The Soluble Periplasmic Domains of Escherichia coli Cell Division Proteins FtsQ/FtsB/FtsL Form a Trimeric Complex with Submicromolar Affinity.
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Cell division in Escherichia coli involves a set of essential proteins that assembles at midcell to form the so-called divisome. The divisome regulates the invagination of the inner membrane, cell wall synthesis, and inward growth of the outer membrane. One of the divisome proteins, FtsQ, plays a central but enigmatic role in cell division. This protein associates with FtsB and FtsL, which, like FtsQ, are bitopic inner membrane proteins with a large periplasmic domain (denoted FtsQ<inf>p</inf>, FtsB<inf>p</inf>, and FtsL<inf>p</inf>) that is indispensable for the function of each protein. Considering the vital nature and accessible location of the FtsQBL complex, it is an attractive target for protein-protein interaction inhibitors intended to block bacterial cell division. In this study, we expressed FtsQ<inf>p</inf>, FtsB<inf>p</inf>, and FtsL<inf>p</inf> individually and in combination. Upon co-expression, FtsQ<inf>p</inf> was co-purified with FtsB<inf>p</inf> and FtsL<inf>p</inf> from E. coli extracts as a stable trimeric complex. FtsB<inf>p</inf> was also shown to interact with FtsQ<inf>p</inf> in the absence of FtsL<inf>p</inf> albeit with lower affinity. Interactions were mapped at the C terminus of the respective domains by site-specific cross-linking. The binding affinity and 1:1:1 stoichiometry of the FtsQ<inf>p</inf>B<inf>p</inf>L<inf>p</inf> complex and the FtsQ<inf>p</inf>B<inf>p</inf> subcomplex were determined in complementary surface plasmon resonance, analytical ultracentrifugation, and native mass spectrometry experiments.