1 Department of Structural Biology, Stanford University School of Medicine, MSLS Building, Room P109, 1201 Welch Road, Stanford, CA 94305-5489, USA. Electronic address: tjardetz@stanford.edu.
2 Department of Biological Sciences, Purdue Institute of Inflammation, Immunology and Infectious Disease, Purdue University, West Lafayette, IN, USA. Electronic address: kuhnr@purdue.edu.
3 Howard Hughes Medical Institute, John Evans Professor of Molecular and Cellular Biology and Chair, Department of Molecular Biosciences, Northwestern University, 2205 Tech Drive, Evanston, IL 60208-3500, USA. Electronic address: ralamb@northwestern.edu.