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The structure of serum resistance-associated protein and its implications for human African trypanosomiasis

Abstract

Only two trypanosome subspecies are able to cause human African trypanosomiasis. To establish an infection in human blood, they must overcome the innate immune system by resisting the toxic effects of trypanolytic factor 1 and trypanolytic factor 2 (refs. 1,2). These lipoprotein complexes contain an active, pore-forming component, apolipoprotein L1 (ApoL1), that causes trypanosome cell death3. One of the two human-infective subspecies, Trypanosoma brucei rhodesiense, differs from non-infective trypanosomes solely by the presence of the serum resistance-associated protein, which binds directly to ApoL1 and blocks its pore-forming capacity3,4,5. Since this interaction is the single critical event that renders T. b. rhodesiense human- infective, detailed structural information that allows identification of binding determinants is crucial to understand immune escape by the parasite. Here, we present the structure of serum resistance-associated protein and reveal the adaptations that occurred as it diverged from other trypanosome surface molecules to neutralize ApoL1. We also present our mapping of residues important for ApoL1 binding, giving molecular insight into this interaction at the heart of human sleeping sickness.

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Acknowledgements

This work was supported by Wellcome Trust grant 093008/Z/10/Z and MRC grant MR/L008246/1. M.K.H. is a Wellcome Trust Investigator. We thank K. Gull and S. Dean for assistance with monoclonal antibody generation. We also thank the beamline scientists at Diamond Light Source beamline I03, BioSAXS P12 beamline EMBL Hamburg and D. Staunton for assistance with biophysics equipment. S.M. and C.V.R. acknowledge with thanks funding from a MRC programme grant (MR/N020413/1) and a Wellcome Trust Instrument Grant (WT 104923/Z/14/Z) for the HDX platform.

Author information

M.K.H. and S.Z. conceived and designed experiments. M.C. conducted trypanosome killing experiments. H.L.S. and S.Z. established protein purification strategies and purified proteins. J.S. cloned and purified mutant proteins. S.M. and S.Z. carried out the HDX experiments. S.M. and C.V.R. analysed and evaluated the HDX data. S.Z. collected all X-ray diffraction data (SAXS and crystallography), solved, refined and analysed the structures. S.Z. carried out the microscale thermophoresis experiments. M.K.H. and S.Z. wrote the paper with input from co-authors.

Competing interests

The authors declare no competing financial interests.

Correspondence to Matthew K. Higgins.

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Supplementary Information

Supplementary Table 1, Supplementary Figures 1–13, Supplementary Tables 1–4.

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Fig. 1: The structure of SRA.
Fig. 2: Structural comparison of SRA with known trypanosome surface proteins.
Fig. 3: Mapping of the ApoL1 binding site of SRA by HDX-MS.
Fig. 4: Mutational analysis of SRA binding to ApoL1.