VOLUME 284 (2009) PAGES 455–464
This article has been withdrawn by the authors. In June 2017, the Journal raised questions about Figs. 1A and 5D. The original data and originally submitted figures were not available for evaluation. The authors were able to locate data for a repeat experiment conducted at the time of the original work for Fig. 1A and offered to prepare a new figure. The authors also offered to repeat the experiment reflected in Fig. 5D and to publish an amended figure. The Journal declined both offers, a decision with which the authors disagree. The authors stand by the reproducibility of the experimental data and the conclusions of the paper. The paper, with confirmatory data supporting the results, can be obtained by contacting the authors.