Using cryoEM to tackle heterogeneity and conformational change in bacterial nutrient transporters

Speaker Professor Neil Ranson, The Astbury Centre for Structural Molecular Biology, University of Leeds

Title Using cryoEM to tackle heterogeneity and conformational change in bacterial nutrient transporters

Host Julien Bergeron

 

Abstract The Bacteroidetes are abundant bacteria in the human microbiota, occupying a niche in the distal gut where they exploit a myriad of diet and host-derived glycans. Glycan transport across the outer membrane (OM) of these bacteria is mediated by SusCD protein complexes, comprising a membrane-embedded barrel and a lid, that are thought to operate together via a ‘pedal-bin’ mechanism in which the lids open and close to facilitate substrate binding. However, additional lipoproteins are expressed on the cell surface, including glycan binding proteins and glycoside hydrolases, and these play critical roles in the capture of large glycan chains and their processing into transport competent substrates. I will discuss recent working using cryo-EM that shows for two key nutrient transporters in Bacteroides thetaiotaomicron, these additional components assemble around the core SusCD transporter, forming a stable, glycan-utilising macromolecular machine. I will also discuss some of the biochemical and image processing strategies necessary to extract near-atomic resolution structures from complex, heterogeneous datasets, yielding cryo-EM structures in the absence and presence of substrate that reveal concerted conformational changes. These conformational changes rationalise the position of each component for efficient nutrient capture, as well as providing a direct demonstration of the pedal-bin mechanism of substrate capture in the intact OM transporter.