structures like stress fibers, cilia, and neuronal processes, our understanding of the regulation of septin filament dynamics and the organized assembly of higher-order structures remains limited. However, recent insights into the architecture of septin filaments, the structure of crucial septin domains, and their interactions with other cellular components (F-actin, microtubules, membranes) and regulatory proteins may now pave the way for rapid progress.

Septins regulate heart contractility through modulation of cardiomyocyte store-operated calcium entry

Highly regulated cardiomyocyte Ca 2+ fluxes drive heart contractions. Recent findings from multiple organisms demonstrate that the specific Ca 2+ transport mechanism known as store-operated Ca 2+ entry (SOCE) is essential in cardiomyocytes for proper heart function, and SOCE dysregulation results in cardiomyopathy. Mechanisms that regulate SOCE in cardiomyocytes are poorly understood. Here we tested the role of cytoskeletal septin proteins in cardiomyocyte SOCE regulation. Septins are essential SOCE modulators in other cell types, but septin functions in cardiomyocytes are nearly completely unexplored. We show using targeted genetics and intravital imaging of heart contractility in Drosophila that cardiomyocyte-specific depletion of septins 1, 2, and 4 results in heart dilation that phenocopies the effects of SOCE suppression. Heart dilation caused by septin 2 depletion was suppressed by SOCE upregulation, supporting the hypothesis that septin 2 is required in cardiomyocytes for sufficient SOCE function. A major function of SOCE is to support SERCA-dependent sarco/endoplasmic reticulum (S/ER) Ca 2+ stores, and augmenting S/ER store filling by SERCA overexpression also suppressed the septin 2 phenotype. We also ruled out several potential SOCE-independent septin functions, as septin 2 phenotypes were not due to septin function during development and septin 2 was not required for z-disk organization as defined by α-actinin labeling. These results demonstrate, for the first time, an essential role of septins in cardiomyocyte physiology and heart function that is due, at least in part, to septin regulation of SOCE function.

A single septin from a polyextremotolerant yeast recapitulates many canonical functions of septin hetero-oligomers

Morphological complexity and plasticity are hallmarks of polyextremotolerant fungi. Septins are conserved cytoskeletal proteins and key contributors to cell polarity and morphogenesis. They sense membrane curvature, coordinate cell division, and influence diffusion at the plasma membrane. Four septin homologues are conserved from yeasts to humans, the systems in which septins have been most studied. But there is also a fifth family of opisthokont septins that remain biochemically mysterious. Members of this family, Group 5 septins, appear in the genomes of filamentous fungi, but are understudied due to their absence from ascomycete yeasts. Knufia petricola is an emerging model polyextremotolerant black fungus that can also serve as a model system for Group 5 septins. We have recombinantly expressed and biochemically characterized KpAspE, a Group 5 septin from K. petricola. This septin--by itself in vitro--recapitulates many functions of canonical septin hetero-octamers. KpAspE is an active GTPase that forms diverse homo-oligomers, binds shallow membrane curvatures, and interacts with the terminal subunit of canonical septin hetero-octamers. These findings raise the possibility that Group 5 septins govern the higher-order structures formed by canonical septins, which in K. petricola cells form extended filaments, and provide insight into how septin hetero-oligomers evolved from ancient homomers.

Novel lipid-interaction motifs within the C-terminal domain of Septin10 from Schistosoma mansoni

