Septins: Key Players in Cytoskeletal Structure and Function

Cells rely on a dynamic cytoskeleton to maintain their shape, organize internal components, and facilitate essential processes like division and transport. While actin filaments and microtubules are well-known structural elements, septins are a less familiar but equally important component. These GTP-binding proteins assemble into filaments and contribute to diverse cellular functions.

Understanding septins is crucial as they influence key biological processes, from cell division to membrane organization. Researchers continue to explore how these proteins interact with other cytoskeletal elements and their potential roles in disease.

Structural Organization In Cells

Septins are unique cytoskeletal components, forming hetero-oligomeric complexes that assemble into filaments. Unlike actin and microtubules, which polymerize linearly, septins exhibit a modular organization where monomers associate into ordered complexes before integrating into larger structures. These complexes typically consist of specific septin isoforms arranged in a defined sequence, dictating their assembly properties and functions.

X-ray crystallography and cryo-electron microscopy have shown that septin monomers interact through conserved GTP-binding domains, stabilizing filaments and regulating their dynamics in response to cellular cues. Their arrangement within cells varies by organism and cell type. In yeast, septins form a ring at the bud neck, serving as a scaffold for division-related proteins. In mammalian cells, they localize to structures such as the cleavage furrow and cell cortex, influenced by post-translational modifications like phosphorylation and sumoylation.

Septins also contribute to intracellular compartmentalization by acting as diffusion barriers. In polarized cells, they create boundaries that restrict membrane-associated protein movement, ensuring proper localization of signaling molecules and transport vesicles. In neurons, septins help maintain distinct axonal and dendritic membrane compositions. Disrupting septin filaments leads to aberrant protein distribution, underscoring their role in cellular organization. Additionally, septins preferentially bind to membranes with specific curvature, affecting their localization and function.

Formation Of Higher-Order Assemblies

Septins organize into higher-order assemblies beyond individual filaments, forming bundled arrays, rings, and meshwork structures that support cellular architecture. These arrangements depend on septin subunit composition, binding partners, and biophysical factors like membrane curvature and tension. Structural studies show that septin filaments align in parallel or form orthogonal networks, allowing them to act as scaffolds that reinforce cell shape and compartmentalize cytoplasmic regions.

Their assembly is regulated by nucleotide binding and hydrolysis, with GTP playing a central role in filament stability. Unlike actin and tubulin, which rely on ATP and GTP hydrolysis for polymerization, septins exhibit a more nuanced relationship with nucleotide state. Some subunits retain GTP, while others cycle between GTP- and GDP-bound forms, influencing filament interactions. Mutations in GTP-binding sites disrupt ordered assembly, leading to fragmented filaments.

Membrane association further refines septin organization by providing anchoring sites. Septins preferentially bind curved membranes, concentrating at regions of cellular constriction. Studies using lipid bilayers and artificial vesicles show that septin filaments align along membrane surfaces with specific curvature thresholds, reinforcing their role in stabilizing dynamic cellular interfaces. In cells, septins localize at membrane invaginations, contributing to vesicle trafficking and cell division. Their recruitment is guided by phosphoinositides, ensuring septin assemblies form where structural support is needed.

Function In Cytokinesis

Septins are essential in cytokinesis, the final step of cell division where the cytoplasm is physically separated into two daughter cells. They localize to the division site early, establishing a framework that coordinates membrane remodeling and contractile forces. In yeast and mammalian cells, septins assemble into a ring at the cleavage furrow, serving as a scaffold for actomyosin ring constriction.

Live-cell imaging shows septins remain closely associated with the contractile apparatus, providing mechanical support as the ring tightens. Disrupting septin function through genetic knockouts or pharmacological inhibition leads to defects in furrow ingression, often resulting in incomplete division. Septins also influence membrane trafficking, ensuring targeted vesicle delivery for abscission.

