Is a Cytoskeleton Present in Prokaryotes?

Prokaryotes do have a cytoskeleton, and the discovery that they possess one ranks among the bigger paradigm shifts in cell biology over the past few decades. Until 1992, the widespread assumption in textbook biology was that internal structural filaments belonged exclusively to eukaryotic cells. Then researchers found that bacteria carry proteins structurally related to tubulin, actin, and even intermediate filaments, the three pillars of the eukaryotic cytoskeleton. The prokaryotic versions are not identical copies, but their evolutionary kinship and functional overlap are now firmly established.

Why Textbooks Used to Say No

For most of the twentieth century, introductory biology courses drew a clean line: eukaryotic cells have a cytoskeleton; prokaryotic cells do not. The reasoning seemed sound. Bacteria are tiny, lack membrane-bound organelles, and appear structurally simple under a standard light microscope. Nobody could see internal filaments, and nobody had found proteins that obviously resembled tubulin or actin in bacterial genomes. So the absence was treated as fact, and the cytoskeleton was filed as one of the defining features that separated eukaryotes from everything else.

That changed in 1992, when three independent research groups recognized that FtsZ, a protein already known to be essential for bacterial cell division, contained a sequence motif found in all forms of tubulin. The same year, computational searches turned up candidates for bacterial actins. Within about a decade, X-ray crystallography confirmed these proteins as genuine structural relatives of their eukaryotic counterparts.

1PubMed Central. The discovery of the prokaryotic cytoskeleton: 25th anniversary The old textbook claim was not just incomplete; it was wrong.

FtsZ, the Bacterial Tubulin

FtsZ is probably the best-studied prokaryotic cytoskeletal protein. It assembles into a dynamic ring at the middle of a dividing bacterial cell, a structure researchers call the Z-ring. This ring marks the future division site, recruits the rest of the cell-division machinery, and helps guide the construction of new cell wall material that pinches the cell in two.

2PubMed Central. FtsZ dynamics in bacterial division: What, how, and why?

What makes FtsZ especially interesting is that it does not just sit still. Individual filaments within the Z-ring treadmill, meaning they grow at one end while shrinking at the other, so the filament appears to move even though individual protein subunits stay in place. This treadmilling is powered by the energy molecule GTP and is crucial for distributing division-related proteins evenly around the ring.

3PubMed Central. Cooperative ordering of treadmilling filaments in cytoskeletal networks of FtsZ and its crosslinker ZapA Recent work has shown that treadmilling alone can drive FtsZ filaments to align and self-organize into the ring structure, both in living cells and in purified systems on a glass surface.

4PubMed Central. Self-organization of mortal filaments and its role in bacterial division ring formation

Bacteria also need to make sure the Z-ring forms in the right place. In Escherichia coli, the Min protein system handles this. Min proteins oscillate back and forth between the two cell poles, which prevents FtsZ from assembling near the ends and steers the ring toward the cell’s midpoint.

5PubMed Central. Changes in the Min oscillation pattern before and after cell birth6PubMed. Spatial regulators for bacterial cell division self-organize into surface waves in vitro The end result is a remarkably precise positioning system, all built on a cytoskeletal backbone.

MreB, the Bacterial Actin

If FtsZ handles division, MreB handles shape. MreB is an actin-like protein found in most rod-shaped bacteria. It assembles into short filaments that run along the inner face of the cell membrane and coordinate where new cell wall material gets inserted. When researchers depleted MreB in the bacterium Caulobacter crescentus, the cells lost their rod shape and became bloated, lemon-like blobs with obvious defects in their cell walls.

7PubMed. MreB, the cell shape-determining bacterial actin homologue, co-ordinates cell wall morphogenesis in Caulobacter crescentus

MreB’s job goes beyond basic shape maintenance. It also plays a role in coordinating chromosome segregation, the process of pulling copied DNA molecules apart so each daughter cell gets a full copy of the genome. Because of its involvement in both shape and division, MreB has attracted attention as a potential antibiotic target: disrupt MreB and you could undermine a bacterium’s structural integrity and its ability to reproduce at the same time.

8PubMed Central. MreB: unraveling the molecular mechanisms of bacterial shape, division, and environmental adaptation

Round bacteria, like the cocci that cause strep throat and staph infections, generally lack MreB. They maintain their spherical shape through other means. The pattern is consistent enough that scientists view MreB as one of the chief determinants of rod-shaped morphology across the bacterial world.

Crescentin and the Intermediate Filament Connection

The eukaryotic cytoskeleton has three main filament types: microtubules (made of tubulin), microfilaments (made of actin), and intermediate filaments. For years, bacteria seemed to possess relatives of only the first two. Then researchers studying Caulobacter crescentus, a crescent-shaped freshwater bacterium, identified a protein called crescentin that shares key structural features with eukaryotic intermediate filament proteins.

