[ODP-034/WS4-4] Structure of the helical cytoskeleton Fibril involved in Spiroplasma swimming revealed by cryoEM
Spiroplasma are helical-shaped bacteria lacking peptidoglycan, and pathogenic for arthropods and plants. They swim by switching cell helicity between right- and left-handed. We solved the structure of the fibril protein which forms the core part of the swimming machinery by single-particle electron cryomicroscopy last year. This year, we obtained the following new findings. (1) We performed phylogenetic analyses to explore the origin of the fibril acquisition which determined the speciation. The result showed that the fibril protein has evolved from MTA/Adohcy nucleosidase, an enzyme essential for bacterial survival. (2) We revealed another conformation of the fibril filament by single particle analysis to understand the structural changes causing the cell swimming. The fibril protein monomer could be divided into four domains linked by three hinges which can move in three independent angles. The dihedral angles between two domains showed that each of the hinge can give torsions of 2-5 degrees. The integrated torsions can cause changes in the helical pitch and helical diameter as much as two and three folds, respectively, depending on the energy state. Based on these results, we propose a scheme to explain the mechanism of cell swimming as an accumulation of the hinge movements.