Septins are cytoskeletal proteins and their interaction with membranes is crucial for their role in various cellular processes. Septins have polybasic regions (PB1 and PB2) which are important for lipid interaction. Earlier, we and others have highlighted the role of the septin C-terminal domain (CTD) to membrane interaction. However, detailed information on residues/group of residues important for such feature is lacking. In this study, we investigate the lipid-binding profile of Schistosoma mansoni Septin10 (SmSEPT10) using PIP strip and Langmuir monolayer adsorption assays. Our findings highlight the CTD as the primary domain responsible for lipid interaction in SmSEPT10, showing binding to phosphatidylinositol phosphates. SmSEPT10 CTD contains a conserved polybasic region (PB3) present in both animals and fungi septins, and a Lys (K367) within its putative amphipathic helix (AH) that we demonstrate as important for lipid binding. PB3 deletion or mutation of this Lys (K367A) strongly impairs lipid interaction. Remarkably, we observe that the AH within a construct lacking the final 43 amino acid residues is insufficient for lipid binding. Furthermore, we investigate the homocomplex formed by SmSEPT10 CTD in solution by cross-linking experiments, CD spectroscopy, SEC-MALS and SEC-SAXS. Taken together, our studies define the lipid-binding region in SmSEPT10 and offer insights into the molecular basis of septin-membrane binding. This information is particularly relevant for less-studied non-human septins, such as SmSEPT10.

Cytokinetic contractile ring structural progression in an early embryo: positioning of scaffolding proteins, recruitment of α-actinin, and effects of myosin II inhibition

Our knowledge of the assembly and dynamics of the cytokinetic contractile ring (CR) in animal cells remains incomplete. We have previously used super-resolution light microscopy and platinum replica electron microscopy to elucidate the ultrastructural organization of the CR in first division sea urchin embryos. To date, our studies indicate that the CR initiates as an equatorial band of clusters containing myosin II, actin, septin and anillin, which then congress over time into patches which coalesce into a linear array characteristic of mature CRs. In the present study, we applied super-resolution interferometric photoactivated localization microscopy to confirm the existence of septin filament-like structures in the developing CR, demonstrate the close associations between septin2, anillin, and myosin II in the CR, as well as to show that septin2 appears consistently submembranous, whereas anillin is more widely distributed in the early CR. We also provide evidence that the major actin cross-linking protein α-actinin only associates with the linearized, late-stage CR and not with the early CR clusters, providing further support to the idea that α-actinin associates with actomyosin structures under tension and can serve as a counterbalance. In addition, we show that inhibition of actomyosin contraction does not stop the assembly of the early CR clusters but does arrest the progression of these structures to the aligned arrays required for functional cytokinesis. Taken together our results reinforce and extend our model for a cluster to patch to linear structural progression of the CR in sea urchin embryos and highlight the evolutionary relationships with cytokinesis in fission yeast.

SEPT9_i1 and Septin Dynamics in Oncogenesis and Cancer Treatment

Despite significant advancements in the field of oncology, cancers still pose one of the greatest challenges of modern healthcare. Given the cytoskeleton's pivotal role in regulating mechanisms critical to cancer development, further studies of the cytoskeletal elements could yield new practical applications. Septins represent a group of relatively well-conserved GTP-binding proteins that constitute the fourth component of the cytoskeleton. Septin 9 (SEPT9) has been linked to a diverse spectrum of malignancies and appears to be the most notable septin member in that category. SEPT9 constitutes a biomarker of colorectal cancer (CRC) and has been positively correlated with a high clinical stage in breast cancer, cervical cancer, and head and neck squamous cell carcinoma. SEPT9_i1 represents the most extensively studied isoform of SEPT9, which substantially contributes to carcinogenesis, metastasis, and treatment resistance. Nevertheless, the mechanistic basis of SEPT9_i1 oncogenicity remains to be fully elucidated. In this review, we highlight SEPT9's and SEPT9_i1's structures and interactions with Hypoxia Inducible Factor α (HIF-1 α) and C-Jun N-Terminal Kinase (JNK), as well as discuss SEPT9_i1's contribution to aneuploidy, cell invasiveness, and taxane resistance-key phenomena in the progression of malignancies. Finally, we emphasize forchlorfenuron and other septin inhibitors as potential chemotherapeutics and migrastatics.

