Nat. Chem. Biol. 8, 375–382 (2012); published online 26 February 2012; corrected after print 19 April 2013

In the version of this article initially published, the domain schemes in Figure 1a were incorrectly labeled sACt (aa 1–146). The correct text is sACt (aa 1–469). The error has been corrected in the HTML and PDF versions of the article.