Luis Orenday Tapia "Elaborated Outer Membrane architectures in Bacteroidota: an enhanced BAM complex"

Bacteroidota are prominent human commensals and the most abundant Gram-negative bacteria in the human gut. Some species are associated with diseases, while others participate in nutrient uptake by degradation of complex polysaccharides. Many of these functions are associated to their multilayered envelope. The Bacteroidota outer membrane (OM) differs to the well-characterized OM of gamma-proteobacteria (e.g. Escherichia coli). The Bacteroidota OM is characterized by the presence of unusually elaborate outer membrane proteins (OMPs), many of which bind surface lipoproteins (SLPs).

Folding and insertion of OMPs is mediated by the beta-barrel assembly machinery (BAM) complex. In E. coli, the well-characterized BAM complex is composed by the BamA (the main and catalytic subunit) and four lipoprotein subunits (BamBCDE) are bound to the periplasmic domain of BamA.

To study how to machinery has adapted in the Bacteroidota, the BamA homologue of Flavobacterium johnsoniae was tagged. After purification, the BAM complex was structurally characterized. Of the subunits found in the E. coli complex only BamA and BamD are retained, and four new proteins are present. These include two SLPs and a second OMP that together form an extensive extracellular canopy above the BamA lateral seam. Two of the additional subunits are essential, demonstrating their importance in Bacteroidota outer membrane biogenesis. Depletion of these essential subunits allowed identification of BAM substrates, for which we are currently characterizing their importance in the OM of F. johnsoniae. Furthermore, isolation of a suppressor mutation of one of these subunits allowed the separation of their essential and non-essential functions.
We propose the BAM complex in Bacteroidota is result of the need of a highly remodelled and enhanced machinery that allows protein folding of unusually complex substrates.