Elucidation of fibril helix structure responsible for swimming in Spiroplasma using electron microscopy

Abstract

Spiroplasma, known pathogens and commensals of arthropods and plants, are helical-shaped bacteria lacking the peptidoglycan layer. They swim by alternating between left- and right-handed cell helicity, which is driven by an internal structure called the ribbon. This system is unrelated to flagellar motility that is widespread in bacteria. The ribbon comprises the bacterial actin homolog MreB and fibril, the protein specific to Spiroplasma. Here, we isolated the ribbon and its core, the fibril filament, and using electron microscopy, found that the helicity of the ribbon and the cell is linked to the helicity of the fibril. Single particle analysis using the negative-staining method revealed that the three-dimensional structures of the fibril filament comprise a repeated ring structure twisting along the filament axis. The handedness of these structures were verified by the structures reconstituted through tomogram from quick-freeze deep-etch replica electron microscopy. Based on these observations, we propose a scheme for the helicity-switching mechanism in which the twists caused by the conformational changes in the fibril filament are accumulated, transmitted to the ribbon, and then propel the cells by rotating the cell body like a screw. Significance Statement Spiroplasma are commensal and pathogenic bacteria widespread globally in animals and plants. A few species are also recognized as male-killing bacteria of insects. Their special swimming mechanism is caused by helicity switching, which could be the simplest swimming mechanism, because helicity shift can be caused by contraction and extension of paired filaments. This mechanism has attracted research attention for many years because of the possible application in the field of nano actuators; however, the details of this mechanism remain to be clarified. Here, we propose a model for the swimming mechanism based on the structure of the core of the Spiroplasma ribbon.