Prokaryotes absolutely have a cytoskeleton, and the discovery of that fact overturned one of biology’s most confidently held assumptions. Until the early 1990s, textbooks treated the cytoskeleton as a defining feature of complex cells, something bacteria and archaea simply lacked. Then, in 1992, three independent labs showed that a well-known bacterial cell-division protein called FtsZ carried a signature sequence found in all tubulins, and candidates for bacterial actins were identified in the same year. Since then, crystal structures have confirmed that bacteria and archaea possess proteins related to all three major branches of the eukaryotic cytoskeleton, plus at least one class of filament-forming protein that has no counterpart in animal or plant cells.
How the Old Textbook Story Fell Apart
For decades, the cytoskeleton was presented as one of the clearest dividing lines between prokaryotes and eukaryotes. Eukaryotic cells had actin filaments, microtubules, and intermediate filaments to maintain shape, move cargo, and divide. Prokaryotic cells were supposedly too small and simple to need any of that internal scaffolding. The story was tidy and wrong. In 1992, researchers recognized that FtsZ, a protein bacteria had been using to divide all along, shared a critical amino-acid motif with all known forms of tubulin. That same year, computational searches flagged several bacterial proteins as possible actin relatives. X-ray crystallography later confirmed these structural relationships beyond any reasonable doubt.
FtsZ and Other Tubulin Relatives
FtsZ is the most widespread tubulin-like protein in bacteria. It assembles into a ring, called the Z ring, at the future site where a bacterial cell will split in two. That ring forms the core of the division machinery, and its assembly is carefully regulated throughout the cell cycle. The Z ring, along with helper proteins FtsA and ZipA, generates the contractile force needed to pinch the cell in half.
Nearly all bacteria carry at least one form of FtsZ, making it one of the most conserved proteins on Earth. But bacteria of the genus Prosthecobacter carry something even more surprising: genes for proteins called BtubA and BtubB that are closer in sequence to eukaryotic alpha and beta tubulin than they are to FtsZ. Structural studies showed that BtubA and BtubB have a fold strikingly similar to tubulin, including surface loops, and the two proteins can form tubulin-like filaments. Their properties differ from standard tubulin in some ways, including weaker pairing and the ability to fold without the chaperone proteins eukaryotic tubulin requires. The leading explanation is that these genes were picked up through horizontal gene transfer from a eukaryote at some point in Prosthecobacter’s evolutionary past rather than inherited from a common ancestor.
MreB and the Actin Side of the Family
If FtsZ is the bacterial tubulin, MreB is the bacterial actin. MreB is found in most rod-shaped bacteria, where it plays a central role in maintaining that elongated form. Without MreB, rod-shaped bacteria become bloated, lemon-shaped, or spherical, and their cell walls lose structural integrity. MreB assembles into short filaments that move along the inner face of the cell membrane, and this motion helps guide where new cell-wall material gets inserted. Simulations suggest that by distributing new wall-building sites evenly around the cell, MreB rotation keeps the cell growing as a uniform rod rather than bulging unevenly.
In the bacterium Caulobacter crescentus, removing MreB caused cells to lose their rod shape and develop defective cell walls. MreB filaments in that organism appear to serve as a scaffold that positions the enzymes responsible for building the peptidoglycan layer, the tough mesh that gives bacterial cell walls their strength. This organizing role mirrors how actin filaments in animal cells provide tracks and scaffolding for a wide range of molecular machinery. Beyond shape, MreB also participates in chromosome segregation and is increasingly viewed as an attractive target for new antibiotics precisely because it is essential and has no close equivalent in human cells.
ParM and DNA Segregation
MreB is not the only bacterial actin. ParM is another actin-like protein, but it does something quite different: it segregates DNA. Specifically, ParM pushes copies of certain low-copy-number plasmids to opposite ends of the cell so that each daughter cell inherits one after division. ParM filaments grow bidirectionally and display dynamic instability, meaning they switch between phases of rapid growth and rapid shrinkage. That behavior is strikingly reminiscent of how microtubules behave in eukaryotic cells during chromosome separation, yet ParM is structurally an actin relative, not a tubulin one. The dynamic instability appears to have arisen independently in the two protein families, a case of convergent evolution driven by the shared physical problem of reliably partitioning DNA.
In the ParMRC system of the E. coli R1 plasmid, ParM forms a bipolar spindle of at least two antiparallel filaments that elongates to push paired plasmid clusters toward opposite cell poles. The resemblance to a eukaryotic mitotic spindle is hard to miss, even though the proteins involved are unrelated to tubulin. This is one of the clearest examples of how evolution can arrive at similar mechanical solutions through completely different molecular starting points.
Crescentin and Intermediate Filament-Like Proteins
The third major class of eukaryotic cytoskeletal protein, the intermediate filament, also has bacterial counterparts. The best-studied example is crescentin, found in Caulobacter crescentus. Without crescentin, these cells lose their characteristic curved or helical shape and become straight rods. Crescentin shares key structural features with eukaryotic intermediate filament proteins, including the ability to self-assemble into filaments without needing energy input or cofactors. Inside the cell, crescentin forms a structure along one side of the inner membrane, and by imposing its own helical geometry on the growing cell, it bends the cell into a curved or corkscrew shape depending on the cell’s length.
