Nat. Chem. Biol. 7, 310–318 (2011); published online 9 March 2011; corrected after print 1 July and 1 August 2011

In the previous version of this article, a water molecule was mislabeled in Figure 7 and an error was inadvertently introduced into the journal title. These errors have been corrected in the HTML and PDF versions of this article.