Septin Organization and Dynamics for Budding Yeast Cytokinesis

Cytokinesis, the process by which the cytoplasm divides to generate two daughter cells after mitosis, is a crucial stage of the cell cycle. Successful cytokinesis must be coordinated with chromosome segregation and requires the fine orchestration of several processes, such as constriction of the actomyosin ring, membrane reorganization, and, in fungi, cell wall deposition. In Saccharomyces cerevisiae, commonly known as budding yeast, septins play a pivotal role in the control of cytokinesis by assisting the assembly of the cytokinetic machinery at the division site and controlling its activity. Yeast septins form a collar at the division site that undergoes major dynamic transitions during the cell cycle. This review discusses the functions of septins in yeast cytokinesis, their regulation and the implications of their dynamic remodelling for cell division.

Structural Insights into Ciona intestinalis Septins: Complexes Suggest a Mechanism for Nucleotide-dependent Interfacial Cross-talk

Septins are filamentous nucleotide-binding proteins which can associate with membranes in a curvature-dependent manner leading to structural remodelling and barrier formation. Ciona intestinalis, a model for exploring the development and evolution of the chordate lineage, has only four septin-coding genes within its genome. These represent orthologues of the four classical mammalian subgroups, making it a minimalist non-redundant model for studying the modular assembly of septins into linear oligomers and thereby filamentous polymers. Here, we show that C. intestinalis septins present a similar biochemistry to their human orthologues and also provide the cryo-EM structures of an octamer, a hexamer and a tetrameric sub-complex. The octamer, which has the canonical arrangement (2-6-7-9-9-7-6-2) clearly shows an exposed NC-interface at its termini enabling copolymerization with hexamers into mixed filaments. Indeed, only combinations of septins which had CiSEPT2 occupying the terminal position were able to assemble into filaments via NC-interface association. The CiSEPT7-CiSEPT9 tetramer is the smallest septin particle to be solved by Cryo-EM to date and its good resolution (2.7 Å) provides a well-defined view of the central NC-interface. On the other hand, the CiSEPT7-CiSEPT9 G-interface shows signs of fragility permitting toggling between hexamers and octamers, similar to that seen in human septins but not in yeast. The new structures provide insights concerning the molecular mechanism for cross-talk between adjacent interfaces. This indicates that C. intestinalis may represent a valuable tool for future studies, fulfilling the requirements of a complete but simpler system to understand the mechanisms behind the assembly and dynamics of septin filaments.

Septins promote macrophage pyroptosis by regulating gasdermin D cleavage and ninjurin-1-mediated plasma membrane rupture

The septin cytoskeleton is primarily known for roles in cell division and host defense against bacterial infection. Despite recent insights, the full breadth of roles for septins in host defense is poorly understood. In macrophages, Shigella induces pyroptosis, a pro-inflammatory form of cell death dependent upon gasdermin D (GSDMD) pores at the plasma membrane and cell surface protein ninjurin-1 (NINJ1) for membrane rupture. Here, we discover that septins promote macrophage pyroptosis induced by lipopolysaccharide (LPS)/nigericin and Shigella infection, but do not affect cytokine expression or release. We observe that septin filaments assemble at the plasma membrane, and cleavage of GSDMD is impaired in septin-depleted cells. We found that septins regulate mitochondrial dynamics and the expression of NINJ1. Using a Shigella-zebrafish infection model, we show that septin-mediated pyroptosis is an in vivo mechanism of infection control. The discovery of septins as a mediator of pyroptosis may inspire innovative anti-bacterial and anti-inflammatory treatments.

Phosphorylation of the F-BAR protein Hof1 drives septin ring splitting in budding yeast

A double septin ring accompanies cytokinesis in yeasts and mammalian cells. In budding yeast, reorganisation of the septin collar at the bud neck into a dynamic double ring is essential for actomyosin ring constriction and cytokinesis. Septin reorganisation requires the Mitotic Exit Network (MEN), a kinase cascade essential for cytokinesis. However, the effectors of MEN in this process are unknown. Here we identify the F-BAR protein Hof1 as a critical target of MEN in septin remodelling. Phospho-mimicking HOF1 mutant alleles overcome the inability of MEN mutants to undergo septin reorganisation by decreasing Hof1 binding to septins and facilitating its translocation to the actomyosin ring. Hof1-mediated septin rearrangement requires its F-BAR domain, suggesting that it may involve a local membrane remodelling that leads to septin reorganisation. In vitro Hof1 can induce the formation of intertwined septin bundles, while a phosphomimetic Hof1 protein has impaired septin-bundling activity. Altogether, our data indicate that Hof1 modulates septin architecture in distinct ways depending on its phosphorylation status.

