Annexins and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 82

ATP dependent reaction(s). Docked and primed secretory vesicles are
inhibited from spontaneous fusion by a molecular “fusion” clamp, which is
only released by a rise in cytosolic free Ca 2+ , usually evoked by secreta-
gogues. This arrangement allows secretion to occur almost instantaneously
once cellular Ca 2+ starts to rise and is of crucial importance in synaptic exo-
cytosis where rapid release of neurotransmitter is essential.
In other cell types, exocytotic secretion is not only regulated by Ca 2+ ,
but also by guanosine triphosphate (GTP) and protein kinase C activation
(Pollard et al., 1998) arguing for the involvement of either a membrane
fusion protein that is activated by both Ca 2+ , GTP and protein kinase C, or
multiple proteins, each of which acts as a sensor for one of these regulatory
elements and whose activities are co-ordinated during secretion. Some cell
types, e.g. chromaffin cells, possess a cortical actin cytoskeleton, which pre-
vents secretory vesicles from direct interaction with the plasma membrane.
Therefore, dissolution of the cortical actin web is required for secretion to
ensue. In compound exocytosis, an intact secretory granule makes contact
and fusion with the membrane residue of a granule which has just
completed a fusion event (Pollard et al., 1992). Compound exocytosis is
distinguished from unitary granule exocytosis, as seen for example in neu-
rotransmitter release and tends to occur more slowly.
Annexins were first suggested by Zaks and Creutz (1990a) to meet
some important prerequisites to play a role in exocytosis. They are
expressed in all exocytotic tissues, they interact with both vesicular and
plasma membranes and they are Ca 2+ -dependent molecules. From the
above considerations, taken together with what we know about exocytosis,
there are multiple sites where annexins may have roles. In sections 2.3.1 to
2.3.7 we consider the experimental evidence that supports roles for annex-
ins in exocytosis.
2.3.1. A Membrane Fusion Protein Activated by Ca 2+ , GTP and
Protein Kinase C
Annexins are Ca 2+ -binding proteins and as such they have a key
attribute to function as Ca 2+ -responsive elements linking the rise of cytoso-
lic free Ca 2+ to membrane fusion in regulated exocytosis. Some annexins
(like annexins V and VII) require Ca 2+ in the high micromolar range for
half-maximal binding to phospholipids. However, of all known annexin
family members annexin II, in its heterotetrameric configuration, has the
lowest Ca 2+ requirement (in the submicromolar range) for binding to phos-
pholipid vesicles. The Ca 2+ concentration at which annexins bind Ca 2+ or
achieve half-maximal membrane binding covers a wide range and depends
on the phospholipid composition. The process of exocytosis probably