Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-020-0496-3, published online 14 September 2020.

In the version of this article initially published online, in Table 1, the PDB accession code for the S20G 2PF cryo-EM structure was incorrect. The correct accession code is PDB 6ZRQ. The error has been corrected in the print, PDF and HTML versions of the article.