Nat. Struct. Mol. Biol. 18, 56–60 (2011); published online 12 December 2010; corrected after print 9 August 2011

In the version of this article initially published, the legend for Figure 2d,e did not include the source of the data in those panels. These data originally appeared in ref. 20. The error has been corrected in the HTML and PDF versions of the article.