Detailed structural work has shown that the rod domain of crescentin is critical for filament assembly, and that mutations disrupting specific regions of the protein result in partial or complete loss of cell curvature. When crescentin was expressed in E. coli, which normally grows as a straight rod, those cells became curved, confirming that crescentin alone is sufficient to impose curvature on a cell that would otherwise grow straight.
Crescentin is not a one-off. A computational survey across diverse bacterial genomes found proteins with the hallmark structural feature of intermediate filaments, a segmented coiled-coil rod domain, in at least 21 genomes spanning 26 phylogenetically diverse species. In the branching bacterium Streptomyces coelicolor, one such protein called FilP forms cytoskeletal networks in the growing tips of the organism’s filamentous hyphae. Atomic force microscopy of living cells showed that FilP contributes measurably to the mechanical stiffness of the hyphae, closely resembling the structural role of intermediate filaments in animal tissues. FilP can self-assemble into a regular, interconnected network in the lab, and in vivo it localizes to the hyphal tip, where the cell wall is newest and most mechanically vulnerable.
Bactofilins, a Class of Their Own
Not every bacterial cytoskeletal protein fits neatly into a eukaryotic category. Bactofilins are a class of small, beta-helical proteins found broadly across bacteria that polymerize spontaneously into stable ribbons or rod-like filament bundles without needing any cofactors. In Caulobacter crescentus, two bactofilin proteins cooperate to form a sheet-like structure lining the cytoplasmic membrane near the stalked pole of the cell, where they recruit a peptidoglycan-building enzyme involved in stalk formation. In other bacteria, bactofilins associate with the cell-division apparatus or form rod-shaped filaments involved in chromosome segregation and motility.
Bactofilins have been described as an alternative to intermediate filaments, serving as versatile molecular scaffolds across a range of cellular processes. Their wide distribution suggests they evolved early in bacterial history and have been repurposed many times for different jobs. The structural basis for their filament formation, a beta-helical fold, is unlike anything in the eukaryotic cytoskeletal toolkit, which makes bactofilins genuinely novel rather than distant cousins of a familiar protein family.
Organizing the Division Site
One of the most sophisticated things the bacterial cytoskeleton does is ensure that cell division happens in the right place. In E. coli, this is handled by the Min system, a set of three proteins (MinC, MinD, and MinE) that oscillate back and forth inside the cell from pole to pole. This oscillation creates a time-averaged concentration gradient that is lowest at the cell’s midpoint. Because MinC inhibits Z-ring assembly, the net effect is that the Z ring can only form at midcell, exactly where division should occur. The Min system is closely related to ParA-type proteins involved in spatial organization throughout bacteria, suggesting that this kind of reaction-diffusion positioning mechanism is a recurring theme in prokaryotic cell biology.
Archaea Have Their Own Cytoskeletal Toolkit
Bacteria are not the only prokaryotes with a cytoskeleton. Archaea, the other major domain of prokaryotic life, have cytoskeletal proteins too, and some of these are even more closely related to eukaryotic versions than their bacterial counterparts are.
Crenactin, found in certain members of the TACK superphylum of archaea, forms helical structures inside cells and is the closest known prokaryotic relative of eukaryotic actin. In Pyrobaculum calidifontis, immunostaining revealed crenactin organized into helical filaments within the cell, and the gene belongs to a conserved cluster correlated with rod-shaped and filamentous cell forms. The phylogenetic distribution of crenactin supports the idea that the eukaryotic actin cytoskeleton has deep roots in the archaeal lineage.
Even more striking are the Asgard archaea, a group that sits on the evolutionary branch closest to eukaryotes. Imaging of an Asgard archaeon called Candidatus Lokiarchaeum ossiferum revealed cells with elaborate branching protrusions and a long-range cytoskeleton extending throughout the cell body and into those protrusions. The filaments had a twisted double-stranded architecture consistent with F-actin, the polymerized form of actin that builds the cytoskeleton in eukaryotic cells. The organism’s genome encodes four actin homologues, and immunostaining pointed to one called Lokiactin as a major component of its filament network. The researchers proposed that a complex actin-based cytoskeleton predated the emergence of the first eukaryotic cells and was crucial in the evolution of the Asgard lineage by supporting elaborate cellular structures.
Cell division in archaea also tells an interesting story. While most prokaryotes divide using FtsZ rings, some archaea use an entirely different system based on ESCRT-III-like proteins, the Cdv system. These proteins are genuine structural relatives of the ESCRT-III machinery that eukaryotic cells use during membrane remodeling and the final step of cell division. In crenarchaea, the CdvA protein forms double-helical filaments that contain DNA and interact with the ESCRT-III-like CdvB protein, potentially helping to coordinate genome segregation with cell splitting. This means that within the prokaryotic world, at least two fundamentally different cytoskeletal strategies for cell division have evolved.
