Journal Article

Insights into SusCD-mediated glycan import by a prominent gut symbiont

Baslé, Arnaud, Hiller, Sebastian, Glenwright, Amy J., van den Berg, Bert, Gray, Declan A., Mazur, Adam, Oluwole, Abraham O., Evans, Sasha L., Ranson, Neil A., Cartmell, Alan, White, Joshua B. R., Zahn, Michael, Bolam, David N., Robinson, Carol V., Morland, Carl, Rath, Parthasarathi

2021

Overview

In Bacteroidetes, one of the dominant phyla of the mammalian gut, active uptake of large nutrients across the outer membrane is mediated by SusCD protein complexes via a “pedal bin” transport mechanism. However, many features of SusCD function in glycan uptake remain unclear, including ligand binding, the role of the SusD lid and the size limit for substrate transport. Here we characterise the β2,6 fructo-oligosaccharide (FOS) importing SusCD from Bacteroides thetaiotaomicron (Bt1762-Bt1763) to shed light on SusCD function. Co-crystal structures reveal residues involved in glycan recognition and suggest that the large binding cavity can accommodate several substrate molecules, each up to ~2.5 kDa in size, a finding supported by native mass spectrometry and isothermal titration calorimetry. Mutational studies in vivo provide functional insights into the key structural features of the SusCD apparatus and cryo-EM of the intact dimeric SusCD complex reveals several distinct states of the transporter, directly visualising the dynamics of the pedal bin transport mechanism. In Bacteroidetes, SusCD complexes mediate uptake of large nutrients across the outer membrane. SusCD structures in the apo state and in complex with β2,6 fructo-oligosaccharides reveal several substrate molecules in the binding cavity and suggest details of the pedal bin mechanism employed in glycan import.

• Publisher: Nature Publishing Group UK,Nature Publishing Group,Nature Portfolio
• Subject: 101/28 / 101/6 / 631/326/1320 / 631/535/1258/1259 / 631/535/1266 / 631/92/577 / Bacterial Proteins - chemistry / Bacterial Proteins - metabolism / Bacteroidetes / Binding / Binding sites / Calorimetry / Chromatography / Cryoelectron Microscopy / Crystal structure / Fructooligosaccharides / Gastrointestinal Microbiome / Glycan / Humanities and Social Sciences / Imports / In vivo methods and tests / Ligands / Magnetic Resonance Spectroscopy / Mass spectrometry / Mass spectroscopy / Membranes / Microbiota / Models, Molecular / multidisciplinary / Nutrients / Oligosaccharides - chemistry / Polysaccharides - chemistry / Polysaccharides - metabolism / Protein Conformation / Science / Science (multidisciplinary) / Scientific imaging / Structure-Activity Relationship / Substrates / Symbiosis / Titration / Titration calorimetry