We thank J. Briggs for discussions and reading of the manuscript. This work was supported by the National Institutes of Health (NIH) grants R01-GM107013 (V.M.V.), R01-GM105684 (G. W. Feigenson), P30-GM110758 and P50-GM082251 (J.R.P.), R01-AI129678 (O.P. and B.K.G.-P.), U54-GM103297 (O.P.), and R01-GM110776 (M.C.J.). F.K.M.S. was supported by Deutsche Forschungsgemeinschaft grant BR 3635/2-1 awarded to J. A. G. Briggs. J.M.W. was supported by NIH postdoctoral fellowship grant F32-GM115007. Anton computer time was provided by the Pittsburgh Supercomputing Center (PSC) through NIH grant R01-GM116961. The Anton machine at PSC was generously made available by D. E. Shaw Research. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation (NSF) grant number OCI-1053575. Specifically, it used the Bridges system, which is supported at PSC by NSF award number ACI-1445606. Some of The EM work was conducted at the Molecular Electron Microscopy Core facility at the University of Virginia.

Reviewer information

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