Abstract
Respiratory complex II (CII), or succinate dehydrogenase, couples succinate oxidation in the Krebs cycle with electron transfer to the respiratory chain. Owing to this pivotal role, CII inhibitors are widely used fungicides globally; however, their development has largely proceeded without structural insights from fungal targets. Here, we report cryo-electron microscopy structures of the 128 kDa mitochondrial CII from Saccharomyces cerevisiae in two states: active, with endogenous ubiquinone-6 bound (3.15 Å), and inhibited with the fungicide bixafen (3.00 Å). Although closely related to the mammalian type C enzyme, our structures show that the yeast CII has lost the canonical heme cofactor. They also reveal how clade-specific sequence extensions of the membrane subunits Sdh3 and Sdh4 - conserved in pathogenic fungi - uniquely contribute to complex stability and fungicide binding. Our findings provide a foundation for rational design of next-generation CII inhibitors and combatting resistance, in both agriculture and human health.
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Data availability
E.M. maps and model coordinates for CII have been deposited in the EM Data Bank and Protein Data Bank, respectively, under accession numbers EM-53029 and PDB-9QDL for CII as purified (CII-nat), and accession numbers EM-53030 and PDB-9QLM for CII with bixafen bound (CII-bix). Raw data presented in Supplementary Fig. 1 for the characterization of the final purified CII sample used for structure determination are available in the Supplementary Data file, with uncropped gels provided in Supplementary Fig. 15.
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Acknowledgements
This work was supported by the Medical Research Council UK (Transition Support MR/T032154/1 to A.M.). Cryo-EM data were collected at the ISMB EM facility (Birkbeck College, University of London) with financial support from the Wellcome Trust (202679/Z/16/Z and 206166/Z/17/Z). We thank Dr D. Houldershaw for his support with computing.
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A.M. funded, designed, and supervised the research. B.M. produced the yeast mutant strain. C.B.-L. grew cells, prepared mitochondria, and purified complex II, with support from S.J. C.B.-L. performed activity measurements with input from E.A.B., S.C., and N.L., prepared and optimized cryo-EM grids, and collected cryo-EM data. N.P. and A.M. processed the cryo-EM images. N.P. and E.A.B. built the models. A.M., N.P., and E.A.B. wrote the manuscript with contributions from all authors.
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Pinotsis, N., Burn-Leefe, C., Jones, S. et al. Cryo-EM structure of bixafen-bound S. cerevisiae complex II unravels SDHI specificity against pathogenic fungi. Commun Biol (2026). https://doi.org/10.1038/s42003-026-09617-8
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DOI: https://doi.org/10.1038/s42003-026-09617-8
