Nat. Struct. Mol. Biol. 20, 918–919 (2013); published online 5 August 2013; corrected after print 23 August 2013

In the version of this article initially published, “Lys251” was incorrectly substituted for “Leu251” in Figure 1 and in the text. The error has been corrected in the HTML and PDF versions of the article.