The Transmembrane Domains of the Bacterial Cell Division Proteins FtsB and FtsL Form a Stable High-Order Oligomer

FtsB and FtsL are two essential integral membrane proteins of the bacterial division complex or divisome, both characterized by a single transmembrane helix and a juxtamembrane coiled coil domain. The two domains are important for the association of FtsB and FtsL, a key event for their recruitment to the divisome, which in turn allows the recruitment of the late divisomal components to the Z-ring and subsequent completion of the division process. Here we present a biophysical analysis performed in vitro that shows that the transmembrane domains of FtsB and FtsL associate strongly in isolation. Using Forster resonance energy transfer, we have measured the oligomerization of fluorophore-labeled transmembrane domains of FtsB and FtsL in both detergent and lipid. The data indicate that the transmembrane helices are likely a major contributor to the stability of the FtsB-FtsL complex. Our analyses show that FtsB and FtsL form a 1:1 higher-order oligomeric complex, possibly a tetramer. This finding suggests that the FtsB-FtsL complex is capable of multivalent binding to FtsQ and other divisome components, a hypothesis that is consistent with the possibility that the FtsB-FtsL complex has a structural role in the stabilization of the Z-ring.