Bacillus anthracis tubulin-related protein Ba-TubZ assembles force-generating polymers

Pathogenicity of Bacillus anthracis depends on the faithful inheritance of plasmid pXO1, in a process that requires the plasmid encoded tubulin-related protein Ba-TubZ. Here we show, using heterologous expression in Schizosaccharomyces pombe, that Ba-TubZ assembles into a dynamic polymer in the absence of other B. anthracis proteins and can generate force capable of deforming the fission yeast nuclear envelope. The polymer bundles contain 27 ± 15 protofilaments/μm assuming that each protofilament spans the entire length. Thinner appearing buckled and thicker appearing straight filaments of Ba-TubZ were both capable of inducing nuclear envelope deformation. Unlike the related protein Bt-TubZ from Bacillus thuringiensis, which undergoes treadmilling upon expression in fission yeast, Ba-TubZ polymers did not undergo detectable treadmilling. Instead, in fluorescence recovery after photobleaching experiments, it displayed a different turnover behavior characterized by moderate fluorescence recovery along the entire length of the polymer. Modeling Ba-TubZ bundles as Euler-Bernoulli beams that buckle under compressive loads when pushed against the nuclear envelope allowed us to estimate that Ba-TubZ generates forces in the order of 1-10 nN. We propose that polymerization based filament elongation and force generation might aid faithful segregation of the virulence plasmid.