Figure 13.

CTL regulates lateral interactions between FtsZ protofilaments. Schematic describing polymerization dynamics of FtsZ and highlighting the proposed roles for the CTL in regulating longitudinal and lateral interactions. FtsZ monomers bind GTP, nucleate, and assemble into short protofilaments that can elongate further and/or anneal together. GTP-bound monomers within the protofilaments can undergo GTP hydrolysis leading to a conformational change that results in destabilization of longitudinal interactions. The CTL might play a role in regulating monomer–monomer interactions that contribute to this destabilization. This destabilization leads to fragmentation and/or depolymerization. At steady state, the forward reactions, nucleation, elongation, and annealing, balance out the reverse reactions, depolymerization and fragmentation, as long as GTP-bound FtsZ monomers are available. FtsZ protofilaments can also form lateral interactions that are most likely disrupted prior to fragmentation and/or depolymerization. We propose that the CTL plays a role in regulating this lateral interaction by functioning as a repulsive brush around FtsZ protofilaments. In the absence of the CTL (or both the CTL and the CTC), lateral interaction between protofilaments is stronger and/or less specific, thereby favoring the assembly of protofilaments into stable bundles. Furthermore, bundle formation could reduce protofilament turnover by stabilizing protofilaments against fragmentation/depolymerization.