The Secretory Pathway From History to the State of the Art - Fusion of Biological Membranes and Related Problems - page 20

complex between the membrane-bound ARF1-GTP and the cytoplasmic
tails of p24 family members (Figure 2).

4.3. Polymerization of Coatomer and COPI Bud Formation

With the knowledge of the minimal requirements for COPI budding,
a still open question is: how does recruitment of a coat drive formation of
a curvature in a membrane?
An answer to this basic question may come from in vitro binding
studies with coatomer and the cytoplasmic tail peptide of p23. The results
of these studies suggest that a tetramer of p23 induces specific polymeri-
zation of the coatomer complex (Reinhard et al., 1999). Polymerization
of coatomer is accompanied by a conformational change of the complex
resulting in an increased susceptibility to protease of its -subunit (the direct

binding partner of p24 family cytoplasmic domains). This polymerization
on the surface of a membrane might be the driving force to shape a mem-
brane into a coated bud.
Strikingly, the conformational change and polymerization of coatomer
is specifically induced by some p24 cytoplasmic domains, but not by a
peptide with a characteristic KKXX ER-retrieval motif that binds coatomer
with similar efficiency and via the same subunit as p23 (see 3.4 and 3.5.1).
Thus, the two classes of cytoplasmic domains of coatomer binding proteins
seem to have two distinct and different functions: interaction of a retrieval
motif might serve the sorting of membrane cargo into retrograde COPI
vesicles. p23 and p24, however, represent part of the machinery of a COPI
vesicle involved in vesicle budding (Figure 3).

γ

5. MECHANISM OF VESICLE FUSION

Delivery of cargo to its destination by a transport vesicle criti-
cally depends on the accurate and specific membrane recognition and
fusion. This involves membrane machinery for specific pairing of the mem-
branes to be fused and cytosolic machinery that is involved in the timing
and regulation of the fusion event. During the past few years, many com-
ponents involved in membrane fusion have been identified at the molecu-
lar level. However, their precise function and the order of events are a
matter of current debate. In the following section we will summarize the
current view over heterotypic membrane fusion, e.g. fusion between a
vesicle membrane and a target membrane and the components involved
(Figure 4).