The lipopolysaccharide transporter complex LptB2FG also displays adenylate kinase activity in vitro dependent on binding partners LptC/LptA

Lipopolysaccharide (LPS) is an essential glycolipid that covers the surface of gram-negative bacteria. The transport of LPS involves a dedicated seven-protein transporter system called the Lpt machinery that physically spans the entire cell envelope. The LptB2FG complex is an ABC transporter that hydrolyses ATP to extract LPS from the inner membrane (IM) for transport to the outer membrane. Here we extracted LptB2FG directly from the IM with its original lipid environment using Styrene-Maleic acid polymers (SMA). We found that SMA-LptB2FG in nanodiscs display not only ATPase activity but a previously uncharacterized Adenylate Kinase (AK) activity, as it catalyzed phosphotransfer between two ADP molecules to generate ATP and AMP. The ATPase and AK activities of LptB2FG were both stimulated by the interaction on the periplasmic side with the periplasmic LPS transport proteins LptC and LptA and inhibited by the presence of the LptC transmembrane helix. We determined that the isolated ATPase module (LptB) had weak AK activity in absence of transmembrane proteins LptF and LptG; one mutation in LptB that weakens its affinity for ADP led to AK activity similar to that of fully assembled complex. Thus, we conclude that LptB2FG is capable of producing ATP from ADP, depending on the assembly of the Lpt bridge, and that this AK activity might be important to ensure efficient LPS transport in the fully assembled Lpt system.