We thank G. Cannone and S. Chen for cryo-EM assistance at the MRC-LMB; Y. Chaban and K. Dent for cryo-EM assistance at the Electron Bio-Imaging Centre (eBIC), Diamond Light Source; J. García-Nafría, L. Dong, T. Nakane and S. Scheres for advice on cryo-EM data processing; J. Grimmett and T. Darling for IT and computing support; and members of the Aricescu laboratory for discussions and comments on the manuscript. This work was supported by the UK Medical Research Council grants MR/L009609/1, MC_UP_1201/15 (A.R.A., S.M. and D.L.) and MC_UP_A025_1013 (J.Z.); UK Biotechnology and Biological Sciences Research Council grant BB/M024709/1 (A.R.A. and D.L.); Human Frontier Science Program grant RGP0065/2014 (A.R.A.); Cancer Research UK grant C20724/A14414 (T.M.); and Swiss National Science Foundation fellowship 168735 (J.Z.). R.D. and K.W.M. were supported by a grant from the National Institute for General Medical Sciences (GM 58448) and by the Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital. We acknowledge the support and the use of resources of Instruct-ERIC (PID1271), part of the European Strategy Forum on Research Infrastructures (ESFRI), and the Research Foundation-Flanders (FWO) for their support of nanobody discovery, and FWO for a doctoral fellowship to T.U.

Reviewer information

Nature thanks M. Chebib, M. Jansen and H. Nury for their contribution to the peer review of this work.