Nat. Chem. Biol. 7, 437–444 (2011); published online 12 June 2011; corrected after print 15 June 2012

In the version of this article initially published, the residue numbers indicated in the y-axis labels in Figure 3c were incorrectly written as 24, 53, 55 and 53 from the origin but should have read 24, 52, 53 and 55. The error has been corrected the PDF and HTML versions of this article.