[1] Univ Calif San Diego, Div Biol Sci, San Diego, CA 92103 USA
[2] Univ Groningen, Dept Cell Biochem, Groningen, Netherlands
[3] MRC, Mol Biol Lab, Cambridge, England
[4] Univ Groningen, Dept Biochem, Groningen, Netherlands
[5] Univ Groningen, Netherlands Prote Ctr, Groningen, Netherlands