X-ray structure of the metastable SEPT14-SEPT7 coiled coil reveals a hendecad region crucial for heterodimerization

Septins are membrane-associated, GTP-binding proteins that are present in most eukaryotes. They polymerize to play important roles as scaffolds and/or diffusion barriers as part of the cytoskeleton. α-Helical coiled-coil domains are believed to contribute to septin assembly, and those observed in both human SEPT6 and SEPT8 form antiparallel homodimers. These are not compatible with their parallel heterodimeric organization expected from the current model for protofilament assembly, but they could explain the interfilament cross-bridges observed by microscopy. Here, the first structure of a heterodimeric septin coiled coil is presented, that between SEPT14 and SEPT7; the former is a SEPT6/SEPT8 homolog. This new structure is parallel, with two long helices that are axially shifted by a full helical turn with reference to their sequence alignment. The structure also has unusual knobs-into-holes packing of side chains. Both standard seven-residue (heptad) and the less common 11-residue (hendecad) repeats are present, creating two distinct regions with opposite supercoiling, which gives rise to an overall straight coiled coil. Part of the hendecad region is required for heterodimerization and therefore may be crucial for selective septin recognition. These unconventional sequences and structural features produce a metastable heterocomplex that nonetheless has enough specificity to promote correct protofilament assembly. For instance, the lack of supercoiling may facilitate unzipping and transitioning to the antiparallel homodimeric state.

Reconstituted in vitro systems to reveal the roles and functions of septins

Septins are essential cytoskeletal proteins involved in key cellular processes and have also been implicated in diseases from cancers to neurodegenerative pathologies. However, they have not been as thoroughly studied as other cytoskeletal proteins. In vivo, septins interact with other cytoskeletal proteins and with the inner plasma membrane. Hence, bottom-up in vitro cell-free assays are well suited to dissect the roles and behavior of septins in a controlled environment. Specifically, in vitro studies have been invaluable in describing the self-assembly of septins into a large diversity of ultrastructures. Given that septins interact specifically with membrane, the details of these septin-membrane interactions have been analyzed using reconstituted lipid systems. In particular, at a membrane, septins are often localized at curvatures of micrometer scale. In that context, in vitro assays have been performed with substrates of varying curvatures (spheres, cylinders or undulated substrates) to probe the sensitivity of septins to membrane curvature. This Review will first present the structural properties of septins in solution and describe the interplay of septins with cytoskeletal partners. We will then discuss how septins interact with biomimetic membranes and induce their reshaping. Finally, we will highlight the curvature sensitivity of septins and how they alter the mechanical properties of membranes.

Blebs and former blebs: From surface protrusions to extracellular vesicles in cancer signalling, anoikis resistance and beyond

Associations between plasma membrane blebbing and metastatic progression have been widely reported. There are also reports of increased extracellular vesicle release from cancer cells. Yet the ties between these closely related phenomena are incompletely understood. In this commentary, we remark on a recent finding on cellular membrane blebs in melanoma signaling. We discuss possible implications for cancer biology and draw parallels to knowns and unknowns in the relationships of extracellular vesicles and cancer progression.

Septins regulate border cell surface geometry, shape, and motility downstream of Rho in Drosophila

Septins self-assemble into polymers that bind and deform membranes in vitro and regulate diverse cell behaviors in vivo. How their in vitro properties relate to their in vivo functions is under active investigation. Here, we uncover requirements for septins in detachment and motility of border cell clusters in the Drosophila ovary. Septins and myosin colocalize dynamically at the cluster periphery and share phenotypes but, surprisingly, do not impact each other. Instead, Rho independently regulates myosin activity and septin localization. Active Rho recruits septins to membranes, whereas inactive Rho sequesters septins in the cytoplasm. Mathematical analyses identify how manipulating septin expression levels alters cluster surface texture and shape. This study shows that the level of septin expression differentially regulates surface properties at different scales. This work suggests that downstream of Rho, septins tune surface deformability while myosin controls contractility, the combination of which governs cluster shape and movement.

