Prokaryotes, such as bacteria and archaea, do not possess true microtubules like those found in eukaryotic cells. Microtubules are a defining feature of the eukaryotic cytoskeleton. While prokaryotes do have an internal framework, known as a cytoskeleton, it is constructed from proteins distinct from eukaryotic tubulin. This prokaryotic cytoskeleton performs similar functions in cell shape and division using its own unique set of structural proteins.
Defining Microtubules and Their Role in Eukaryotes
Microtubules are dynamic, hollow tubes that serve as a major component of the cytoskeleton in every eukaryotic cell. Each microtubule is a polymer constructed from repeating units of tubulin, which exists as a dimer of alpha-tubulin and beta-tubulin subunits. These dimers stack together to form long, linear chains called protofilaments. Typically, thirteen protofilaments align laterally to create the hollow, cylindrical structure. Microtubules are highly polar: the plus end grows and shrinks rapidly, while the minus end is generally more stable.
Microtubules function as cellular highways for intracellular transport. Motor proteins like kinesin and dynein move along these tracks, hauling organelles, vesicles, and other cellular cargo. During cell division, microtubules form the mitotic spindle. This spindle accurately separates the duplicated chromosomes into the two new daughter cells, ensuring genetic integrity. Microtubules also provide the structural core for specialized motility structures like cilia and flagella.
The Components of the Prokaryotic Cytoskeleton
Prokaryotes rely on a sophisticated internal framework made of structurally similar but chemically distinct proteins. The most well-studied are FtsZ and MreB, which parallel the main eukaryotic cytoskeletal components. FtsZ (Filamenting temperature-sensitive mutant Z) is a homologue of eukaryotic tubulin. Although they have weak similarity in their amino acid sequence, FtsZ and tubulin share a remarkably similar three-dimensional structure.
FtsZ polymerizes in the presence of GTP, much like tubulin, and forms the Z-ring at the cell’s midsection, marking the future division site. The other prominent component is MreB, a homologue of eukaryotic actin. MreB polymerizes into filaments structurally similar to actin microfilaments and is typically found just beneath the cell membrane. MreB is present in most non-spherical bacteria, such as rod-shaped species, and is absent in naturally spherical bacteria. A third protein, Crescentin (CreS), is an analogue of eukaryotic intermediate filaments and is found in bacteria with a curved shape.
How Prokaryotes Achieve Cellular Structure and Division
The prokaryotic cytoskeletal components enable cell structure and division without the complex machinery associated with true microtubules. The FtsZ protein is the primary organizer of cell division in bacteria, a process known as binary fission. The Z-ring, a dynamic collection of FtsZ protofilaments, serves as a scaffold for recruiting all other necessary cell division proteins. This assembly, called the divisome, directs the synthesis of new cell wall material, leading to the constriction and separation of the two daughter cells.
MreB plays a central role in determining and maintaining the shape of rod-shaped bacteria. The protein filaments align circumferentially under the cytoplasmic membrane and guide the insertion of new cell wall material during growth and elongation. This organized, directional insertion of peptidoglycan prevents the cell from becoming spherical, promoting the rod-like shape. In the curved bacterium Caulobacter crescentus, Crescentin forms a continuous filament along the inner, concave side of the cell. This asymmetric localization is thought to impede growth on that side, effectively bending the cell and producing its characteristic crescent shape.