1. L. Song, M. R. Hobaugh, and J. E. Gouaux are in the Department of Biochemistry and Molecular Biology, The University of Chicago, 920 East 58 Street, Chicago, IL 60637, USA.
2. C. Shustak, S. Cheley, and H. Bayley are with the Worcester Foundation for Biomedical Research, 222 Maple Avenue, Shrewsbury MA 01545, USA.