A highly active bacterial actin actuates the polymerization of another isoform essential for swimming motility of Spiroplasma

Spiroplasma is a wall−less helical bacterium possessing five isoforms of bacterial actin MreBs (SMreB1−5) for its swimming, speculated to be the sole motility system driven by endogenous bacterial actin proteins. Its detailed molecular mechanism remains elusive due to the lack of soluble constructs of SMreB1 essential for Spiroplasma swimming. Here, we isolated soluble SMreB1 of Spiroplasma eriocheiris (SpeMreB1) and evaluated its activity. The phosphate release rate and fold changes of polymerization−critical concentrations over the nucleotide states of SpeMreB1 are the highest among the MreB family proteins. SpeMreB1 interacts with polymerized SpeMreB5, another SMreB essential for Spiroplasma swimming, and decreases SpeMreB5 filament amount depending on the nucleotide state. A decrease in SpeMreB5 filament amount is independent of SpeMreB1 polymerization, although it is essential for swimming motility. SpeMreB1 binds to negatively charged lipids, regardless of their nucleotide state. These results suggest that SpeMreB1 manages SpeMreB5 filaments to drive Spiroplasma swimming.