Outer membrane biogenesis in Gram-negative bacteria

Gram-negative bacteria have become increasingly resistant to available antibiotics resulting in more illness, healthcare costs, and deaths. Their cell envelope consists of the inner membrane (IM) surrounding the cytoplasm and an outer membrane (OM) that protects the cells from harsh conditions. The OM is an asymmetric bilayer made up of phospholipids (PL) and lipopolysaccharides (LPS). Also, the OM harbors numerous β- barrel proteins (outer membrane proteins, OMPs). Both LPS and OMPs are synthesized in the cytoplasm and subsequently transported across the periplasm into the OM. In E. coli,  seven essential proteins LptABCDEFG spanning the entire cell envelope form the LPS transport system. Similarly, The β-Barrel Assembly Machinery (BAM), which consists of the BamABCDE subunits mediates folding and insertion of OMP precursors from periplasm into the OM.

 

The BAM complex - β-barrel folding and insertion mechanism

In the two available structures for BAM full-complex, the central β-barrel BamA exists either in an inward-open or lateral open conformation. It is considered that those conformational changes might be coupled to OMP folding and insertion through an unknown mechanism. As the asymmetric outer membrane is an integral part of the BAM complex, mechanistic investigations must be performed in whole cells or native membranes. Using the in-situ ESR approach, which we demonstrated over the past years, we are elucidating how the conformational changes in BAM are coupled to protein folding and insertion.