The Secretory Pathway From History to the State of the Art - Fusion of Biological Membranes and Related Problems - page 21

FIGURE 3. Model for the formation of a COPI-coated bud. A , Coated vesicle formation is
initiated by the recruitment to the Golgi membrane of ARF1 from the cytoplasm.This requires
exchange of GDP for GTP, a reaction catalyzed by a nucleotide exchange factor. B , Subse-
quent binding of coatomer is mediated by its interaction with the cytoplasmic domain of a
tetramer of p24 family members, e.g. p23. C , Interaction of coatomer with the p24 tetramer
induces a conformational change and polymerization of the complex that shapes the mem-
brane into a coated bud.

5.1. SNARE Proteins

It is generally accepted that specificity in membrane recognition relies
on the pairing of SNARE proteins, a family of soluble NSF attachment
protein (SNAP) receptors (Figure 4B). The concept of a pair-wise recogni-
tion of SNARE proteins that reside in opposite membranes-the SNARE
hypothesis-was first formulated by Rothman and colleagues for vesicle
docking at the synapse (Soellner et al., 1993b; reviewed in: Rothman and
Warren, 1994). This hypothesis was originally based on the discovery of
a core complex consisting of v(esicle)-SNAREs and t(arget)-SNARES
that have been identified as constituents of synaptic vesicles and the pre-
synaptic plasma membrane, respectively (Soellner et al., 1993b). In the
nerve terminal, the core complex consists of vesicle-associated proteins,
VAMP/synaptobrevin, from the synaptic vesicle, and of target membrane-
associated proteins, syntaxin and SNAP-25 (which is unrelated to α -, β -,

γ -SNAP) from the plasma membrane (Scheller, 1995).

Many v- and t-SNARES have been characterized now in plants, yeast
and mammals and it has been shown that they belong to gene families
each of which comprising various numbers of isoforms. Specific isoforms