The transmembrane α-helix of LptC participates in LPS extraction by the LptB 2 FGC transporter.

Lipopolysaccharide (LPS) is an essential component of the outer membrane of most Gram-negative bacteria that provides resistance to various toxic compounds and antibiotics. Newly synthesized LPS is extracted from the inner membrane by the ATP-binding cassette (ABC) transporter LptB 2 FGC, which places the glycolipid onto a periplasmic protein bridge that connects to the outer membrane. This ABC transporter is structurally unusual in that it associates with an additional protein, LptC. The periplasmic domain of LptC is part of the transporter's bridge while its transmembrane α-helix intercalates into the LPS-binding cavity of the core LptB 2 FG transporter. LptC's transmembrane helix affects the in vitro ATPase activity of LptB 2 FG, but its role in LPS transport in cells remains undefined. Here, we describe two roles of LptC's transmembrane helix in Escherichia coli. We demonstrate that it is required to maintain proper levels of LptC and participates in coupling the activity of the ATPase LptB to that of its transmembrane partners LptF/LptG prior to loading LPS onto the periplasmic bridge. Our data support a model in which the association of LptC's transmembrane helix with LptFG creates a nonessential step that slows down the LPS transporter.