We, the authors, are retracting this Letter owing to anomalies in Figs. 2j, 3a and 3d. We and the Nature editors reviewed the data and found that these anomalies were present in the digital scans of the original western blot films, which could not be located for further examination. Although repeat experiments at the time of writing the paper have reproduced the data, the anomalies undermine confidence in the published Letter. We therefore believe that the most appropriate course of action is to retract this Letter in its entirety. We thank the readers who brought the issues to our attention, and apologize to the scientific community. We remain confident that the key findings of the Letter are valid and reproducible. All authors agree with the Retraction.