Department of Cell Biology, Max Planck Institute of Biochemistry, Martinsried D-82152, Germany. malik@biochem.mpg.de|Department of Cell Biology, Max Planck Institute of Biochemistry, Martinsried D-82152, Germany. malik@biochem.mpg.de|Department of Cell Biology, Max Planck Institute of Biochemistry, Martinsried D-82152, Germany. malik@biochem.mpg.de