Conservation and divergence of the G-interfaces of Drosophila melanogaster septins

Septins possess a conserved guanine nucleotide-binding (G) domain that participates in the stabilization of organized hetero-oligomeric complexes which assemble into filaments, rings and network-like structures. The fruit fly, Drosophila melanogaster, has five such septin genes encoding Sep1, Sep2, Sep4, Sep5 and Pnut. Here, we report the crystal structure of the heterodimer formed between the G-domains of Sep1 and Sep2, the first from an insect to be described to date. A G-interface stabilizes the dimer (in agreement with the expected arrangement for the Drosophila hexameric particle) and this bears significant resemblance to its human counterparts, even down to the level of individual amino acid interactions. On the other hand, a model for the G-interface formed between the two copies of Pnut which occupy the centre of the hexamer, shows important structural differences, including the loss of a highly favourable bifurcated salt-bridge network. Whereas wild-type Pnut purifies as a monomer, the reintroduction of the salt-bridge network results in stabilizing the dimeric interface in solution as shown by size exclusion chromatography and thermal stability measurements. Adaptive steered molecular dynamics reveals an unzipping mechanism for dimer dissociation which initiates at a point of electrostatic repulsion within the switch II region. Overall, the data contribute to a better understanding of the molecular interactions involved in septin assembly/disassembly.

Dissecting the binding interface of the septin polymerization enhancer Borg BD3

The molecular basis for septin filament assembly has begun to emerge over recent years. These filaments are essential for many septin functions which depend on their association with biological membranes or components of the cytoskeleton. Much less is known about how septins specifically interact with their binding partners. Here we describe the essential role played by the C-terminal domains in both septin polymerization and their association with the BD3 motif of the Borg family of Cdc42 effector proteins. We provide a detailed description, at the molecular level, of a previously reported interaction between BD3 and the NC-interface between SEPT6 and SEPT7. Upon ternary complex formation, the heterodimeric coiled coil formed by the C-terminal domains of the septins becomes stabilized and filament formation is promoted under conditions of ionic strength/protein concentration which are not normally permissible, likely by favouring hexamers over smaller oligomeric states. This demonstrates that binding partners, such as Borg's, have the potential to control filament assembly/disassembly in vivo in a way which can be emulated in vitro by altering the ionic strength. Experimentally validated models indicate that the BD3 peptide lies antiparallel to the coiled coil and is stabilized by a mixture of polar and apolar contacts. At its center, an LGPS motif, common to all human Borg sequences, interacts with charged residues from both helices of the coiled coil (K368 from SEPT7 and the conserved E354 from SEPT6) suggesting a universal mechanism which governs Borg-septin interactions.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

The septin cytoskeleton: Heteromer composition defines filament function

Septins are an evolutionarily conserved protein family whose members form hetero-oligomeric complexes that assemble into filaments and higher-order structures. In this issue, Martins et al. (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202203016) and Cannon et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202204063) report that heteromer composition impacts the physiological role of septin filaments in yeast and human cells.

Human septins organize as octamer-based filaments and mediate actin-membrane anchoring in cells

Septins are cytoskeletal proteins conserved from algae and protists to mammals. A unique feature of septins is their presence as heteromeric complexes that polymerize into filaments in solution and on lipid membranes. Although animal septins associate extensively with actin-based structures in cells, whether septins organize as filaments in cells and if septin organization impacts septin function is not known. Customizing a tripartite split-GFP complementation assay, we show that all septins decorating actin stress fibers are octamer-containing filaments. Depleting octamers or preventing septins from polymerizing leads to a loss of stress fibers and reduced cell stiffness. Super-resolution microscopy revealed septin fibers with widths compatible with their organization as paired septin filaments. Nanometer-resolved distance measurements and single-protein tracking further showed that septin filaments are membrane bound and largely immobilized. Finally, reconstitution assays showed that septin filaments mediate actin-membrane anchoring. We propose that septin organization as octamer-based filaments is essential for septin function in anchoring and stabilizing actin filaments at the plasma membrane.

