A cytoskeleton, a network of protein filaments, was once thought to be exclusive to eukaryotic cells. However, scientific advancements reveal that prokaryotic cells, including bacteria and archaea, also possess a dynamic cytoskeletal system. While these prokaryotic structures differ in complexity from their eukaryotic counterparts, they perform analogous functions crucial for cell survival and organization. They provide essential structural support and facilitate various cellular processes.
Historical Perspective
For many years, scientists believed prokaryotic cells, due to their simpler structure, lacked the internal cytoskeletal networks found in eukaryotes. This understanding was based on the absence of readily identifiable structures comparable to the prominent microtubules, microfilaments, and intermediate filaments. The paradigm shifted in the early 1990s with key discoveries. Researchers identified bacterial proteins sharing structural and functional similarities with eukaryotic tubulin and actin.
The discovery of FtsZ in 1992, a protein involved in bacterial cell division, was a turning point. Research groups found FtsZ possessed a “tubulin signature sequence” and could assemble into filamentous structures, much like eukaryotic tubulin. Shortly after, MreB proteins were identified, showing homology to actin and demonstrating their role in maintaining bacterial cell shape. These findings revolutionized prokaryotic cell biology.
Core Components
The prokaryotic cytoskeleton is built from several protein families, each contributing to distinct cellular architecture and processes. Well-characterized components include FtsZ, MreB, and CreS. FtsZ, or “Filamenting temperature-sensitive mutant Z,” is a widely distributed protein across most bacteria and archaea, serving as a prokaryotic homolog of eukaryotic tubulin. It forms protofilaments that assemble into larger structures, often appearing as a ring-like scaffold within the cell.
MreB is another cytoskeletal protein, recognized as an actin homolog due to similarities in its tertiary structure and active site. This protein is found in rod-shaped and helical bacteria, forming helical filaments that extend along the cell’s length, usually beneath the cytoplasmic membrane.
CreS, or crescentin, represents a third class of prokaryotic cytoskeletal proteins, sharing characteristics with eukaryotic intermediate filaments. Crescentin forms stable filaments and is found in crescent-shaped bacteria, such as Caulobacter crescentus.
Diverse Cellular Functions
Prokaryotic cytoskeletal elements perform various functions, orchestrating cellular processes from division to shape maintenance. FtsZ plays a central role in bacterial cell division, assembling into a Z-ring at the cell’s future division site. This Z-ring acts as a scaffold, recruiting other proteins necessary for synthesizing the new cell wall (septum) that divides the parent cell. Its dynamic polymerization and depolymerization contribute to the constriction force during cytokinesis.
MreB maintains the characteristic rod shape of many bacteria. It forms helical filaments that guide the insertion of new cell wall material, ensuring the cell elongates properly. Beyond shape determination, MreB also participates in chromosome segregation, influencing genetic material distribution to daughter cells. Mutations in MreB can lead to defects in cell shape and chromosome partitioning.
Crescentin (CreS) is responsible for the curved or crescent shape observed in certain bacteria. By forming filaments along the inner curvature of the cell, crescentin helps to establish and maintain this distinct morphology.
Another actin-like protein, ParM, segregates plasmids, small circular DNA molecules found in many bacteria. ParM forms dynamic filaments that push plasmids to opposite ends of the cell before division, ensuring each daughter cell receives a copy. This system is analogous to the microtubule spindle in eukaryotes.
Distinctions from Eukaryotic Cytoskeletons
While prokaryotic and eukaryotic cytoskeletons share functional parallels, their organization and complexity differ. Eukaryotic cytoskeletons are more extensive, comprising a complex network of microtubules, microfilaments, and intermediate filaments that permeate the entire cytoplasm. This elaborate system facilitates large-scale intracellular transport, organelle positioning, and complex cell movements.
In contrast, prokaryotic cytoskeletons involve fewer protein types and exhibit simpler overall organization. Prokaryotic cytoskeletal elements are primarily involved in specific, localized processes such as cell division, shape determination, and DNA segregation. They do not engage in the broad, coordinated movements or extensive intracellular transport seen in eukaryotes, which lack membrane-bound organelles that require such transport mechanisms.