1 Department of Cell Biology, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, Baltimore, MD 21205, USA. Electronic address: klwilson@jhmi.edu.
2 Department of Biology, Institute of Biochemistry, ETH Zurich, Otto-Stern-Weg 3, CH8903 Zurich, Switzerland. Electronic address: karsten.weis@bc.biol.ethz.ch.