Discovering the bacterial cytoskeleton that shapes cellular architecture and offers new antibiotic targets
Imagine a world where buildings constructed themselves, guided by an internal skeleton that sensed and responded to the environment. This isn't science fiction—it's the daily reality inside countless bacterial cells. For centuries, we viewed bacteria as simple bags of enzymes, but revolutionary discoveries have revealed they possess a sophisticated internal skeleton much like our own cells. Among the most fascinating components of this cytoskeleton is MreB, a protein that acts as both architect and engineer of bacterial shape.
Bacteria exhibit remarkable morphological diversity, from rods to spheres to spirals, each shape optimized for specific environmental niches.
MreB proteins monitor and maintain cell form against tremendous internal pressure while sensing environmental changes.
For decades, scientists believed that cytoskeletons—internal protein frameworks that shape cells—were exclusive to complex eukaryotic organisms. This assumption collapsed in 2001 when researchers discovered that bacteria contain their own structural proteins, with MreB emerging as a striking analog of eukaryotic actin2 .
Though MreB shares only about 15% of its amino acid sequence with actin, their three-dimensional structures are remarkably similar. Both proteins contain conserved ATP-binding motifs and form filaments through polymerization. However, unlike actin's parallel double helix, MreB assembles into antiparallel double protofilaments that generate mechanical forces on bacterial membranes2 .
Structural and functional comparison between MreB and eukaryotic actin
At the molecular level, MreB exhibits both elegant simplicity and sophisticated adaptability. The protein contains ATP-binding pockets with critical residues (T15, A16, N17, G161, G162, T163, E209, K212, G289) spatially arranged to drive polymerization2 . This nucleotide-dependent assembly allows MreB filaments to dynamically form and disassemble as cellular conditions change.
Bacillus subtilis presents a particularly interesting case with not one but three MreB paralogs: MreB proper, Mbl (MreB-like), and MreBH (MreB homolog). These variants form mixed filaments in vivo and collaborate to maintain cell shape, though each may have specialized functions1 5 . This redundancy provides robustness to the system while allowing functional diversification—a hallmark of evolutionary success.
MreB's primary role in Bacillus subtilis resembles that of a construction foreman coordinating a building project. It organizes the elongasome complex—a team of proteins including MreC, MreD, RodA, and PBP2 that work together to insert new peptidoglycan (the mesh-like material comprising the bacterial cell wall) in an organized manner3 .
Without MreB's guiding influence, this construction crew loses direction. Depleting MreB or disrupting its function with inhibitors like A22 causes rod-shaped cells to grow as spheres instead of maintaining their elongated form3 4 . These spherical cells eventually rupture, unable to withstand their internal pressure—clear evidence that MreB is essential for viability under normal conditions.
One of the most fascinating discoveries about MreB is its dynamic behavior within living cells. Super-resolution microscopy has revealed that MreB filaments don't remain stationary but instead move circumferentially around the cell width, perpendicular to the long axis7 .
This directional motion isn't random—it follows the path of greatest membrane curvature. In rod-shaped cells, this means moving around the circumference, but when cells are artificially rounded, MreB motion becomes isotropic. Restore the rod shape, and MreB immediately resumes its circumferential travel7 .
This elegant feedback system allows MreB to both sense and reinforce cell shape, creating a self-organizing mechanism for morphogenesis.
| Paralog Name | Key Characteristics | Primary Functions |
|---|---|---|
| MreB | Primary cytoskeletal element | Cell shape maintenance, coordinating elongasome |
| Mbl (MreB-like) | Forms dynamic filaments | Cell wall synthesis, chromosome segregation |
| MreBH (MreB homolog) | Membrane-associated | Cell wall synthesis, possibly other specialized roles |
To understand how MreB proteins self-organize, researchers designed an elegant experiment using a heterologous cell system. They expressed Bacillus subtilis MreB paralogs in Drosophila S2 Schneider cells—a completely different cellular environment lacking bacterial factors1 . This approach allowed them to determine whether MreB could form filaments without its normal bacterial partners.
The research team engineered the fly cells to produce YFP-tagged MreB, CFP-tagged Mbl, and mCherry-tagged MreBH, either individually or in combination. Using advanced fluorescence microscopy, they tracked the localization and behavior of these proteins in real-time1 . They also tested how modifying the ATPase domain or depleting cellular ATP affected filament formation.
Express MreB paralogs in Drosophila cells
Use fluorescent protein tags (YFP, CFP, mCherry)
Track localization with advanced microscopy
Modify ATPase domain and deplete ATP
The results were striking. Despite being in an alien environment, all three MreB paralogs assembled into filamentous structures underneath the cell membrane. When the ATPase motif was modified, these filaments became highly stable, while ATP depletion caused rapid filament dissociation—clear evidence of ATP-dependent polymerization similar to eukaryotic actin1 .
Even more remarkably, when co-expressed, the three paralogs formed mixed filaments, demonstrating their ability to directly interact and co-polymerize. Extended MreB expression created membrane protrusions, proving that MreB can generate mechanical force against membranes1 . Perhaps most importantly, the bacterial membrane protein RodZ, which localized to endosomes when expressed alone, was recruited to the cell membrane when co-expressed with Mbl—showing that MreB structures can actively reposition other proteins.
