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Proc Natl Acad Sci U S A. 2015 Jul 28;112(30):9340-5. doi: 10.1073/pnas.1509069112. Epub 2015 Jun 29.

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

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Institute of Complex Systems, Forschungszentrum Jülich, 52425 Julich, Germany; Physics Department, University of Düsseldorf, 40225 Dusseldorf, Germany;
Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, VA 22908;
Department of Cell and Molecular Biology, Uppsala University, 75124 Uppsala, Sweden;
Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, 106 91 Stockholm, Sweden.
Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, VA 22908;


The prokaryotic origins of the actin cytoskeleton have been firmly established, but it has become clear that the bacterial actins form a wide variety of different filaments, different both from each other and from eukaryotic F-actin. We have used electron cryomicroscopy (cryo-EM) to examine the filaments formed by the protein crenactin (a crenarchaeal actin) from Pyrobaculum calidifontis, an organism that grows optimally at 90 °C. Although this protein only has ∼ 20% sequence identity with eukaryotic actin, phylogenetic analyses have placed it much closer to eukaryotic actin than any of the bacterial homologs. It has been assumed that the crenactin filament is double-stranded, like F-actin, in part because it would be hard to imagine how a single-stranded filament would be stable at such high temperatures. We show that not only is the crenactin filament single-stranded, but that it is remarkably similar to each of the two strands in F-actin. A large insertion in the crenactin sequence would prevent the formation of an F-actin-like double-stranded filament. Further, analysis of two existing crystal structures reveals six different subunit-subunit interfaces that are filament-like, but each is different from the others in terms of significant rotations. This variability in the subunit-subunit interface, seen at atomic resolution in crystals, can explain the large variability in the crenactin filaments observed by cryo-EM and helps to explain the variability in twist that has been observed for eukaryotic actin filaments.


crenactin; cytoskeletal filaments; helical polymers; variable twist

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