1. Cell Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, and Weill Graduate School of Medical Sciences of Cornell University, New York, New York 10021, Institute of Biochemistry, University Clinics Charite, Humboldt University, Monbijoustrasse 2, D-10117 Berlin, Germany, and Neurological Sciences Institute, Oregon Health and Science University, Portland, Oregon 97239