We thank Bob Lesch and Crystal Chan for help with protein purifications, James Riches for help with cryo-EM, Holly Aaron (MIC, Berkeley) for assistance with confocal and Gerd Hause (Halle) with electron microscopy. We thank Lelio Orci (Faculty of Medicine, University of Geneva) for providing Fig. 3I. We are grateful to current and former members of the labs of Randy Schekman, Jay Groves, George Oster and David Drubin for help and discussions, especially Chao-Wen Wang, Raghuveer Parthasarathy, John Tran, and Bruno Antonny. K.B. acknowledges a postdoctoral fellowship of the Deutsche Akademie der Naturforscher Leopoldina (BMBF-LPD-9901/8-160). K.B., A.M, S.D. and D.G. were supported by the BMBF ZIK program (FKZ 03Z2HN22) and ERDF grant 1241 09 0001. R.S. and K.B. were supported by the Howard Hughes Medical Institute.