# A bacterial actin with high ATPase activity regulates the polymerization of a partner MreB isoform essential for Spiroplasma swimming motility

## TL;DR

This study identifies a bacterial actin, SpeMreB1, that regulates another MreB isoform to enable the unique swimming motility of Spiroplasma bacteria.

## Contribution

The purification and characterization of SpeMreB1, revealing its high ATPase activity and regulatory role in MreB5 filament dynamics for motility.

## Key findings

- SpeMreB1 has the highest phosphate release rate and alters polymerization critical concentrations among MreB proteins.
- SpeMreB1 interacts with and potentially disassembles SpeMreB5 filaments in nucleotide-bound states.
- SpeMreB1 binds to negatively charged lipids regardless of its nucleotide state.

## Abstract

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 localize the cell wall synthesis complex by forming filaments that slowly turn over to maintain the cell shape. 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 is essential for 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, which allowed us to purify it in the soluble fraction. SpeMreB1 exhibited the highest phosphate (Pi) 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.