Nature Chemical Biology; doi:10.1038/nchembio.1849; corrected online 7 July 2015 and 21 July 2015

In the version of this article initially published, the PDB code for the structure of BamBC as isolated was listed as 4Z4O, which was incorrect. The correct code is 4Z40. The error has been corrected for the PDF and HTML versions of this article.