Fig. 1.

Diagrammatic representation of the cell envelope of E. coli. The cell envelope consists of three layers: OM, PG, and IM. PG is stapled to the OM by Lpp or Braun’s lipoprotein (red helix), which exists in either bound or free form (5−9). In the bound form, the N-terminal end of Lpp is anchored to the OM, whereas the C-terminal lysine residue (purple circle) is covalently attached to an mDAP residue (dark green) of the PG stem peptides. The free form of Lpp spans the OM and is exposed to the surface (11). LdtF is identified in this study as an endopeptidase that cleaves the PG−Lpp cross-links and as a glycine-specific carboxypeptidase.