Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-020-0430-8, published online 1 June 2020.

In the version of this article initially published online, the grant GM125001, awarded to D.H.C., was incorrectly attributed to the National Institute of Diabetes and Digestive and Kidney Diseases; the grant was awarded by the National Institute of General Medical Sciences. The error has been corrected in the print, PDF and HTML versions of the article.