An actin-like filament from Clostridium botulinum exhibits a novel mechanism of filament dynamics

Here, we report the discovery of a ParM protein from Clostridium botulinum (CBg-ParM), which forms a double-stranded polar filament. CBg-ParM is promiscuous in hydrolyzing nucleotides, showing robust activities for GDP, ADP, and AMPPNP in addition to ATP and GTP. CBg-ParM shares many similarities in its basic filament architecture with actin, however it does not depolymerize after nucleotide hydrolysis and Pi release, as observed for actin or other ParMs. Instead of destabilizing the filament, the Pi release induces a large lateral strand shift of ~2.5 nm. We identified the ParR (CBg-ParR) that acts as a nucleation factor in the initial stage of polymerization, similar to ParR from the Escherichia coli ParM-R1 plasmid. CBg-ParR also functions as a depolymerization factor, probably by recognizing the structural change in the CBg-ParM filament after Pi release, exhibiting a new mechanism for accelerated turnover of the ParM filament. Thus, CBg-ParM is thus a unique actin homolog with novel filament dynamics and nucleotide hydrolysis mechanisms.