Force generation by a dynamic Z-ring in Escherichia coli cell division

Abstract

FtsZ, a bacterial homologue of tubulin, plays a central role in bacterial cell division. It is the first of many proteins recruited to the division site to form the Z-ring, a dynamic structure that recycles on the time scale of seconds and is required for division to proceed. FtsZ has been recently shown to form rings inside tubular liposomes and to constrict the liposome membrane without the presence of other proteins, particularly molecular motors that appear to be absent from the bacterial proteome. Here, we propose a mathematical model for the dynamic turnover of the Z-ring and for its ability to generate a constriction force. Force generation is assumed to derive from GTP hydrolysis, which is known to induce curvature in FtsZ filaments. We find that this transition to a curved state is capable of generating a sufficient force to drive cell-wall invagination in vivo and can also explain the constriction seen in the in vitro liposome experiments. Our observations resolve the question of how FtsZ might accomplish cell division despite the highly dynamic nature of the Z-ring and the lack of molecular motors.bacterial cell division ͉ constriction force ͉ FtsZ ͉ mathematical model ͉ cytokinesis

Nucleotide-dependent conformations of FtsZ dimers and force generation observed through molecular dynamics simulations

“…Our data suggest that during hydrolysis, FtsZ filaments can generate a significant amount of force, providing a molecular basis for the previously proposed "hydrolyze and bend" model for the force generation of FtsZ (11)(12)(13)16). In the absence of hydrolysis, FtsZ can still produce a smaller mechanical force, which could be the constriction mechanism in the occasional in vivo division events observed in cells with a mutant FtsZ that exhibits very low hydrolysis activity (10,34).…”

Defining the rate-limiting processes of bacterial cytokinesis

“…Proposed Z-ring force generation models predict that Z-ring structure would be remodeled in different ways as it contracts [i.e., thickening (46), widening (44), condensing (43), or disassembling (38,39,42)]. To determine whether the Z-ring undergoes such structural remodeling in vivo, we examined Z-ring structures using the single-molecule-based superresolution technique, photoactivated localization microscopy (PALM) (47).…”

“…The duration of visible cell constriction, τ c , can thus be divided into phase I (from the first visible cell wall indentation to the initiation of Z-ring disassembly) and phase II (Z-ring disassembly). The substantial duration of constriction phase I suggests that, although theoretical models suggest that Z-ring disassembly can drive constriction (38,39,42), constriction can both initiate and proceed without Z-ring disassembly in vivo.…”

“…Having established methods to measure septum closure rates, we next investigated whether the average rate would be altered under perturbed Z-ring properties, which would be expected if Z-ring contraction drives septum closure. We first examined the effects of reduced FtsZ GTPase activity, because GTP hydrolysis-dependent FtsZ protofilament bending and/or disassembly have been prominent hypotheses for force generation by the Z-ring (38,39,42).…”

“…For example, purified, membrane-tethered FtsZ was shown to assemble into ring-like structures that deform and constrict liposome membranes (30)(31)(32)(33)(34)(35). Mechanistically, it has been proposed that a constrictive force could be generated by the bending of FtsZ protofilaments because of their preferred curvature or GTP hydrolysis-induced conformation change (36)(37)(38)(39)(40)(41), immediate reannealing of FtsZ protofilaments upon GTP hydrolysis-induced subunit loss (42), condensation of FtsZ protofilaments caused by their lateral affinity (43), or a combination of these mechanisms (38,42,44,45). However, these proposed mechanisms have been difficult to test in vivo because of the essentiality of FtsZ, the limited ability to spatially resolve the Z-ring structure in small bacterial cells, and the lack of sensitive methods to monitor Z-ring contraction and the rate of septum closure.…”