The 2019 Cytoskeleton Paper of the Year revealed a fundamental revision in mammalian septin complex structure
Imagine a master carpenter who has meticulously built furniture for years, only to discover that the fundamental blueprint of every chair and table was backwards. This is exactly what happened in the world of cell biology in 2019, when a team of scientists made a startling discovery about septins—essential proteins that form a crucial part of the cellular "skeleton." Their finding, honored as the 2019 Cytoskeleton Paper of the Year, revealed that the long-accepted arrangement of subunits in mammalian septin complexes was precisely opposite to what researchers had believed for nearly two decades. This reversal wasn't just an academic correction—it fundamentally reshaped our understanding of how cells build their internal structures, with far-reaching implications for everything from cancer research to fertility studies.
"The correct assembly for the canonical combination of septins 2-6-7 is therefore established to be SEPT2-SEPT6-SEPT7-SEPT7-SEPT6-SEPT2, implying the need for revision of the mechanisms involved in filament assembly" 6 .
The revision emerged from two synchronized studies that independently reached the same startling conclusion. This inside-out rearrangement solved mysteries that had puzzled scientists for years and revealed that the structure of these essential cellular components is far more conserved across species than anyone had suspected.
Most of us learn in biology class that cells have three main types of "skeletal" structures that give them shape and organization: microfilaments, intermediate filaments, and microtubules. But there's a fourth player—septins—that has emerged as equally crucial, though less famous. These GTP-binding proteins act as the architects and construction managers of the cell, assembling into sophisticated scaffolds that guide cellular organization 2 .
Recruit and organize other proteins 2
Compartmentalize the plasma membrane into specialized domains 2
Recognize and bind to curved cellular membranes 8
Guide the movement of vesicles and other cellular components
When septins malfunction, the consequences can be severe. These proteins have been linked to a startling array of diseases, including male infertility, cancer, and neurodegenerative diseases 2 . For example, in neurons, septins regulate calcium entry channels at the plasma membrane, affecting neurotransmitter release 8 . Mutations in septin genes can disrupt fundamental processes like cell division—a hallmark of cancer—or interfere with the precise cellular architecture required for sperm function or brain activity.
For years, textbooks described the mammalian septin hexamer (a six-part complex) with the order: SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7 5 6 . This arrangement placed SEPT7 proteins at both ends of the rod-shaped complex. The octamer (eight-part complex), which includes SEPT9, was thought to follow a similar pattern with SEPT9 at the termini 5 .
But this model had troubling inconsistencies. First, it placed evolutionarily similar septins from different species in opposite positions within their respective complexes. SEPT9 in mammals is structurally similar to Cdc10 in yeast, yet the accepted models placed them at opposite ends of their complexes 5 . Second, the model created a logistical nightmare for filament formation—if hexamers ended with SEPT7's G interface and octamers ended with SEPT9's NC interface, they wouldn't be able to connect properly, suggesting cells would need two separate filament systems 5 . This seemed unnecessarily complicated for efficient cellular organization.
Two research groups independently stumbled upon the same revolutionary discovery using complementary approaches. The critical breakthrough came when scientists decided to directly visualize the location of specific septins within the complexes using electron microscopy and other biochemical techniques 5 6 .
Researchers created engineered versions of SEPT2, SEPT5, and SEPT7 with molecular "tags" (like MBP or GFP) that would be visible under electron microscopy 6 .
They isolated complete septin complexes from human cells, ensuring they were working with intact, properly assembled structures 5 .
Using transmission electron microscopy (TEM), they directly visualized where each tagged septin located within the complexes 6 .
They tested whether different complexes (hexamers and octamers) could copolymerize into filaments, which shouldn't be possible under the old model 5 .
The results were unequivocal—and startling. The tags consistently showed that SEPT2 (or its substitute SEPT5) occupied the terminal positions, not SEPT7 as previously thought 6 . This meant the complex was actually arranged in the reverse order: SEPT2-SEPT6-SEPT7-SEPT7-SEPT6-SEPT2 6 .
