Nat. Struct. Mol. Biol. 18, 423–431 (2011); published online 27 March 2011; corrected after print 21 September 2011

In the version of this article initially published, panels in Figures 2b (middle row, Dex-LWH), 3b (0.1mM CPT) and 3d (0.1mM CPT) were mistakenly replaced with duplicates of adjacent panels. The errors have been corrected in the HTML and PDF versions of the article.