Structural analyses of a heterodimeric SusCD complex captures intermediate states of maltooligosaccharide transport
- 1University of Michigan Microbiology and Immunology
Abstract
Bacteroides are abundant gut bacteria with diverse polysaccharide metabolizing capabilities encoded by co-transcribed polysaccharide utilization loci (PULs). The starch utilization system (Sus) is required for starch and a-glucan metabolism in Bacteroides thetaiotaomicron and has served as a model for PUL studies. However, the mechanism of maltooligosaccharide (MOS) transport and the architecture of the SusCD complex remain incompletely understood. Here, we used cryo-electron microscopy to capture multiple transport-relevant conformations of the SusD lipoprotein and the TonB-dependent transporter SusC in both unliganded and ligand-bound states. In the absence of ligand, SusCD adopts three conformations including an open state in which SusD is displaced from SusC, a state containing the SusC plug, and a state lacking density for the SusC plug. Three-dimensional variability analysis of the structure where SusD is displaced from SusC reveals continuous SusD mobility coupled to movement of the SusC N-terminus. The maltoheptaose-bound SusCD structure reveals two binding sites, one at the SusD-SusC interface and a second within SusC, indicating how MOS stabilizes a closed SusCD assembly. Notably, all structures show that SusCD is a heterodimer, unlike other Sus-like transporters that assemble into SusCD heterotetramers. Native mass spectrometry confirms this heterodimeric stoichiometry, and comparisons with other SusCD-like complexes suggest a structural basis for this distinct assembly of SusCD. Together, these data define substrate binding and intermediate transport states in a canonical PUL system and reveal architectural divergence among Sus-like carbohydrate transport complexes in Bacteroides.
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BibTeX
@misc{cadigan2026structural,
title = {{Structural analyses of a heterodimeric SusCD complex captures intermediate states of maltooligosaccharide transport}},
author = {M. C. Cadigan and N. Rivera-Fuentes and W. Salmen and J. R. Roberts and B. Ruotolo and M. D. Ohi and N. M. Koropatkin},
howpublished = {bioRxiv},
year = {2026},
month = sep,
doi = {10.64898/2026.09.16.752182},
url = {https://www.biorxiv.org/content/10.64898/2026.09.16.752182v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Structural analyses of a heterodimeric SusCD complex captures intermediate states of maltooligosaccharide transport
AU - M. C. Cadigan
AU - N. Rivera-Fuentes
AU - W. Salmen
AU - J. R. Roberts
AU - B. Ruotolo
AU - M. D. Ohi
AU - N. M. Koropatkin
PY - 2026
DA - 2026/09/16
PB - bioRxiv
DO - 10.64898/2026.09.16.752182
UR - https://www.biorxiv.org/content/10.64898/2026.09.16.752182v1
AB - Bacteroides are abundant gut bacteria with diverse polysaccharide metabolizing capabilities encoded by co-transcribed polysaccharide utilization loci (PULs). The starch utilization system (Sus) is required for starch and a-glucan metabolism in Bacteroides thetaiotaomicron and has served as a model for PUL studies. However, the mechanism of maltooligosaccharide (MOS) transport and the architecture of the SusCD complex remain incompletely understood. Here, we used cryo-electron microscopy to capture multiple transport-relevant conformations of the SusD lipoprotein and the TonB-dependent transporter SusC in both unliganded and ligand-bound states. In the absence of ligand, SusCD adopts three conformations including an open state in which SusD is displaced from SusC, a state containing the SusC plug, and a state lacking density for the SusC plug. Three-dimensional variability analysis of the structure where SusD is displaced from SusC reveals continuous SusD mobility coupled to movement of the SusC N-terminus. The maltoheptaose-bound SusCD structure reveals two binding sites, one at the SusD-SusC interface and a second within SusC, indicating how MOS stabilizes a closed SusCD assembly. Notably, all structures show that SusCD is a heterodimer, unlike other Sus-like transporters that assemble into SusCD heterotetramers. Native mass spectrometry confirms this heterodimeric stoichiometry, and comparisons with other SusCD-like complexes suggest a structural basis for this distinct assembly of SusCD. Together, these data define substrate binding and intermediate transport states in a canonical PUL system and reveal architectural divergence among Sus-like carbohydrate transport complexes in Bacteroides.
ER -