Abstract
FtsZ is an essential bacterial cytoskeletal protein that assembles into a ring-like structure (Z-ring) at midcell to carry out cytokinesis. In vitro, FtsZ exhibits polymorphism in polymerizing into different forms of filaments based on its GTPase activity, concentration, and buffer condition. In vivo, the Z-ring appeared to be punctate and heterogeneously organized, although continuous, homogenous Z-ring structures have also been observed. Understanding how the Z-ring is organized in vivo is important because it provides a structural basis for the functional role of the Z-ring in cytokinesis. Here, we assess the effects of both GTPase activity and FtsZ concentration on the organization of the Z-ring in vivo using three-dimensional (3D) superresolution microscopy. We found that the Z-ring became more homogenous when assembled in the presence of a GTPase-deficient mutant, and upon overexpression of either wt or mutant FtsZ. These results suggest that the in vivo organization of the Z-ring is largely dependent on the intrinsic polymerization properties of FtsZ, which are significantly influenced by the GTPase activity and concentration of FtsZ. Our work provides a unifying theme to reconcile previous observations of different Z-ring structures, and supports a model in which the wt Z-ring comprises loosely associated, heterogeneously distributed FtsZ clusters. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 725-734, 2016.