Nat. Struct. Mol. Biol. https://doi.org/10.1038/s41594-017-0020-6; published online 22 January 2018.

In the version of this article initially published online, an incorrect accession code PDB 6FN9 was introduced in Methods, in the ‘Model building’ section, line 2. This has been corrected to PDB 6F9N. The error has been corrected in the PDF and HTML versions of this article.