Curvature sensing as an emergent property of multiscale assembly of septins

The ability of cells to sense and communicate their shape is central to many of their functions. Much is known about how cells generate complex shapes, yet how they sense and respond to geometric cues remains poorly understood. Septins are GTP-binding proteins that localize to sites of micrometer-scale membrane curvature. Assembly of septins is a multistep and multiscale process, but it is unknown how these discrete steps lead to curvature sensing. Here, we experimentally examine the time-dependent binding of septins at different curvatures and septin bulk concentrations. These experiments unexpectedly indicated that septins' curvature preference is not absolute but rather is sensitive to the combinations of membrane curvatures present in a reaction, suggesting that there is competition between different curvatures for septin binding. To understand the physical underpinning of this result, we developed a kinetic model that connects septins' self-assembly and curvature-sensing properties. Our experimental and modeling results are consistent with curvature-sensitive assembly being driven by cooperative associations of septin oligomers in solution with the bound septins. When combined, the work indicates that septin curvature sensing is an emergent property of the multistep, multiscale assembly of membrane-bound septins. As a result, curvature preference is not absolute and can be modulated by changing the physicochemical and geometric parameters involved in septin assembly, including bulk concentration, and the available membrane curvatures. While much geometry-sensitive assembly in biology is thought to be guided by intrinsic material properties of molecules, this is an important example of how curvature sensing can arise from multiscale assembly of polymers.

The compass to follow: Focal adhesion turnover

How cells move is a fundamental biological question. The directionality of adherent migrating cells depends on the assembly and disassembly (turnover) of focal adhesions (FAs). FAs are micron-sized actin-based structures that link cells to the extracellular matrix. Traditionally, microtubules have been considered key to triggering FA turnover. Through the years, advancements in biochemistry, biophysics, and bioimaging tools have been invaluable for many research groups to unravel a variety of mechanisms and molecular players that contribute to FA turnover, beyond microtubules. Here, we discuss recent discoveries of key molecular players that affect the dynamics and organization of the actin cytoskeleton to enable timely FA turnover and consequently proper directed cell migration.

The Syp1/FCHo2 protein induces septin filament bundling through its intrinsically disordered domain

The septin collar of budding yeast is an ordered array of septin filaments that serves a scaffolding function for the cytokinetic machinery at the bud neck and compartmentalizes the membrane between mother and daughter cell. How septin architecture is aided by septin-binding proteins is largely unknown. Syp1 is an endocytic protein that was implicated in the timely recruitment of septins to the newly forming collar through an unknown mechanism. Using advanced microscopy and in vitro reconstitution assays, we show that Syp1 is able to align laterally and tightly pack septin filaments, thereby forming flat bundles or sheets. This property is shared by the Syp1 mammalian counterpart FCHo2, thus emphasizing conserved protein functions. Interestingly, the septin-bundling activity of Syp1 resides mainly in its intrinsically disordered region. Our data uncover the mechanism through which Syp1 promotes septin collar assembly and offer another example of functional diversity of unstructured protein domains.

The structure of the human LACTB filament reveals the mechanisms of assembly and membrane binding

