Dynamics and architecture of Bacillus subtilis cell division

Abstract

Cells generate more cells. This proliferation requires multiple cellular-scale morphological changes from one generation to the next. One cell must physically separate into two daughters in cytokinesis. In bacteria, this change represents the generation not only of separate cells, but also of separate organisms. However, the mechanisms by which nanometer scale proteins coordinate this micron scale reorganization is not understood. The filament-forming protein FtsZ organizes the division site, forming a Z-ring that recruits cell wall synthesis enzymes to build a septum between daughters. Understanding how FtsZ organizes division’s cellular-scale change requires studying 1) what spatiotemporal patterns are established by FtsZ, 2) how these patterns are regulated by other factors, and 3) how these patterns effect physiology downstream of the Z-ring. In addition to forming a ring at midcell, FtsZ filaments treadmill around the division site; this treadmilling is required for the coincident motion of the cell wall synthesis enzyme Pbp2B, as well as efficient cell division. To understand how the division machinery collectively functions, here I present single-molecule imaging of the dynamics of the entire Bacillus subtilis division machinery using TIRF microscopy. The proteins previously shown to bind FtsZ (ZapA, SepF, and EzrA) remain stationary, associated with their bound FtsZ subunits. Meanwhile, Pbp2B moves in complex with the cell wall synthesis protein FtsW and the DivIB-DivIC-FtsL complex. The division complex is therefore actually made of two distinct subcomplexes: one stationary and the other moving around the cell. Further, I present a characterization of Z-ring architecture: FtsZ condensation into narrow rings by the FtsZ binding proteins ZapA, SepF, and EzrA. Removing synthetically lethal combinations of these proteins results in FtsZ being unable to bundle into narrow rings as cells die. This lethality cannot be explained solely by decreased recruitment of Pbp2B, and cell wall synthesis dynamics are unperturbed in uncondensed Z-rings. Taken together, these results show that a subset of stationary divisome proteins coordinate essential changes in FtsZ architecture, while another subset of divisome proteins—cell wall synthesis proteins and their putative regulators—move collectively dependent on FtsZ treadmilling.