In the Beginning, Escherichia coli Assembled the Proto-ring: An Initial Phase of Division

Abstract

Cell division in Escherichia coli begins by assembling three proteins, FtsZ, FtsA, and ZipA, to form a proto-ring at midcell. These proteins nucleate an assembly of at least 35 components, the divisome. The structuring of FtsZ to form a ring and the processes that effect constriction have been explained by alternative but not mutually exclusive mechanisms. We discuss how FtsA and ZipA provide anchoring of the cytoplasmic FtsZ to the membrane and how a temporal sequence of alternative protein interactions may operate in the maturation and stability of the proto-ring. How the force needed for constriction is generated and how the proto-ring proteins relate to peptidoglycan synthesis remain as the main challenges for future research. Overview of SeptationCell division is the last event in the bacterial cell cycle. It takes place by producing a rigid transversal septum after the growth processes fully duplicate the contents of the cell. Septation is always preceded by segregation of the duplicated nucleoids, ensuring that each of the daughter cells contains a fully replicated chromosome. Specialized molecules control the positioning of the division machinery at midcell, preventing any harmful fragmentation that septum progression could cause on unsegregated nucleoids.The division machinery, the divisome, establishes a complex interacting network together with DNA, lipids, and peptidoglycan components. It comprises a variable number of proteins, depending on the bacterial species. These proteins have redundant multiple functions and connections serving as safeguards to complete division and to guarantee the efficiency and versatility of the process. Altogether, an efficient division process allows the proliferation of bacteria under a variety of conditions, and it is one of the reasons for their success in the environment. Free-living bacteria, such as Escherichia coli, have more elaborated divisomes, whereas those that need to grow as intracellular parasites, as such Mycoplasma genitalium, have streamlined ones (1, 2).The structure of the E. coli divisome as revealed by immunofluorescence images of dividing cells is called the division ring (3). For its assembly, three essential proteins, FtsZ, FtsA, and ZipA, are first gathered at midcell, forming a proto-ring attached to the inner membrane ( Fig. 1) (4). In the proto-ring structure, the cytoplasmic GTPase FtsZ is anchored to the membrane by its interaction with ZipA and FtsA. ZipA is a bitopic protein containing a transmembrane domain (5). FtsA is a protein of the actin family that associates with the membrane by a short amphipathic helix (6, 7). The assembly of the FtsZ ring is dependent on a continuous energy supply. The FtsZ ring requires either ZipA or FtsA (8). Although it can be formed in the presence of either of them, division is affected unless both are present (9). A maturation period takes place after the protoring is assembled and before the rest of the divisome proteins become incorporated (10). During maturation, the proto-ring assembly is not stable...

Splitsville: structural and functional insights into the dynamic bacterial Z ring

“…Therefore, FtsZ polymers require at least one membrane-associated factor to tether them to the inner surface of the cytoplasmic membrane. In E. coli , this task is accomplished by a partnership between FtsA and ZipA proteins, which are both essential for cytokinesis and simultaneously associate with the membrane and bind the conserved carboxyl-terminal core peptide of FtsZ 12 . The assembly of these three proteins at the membrane defines the essential part of the proto-ring.…”

Functional Modules of Minimal Cell Division for Synthetic Biology

“…[70][71][72] These proteins associate at the cytoplasmic membrane, forming a scaffold structure into which the other proteins are incorporated. [73][74][75] The attachment to the membrane can also be achieved in vitro by replacing the second player, FtsA or ZipA, by an N-or C-terminal membrane-targeting sequence (mts) added to FtsZ. It is highly conserved in the bacterial domain and the ancestor of eukaryotic tubulin.…”

“…[105] However, it has to be stated that this is only one out of many possible factors that could contribute to the required force. ZipA-FtsZ binding also increases membrane fluidity, [75] while MreB (gray) structures colocalize with Z-ring at mid-cell. Arumugam et al showed that Z-rings contain pre-curved FtsZ filaments, which preferentially orient themselves along curvatures similar to the inner diameter of E. coli cells, but at the same time avoid smaller diameters resembling constriction stages during division.…”

Lessons from a Minimal Genome: What Are the Essential Organizing Principles of a Cell Built from Scratch?

“…[64] As mentioned above, FtsZ has multiple spatialr egulators that allow it to coalesce in the form of aZring on the inner membrane at mid-cell (reviewed in Monahan et al [50] ). [66,67] FtsZ polymerization and treadmilling dynamics have been successfully reconstituted on supported lipid bilayersb yu sing FtsZ's native membrane anchors FtsA and ZipA purifiedf rom E. coli. FtsZ cannot bind to the membrane by itself but depends on adaptor proteins such as FtsA and ZipA that recruit FtsZ to the membrane.…”