Multiple Interactions between the Transmembrane Division Proteins ofBacillus subtilisand the Role of FtsL Instability in Divisome Assembly

About 11 essential proteins assemble into a ring structure at the surface of the cell to bring about cytokinesis in bacteria. Several of these proteins have their major domains located outside the membrane, forming an assembly that we call the outer ring (OR). Previous work on division inBacillus subtilis has shown that four of the OR proteins—FtsL, DivIC, DivIB, and PBP 2B—are interdependent for assembly. This contrasts with the mainly linear pathway for the equivalent proteins inEscherichia coli . Here we show that the interdependent nature of theB. subtilis pathway could be due to effects on FtsL and DivIC stability and that DivIB is an important player in regulating this turnover. Two-hybrid approaches suggest that a multiplicity of protein-protein interactions contribute to the assembly of the OR. DivIC is unusual in interacting strongly only with FtsL. We propose a model for the formation of the OR through the mutual association of the membrane proteins directed by the cytosolic inner-ring proteins.