A new class of alternating co-polymeric cytomotive filament in bacteria

Deviant Walker A Cytoskeletal ATPases (WACAs) are widely conserved bacterial proteins that play critical roles in septum site determination, plasmid and chromosomal DNA segregation, regulation of development, positioning of chemotaxis proteins and diverse other functions. It has been suggested that at least some of these functions are mediated by the constant oscillation of these ATPases within the cell. The bacterial MinCDE septum-site selection system, containing MinC, MinD and MinE, is an example of one such oscillation system. In Gram-negative bacteria, the Min proteins continuously oscillate between the two cell poles in order to determine the cell division plane. MinD, the deviant Walker A ATPase, along with its activator protein MinE is thought to generate the oscillation, while MinC has been suggested to be a passenger in this reaction diffusion process. The source of cooperativity that is required to generate the oscillation has remained largely elusive, although several reports have described polymers of the ATPases themselves. Here, I show that MinD does not form filaments on its own. Instead it forms nucleotide-dependent, alternating copolymeric filaments with its partner protein MinC and this is investigated biochemically. Furthermore, I describe the MinCD co-crystal structure and together with EM analysis, reveal the architecture of MinCD filaments. Finally, I show MinE-dependent disassembly of MinCD filaments and propose that this is at least partly caused by the overlap of the binding sites of MinC and MinE on MinD. Taken together, this suggests that MinCD filaments belong to a new class of nucleotide-regulated cytomotive filaments, in addition to the well-known actin and tubulin-like systems present in many bacteria.