9PubMed. The bacterial cytoskeleton: an intermediate filament-like function in cell shape

Crescentin forms a filamentous structure that runs along the inner curvature of the cell, just beneath the membrane. Without it, Caulobacter cells straighten out into ordinary rods, losing their signature crescent shape. The protein assembles into filaments without requiring energy or cofactors, another hallmark it shares with eukaryotic intermediate filaments.

10PubMed Central. Bacterial cell curvature through mechanical control of cell growth Structural studies have confirmed the resemblance in molecular detail: crescentin is built around long parallel coiled coils and even contains a specific structural feature called a “stutter” that is characteristic of eukaryotic intermediate filament proteins.

11PubMed Central. Filament structure and subcellular organization of the bacterial intermediate filament-like protein crescentin

The mechanism is elegant. By applying its own curved geometry to the growing cell wall, crescentin physically bends the cell. Think of it like a stiff wire glued to one side of an inflating balloon: the balloon cannot expand evenly because the wire constrains one side. Crescentin does the same thing to the bacterium, generating curvature through mechanical asymmetry rather than through any enzymatic activity.

Filaments Without Eukaryotic Counterparts

Not every prokaryotic cytoskeletal protein is a relative of tubulin, actin, or intermediate filaments. Bacteria have also evolved filament-forming proteins with no clear eukaryotic counterpart. The most prominent of these are the bactofilins, small proteins that fold into a distinctive beta-helical shape and spontaneously assemble into filaments.

12PubMed Central. Structure of the Bacterial Cytoskeleton Protein Bactofilin by NMR Chemical Shifts and Sequence Variation

Bactofilins are widespread across bacterial species and have been linked to a surprisingly varied set of functions. In Caulobacter crescentus, they help form the stalk, a thin appendage the bacterium uses to attach to surfaces. In Myxococcus xanthus, a social bacterium known for swarming in coordinated groups, bactofilins contribute to both chromosome segregation and the gliding motility the organism uses to move across surfaces.

13PubMed Central. The structure of bactofilin filaments reveals their mode of membrane binding and lack of polarity The existence of bactofilins makes an important conceptual point: the prokaryotic cytoskeleton is not merely a stripped-down version of its eukaryotic counterpart. Bacteria have also innovated their own unique structural solutions.

Scaffold proteins add another layer. In Caulobacter, a protein called PopZ forms gel-like assemblies at the cell poles that help organize signaling proteins and contribute to asymmetric cell division, where the two daughter cells intentionally differ from each other.

14PubMed Central. Scaffold-Scaffold Interaction Facilitates Cell Polarity Development in Caulobacter crescentus These polar scaffolds are not classical filaments, but they serve the same organizing role that cytoskeletal structures fulfill in larger cells.

Moving DNA and Aligning Organelles

One of the more striking prokaryotic cytoskeletal systems handles a problem most people associate only with eukaryotic cells: segregating genetic material. Many bacteria carry plasmids, small circular DNA molecules separate from the main chromosome, and some of these plasmids encode their own miniature segregation machinery built around an actin-like protein called ParM.

ParM filaments display dynamic instability, rapidly growing and then catastrophically collapsing, similar to the behavior of eukaryotic microtubules during cell division.

15PubMed. Dynamic instability in a DNA-segregating prokaryotic actin homolog When two copies of a plasmid each bind to one end of a ParM filament bundle, the filaments push the plasmid copies toward opposite poles of the cell, forming what is essentially a bipolar spindle. The architecture involves pairs of filaments arranged in an antiparallel orientation, mechanically resembling the mitotic spindle eukaryotes use to separate chromosomes.

16PubMed Central. A bipolar spindle of antiparallel ParM filaments drives bacterial plasmid segregation

Bacteria also use cytoskeletal filaments to organize membrane-bound compartments. Magnetotactic bacteria, which navigate using Earth’s magnetic field, contain chains of tiny iron-mineral crystals housed in membrane-enclosed vesicles called magnetosomes. Keeping these magnetosomes aligned in a straight chain is essential for the bacterium’s internal compass to work. An actin-like protein called MamK forms filaments that anchor the magnetosomes and hold the chain together.

17PubMed Central. Interplay between two bacterial actin homologs, MamK and MamK-Like, is required for the alignment of magnetosome organelles in Magnetospirillum magneticum AMB-118PubMed Central. Structure of the magnetosome-associated actin-like MamK filament at subnanometer resolution This is a case where the prokaryotic cytoskeleton does something often assumed to be a eukaryotic specialty: organizing membrane-bound organelles in space.

The Archaeal Cytoskeleton

The prokaryotic world includes not just bacteria but also archaea, the other major domain of life that lacks a nucleus. Archaea were long treated as footnotes in cytoskeleton research, but they too have turned out to possess their own structural filaments.

On the actin side, a protein called crenactin was discovered in certain archaea and shown to belong to a conserved gene cluster involved in cell shape determination.

