Nat. Chem. Biol.; doi:https://doi.org/10.1038/nchembio.2548; published online 15 January 2018; corrected online 22 January 2018

In the version of this article initially published online, the submission date was incorrectly stated as 21 June 2016. The correct date is 15 June 2017. The error has been corrected in the PDF and HTML versions of this article.