Essential cell division protein. May link together the upstream cell division proteins, which are predominantly cytoplasmic, with the downstream cell division proteins, which are predominantly periplasmic. Can also mediate Zn(2+)-sensitivity probably by increasing the permeability of the inner membrane. Part of a complex composed of FtsB, FtsL and FtsQ. The complex can be formed before its localization to the division site. This tripartite complex can be divided further into a subcomplex of FtsB and FtsL, which forms in the absence of FtsQ. Interacts also with FtsI, FtsN and YmgF. Homodimer in vitro. May form multimers. Localizes to the division septum where it forms a ring structure. Localization requires FtsZ, FtsA, ZipA, FtsK, FtsQ and FtsB, but not FtsI and FtsN. Localization of FtsB and FtsL is codependent. The cytoplasmic region is involved in recruitment of the downstream cell division protein FtsW. The transmembrane segment and the membrane-proximal periplasmic region form a coiled-coil structure and are required for septal localization and interaction with FtsB. The C-terminal region is required for interaction with FtsQ. Contains a leucine zipper-like (LZ) motif, which is required for optimal interaction with FtsB. Mutants show inhibition of cell division, formation of long, nonseptate filaments, ultimate cessation of growth and lysis. Belongs to the FtsL family.