Models for the arrangement of FtsZ protofilaments inside the Z ring.
(A) We propose two models to describe the architecture of the Z ring. The overlapping model predicts that short FtsZ protofilaments begin to bundle in both lateral (circumferential) and radial directions of the cell. Alternatively, FtsZ protofilaments could be arranged into a single layer in the radial direction and only bundle in the lateral direction. (B) The amount of FtsZ inside the Z ring is constant throughout constriction, but the distribution of FtsZ inside the rings fluctuates over time. As constriction begins the Z ring exerts a localized pinching force on the membrane where increased levels of FtsZ are found. Gaps (indicated by red brackets) in the Z ring allow constriction to occur continuously as the circumference of this structure becomes reduced (indicated by the decrease in height of the blue rectangle).