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Archaeal actin from a hyperthermophile forms a single-stranded filament.

Tatjana Braun1, Albina Orlova2, Karin Valegård3

  • 1Institute of Complex Systems, Forschungszentrum Jülich, 52425 Julich, Germany; Physics Department, University of Düsseldorf, 40225 Dusseldorf, Germany;

Proceedings of the National Academy of Sciences of the United States of America
|July 1, 2015
PubMed
Summary

Crenactin, a high-temperature archaeal actin, forms a single-stranded filament, unlike typical double-stranded eukaryotic actin filaments. This structural difference, explained by sequence variations, reveals insights into actin filament diversity.

Keywords:
crenactincytoskeletal filamentshelical polymersvariable twist

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Area of Science:

  • Cell Biology
  • Biophysics
  • Structural Biology

Background:

  • Prokaryotic actin origins are established, but bacterial actins form diverse filaments distinct from eukaryotic F-actin.
  • Crenactin from Pyrobaculum calidifontis, an archaeal actin, shows low sequence identity to eukaryotic actin but is phylogenetically closer.
  • Previous assumptions suggested crenactin filaments were double-stranded for thermal stability.

Purpose of the Study:

  • To investigate the filament structure formed by crenactin using electron cryomicroscopy (cryo-EM).
  • To understand the structural basis for crenactin filament formation and its relationship to eukaryotic actin.
  • To explore the implications of crenactin structure for actin filament diversity.

Main Methods:

  • Electron cryomicroscopy (cryo-EM) to visualize crenactin filaments.
  • Phylogenetic analysis to determine evolutionary relationships.
  • Analysis of existing crystal structures to examine subunit-subunit interfaces.

Main Results:

  • Crenactin filaments are single-stranded, not double-stranded as previously assumed.
  • The single-stranded crenactin filament structure closely resembles the individual strands of eukaryotic F-actin.
  • A sequence insertion in crenactin prevents the formation of a double-stranded F-actin-like filament.
  • Analysis of crystal structures revealed six distinct filament-like subunit-subunit interfaces with varying rotations.
  • This interface variability explains observed diversity in crenactin filament structures and contributes to understanding eukaryotic actin filament twist variability.

Conclusions:

  • Crenactin forms a single-stranded filament, challenging previous assumptions about high-temperature actin stability.
  • Structural variations, including sequence insertions and diverse subunit interfaces, drive the wide array of actin filament structures observed across different organisms.
  • The study provides crucial insights into the evolution and structural plasticity of the actin cytoskeleton.