[1] Univ Calif Berkeley, Howard Hughes Med Inst, Berkeley, CA 94720 USA
[2] Univ Calif Berkeley, Dept Mol & Cell Biol, Berkeley, CA 94720 USA
[3] Univ Calif Los Angeles, Sch Med, Dept Biol Chem, Los Angeles, CA 90095 USA
[4] Univ Geneva, Ctr Med, Dept Biochim Med, CH-1211 Geneva 4, Switzerland