1. Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University,1 and
2. Division of Neurology, Evanston Hospital,2 Evanston, Illinois;
3. Department of Biochemistry, University of Colorado Health Sciences Center, Denver, Colorado 802623
4. Center for Macromolecular Crystallography, University of Alabama at Birmingham, Birmingham, Alabama 35294-00054; and