Mitochondria are complex organelles that play a central role in metabolism. Dynamic membrane-associated processes regulate mitochondrial morphology and bioenergetics in response to cellular demand. In tumor cells, metabolic reprogramming requires active mitochondrial metabolism for providing key metabolites and building blocks for tumor growth and rapid proliferation. To counter this, the mitochondrial serine beta-lactamase-like protein (LACTB) alters mitochondrial lipid metabolism and potently inhibits the proliferation of a variety of tumor cells. Mammalian LACTB is localized in the mitochondrial intermembrane space (IMS), where it assembles into filaments to regulate the efficiency of essential metabolic processes. However, the structural basis of LACTB polymerization and regulation remains incompletely understood. Here, we describe how human LACTB self-assembles into micron-scale filaments that increase their catalytic activity. The electron cryo-microscopy (cryoEM) structure defines the mechanism of assembly and reveals how highly ordered filament bundles stabilize the active state of the enzyme. We identify and characterize residues that are located at the filament-forming interface and further show that mutations that disrupt filamentation reduce enzyme activity. Furthermore, our results provide evidence that LACTB filaments can bind lipid membranes. These data reveal the detailed molecular organization and polymerization-based regulation of human LACTB and provide new insights into the mechanism of mitochondrial membrane organization that modulates lipid metabolism.

Actomyosin-Driven Division of a Synthetic Cell

One of the major challenges of bottom-up synthetic biology is rebuilding a minimal cell division machinery. From a reconstitution perspective, the animal cell division apparatus is mechanically the simplest and therefore attractive to rebuild. An actin-based ring produces contractile force to constrict the membrane. By contrast, microbes and plant cells have a cell wall, so division requires concerted membrane constriction and cell wall synthesis. Furthermore, reconstitution of the actin division machinery helps in understanding the physical and molecular mechanisms of cytokinesis in animal cells and thus our own cells. In this review, we describe the state-of-the-art research on reconstitution of minimal actin-mediated cytokinetic machineries. Based on the conceptual requirements that we obtained from the physics of the shape changes involved in cell division, we propose two major routes for building a minimal actin apparatus capable of division. Importantly, we acknowledge both the passive and active roles that the confining lipid membrane can play in synthetic cytokinesis. We conclude this review by identifying the most pressing challenges for future reconstitution work, thereby laying out a roadmap for building a synthetic cell equipped with a minimal actin division machinery.

An anillin-CIN85-SEPT9 complex promotes intercellular bridge maturation required for successful cytokinesis

Cleavage of one cell into two is the most dramatic event in the life of a cell. Plasma membrane fission occurs within a narrow intercellular bridge (ICB) between the daughter cells, but the mechanisms underlying ICB formation and maturation are poorly understood. Here we identify CIN85 as an ICB assembly factor and demonstrate its requirement for robust and timely cytokinesis. CIN85 interacts directly with the N-terminal region of anillin and SEPT9 and thereby facilitates SEPT9-containing filament localization to the plasma membrane of the ICB. In contrast, the C-terminal pleckstrin homology (PH) domain of anillin binds to septin units lacking SEPT9 but enriched in SEPT11. Anillin's interactions with distinct septin units are required to promote ICB elongation and maturation that, we propose, generate the physical space into which the abscission machinery is recruited to drive the final membrane scission event releasing two independent daughter cells.

Contribution of septins to human platelet structure and function

Although septins have been well-studied in nucleated cells, their role in anucleate blood platelets remains obscure. Here, we elucidate the contribution of septins to human platelet structure and functionality. We show that Septin-2 and Septin-9 are predominantly distributed at the periphery of resting platelets and co-localize strongly with microtubules. Activation of platelets by thrombin causes clustering of septins and impairs their association with microtubules. Inhibition of septin dynamics with forchlorfenuron (FCF) reduces thrombin-induced densification of septins and lessens their colocalization with microtubules in resting and activated platelets. Exposure to FCF alters platelet shape, suggesting that septins stabilize platelet cytoskeleton. FCF suppresses platelet integrin αIIbβ3 activation, promotes phosphatidylserine exposure on activated platelets, and induces P-selectin expression on resting platelets, suggesting septin involvement in these processes. Inhibition of septin dynamics substantially reduces platelet contractility and abrogates their spreading on fibrinogen-coated surfaces. Overall, septins strongly contribute to platelet structure, activation and biomechanics.

