Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-019-0199-9 (2019), published online 14 Month 2019.

In the version of this article initially published, the date of publication (13 March 2019) was incorrect. The correct date is 14 March 2019. The error has been corrected in the HTML and PDF versions of the article.