Fig. 1: LetA and LetB form a complex.

a, Model of LetA and LetB in the cell envelope. A cross-section of LetB (PDB 6V0C) is oriented in the context of the inner membrane (IM) and outer membrane (OM), with phospholipids (PL) and LPS indicated. b, Schematic of the letAB operon. c, Western blot from a pull-down assay to assess the interaction between LetA and LetB. His–LetA was used as the bait, and the interaction with untagged LetB was assessed using anti-LetA (clone 72) and anti-LetB antibodies. Three independent purifications were performed starting with three different colonies, with similar results. d, 2D class averages from negative-stain electron microscopy data for full-length LetAB or the soluble periplasmic domain of LetB alone. e, Residues in LetB that were targeted for incorporation of photocrosslinking amino acid, BPA (red sphere for the inside tunnel, and blue spheres for the outside tunnel). f, SDS–PAGE analysis of purified LetAB without BPA incorporation (WT) or with BPA incorporated at positions indicated in panel e. Samples were either UV crosslinked in vivo or uncrosslinked, and the SDS–PAGE gel was stained with Coomassie (LetB) and phosphor-imaged (³²P signal). Three replicates were performed starting with three different colonies, on different days, with similar results. g, SDS–PAGE analysis of purified LetB with BPA incorporated at position F468, with or without co-expression of LetA, prepared as in panel f. Three replicates were performed starting with three different colonies, on different days, with similar results. h, Surface representation of our LetAB cryo-EM structure oriented in the context of the inner membrane and outer membrane. LetB monomers are depicted in different colours. i, Views of the LetAB complex from the cytoplasm (top) and outer membrane (bottom), shown as surface representations. The LetA surface is partially transparent (blue). Gel source data for panels c,f,g are provided in Supplementary Fig. 1a,c,d, respectively.