Glycan recognition by the Bacteroidetes Sus-like systems

The human gut Bacteroidetes employ multiple cell-envelope associated protein complexes, termed Sus-like systems, to capture and degrade glycans. Recently the structures of key glycan-binding Sus proteins, the surface located SusD proteins and the periplasmic sensor domains of the membrane spanning hybrid two-component systems (HTCS) controlling Sus expression, have been solved, providing insight into glycan acquisition and sensing in these symbionts. The α-helical SusD proteins bind glycans at the cell surface, and likely facilitate the shuttling of oligosaccharides to an associated TonB-dependent porin. The HTCS sensor domains adopt two distinct folds and the structures of these sensors with glycan suggest that signal transduction across the cytoplasmic membrane is different from classical two component systems.