Volume 390 (2005), pp. 681–688
We now realize that some inadvertent errors were introduced into the above paper, resulting in misrepresentation of certain lanes in the original Figures 4A (lanes 7–10), 5 (lane 7) and 6B (lanes 5, 7 and 13). Although this influenced neither the main observations nor the resulting conclusions, we are retracting this paper. We thank the Editorial Board of the Biochemical Journal for pointing out the errors.