Characterization of interactions between LPS transport proteins of the Lpt system

The lipopolysaccharide transport system (Lpt) in Gram-negative bacteria is responsible for transporting lipopolysaccharide (LPS) from the cytoplasmic surface of the inner membrane, where it is assembled, across the inner membrane, periplasm and outer membrane, to the surface where it is then inserted in the outer leaflet of the asymmetric lipid bilayer. The Lpt system consists of seven known LPS transport proteins (LptA-G) spanning from the cytoplasm to the cell surface. We have shown that the periplasmic component, LptA is able to form a stable complex with the inner membrane anchored LptC but does not interact with the outer membrane anchored LptE. This suggests that the LptC component of the LptBFGC complex may act as a dock for LptA, allowing it to bind LPS after it has been assembled at the inner membrane. That no interaction between LptA and LptE has been observed supports the theory that LptA binds LptD in the LptDE homodimeric complex at the outer membrane.

Research highlights
► Expressed and purified three proteins from the Lpt system: LptC, LptE and LptA.
► Showed by pull-down and SPR that LptA interacts with the IM anchored LptC.
► Determined binding constant for this interaction by SPR.
► Showed that the LptA does not interact with the OM component LptE.
► We postulate that LptC acts as an IM docking site for LptA.
► We postulate that LptD and not LptE provides the OM docking site for LptA.