Do Prokaryotic Cells Have a Cytoskeleton? Cutting-Edge Insights

For a long time, scientists believed prokaryotic cells lacked a cytoskeleton, a feature thought to be exclusive to eukaryotes. However, recent discoveries have revealed that bacteria and archaea possess filamentous proteins with structural and functional similarities to the eukaryotic cytoskeleton.

These findings reshape our understanding of bacterial cell biology, highlighting protein networks that contribute to shape, division, and organization. Scientists continue to uncover new roles for these structures, expanding knowledge of their importance in cellular processes.

Discovery of Prokaryotic Filaments

The absence of membrane-bound organelles in prokaryotic cells once led researchers to assume bacteria and archaea lacked an internal scaffold akin to the eukaryotic cytoskeleton. This perspective shifted in the late 20th century when electron microscopy and fluorescence imaging techniques revealed filamentous structures within bacterial cells. Early observations hinted at their role in cellular organization, but their molecular identity remained unknown until genetic and biochemical analyses identified specific proteins forming these structures.

One breakthrough was the discovery of actin-like and tubulin-like proteins in bacterial species. These proteins polymerized into dynamic filaments, resembling their eukaryotic counterparts in both structure and function. Their identification provided compelling evidence that cytoskeletal elements had deep evolutionary roots, predating the divergence of prokaryotic and eukaryotic lineages. This realization spurred studies characterizing bacterial cytoskeletal proteins and their distinct roles in cellular architecture.

Advancements in super-resolution microscopy and cryo-electron tomography further refined our understanding, revealing the intricate organization of these structures. Unlike the rigid frameworks once assumed, these filaments dynamically assemble and disassemble in response to cellular needs. This adaptability suggests bacterial cytoskeletal elements actively participate in shape maintenance, intracellular transport, and spatial organization. Their discovery also raised questions about their evolutionary significance, prompting comparative studies across bacterial and archaeal species.

MreB Proteins and Structural Integrity

MreB proteins are crucial for maintaining bacterial cell shape, forming a dynamic cytoskeletal network beneath the cytoplasmic membrane. These actin homologs guide the synthesis and distribution of peptidoglycan, the primary component of the bacterial cell wall. Rather than serving as rigid scaffolds, MreB filaments continuously remodel, aligning with regions of active cell wall synthesis to ensure uniform growth and structural stability. In rod-shaped bacteria like Escherichia coli and Bacillus subtilis, this process prevents shape loss that could impair function.

MreB interacts with peptidoglycan-synthesizing enzymes to coordinate cell wall expansion. High-resolution fluorescence microscopy has shown that MreB filaments move circumferentially around the cell, driven by the activity of penicillin-binding proteins and other enzymes. This movement ensures controlled peptidoglycan insertion, preserving cell shape while allowing growth. Disruptions to MreB function, whether by genetic mutations or inhibitors like A22, result in severe morphological defects, often causing cells to become spherical due to unregulated peptidoglycan deposition.

Beyond structural integrity, MreB influences the localization of membrane-associated proteins involved in chromosome segregation and membrane transport. This positioning function suggests MreB acts as a scaffold for intracellular organization. Loss of MreB not only leads to shape defects but also disrupts the spatial arrangement of critical proteins, highlighting its broader role in bacterial physiology.

FtsZ and Cell Division

FtsZ, a tubulin homolog, is essential for bacterial cell division, assembling into a ring-like structure at the future site of cytokinesis. This Z-ring serves as a scaffold that recruits division proteins, orchestrating septum formation to partition the cell into two daughter cells. Unlike eukaryotic tubulin, which forms microtubules, FtsZ polymerizes into dynamic filaments that remodel in response to cellular cues.

The Z-ring’s positioning is regulated by proteins that prevent misplaced septation. MinCDE proteins oscillate between the poles of rod-shaped bacteria like Escherichia coli, inhibiting FtsZ polymerization in these regions and ensuring midcell assembly. The nucleoid occlusion system prevents premature Z-ring formation over unsegregated chromosomes, maintaining genetic integrity during division.

Once established, the Z-ring anchors the divisome, a multi-protein complex responsible for synthesizing new cell wall material at the division site. FtsZ filaments generate constriction forces through GTP hydrolysis, driving membrane invagination. Accessory proteins such as FtsA and ZipA tether FtsZ to the inner membrane and facilitate interactions with peptidoglycan remodeling enzymes. This coordinated action ensures controlled septal closure, preventing structural defects that could compromise viability.

Crescentin and Cell Curvature

Crescentin, an intermediate filament-like protein, shapes the curved morphology of bacteria like Caulobacter crescentus. Unlike MreB and FtsZ, which influence cell shape through peptidoglycan synthesis, crescentin assembles into filaments that align along the inner curvature of the cell. This asymmetric positioning generates mechanical tension, influencing cell wall growth and reinforcing curvature. In aquatic environments, this shape enhances surface attachment and nutrient acquisition.

Crescentin filaments resemble eukaryotic intermediate filaments but lack the dynamic polymerization and depolymerization seen in actin- and tubulin-like proteins. Instead, they form stable bundles along one side of the cell, biasing the insertion of new peptidoglycan. This differential growth leads to curvature, with the convex side elongating faster than the concave side. Disrupting crescentin function results in straight, rod-like cells, underscoring its role as a structural determinant.

Roles in Growth and Cell Cycle

The bacterial cytoskeleton coordinates growth and cell cycle progression, ensuring structural integrity while distributing cellular components. Unlike eukaryotic cells, which use microtubules, actin filaments, and intermediate filaments, bacteria rely on a streamlined set of cytoskeletal proteins with analogous functions. These proteins guide cell wall synthesis, chromosome positioning, intracellular transport, and division timing.

MreB regulates cell elongation by directing peptidoglycan synthesis along the cell periphery, ensuring uniform growth. FtsZ dictates cytokinesis, assembling into a contractile ring that initiates septation at the appropriate stage. Crescentin, while primarily influencing curvature, also affects the spatial pattern of cell wall deposition. Together, these cytoskeletal elements integrate mechanical stability with the biochemical processes driving bacterial replication.

Distribution Across Various Bacterial Species

The presence and function of cytoskeletal proteins vary among bacterial species, reflecting diverse morphologies and ecological niches. While MreB, FtsZ, and crescentin are well-characterized in model organisms like Escherichia coli, Bacillus subtilis, and Caulobacter crescentus, their homologs exhibit distinct adaptations in other bacterial lineages. Some species possess additional cytoskeletal components for specialized processes like intracellular motility or host cell invasion.

Rod-shaped bacteria almost universally rely on MreB for their elongated structure, though variations in MreB organization influence specific growth patterns. In contrast, spherical bacteria like Staphylococcus aureus lack MreB, relying on alternative mechanisms for cell wall synthesis. FtsZ is nearly ubiquitous, serving as the primary driver of division across most bacterial species. Its conservation underscores its fundamental role in bacterial proliferation, though variations in its regulatory network allow for species-specific adaptations. Crescentin, by comparison, is more restricted, primarily found in curved or helical bacteria where it serves a specialized structural role.