Nature 542, 255–259 (2017); doi:10.1038/nature21061

In Fig. 3c of this Letter, the bottom left helix should have been labelled ‘αE’ rather than ‘αA’. This helix is also mentioned in the main text, where “positioned near the N terminus of helix αA in CD” should have read “positioned near the N terminus of helix αE in CD”. The original Letter has been corrected online.