The original version of this Article contained an error in the spelling of the author Ambrosius P. Snijders, which was incorrectly given as Ambrosious P. Snijders. This has now been corrected in both the PDF and HTML versions of the Article.
Due to a typesetting error, Figure 1 of the above article was published incorrectly.Correction to: The EMBO Journal (2005) 24, 63-72. doi:10.1038/sj.emboj.7600493.