#bacterial cytoskeleton

Microbial Physiology

The MreB cytoskeletal protein plays a role in:

MreB is a widely distributed bacterial homolog of eukaryotic actin conserved across rod-shaped bacteria including Escherichia coli, Bacillus subtilis, and Caulobacter crescentus, essential for maintaining cylindrical morphology and coordinating growth. Unlike eukaryotic actin that forms long helical filaments, MreB assembles into short, antiparallel double protofilaments associated with inner face of cytoplasmic membrane in an ATP-dependent manner, often guided by membrane regions enriched in specific phospholipids. Its primary morphogenetic function is to act as a dynamic scaffold organizing cell wall synthetic complexes, recruiting penicillin-binding protein PBP2, rod shape-determining proteins RodA and RodZ, and class A bifunctional transglycosylases, directing their processive circumferential motion around cell long axis tracked by single-molecule imaging. This coordinated movement ensures new peptidoglycan glycan strands are inserted in ordered hoops rather than randomly, preserving width and straightness during elongation. Pharmacological depolymerization with compound A22 or genetic depletion converts rods into spheres, reduces growth rate, and disorganizes wall insertion. FtsZ instead assembles Z-ring for division, illustrating division of labor between elongation and cytokinesis.

Ref: Burkman et al., Ann Rev Microbiol 2018, MreB and Rod Shape; Errington, Nature Rev Microbiol 2015, Bacterial Cytoskeleton.

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Microbial Physiology

The cytoskeletal protein MamK is involved in:

Magnetotactic bacteria such as Magnetospirillum magneticum AMB-1 and gryphiswaldense biomineralize magnetic nanoparticles within lipid vesicles called magnetosomes to navigate chemical gradients in stratified water columns. Cytoskeletal protein MamK actin homolog is essential for organizing magnetosomes into linear chain maximizing dipole moment for efficient orientation. MamK polymerizes ATP dependently into filaments extending along cell long axis, forms scaffold similar MreB but dedicated. Adaptor protein MamJ interacts with magnetosome membrane proteins MamY and MamK linking vesicles to filament, ensuring alignment at midcell near positive curvature and preventing agglomeration. Without MamK, magnetosomes dispersed clusters reducing magnetic orientation efficiency, cells unable to perform efficient magnetotaxis, chain fragmented after division. MamK dynamic polymerization also controls chain segmentation during division ensuring equal inheritance to daughters. Unlike plasmid segregation protein ParM requiring dynamic instability pushing apart or division FtsZ constricting, MamK specifically magnetosome positioning function. Additional proteins MamE protease, MamO scaffold regulate vesicle formation iron transport and magnetite nucleation precise control.

Ref: Komeili et al., PNAS 2006, MamK involved in magnetosome positioning chain assembly.

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Microbial Physiology

The Elongasome in rod-shaped bacteria is primarily composed of:

In rod shaped bacteria elongation before division mediated by elongasome or Rod complex distinct from divisome. Core organizer is MreB actin homolog that forms short dynamic patches moving around cell circumference perpendicular to long axis. Motion driven by peptidoglycan synthesis not treadmilling alone, powered by RodA transglycosylase activity. MreB interacts with integral membrane proteins MreC, MreD, RodZ cytoplasmic tail linking to peptidoglycan synthases RodA transglycosylase from SEDS family polymerizing lipid II glycan strands and PBP2a PBP2 transpeptidase crosslinking peptides. Additional components PBP1A class A bifunctional, Rod complex proteins also include MreB associated proteins and cytoskeletal linkers. Together complex inserts new peptidoglycan in circumferential hoops ensuring length increase while maintaining constant diameter. Upon assembly of FtsZ ring at midcell MreB delocalizes, elongasome disassembles, divisome takes over septal synthesis. Unlike FtsZ FtsA division proteins, ParA ParB partition machinery, MinC MinD division inhibition system, elongasome specifically composition MreB and associated proteins responsible for rod elongation coordinating shape with growth rate and protecting from osmotic lysis.

Ref: Shi et al., PNAS 2020, Elongasome - MreB and associated proteins direct lateral wall synthesis.

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Microbial Physiology

Which cytoskeletal protein is responsible for maintaining rod shape in bacteria?

Cell shape maintenance in prokaryotes relies on cytoskeletal elements directing wall synthesis. Rod shape requires lateral peptidoglycan insertion along sidewall producing cylinder with hemispherical poles. MreB is bacterial actin homolog sharing actin fold ATP binding pocket polymerizes into short double filaments moving circumferentially around cell coordinated with cell wall synthesis enzymes. MreB associates with MreC MreD transmembrane proteins and RodZ linking to RodA SEDS transglycosylase and PBP2 transpeptidase elongasome complex synthesizing hoop like glycans inserting along length. Depolymerization by drug A22 binding ATP pocket converts rods to spheres demonstrating essentiality, shape defect sensitizes to cell wall antibiotics. FtsZ builds division septum, ParA mediates chromosome plasmid segregation via Walker ATPase, TubZ is tubulin homolog involved in plasmid pBToxis partitioning pushing plasmids. Therefore maintaining rod shape assigned to MreB actin like cytoskeleton, illustrating conservation of actin function in morphogenesis across domains and target for shape altering antibiotics exploring novel antibacterial mechanisms targeting MreB dynamics and Rod complex.

Ref: Cabeen & Jacobs-Wagner, Annu Rev Genetics, 2010, Bacterial cytoskeleton: MreB maintains rod shape.

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