Authors

Discussion

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 the trypanolytic factors TLF1 and TLF2. These lipoprotein complexes contain an active component, apolipoprotein L1, ApoL1, a pore-forming protein that causes trypanosome cell death by a yet not well-characterised mechanism. One of the two human infective subspecies, Trypanosoma brucei rhodesiense, differs from non-infective trypanosomes solely by presence of the serum-resistance-associated protein, SRA, which binds directly to ApoL1 and blocks its pore-forming capacity. 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 of the parasite. This will ultimately create the input needed to drive the development of therapeutics as there is no current vaccine and existing drugs have severe side effects. Here we present the crystal structure of SRA and reveal the adaptations that occurred as it diverged from other trypanosome surface molecules to neutralise ApoL1. In order to determine the binding region with ApoL1 we carried out hydrogen-deuterium exchange mass spectrometry (HDX-MS) using recombinantly expressed proteins. To confirm and to further delineate the region of SRA identified as a hot-spot for ApoL1 binding in HDX-MS, mutational analysis was used.  The binding affinities of wild-type and mutant SRAs to ApoL1 were  then studied using microscale thermophoresis. Our mapping of residues important for ApoL1 binding revealed that the interaction is likely to be predominantly electrostatic in nature. These results give molecular insight into the SRA ApoL1 interaction, which is at the heart of human sleeping sickness.