The structure of a 15-stranded actin-like filament from Clostridium botulinum

Nat Commun. 2019 Jun 28;10(1):2856. doi: 10.1038/s41467-019-10779-9.


Microfilaments (actin) and microtubules represent the extremes in eukaryotic cytoskeleton cross-sectional dimensions, raising the question of whether filament architectures are limited by protein fold. Here, we report the cryoelectron microscopy structure of a complex filament formed from 15 protofilaments of an actin-like protein. This actin-like ParM is encoded on the large pCBH Clostridium botulinum plasmid. In cross-section, the ~26 nm diameter filament comprises a central helical protofilament surrounded by intermediate and outer layers of six and eight twisted protofilaments, respectively. Alternating polarity of the layers allows for similar lateral contacts between each layer. This filament design is stiffer than the actin filament, and has likely been selected for during evolution to move large cargos. The comparable sizes of microtubule and pCBH ParM filaments indicate that larger filament architectures are not limited by the protomer fold. Instead, function appears to have been the evolutionary driving force to produce broad, complex filaments.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actin Cytoskeleton
  • Actins / genetics
  • Actins / metabolism*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Clostridium botulinum / metabolism*
  • Cryoelectron Microscopy
  • Cytoskeleton / physiology*
  • Gene Expression Regulation, Bacterial / physiology
  • Models, Molecular
  • Protein Conformation


  • Actins
  • Bacterial Proteins