Organelle Positioning in Bacteria
One of the more visually dramatic roles for the bacterial cytoskeleton involves magnetosomes, the tiny membrane-bound iron mineral crystals that magnetotactic bacteria use to orient themselves along Earth’s magnetic field. For these compass-like organelles to work, they need to be lined up in a chain rather than scattered randomly through the cell. That alignment is achieved by MamK, an actin-like protein that polymerizes into filaments to which magnetosomes are anchored. In Magnetospirillum magneticum, MamK works together with a related protein called MamK-Like, and both are required for proper chain formation. The result is a string of tiny magnets neatly arranged along the cell’s long axis, giving the bacterium a reliable internal compass.
This is a case where the bacterial cytoskeleton does something with no direct parallel in eukaryotic biology. Eukaryotic cells use their cytoskeletons to position organelles, of course, but no eukaryote builds a magnetic navigation system out of membrane-bound mineral crystals tethered to actin-like filaments. It is a reminder that prokaryotic cytoskeletal proteins have been adapted to fill ecological niches that eukaryotes simply do not occupy.
Driving Bacterial Motility
Some bacteria glide across surfaces without flagella, and their cytoskeleton is involved here too. In Myxococcus xanthus, gliding motility depends on large multiprotein complexes, regulatory proteins, and cytoskeletal filaments working together. Studies combining fluorescence imaging and force microscopy showed that membrane-bound complexes of motor and regulatory proteins are transported directionally along the inside of the cell at a constant velocity. That intracellular motion is somehow transmitted to the cell’s exterior, generating traction against the surface and propelling the cell forward.
The actin-like MreB cytoskeleton plays a direct part in this system. In myxobacteria, a helical protein complex called AgmU rotates within the cell, and that rotation depends on MreB filaments and the proton motive force, the same electrochemical gradient that powers the bacterial flagellar motor. The emerging model is that motor complexes similar to flagellar stator units run along a helical cytoskeletal track, driving its rotation and generating pressure waves in the slime layer that push the cell forward. It is a remarkably sophisticated piece of machinery for an organism with no internal membrane-bound organelles.
How Researchers See Cytoskeletal Filaments in Tiny Cells
Visualizing structures inside cells that are only about a micrometer across has always been a challenge, and the prokaryotic cytoskeleton went undetected for so long partly because the tools to see it did not exist. Fluorescence microscopy with tagged proteins allowed researchers to watch cytoskeletal dynamics in living bacteria for the first time, revealing MreB rotation and Min protein oscillation in real time. But the real breakthrough for structural detail has been cryo-electron tomography, which flash-freezes cells and images them in three dimensions at resolutions of a few nanometers without any chemical fixation or staining that might distort the delicate filaments.
Recent advances in cryo-focused ion beam milling, which shaves frozen bacterial cells down to slices thin enough for high-resolution imaging, have opened up new views of cytoskeletal filament assembly, intracellular organelles, and the spatial relationships between them. It was cryo-electron tomography that provided the detailed images of actin-like filaments in Asgard archaea and the structural architecture of crescentin filaments, findings that would have been impossible with older techniques. As the resolution of these methods continues to improve, more subtle cytoskeletal structures are likely waiting to be discovered in organisms whose internal organization has barely been examined.
Why Bacterial Cytoskeletal Proteins Interest Drug Developers
Because FtsZ and MreB are essential for bacterial survival and have no close counterparts in human cells, they represent appealing targets for new antibiotics. A drug that blocks FtsZ assembly would prevent bacteria from dividing; one that disrupts MreB could cause cells to lose their shape and burst. Several experimental compounds targeting FtsZ are in various stages of development, and MreB inhibitors have shown activity in laboratory settings. The appeal is selectivity: since human cells rely on tubulin and actin rather than FtsZ and MreB, a well-designed inhibitor should, in principle, harm bacteria without damaging the patient’s own cells. Whether that selectivity holds up in practice is the question clinical development is trying to answer.
Evolutionary Roots of the Eukaryotic Cytoskeleton
The prokaryotic cytoskeleton is not just a curiosity for microbiologists. It reshapes how we think about the origin of complex cells. The eukaryotic cytoskeleton evolved from prokaryotic filament-forming proteins, and the diversity of those prokaryotic systems hints at how rich the starting material was. Prokaryotic filament systems show structural and dynamic complexity that, in many respects, foreshadows the self-organizing properties of the eukaryotic cytoskeleton. FtsZ and MreB are deeply diverged from tubulin and actin in sequence, but their core folds and polymerization mechanisms are clearly related. One explanation for the extensive divergence is that tubulin and actin were freed from the specific jobs FtsZ and MreB perform in bacteria, like cell division and wall synthesis, and diversified rapidly as they took on the many new roles demanded by the larger, more complex eukaryotic cell.
The Asgard archaea findings are particularly provocative. If an archaeon closely related to the ancestor of eukaryotes already had a complex actin-based cytoskeleton supporting branching cell protrusions, then much of what we consider characteristically eukaryotic cellular architecture may have been assembled from pre-existing parts rather than invented from scratch. The prokaryotic cytoskeleton, in other words, is not a pale imitation of the eukaryotic one. It is the ancestral toolkit from which the eukaryotic version was built.