The Structural Biology of Septins and Their Filaments: An Update

In order to fully understand any complex biochemical system from a mechanistic point of view, it is necessary to have access to the three-dimensional structures of the molecular components involved. Septins and their oligomers, filaments and higher-order complexes are no exception. Indeed, the spontaneous recruitment of different septin monomers to specific positions along a filament represents a fascinating example of subtle molecular recognition. Over the last few years, the amount of structural information available about these important cytoskeletal proteins has increased dramatically. This has allowed for a more detailed description of their individual domains and the different interfaces formed between them, which are the basis for stabilizing higher-order structures such as hexamers, octamers and fully formed filaments. The flexibility of these structures and the plasticity of the individual interfaces have also begun to be understood. Furthermore, recently, light has been shed on how filaments may bundle into higher-order structures by the formation of antiparallel coiled coils involving the C-terminal domains. Nevertheless, even with these advances, there is still some way to go before we fully understand how the structure and dynamics of septin assemblies are related to their physiological roles, including their interactions with biological membranes and other cytoskeletal components. In this review, we aim to bring together the various strands of structural evidence currently available into a more coherent picture. Although it would be an exaggeration to say that this is complete, recent progress seems to suggest that headway is being made in that direction.

Insights into animal septins using recombinant human septin octamers with distinct SEPT9 isoforms

Septin GTP-binding proteins contribute essential biological functions that range from the establishment of cell polarity to animal tissue morphogenesis. Human septins in cells form hetero-octameric septin complexes containing the ubiquitously expressed SEPT9 subunit (also known as SEPTIN9). Despite the established role of SEPT9 in mammalian development and human pathophysiology, biochemical and biophysical studies have relied on monomeric SEPT9, thus not recapitulating its native assembly into hetero-octameric complexes. We established a protocol that enabled, for the first time, the isolation of recombinant human septin octamers containing distinct SEPT9 isoforms. A combination of biochemical and biophysical assays confirmed the octameric nature of the isolated complexes in solution. Reconstitution studies showed that octamers with either a long or a short SEPT9 isoform form filament assemblies, and can directly bind and cross-link actin filaments, raising the possibility that septin-decorated actin structures in cells reflect direct actin-septin interactions. Recombinant SEPT9-containing octamers will make it possible to design cell-free assays to dissect the complex interactions of septins with cell membranes and the actin and microtubule cytoskeleton.

Mechanistic insight into bacterial entrapment by septin cage reconstitution

Septins are cytoskeletal proteins that assemble into hetero-oligomeric complexes and sense micron-scale membrane curvature. During infection with Shigella flexneri, an invasive enteropathogen, septins restrict actin tail formation by entrapping bacteria in cage-like structures. Here, we reconstitute septin cages in vitro using purified recombinant septin complexes (SEPT2-SEPT6-SEPT7), and study how these recognize bacterial cells and assemble on their surface. We show that septin complexes recognize the pole of growing Shigella cells. An amphipathic helix domain in human SEPT6 enables septins to sense positively curved membranes and entrap bacterial cells. Shigella strains lacking lipopolysaccharide components are more efficiently entrapped in septin cages. Finally, cryo-electron tomography of in vitro cages reveals how septins assemble as filaments on the bacterial cell surface.

Septins are cytoskeletal proteins that assemble into complexes and contribute to immunity by entrapping intracellular bacteria in cage-like structures. Here, Lobato-Márquez et al. reconstitute septin cages in vitro using purified recombinant complexes, and study how these recognize bacterial cells and assemble as filaments on their surface.

Spatial regulation of microtubule-dependent transport by septin GTPases

The intracellular long-range transport of membrane vesicles and organelles is mediated by microtubule motors (kinesins, dynein) which move cargo with spatiotemporal accuracy and efficiency. How motors navigate the microtubule network and coordinate their activity on membrane cargo are fundamental but poorly understood questions. New studies show that microtubule-dependent membrane traffic is spatially controlled by septins - a unique family of multimerizing GTPases that associate with microtubules and membrane organelles. We review how septins selectively regulate motor interactions with microtubules and membrane cargo. We posit that septins provide a novel traffic code that specifies the movement and directionality of select motor-cargo complexes on distinct microtubule tracks.