1. Department of Biochemistry, University of Wisconsin-Madison, 420 Henry Mall, Madison, WI 53706, U.S.A.
2. Cell Biology and Genetics Program, Sloan-Kettering Institute, 1275 York Avenue, New York, NY 10021, U.S.A.
3. Department of Biochemistry and Biophysics, Vilnius University, Ciurlionio 21, Vilnius, Lithuania