wrote the paper.The authors declare no conflict of interest.
This article is a PNAS Direct Submission.Published under the PNAS license.Data deposition: The cryo-EM density maps and structures have been deposited in the Protein Data Bank, www.pdb.org, and the Electron Microscopy Data Bank (EMD) [accession nos.: mature particles in pH 8, 6ADT and EMD-9613; mature particles in pH 6, 6ADS and EMD-9612; SVV(full)-ANTXR1 in pH 8, 6ADR and EMD-9611; SVV(full)-ANTXR1 in pH 6, 6ADM and EMD-9608; and SVV(spent)-ANTXR1, 6ADL and EMD-9607].