A polymeric protein anchors the chromosomal origin/ParB complex at a bacterial cell pole.

Bacterial replication origins move towards opposite ends of the cell during DNA segregation. We have identified a proline-rich polar protein, PopZ, required to anchor the separated Caulobacter crescentus chromosome origins at the cell poles, a function that is essential for maintaining chromosome organization and normal cell division. PopZ interacts directly with the ParB protein bound to specific DNA sequences near the replication origin. As the origin/ParB complex is being replicated and moved across the cell, PopZ accumulates at the cell pole and tethers the origin in place upon arrival. The polar accumulation of PopZ occurs by a diffusion/capture mechanism that requires the MreB cytoskeleton. High molecular weight oligomers of PopZ assemble in vitro into a filamentous network with trimer junctions, suggesting that the PopZ network and ParB-bound DNA interact in an adhesive complex, fixing the chromosome origin at the cell pole.