Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-020-0466-9, published online 3 August 2020.

This paper was originally published under standard Springer Nature copyright (© The Author(s), under exclusive licence to Springer Nature America, Inc.). It is now available as an open-access paper under a Creative Commons Attribution 4.0 International license, © The Author(s). The error has been corrected in the print, HTML and PDF versions of the article.