1. Department of Genetics and Cell Biology, Washington State University, Pullman, Washington 99164-4234;
2. Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, California 94720-3200;
3. Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan;
4. Department of Biology, International Christian University, Tokyo 181, Japan; and
5. Department of Cell Biology, University of Massachusetts Medical Center, Worcester, Massachusetts 01655