Spatial resolution of two bacterial cell division proteins: ZapA recruits ZapB to the inner face of the Z-ring

FtsZ, the essential regulator of bacterial cell division, is a dynamic cytoskeletal protein that forms helices that condense into the Z-ring prior to division. Two small coiled-coil proteins, ZapA and ZapB, are both recruited early to the Z-ring. We show here that ZapB is recruited to the Z-ring by ZapA. A direct interaction between ZapA and ZapB is supported by bacterial two-hybrid and in vitro interaction assays. Using high-resolution 3-D reconstruction microscopy, we find that, surprisingly, ZapB is located inside the Z-ring in virtually all cells investigated. We propose a molecular model in which ZapA increases lateral interactions between FtsZ proto-filaments and ZapB mediates further stabilization of this interaction by cross-linking ZapA molecules bound to adjacent FtsZ proto-filaments. Gene deletion and complementation assays show that ZapB can mitigate cell division and Z-ring assembly defects even in the absence of ZapA, raising the possibility that ZapB stimulates Z-ring assembly by two different mechanisms.