1. Instituto Cajal, CSIC, Doctor Arce 37, Madrid 28002, Spain;
2. Department of Experimental Biomedical Sciences, and CNR Center for the Study of Biomembranes, University of Padova, Padova, 35121 Italy;
3. Department of Zoology and Genetics, Roy J. Carver Laboratory for Ultrahigh Resolution Biological Microscopy, Iowa State University, Ames, Iowa 50011;