1. Harvard University School of Engineering and Applied Sciences, Cambridge, MA 02138, USA.
2. Department of Biochemistry, University of Washington, Seattle, WA 98105, USA.
3. Institute for Protein Design, University of Washington, Seattle, WA 98105, USA.
4. Howard Hughes Medical Institute, University of Washington, Seattle, WA 98105, USA.
5. Kavli Institute for Bionano Science and Technology Harvard University, Cambridge, MA 02138, USA.