Annexins and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 95

vesicles (Hinshaw and Schmid, 1995). In fibroblasts endosomes formed
without annexin VI are unable to move to fuse with late endosomes and
are therefore not degraded. Dynactin is thought to be the physical link
between spectrin that is bound to vesicles and dynein which is bound to
microtubules. This inability to move towards late endosomes may be due to
them lacking spectrin and not being able to link to dynein on microtubules
via dynactin (Holleran et al., 1996).
On large endosomes annexin VI and dynamin were reported to be
tightly associated while on small endosomes they are both present but
without any interaction. It may be that only the large vesicles are able to
support this association (Warnock et al., 1996) or alternatively, there may
be an unidentified protein containing an SH3 domain, that forms a “bridge”
to connect these two proteins together (Shpetner et al., 1996). Experiments
by Turpin et al. (Turpin et al., 1998) have shown that after dissociation of
clathrin annexins II and VI remain associated with the membrane and
cannot be removed by either chelation of Ca 2+ and/or ATP, indicating that
the association is Ca 2+ -independent. This may be an example of annexins
initially binding to membranes in a Ca 2+ -dependent manner, but then
becoming bound Ca 2+ -independently either by stabilization through
binding to a membrane component by a Ca 2+ -independent mechanism (as
has been suggested for the binding of annexin II to early endosomes (Jost
et al., 1997) or by membrane insertion (see section 2). As both annexins
were found with small, medium and large clathrin coated endosomes, they
could be part of the general vesicle trafficking machinery rather than being
involved in more restricted steps (Turpin et al., 1998).
Recent studies on human fibroblasts have shown that annexin VI
bound to the amino-terminal domain of -spectrin (Watanabe et al., 1994)

is as effective as free cytosolic annexin VI at aiding budding of coated vesi-
cles from the membrane (Kamal et al., 1998). When annexin VI is bound in
this way to spectrin, spectrin is no longer able to cross-link actin (Watan-
abe et al., 1994). During exocytosis in chromaffin cells there is depolymer-
ization of F-actin within the submembraneous cytoskeleton, and spectrin
becomes reorganized into small areas on the membrane (Aunis and Bader,
1988). Kamal et al. (1998) found that during annexin VI-dependent coated
pit budding approximately 50% of spectrin from the membrane was lost,
and that the whole process could be stopped by using N-acetyl-leucyl-
leucyl-norleucinal (ALLN), a cysteine protease inhibitor. In fibroblasts
the cells recover one hour after treatment with ALLN and are once again
able to internalize cell surface receptors, but the endosomes formed are not
able to follow their usual route and are therefore not degraded. Under
these conditions new pits are formed at the cell membrane along with the
original ones that are no longer able to invaginate, and the new pits are not

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