Assembly properties of Spiroplasma MreB involved in swimming motility

Bacterial actin MreB forms filaments in which the unit of the structure is an antiparallel double strand. The wall-less helical bacterium Spiroplasma has five MreB homologs (MreB1-5), a part of which is composed of an intra-cellular ribbon for driving its swimming motility. The interaction modes of each ribbon component are unclear, although these are clues for understanding Spiroplasma swimming. Here, we examined the assembly properties of Spiroplasma eriocheiris MreB5 (SpeMreB5), which forms sheets and is a component protein of the ribbon. Electron microscopy (EM) revealed that sheet formation was inhibited under acidic conditions and paracrystal structures were formed under acidic and neutral conditions with low ionic strength. Solution assays found four properties of paracrystals as follows: (I) their formation followed sheet formation, (II) electrostatic interactions were required for their formation, (III) the positively charged and unstructured C-terminal region contributed to the nucleation of their formation, and (IV) their formation required Mg2+ at neutral pH but was inhibited by divalent cations under acidic conditions. During these studies, we found two aggregation modes of SpeMreB5, with distinct responses to ATP. These properties will shed light on SpeMreB5 assembly dynamics at the molecular level.