VOLUME 276 (2001) PAGES 514–521
This article has been withdrawn by the authors.
Figs. 1 and 9C contain images that were taken from previous publications where they were described as representing the results of different experiments. Some of the data were labeled incorrectly, and permission for re-use of the data was not obtained from the copyright holder, although the articles in which the data in Figs. 1 and 9C were originally published were referenced in the text. Therefore, in accordance with accepted scientific practice, the authors have withdrawn the paper.