Cytoskeletal Proteins in Bacteria

Bacterial cells contain four major groups of cytoskeletal proteins: actin homologs like MreB and Mbl, tubulin homologs including FtsZ and BtubA/B, intermediate filament-like proteins such as crescentin, and unique bacterial proteins called MinD-ParA. These proteins regulate cell shape, polarity, division, and plasmid partitioning, demonstrating that bacteria possess complex cytoskeletal organization similar to eukaryotes.

Bacterial actins are highly diverse, classified into up to 35 different families based on phylogenetic analysis and structural features. They exhibit dramatic variations in filament twist, with intersubunit angles varying by approximately ±10 degrees, changes in strand number, and possible antiparallel strands. Despite these structural differences, bacterial actin subunits share similar tertiary structure and evolved from the same ancestor as eukaryotic actins.

MreB and Mbl are actin homologs abundantly found in rod and spiral-shaped bacteria. These proteins form spiral scaffolds that guide the formation of the peptidoglycan cell wall, directly determining bacterial cell shape. Their organization as helical structures ensures proper cell wall synthesis and maintenance of the characteristic rod or spiral morphology in these bacterial species.

FtsZ is a tubulin homolog that polymerizes into filaments and assembles into the Z-ring at the cell's middle to initiate cell division. Unlike eukaryotic microtubules, FtsZ forms single-stranded filaments or twisted filament pairs rather than hollow tubes. This Z-ring structure serves as the contractile apparatus that orchestrates the physical separation of daughter cells during cytokinesis.

ParM is an actin homolog encoded by non-genomic antibiotic-resistant plasmids in some bacteria. It spontaneously assembles into filaments that grow and push plasmid copies apart to opposite ends of the cell during division. This mechanism ensures accurate plasmid partitioning and inheritance by daughter cells, maintaining antibiotic resistance genes across generations.

Crescentin is the only intermediate filament-like protein identified in bacteria, making it extremely rare compared to eukaryotic intermediate filaments. In bacteria like Caulobacter crescentus, crescentin subunits create a distinctive crescent shape. When crescentin is absent, these bacteria lose their curved morphology and become rod-shaped, demonstrating its critical role in determining cell shape.

Bacterial tubulins form structurally different microtubules compared to the 13-protofilament structure common in eukaryotes. FtsZ forms single-stranded filaments or twisted filament pairs, while BtubA/BtubB in Prosthecobacter species form a 5-protofilament structure. These variations suggest bacterial microtubules may have evolved before the eukaryotic 13-protofilament structure, with longitudinal bonds in single-stranded filaments preceding lateral interactions.