Nat. Struct. Mol. Biol. 16, 477–485 (2009); published online 26 April 2009; corrected after print 18 May 2009

In the version of this article initially published, the incorrect accession number for EM map deposition was provided. The correct accession number for the data deposited in EmDep is EMD-1617. The error has been corrected in the HTML and PDF versions of the article.