Study of Brucella abortus / Agrobacterium tumefaciens outer membrane proteins covalently bound to peptidoglycan

Gram negative bacteria are surrounded by the cell envelope, a structure comprised of an outer membrane (OM) and an inner membrane (IM). These two membranes delimit the periplasmic space, a compartment containing the peptidoglycan (PG), a giant polymer surrounding the cell and functioning as an extracytoplasmic skeleton. In the model organism Escherichia coli, covalently anchoring the OM to the peptidoglycan is crucial for envelope integrity. When attachment is prevented, the OM forms blebs and detaches from the cell body. Intriguingly, the only bacterial protein known to covalently attach the OM to the PG (the Braun lipoprotein or Lpp) is present in a small number of γ-proteobacteria, raising the question of OM-PG attachment in other species. Here, we show that in -proteobacteria Brucella abortus and Agrobacterium tumefaciens, the OM is attached to the peptidoglycan via the formation of covalent crosslinks between the N-terminus of integral OM proteins (OMPs; including porins) and peptide stems of peptidoglycan.