We thank the staff of Beamline 8.3.1 at the Advanced Light Source (A LS) at the Lawrence Berkeley National Laborator y, and Dr. Christopher Waddling (University of California, San Francisco) for x-ray support. This work was supported by National Institutes of Heath Grant GM24485 (to R.M.S.).
Abbreviations: AQP(n), aquaporin (n); bAQPn, bovine AQPn; sAQP0, sheep AQP0; OG, octylglucopyranoside; NPA, Asn-Pro-Ala sequence.
Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 1TM8).