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

Spiroplasma is a wall-less helical bacterium that is characterized by a unique swimming motility involving five isoforms of the bacterial actin MreBs (SMreB1-5). The functions of SMreBs are unique in the MreB family proteins, as their counterparts in walled-bacteria are involved in maintaining the cell shape by scaffolding the cell-wall synthesis complex through their static properties. In vitro analyses of individual SMreBs provide clues to understand the detailed molecular mechanism of Spiroplasma swimming. However, the purification difficulties have hampered in vitro analyses of one of the SMreBs, SMreB1, which drives the swimming. Here, we isolated soluble SMreB1 of Spiroplasma eriocheiris (SpeMreB1) and evaluated its activity. SpeMreB1 was expressed as a fusion with a solubilization-tag, ProteinS (PrS), which allowed us to purify it in the soluble fraction. SpeMreB1 exhibited the highest phosphate release rate and the fold changes of critical concentrations for polymerization across the nucleotide states among the MreB family proteins. SpeMreB1 interacted with polymerized SpeMreB5, another SMreB essential for Spiroplasma swimming. In the AMPPNP- or ADP-bound state, SpeMreB1 decreased the amount of SpeMreB5 filaments, possibly reflecting their disassembly. Regardless of the nucleotide state, SpeMreB1 bound to negatively charged lipids. These results suggest that SpeMreB1 utilizes its highest activity to manage SpeMreB5 filaments underneath the cell membrane to drive Spiroplasma swimming. ImportanceIn most bacterial species, MreB is involved in cell-shape maintenance by localizing the bacterial cell-wall synthesis complex with its static properties. In contrast, five isoforms of MreBs in a pathogenic wall-less helical bacterium Spiroplasma are involved in its unique motility system driven by a kink propagation along the helical cell. Our integrated biochemical assays show that one isoform of MreBs involved in the swimming of a crustacean pathogen S. eriocheiris (SpeMreB1) is exceptionally active in the MreB family proteins and manages the polymerization of another MreB essential for the swimming (SpeMreB5). This study sheds light on an evolutionary mystery how Spiroplasma has adapted static MreB proteins to a dynamic phenomenon like its swimming motility.