Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-018-0086-9, published online 2 July 2018.

In the version of this article initially published, the PDB code associated with the study was given as 6F2E but should have been 6F2D in Table 1 and the data availability statement. The error has been corrected in the HTML and PDF versions of the article.