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3 9 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Cryo-EM structures of Trypanosoma brucei gambiense ISG65 with human complement C3 and C3b and their roles in alternative pathway restriction, H. Sülzen (a), J. Began (a), A. Dhillon (a), S. Kereiche (a), P. Pompach (b), J. Votrubova (a), F. Zahedifard (c), A. Subrtova (a), M. Safner (a), M. Hubalek (a), M. Thompson (a), M. Zoltner (c), S. Zoll (a), Nat. Commun. 14, 2403 (2023); https:/doi.org/10.1038/s41467-023-37988-7 (a) Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Prague (Czech Republic) (b) Institute of Biotechnology of the Czech Academy of Sciences, Vestec (Czech Republic) (c) Department of Parasitology, Faculty of Science, Charles University Prague, Biocev, Vestec (Czech Republic)

This flexible connection to the cell surface allows ISG65 to switch between these two states, likely enabling the parasite to avoid detection by the host s immune system while still being able to bind and remove complement factor C3b from the surface.

The findings of this study provide new insights into how trypanosomes interact with host factors in the bloodstream, adding another layer of complexity to our understanding of these parasites, and could lead to the development of new treatments for African sleeping sickness, which affects thousands of people in sub- Saharan Africa. The implications of the findings also extend beyond trypanosome biology to our understanding of the immune system and its interactions with pathogens. While the complexity of the immune system can make it difficult to study, the integration of structural biology can offer unprecedented insights into these interactions, paving the way for a better understanding of immune defence mechanisms.

Fig. 24: Model of complement binding on the surface of T. brucei gambiense. Using SAXS and ensemble optimisation, conformations of the disordered C-terminal linker of ISG65 were calculated in solution. In its compact down position, ISG65 would remain embedded within the VSG coat, with its C3-binding epitopes being concealed and only the disordered head domain exposed to the outside. In this state, ISG65 may bind and inactivate AP convertases via C3b that may pass through the VSG coat, where it could covalently bind to the plasma membrane via Gln1013 of the former thioester (red connecting line). In its extended up position, ISG65 would be able to intercept C3b outside the VSG coat and transport it to the flagellar pocket for uptake and subsequent lysosomal degradation. C3b would hereby either be present in close proximity to the coat or attached covalently or non-covalently to VSGs. C3(H2O) and native C3 are depicted in the solution, as it is currently unknown whether they bind to the trypanosome surface. Surface receptors embedded within the VSG coat are depicted in different colours. The atomic structures of C3(H2O) and the AP C5 convertase have not been determined and are therefore shown as dashed lines.