Nat. Chem. Biol. 6, 534–540 (2010); published online 30 May 2010; corrected after print 17 June 2010.

In the version of this article initially published, one of the corresponding authors (Y-W.W.) was not included in the Additional Information section. The error has been corrected in the HTML and PDF versions of the article to read “Correspondence and requests for materials should be addressed to R.S.G. or Y-W.W.”