Roles In Membrane Dynamics

Septins shape cellular membranes, influencing curvature, stability, and organization. Their selective association with membranes allows them to regulate dynamic remodeling at sites like filopodia, lamellipodia, and intracellular vesicles. This interaction is mediated by their affinity for phosphoinositide-enriched domains, which serve as anchoring points. Studies using reconstituted lipid bilayers show septins preferentially bind to membranes with high curvature, reinforcing tubular or constricted regions under mechanical stress.

Septins also organize protein complexes involved in membrane trafficking, interacting with the exocyst and SNARE machinery to facilitate vesicle docking and fusion. In polarized cells, they regulate vesicular transport to maintain specialized domains. In epithelial cells, for example, septins help establish apical-basal polarity. Their depletion disrupts directed trafficking, leading to mislocalized proteins and altered cell morphology. In neurons, septins influence synaptic vesicle dynamics, ensuring precise neurotransmitter release.

Coordination With Other Cytoskeletal Proteins

Septins coordinate with actin filaments and microtubules to regulate cell shape, transport, and mechanical integrity. They serve as physical barriers that compartmentalize actin networks, restricting filament turnover and influencing processes like migration and adhesion. In fibroblasts, septins localize to the leading edge, organizing actin-rich protrusions. Disrupting septins impairs actin remodeling and directional movement.

Their relationship with microtubules is equally intricate. Septins interact with microtubule-associated proteins, influencing spindle orientation and intracellular trafficking. During mitosis, they localize to centrosomes and spindle poles, ensuring accurate chromosome segregation. They also modulate motor protein activity, affecting organelle and vesicle transport. In neurons, septins regulate synaptic vesicle delivery by coordinating microtubule interactions with motor proteins like kinesin. These findings highlight septins as integrators of cytoskeletal function, bridging actin and microtubule networks.

Potential Links To Disease

Dysregulated septin function is implicated in neurodegenerative disorders, cancer, and infectious diseases. Mutations or altered expression disrupt cytoskeletal organization, leading to cellular dysfunction. In cancer, aberrant septin expression is observed across tumor types. Some isoforms act as oncogenes, while others function as tumor suppressors. SEPT9, for example, is a biomarker for colorectal cancer, where its hypermethylation correlates with disease progression. In breast and ovarian cancers, upregulated septin expression enhances invasive potential by altering cytoskeletal dynamics.

Septins also play a role in neurological disorders. They contribute to synaptic function and neuronal plasticity, and disruptions in their organization are linked to Alzheimer’s disease and schizophrenia. Mislocalized septin aggregates appear in Alzheimer’s patients, suggesting a role in tau pathology and synaptic dysfunction. In schizophrenia, septin-associated gene mutations may contribute to structural abnormalities in neuronal circuits.

Septins are also manipulated by pathogens. Some bacteria and viruses exploit septin filaments to evade immune detection or facilitate intracellular movement. Listeria monocytogenes, for example, uses septin-based structures to form protective cages that shield it from autophagic degradation. These diverse pathological associations highlight septins’ significance in cellular homeostasis and their potential as therapeutic targets.

Methods For Visualization And Analysis

Understanding septins requires advanced visualization and analysis techniques. Fluorescence microscopy, including super-resolution methods like STORM and SIM, provides detailed images of septin filaments. Genetically encoded fluorescent tags, such as GFP-septin fusions, allow researchers to track septin dynamics in live cells. However, fluorescence-based methods can introduce artifacts, necessitating complementary approaches.

Electron microscopy offers high-resolution structural insights into septin filaments. Cryo-electron tomography has been particularly useful in revealing how filaments interact with membranes and other cytoskeletal elements. Biochemical techniques such as co-immunoprecipitation and mass spectrometry identify septin-binding partners and post-translational modifications that regulate function. Single-molecule biophysics approaches, like atomic force microscopy, help probe septin filaments’ mechanical properties. These diverse methodologies continue to refine our understanding of septin biology.