1 Department of Structural Cell Biology, Max-Planck-Institute of Biochemistry, Am Klopferspitz 18, D-82152 Martinsried, Germany.
2 Department of Biology, University of Copenhagen, Universitetsparken 13, DK-2100 Copenhagen, Denmark. Electronic address: stchristensen@bio.ku.dk.
3 Department of Structural Cell Biology, Max-Planck-Institute of Biochemistry, Am Klopferspitz 18, D-82152 Martinsried, Germany. Electronic address: lorentze@biochem.mpg.de.