Molecular Mechanisms of Endocytosis

that is needed to change membrane shape and to form a vesicle is very simple. These studies are in line with previous genetic studies that have identified a limited Howard Riezman,* Philip G. Woodman,† Gerrit van Meer,‡ and Mark Marsh§ *Biozentrum of the University of Basel CH-4056 Basel number of proteins that are required for COP-II-coated Switzerland vesicle formation from the ER. The function of one of †Division of Biochemistry these, Sec12p, which promotes nucleotide exchange School of Biological Sciences on Sar1p, was probably bypassed through the use of University of Manchester GTPgS-loaded Sar1p. Interestingly, the COP-II-coated Oxford Road vesicles formed from phospholipid vesicles did not have Manchester M13 9PT the regular shape and defined size of COP-II-coated United Kingdom vesicles budded from the ER. ‡Academic Medical Center In the endocytic pathway, it is clear that proteins are University of Amsterdam actively sorted into clathrin-coated vesicles for internalDepartment of Cell Biology and Histology ization at the plasma membrane. However, in the early 1105 AZ Amsterdam secretory pathway, clear evidence for this has only reThe Netherlands cently emerged (Kirchhausen et al., 1997; Kuehn and §MRC Laboratory of Molecular Cell Biology Schekman, 1997). Schekman reported that two proteins, University College London the general amino acid permease and pro–a factor, are Gower Street actively sorted into COP-II-coated buds and that a transLondon WC1E 6BT membrane protein which can be crosslinked to both United Kingdom prepro–a factor and Sec23p might be involved in recruitment.