We thank R. Tarkiainen for technical assistance and Diamond Light Source for access to and support with the cryo-EM facilities (EM14856) at the UK national electron bio-imaging centre (eBIC), which is funded by the Wellcome Trust, MRC and BBSRC. We acknowledge the use of the Instruct-HiLIFE Biocomplex unit (University of Helsinki and Instruct-FI), and the support of the Academy of Finland (grant 1306833) for the unit. This work was supported by the European Research Council under the European Union’s Horizon 2020 research and innovation programme (649053 to J.T.H.); a Wellcome Trust administrative support grant (203141/Z/16/Z); a Wellcome Trust four-year PhD studentship (109135/Z/15/A to S.L.I.); the Academy of Finland (grant 272507 to M.M.P.); and the Sigrid Jusélius Foundation (to M.M.P).

Reviewer information

Nature thanks Alex Evilevitch and the other anonymous reviewer(s) for their contribution to the peer review of this work.