Nature Cell Biol. 10, 460–467 (2008); published online 16 March 2008; corrected after print 1 October 2009

In the version of this letter initially published online and in print, the wrong image was displayed for Fig. 4c, upper left. The correct version of this figure is shown below. This error has been corrected in both the HTML and PDF versions of the article. The conclusions are unaltered. We apologize for the mistake.