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This work was published in the The Journal of Cell Biology, 180 (2) 2008 p. 341-355 published by Rockefeller University Press and has be reproduced here under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License http://creativecommons.org/licenses/by-nc-sa/3.0/ this grants to all users a free, irrevocable, worldwide, perpetual right of access to, and a license to copy, use, distribute, perform and display the work publicly and to make and distribute derivative works in any digital medium for any reasonable non-commercial purpose, subject to proper attribution of authorship and ownership of the rights.