The Bactofilin Cytoskeleton Protein BacM of Myxococcus xanthus Forms an Extended [beta]-Sheet Structure Likely Mediated by Hydrophobic Interactions

• Published in: PloS one 2015-03, Vol.10 (3)
• Main Authors: Zuckerman, David M, Boucher, Lauren E, Xie, Kefang, Engelhardt, Harald, Bosch, Jürgen, Hoiczyk, Egbert
• Format: Article
• Language: English
• Subjects: Analysis; Polymerization; Proteins; Soil microbiology
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0121074

Bactofilins are novel cytoskeleton proteins that are widespread in Gram-negative bacteria. Myxococcus xanthus, an important predatory soil bacterium, possesses four bactofilins of which one, BacM (Mxan_7475) plays an important role in cell shape maintenance. Electron and fluorescence light microscopy, as well as studies using over-expressed, purified BacM, indicate that this protein polymerizes in vivo and in vitro into ~3 nm wide filaments that further associate into higher ordered fibers of about 10 nm. Here we use a multipronged approach combining secondary structure determination, molecular modeling, biochemistry, and genetics to identify and characterize critical molecular elements that enable BacM to polymerize. Our results indicate that the bactofilin-determining domain DUF583 folds into an extended [beta]-sheet structure, and we hypothesize a left-handed [beta]-helix with polymerization into 3 nm filaments primarily via patches of hydrophobic amino acid residues. These patches form the interface allowing head-to-tail polymerization during filament formation. Biochemical analyses of these processes show that folding and polymerization occur across a wide variety of conditions and even in the presence of chaotropic agents such as one molar urea. Together, these data suggest that bactofilins are comprised of a structure unique to cytoskeleton proteins, which enables robust polymerization.