19PubMed Central. An archaeal origin for the actin cytoskeleton: Implications for eukaryogenesis On the tubulin side, researchers identified a protein family called CetZ in the salt-loving archaeon Haloferax volcanii. CetZ proteins share the core fold of tubulin and FtsZ but form a distinct evolutionary branch. In living cells, CetZ1 assembled into dynamic structures and was required for the cells to switch from disc-shaped to rod-shaped, suggesting a direct role in controlling cell shape. The researchers who described CetZ proposed that a cytoskeletal function for tubulin-family proteins may predate the evolution of eukaryotic cells entirely.

20PubMed Central. CetZ tubulin-like proteins control archaeal cell shape

These archaeal findings are significant for understanding where the eukaryotic cytoskeleton came from. Current evolutionary models suggest that eukaryotic cells arose from an archaeal ancestor, possibly through a merger with a bacterial partner. If archaea already had functional cytoskeletal proteins, the raw material for the eukaryotic cytoskeleton was in place long before the first eukaryotic cell appeared.

21PubMed Central. Evolution of the cytoskeleton

How Different Is the Prokaryotic Cytoskeleton From the Eukaryotic One?

The evolutionary relationship is real, but so are the differences. Eukaryotic microtubules are hollow tubes built from two types of tubulin subunit arranged in pairs. FtsZ filaments are single-stranded and do not form tubes. Eukaryotic actin forms long, twisted double-helix filaments stabilized by an array of accessory proteins. MreB filaments are typically shorter and interact directly with the membrane. Eukaryotic intermediate filaments form elaborate rope-like bundles layered across the cell interior. Crescentin appears to form simpler structures closely associated with one side of the cell.

Functionally, the eukaryotic cytoskeleton does things prokaryotic cells rarely need to do: haul cargo across long intracellular distances using motor proteins, support cell crawling and amoeboid movement, maintain the shape of enormous cells hundreds of times larger than a bacterium. The prokaryotic versions handle a more compact set of tasks, primarily division, shape, and spatial organization, but they accomplish those tasks with the same basic filament-forming chemistry.

There is also a difference in scale. A typical bacterial cell is about one to two micrometers long. At that size, diffusion can move most molecules where they need to go within seconds. The cytoskeleton’s role is less about transport and more about imposing spatial order: deciding where division happens, controlling how the cell wall is built, keeping DNA copies apart. In much larger eukaryotic cells, the cytoskeleton takes on the additional burden of long-range transport because diffusion alone is too slow.

Antibiotics That Target the Bacterial Cytoskeleton

Because FtsZ is essential for division in most bacteria and has no close equivalent in human cells, it has drawn considerable interest as a drug target. Block FtsZ and, in principle, bacteria cannot divide. Several research groups have spent years searching for small molecules that bind to FtsZ and shut it down, though the path from lab to clinic has proven difficult.

22PubMed Central. The Search for Antibacterial Inhibitors Targeting Cell Division Protein FtsZ at Its Nucleotide and Allosteric Binding Sites

Both synthetic compounds and natural products have been tested as FtsZ inhibitors. The appeal is clear: with antibiotic resistance rising and the pipeline of new antibiotics thinning, a completely new target like FtsZ could open up treatment options against drug-resistant bacteria. MreB has attracted similar attention, since disrupting it compromises both cell shape and wall integrity.

23Biomolecules and Biomedicine. A key bacterial cytoskeletal cell division protein FtsZ as a novel therapeutic antibacterial drug target8PubMed Central. MreB: unraveling the molecular mechanisms of bacterial shape, division, and environmental adaptation No FtsZ-targeting antibiotic has reached widespread clinical use yet, but the research is active and the rationale is strong. The prokaryotic cytoskeleton, once thought not to exist, could eventually become a weak point that medicine exploits.

What Happens When Bacteria Lose Their Wall

An unusual test case for the prokaryotic cytoskeleton comes from so-called L-form bacteria, cells that have lost or been stripped of their rigid cell wall. Without a wall, bacteria become soft, irregularly shaped blobs that can no longer rely on the wall-associated cytoskeletal machinery, like MreB-directed peptidoglycan synthesis, to maintain their form or divide normally.

Remarkably, L-forms can still proliferate. Researchers have observed them dividing through a process of membrane deformation, blebbing, and spontaneous scission that looks nothing like the orderly FtsZ-ring-driven division of walled cells. Studies in both gram-positive and gram-negative bacteria have demonstrated this alternative mode of proliferation, suggesting it is a general capability rather than a quirk of one species.

24PubMed Central. General principles for the formation and proliferation of a wall-free (L-form) state in bacteria L-form division appears to bypass the FtsZ-based cytoskeletal system altogether, relying instead on excess membrane production and physical instability to split cells apart. The existence of L-forms hints at a very ancient mode of cell division that may have predated the evolution of cytoskeletal division machinery, offering a glimpse of what life might have looked like before cells developed internal structural scaffolding.