Targeting of DMinC/MinD and DMinC/DicB complexes to septal rings in Escherichia coli suggests a multistep mechanism for MinC-mediated destruction of nascent FtsZ rings

The MinC protein is an important determinant of septal ring positioning in Escherichia coli. The N-terminal domain ((Z)MinC) suppresses septal ring formation by interfering with FtsZ polymerization, whereas the C-terminal domain ((D)MinC) is required for dimerization as well as for interaction with the MinD protein. MinD oscillates between the membrane of both cell halves in a MinE-dependent fashion. MinC oscillates along with MinD such that the time-integrated concentration of (Z)MinC at the membrane is minimal, and hence the stability of FtsZ polymers is maximal, at the cell center. MinC is cytoplasmic and fails to block FtsZ assembly in the absence of MinD, indicating that recruitment of MinC by MinD to the membrane enhances (Z)MinC function. Here, we present evidence that the binding of (D)MinC to MinD endows the MinC/MinD complex with a more specific affinity for a septal ring-associated target in vivo. Thus, MinD does not merely attract MinC to the membrane but also aids MinC in specifically binding to, or in close proximity to, the substrate of its (Z)MinC domain. MinC-mediated division inhibition can also be activated in a MinD-independent fashion by the DicB protein of cryptic prophage Kim. DicB shows little homology to MinD, and how it stimulates MinC function has been unclear. Similar to the results obtained with MinD, we find that DicB interacts directly with (D)MinC, that the (D)MinC/DicB complex has a high affinity for some septal ring target(s), and that MinC/DicB interferes with the assembly and/or integrity of FtsZ rings in vivo. The results suggest a multistep mechanism for the activation of MinC-mediated division inhibition by either MinD or DicB and further expand the number of properties that can be ascribed to the Min proteins.