Annexins and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 109

chromaffin granules (Gerke and Moss, 1997). Annexin II has been sug-
gested to form physical connections between the plasma-membrane and the
membrane of chromaffin granules (Nakata et al., 1990). High resolution
images obtained by cryo-electron microscopy showed that short intergran-
ular or granule-membrane bridges formed only in the presence of annexin
II, although in this study the molecular composition of these bridges was
not investigated. Nevertheless, such intermembrane junctions could be
formed by annexin II molecules bound to the outer leaflet of each vesicle
linked by a central p11 dimer (Lambert et al., 1997).
The hypothesis that both annexin II and the annexin II 2 /p11 2 -
heterotetramer are involved in exocytosis is supported by the run-down
assays of secretion in permeabilized cells (see below). Stimulation of chro-
maffin cells also leads to phosphorylation of annexin II by protein kinase
C. In vitro, phosphorylation by protein kinase C decreases the granule-
binding affinity of annexin II, induces disassembly of the heterotetramer
and leads to granule fusion (Regnouf et al., 1995). Moreover, Sarafian et al.
(1991) showed that prior phosphorylation of annexin II by PKC was
required to endow annexin II with maximal capacity to arrest secretory run-
down in permeabilized chromaffin cells. Also, a synthetic annexin II peptide
corresponding to amino acids 15-26 (a region of the N-terminal domain
containing phosphorylation sites but not the p11-binding site) blocks the
nicotine-induced translocation of annexin II to the cell periphery and
inhibits Ca 2+ -triggered catecholamine secretion (Chasserot Golaz et al.,
1996). It is possible therefore that N-terminal phosphorylation regulates
translocation of annexin II from the cytosol to the plasma membrane.
Using membrane capacitance tracing to follow exocytotic fusion
events, Koenig et al. (1998) recently discovered that disruption of preformed
annexin II 2 /p11 2 complexes in bovine pulmonary artery endothelial cells
inhibits regulated exocytosis. This was demonstrated by loading the cells
with a synthetic peptide comprising the N-terminal 14 residues of annexin
11. The excess of free peptide displaces the endogenous annexin II by com-
peting for binding sites on p11. One interpretation of these data is that the
monomeric annexin II liberated in this way inhibits exocytosis, but given
the weight of data supporting a role for annexin II in exocytosis, the more
likely explanation is that the annexin II 2 /p11 2 complex has a pro-exocytotic
role. The role of annexin II in Ca 2+ -regulated secretion is thus not restricted
to chromaffin cells (Gerke and Moss, 1997). For example, Ca2+-induced
exocytosis of lamellar bodies in permeabilized alveolar epithelial cells,
is stimulated specifically by monomeric annexin II and the annexin II
heterotetramer, since in the same assay, annexin I, III, IV, V and VI were
ineffective (Liu et al., 1996).