Even more convincing was the polymerization evidence. Researchers found that hexamers and octamers could readily copolymerize into long filaments 5 8 —something that should have been impossible according to the old model. This functional evidence strongly supported the revised arrangement.
| Feature | Old Model (Pre-2019) | Revised Model (2019) |
|---|---|---|
| Hexamer Order | SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7 | SEPT2-SEPT6-SEPT7-SEPT7-SEPT6-SEPT2 |
| Terminal Subunit | SEPT7 | SEPT2 |
| Octamer-Hexamer Compatibility | Predicted incompatible | Compatible, can copolymerize |
| Evolutionary Conservation | Differed between yeast and mammals | Highly conserved from yeast to mammals |
| Filament Polymerization Interface | G interface | NC interface |
Table 1: Comparison of Old vs. Revised Septin Models
Unraveling the septin mystery required a sophisticated set of research tools. Here are some of the essential reagents and methods that enabled this discovery:
| Tool | Function in Septin Research | Key Insight |
|---|---|---|
| Recombinant Septin Complexes | Purified septins assembled into defined complexes for in vitro study | Enabled direct testing of polymerization without cellular variables 5 |
| Transmission Electron Microscopy (TEM) | Visualized septin complexes at molecular resolution | Directly revealed subunit order within complexes 6 |
| Blue Native PAGE | Separated protein complexes by size and shape under native conditions | Confirmed presence of both hexamers and octamers in cells 5 |
| Stable Cell Lines | Engineered cells expressing tagged septin subunits | Allowed purification of specific complex types from cellular environment 5 |
| Borg3 BD3 Domain | Binds septin filaments and causes distinctive clustering | Served as diagnostic tool for different filament types 5 |
Table 2: Essential Research Tools for Septin Studies
Each tool in the researcher's arsenal contributed a unique piece of the puzzle. Blue Native PAGE confirmed that cells contained both hexameric and octameric septin complexes, and that introducing different septin subunits could shift this balance 5 . Stable cell lines expressing tagged septins allowed researchers to pull out specific complexes and analyze their composition, confirming which subunits were present 5 .
Most importantly, electron microscopy provided the visual proof that overturned the old model. When researchers attached bulky tags to different septins and viewed them under the microscope, they could literally see which subunits were at the ends of the complexes 6 . Meanwhile, in vitro polymerization assays with purified components demonstrated that the revised arrangement made functional sense—complexes could now properly assemble into filaments.
The revised septin model did more than just correct a structural diagram—it revealed a profound evolutionary conservation between species. The new arrangement shows that mammalian septins assemble according to the same fundamental principles as their yeast counterparts 5 8 . Specifically, the terminal positions in both yeast and mammalian complexes are occupied by members of the SEPT2 group (Cdc11 in yeast, SEPT2 in mammals), while the central positions are filled by SEPT7 group members (Cdc3 in yeast, SEPT7 in mammals) 8 .
This conservation suggests that the fundamental architecture of septin complexes was established early in eukaryotic evolution and has been maintained across billions of years of evolutionary divergence. What appears at first glance to be a simple correction of a molecular diagram actually represents a significant insight into how evolution conserves successful cellular engineering solutions.
The revised model has practical implications for understanding cellular processes and developing therapeutic interventions:
The corrected architecture means that filament polymerization occurs through NC interfaces rather than G interfaces, explaining why filament formation is sensitive to salt concentrations 6 . This helps researchers understand how cells regulate septin assembly and disassembly.
With the proper architecture in hand, scientists can now more accurately model how disease-causing mutations disrupt septin function. For example, mutations that affect the NC interface would be expected to disrupt filament formation.
The discovery that different septin complexes can copolymerize 5 suggests new strategies for therapeutic intervention. Rather than targeting all septin function, it might be possible to develop compounds that specifically disrupt certain complex types or their interactions.
Understanding the precise rules of septin assembly opens possibilities for designing synthetic septin-like structures that could organize cellular components in engineered systems.
| Biological Process | Impact of Revised Septin Model |
|---|---|
| Cell Division | Clarifies how septin rings guide cytokinesis and split into double rings during cell separation 2 9 |
| Neuronal Function | Provides better understanding of how septins organize membrane domains crucial for neurotransmitter release 8 |
| Fertility | Illuminates structural basis for septin function in sperm formation and motility 8 |
| Infection Response | Reveals how septins recognize and cage invading pathogens 8 |
Table 3: Impact of the Revised Model on Different Biological Processes
The revision of the septin subunit order stands as a powerful reminder that science is a continuously evolving process of discovery and correction. What we "know" today may be refined—or even overturned—by tomorrow's evidence. As one researcher noted, this revision "implies the need for revision of the mechanisms involved in filament assembly" 6 —a statement that applies not just to septins, but to all scientific inquiry.
The 2019 Cytoskeleton Paper of the Year represents both a culmination and a beginning. It solves longstanding puzzles in septin biology while opening new avenues of investigation.
The septin story continues to unfold, with researchers now building on the revised model to explore these questions and more. What remains constant is the spirit of scientific inquiry—the willingness to question established models, the creativity to develop new methods, and the collaboration to piece together evidence from multiple sources. In the end, the inside-out revolution in septin biology isn't just about protein structure; it's about the very process of discovery that drives science forward.