Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-019-0300-4, published online 30 September 2019.

In the supplementary information originally posted for this article, the Supplementary Information PDF file did not contain the Supplementary Figs. or Tables; it contained only a title page with the authors’ names and affiliations. The error has been corrected online.