Nat. Cell Biol. 14, 457–466 (2012); published online 22 April 2012; corrected after print 25 April 2012

In the version of this Article originally published, in the legend for Fig. 4, the statement “Data in a and d represent the mean of two independent experiments” should have read: “Data in a and e represent the mean of two independent experiments”. This error has been corrected in both the HTML and PDF versions of the Article.