A lipoprotein partner for the Escherichia coli outer membrane protein TolC

The outer-membrane protein TolC from Escherichia coli belongs to an extensive superfamily whose members are found throughout the didermal, Gram-negative bacterial lineages. The protein serves as an activated exit duct in multi-drug efflux pumps and protein secretion machinery. Many TolC homologs bear a lipid modification on the N-terminus that embeds into the inner leaflet of the outer membrane and appears to have been a conserved feature for millions of years; however, the moiety is absent entirely in the E. coli TolC. We have discovered that the E. coli lipoprotein YbjP interacts extensively with the periplasmic surface of TolC and its N-terminal lipid moiety is embedded in the membrane, mimicking the intramolecular interactions seen for related proteins. Here, we present cryo-EM structures of the MacA-MacB-TolC and AcrA-AcrB-TolC tripartite pumps complexed to YbjP. We demonstrate that the association occurs spontaneously both in vitro and in vivo and that YbjP facilitates recovery following exposure to bacteriostatic agents. We suggest that the YbjP-TolC interaction may facilitate the assembly of the outer membrane protein in a redundant biogenesis pathway.