This experiment demonstrated that MreB paralogs contain an inherent ability to self-organize into a dynamic, membrane-associated filamentous scaffold that can recruit proteins to specific cellular locations—fundamental properties of a true cytoskeletal system.
| Experimental Condition | Observation | Interpretation |
|---|---|---|
| MreB expressed alone | Formed dynamic membrane-associated filaments | Self-polymerization capability |
| Modified ATPase domain | Highly stable filaments | Normal ATPase activity required for filament dynamics |
| ATP depletion | Rapid filament dissociation | ATP-dependent polymerization |
| Co-expression of paralogs | Formation of mixed filaments | Capacity for co-polymerization |
| Extended MreB induction | Membrane protrusions formed | Ability to generate mechanical force |
| RodZ with Mbl | Recruitment to cell membrane | Capability to organize other proteins |
Bacteria constantly face changing environmental conditions, and MreB plays a key role in adapting to these challenges. When Bacillus subtilis experiences osmotic stress—sudden changes in solute concentration—MreB filaments undergo rapid remodeling9 .
In response to high osmolarity, MreB filaments partially disassemble, releasing monomers that diffuse freely throughout the cell. This change happens in parallel with similar changes in RodZ, an MreB-interacting membrane protein. However, this remodeling isn't mediated by RodZ itself, as it occurs even in RodZ's absence9 .
MreB filament integrity during osmotic stress
The disassembly of MreB filaments following osmotic shock depends directly on potassium ions. Mutant strains defective in potassium uptake fail to disassemble MreB filaments after osmotic shock, showing less perturbed cell wall extension than wild-type cells9 .
This finding aligns beautifully with in vitro studies showing that monovalent ions (like K+) inhibit MreB polymerization, while divalent ions (like Mg2+) promote it5 . The intracellular potassium surge that follows osmotic stress likely directly dissolves MreB polymers, providing a rapid mechanism to slow cell wall extension during adaptation.
This dynamic remodeling represents a sophisticated adaptation strategy: by modifying its cytoskeleton in response to ionic changes, the cell can temporarily reduce elongation rates while it allocates resources to stress survival, then resume normal growth once conditions improve.
| Factor | Effect on MreB | Biological Significance |
|---|---|---|
| ATP | Promotes polymerization | Energy-dependent filament assembly |
| Mg2+ (divalent cation) | Induces/stimulates polymerization | Facilitates filament formation |
| K+ (monovalent cation) | Inhibits polymerization | Prevents excessive filament formation; mediates stress response |
| Membrane binding | Localizes polymerization | Targets activity to cell periphery |
| RodZ interaction | Regulates dynamics | Anchors filaments to membrane |
Studying a complex system like MreB requires specialized tools. Over years of research, scientists have developed a sophisticated toolkit for probing MreB's structure and function:
A third-generation MreB inhibitor with enhanced broad-spectrum activity against Gram-negative pathogens. Inhibits MreB ATPase activity via a noncompetitive mechanism, making it particularly effective4 .
Artificial lipid bilayers that allow in vitro reconstitution of MreB polymerization, revealing how filaments interact with membranes without cellular complexity5 .
Advanced microscopy technique that follows individual MreB molecules, distinguishing between polymerized and freely diffusing populations to quantify assembly states9 .
Reveals detailed ultrastructure of MreB filaments, showing they form bundles approximately 70nm wide that can branch and fuse5 .
The essential role of MreB in bacterial shape maintenance makes it an attractive target for novel antibiotics. As antibiotic resistance reaches crisis levels, targeting bacterial-specific pathways like the MreB-directed elongasome offers hope for tackling multidrug-resistant pathogens2 4 .
Pharmaceutical researchers have developed increasingly sophisticated MreB inhibitors. The evolution from A22 to CBR-4830 to TXH11106 represents a progression of compounds with enhanced bactericidal activity against dangerous Gram-negative pathogens like Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa4 .
Development timeline of MreB inhibitors with increasing efficacy
Perhaps most promisingly, MreB inhibitors show strong synergistic effects when combined with conventional antibiotics. A22 demonstrates significant synergy with ceftazidime and meropenem against P. aeruginosa and with cefoxitin and azithromycin against E. coli6 .
These combinations effectively inhibit biofilm formation and eradicate mature biofilms—particularly valuable since biofilms are notoriously resistant to conventional antibiotics. The combinations show no cytotoxic or hemolytic effects toward human cells, highlighting their potential therapeutic window6 .
Efficacy against P. aeruginosa biofilm formation
The discovery and characterization of MreB proteins in Bacillus subtilis and other bacteria represents a fundamental shift in our understanding of the bacterial cell. Once considered simple undifferentiated bags of enzymes, we now recognize bacteria as possessing sophisticated internal organization, with MreB serving as both structural scaffold and dynamic coordinator of cellular architecture.
As research continues, key questions remain: How exactly do MreB filaments sense membrane curvature? What determines the specific roles of different MreB paralogs? How can we leverage our growing knowledge to develop effective anti-MreB therapeutics that overcome antibiotic resistance?
What's clear is that MreB research has taken a fascinating new turn—one that continues to reshape our understanding of cellular life at its most fundamental level. The humble bacterial cytoskeleton may well hold clues not just to combating pathogenic bacteria, but to understanding the universal principles of cellular organization across all domains of life.