Bacterial lipids traverse the hydrophobic groove of TamB.

The double-membrane envelope of Gram-negative bacteria protects them against environmental stress and antibiotics. Phospholipids are a core component of the bacterial outer membrane (OM). However, how phospholipids are transported to the OM from the inner membrane where they are synthesized is poorly understood. We show that the AsmA-like protein TamB transfers lipids through a large hydrophobic groove that directly bridges the inner and outer membranes. Lipid dissociation at the outer membrane is impeded when TamB is bound to its OM partner protein TamA, but lipids can spontaneously enter the groove in simulations where the C terminus of TamB is embedded in the OM. Other members of the AsmA-like family in E. coli can also accommodate lipids within their hydrophobic grooves, suggesting they may also function as lipid transporters. Our findings highlight the lipid transport ability of TamB and other AsmA-like proteins, suggesting their importance in maintaining bacterial OM integrity.