Nat. Chem. Biol. 7, 925–934 (2011); published online 30 October 2011; corrected after print 2 December 2011

In the version of this article initially published, lanes 6 and 8 in Figure 6b had identical blot images. The blot now shown in lane 8 has been confirmed to come from the same gel as those in the other lanes and has been used to replace the incorrect blot image in the HTML and PDF versions of the article.