Annexins and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 75

into type II and III sites and which are different from helix-loop-helix Ca 2+ -
binding sites (type I) typical of the superfamily of EF-hand Ca 2+ -binding
proteins. The primary structure of all annexins is characterized by four
repeats (I-IV) of highly conserved 70 amino acid domains (the only excep-
tion is annexin VI which has eight repeats), forming the protein core. Each
repeat folds into 5 -helices (A to E), which are themselves wound into a

right-handed superhelix. The helices are connected through short loops or
turns. The four repeats are arranged in a slightly curved cyclic array, giving
the three dimensional structure of the annexin protein core the shape of a
concave disk with a central hydrophobic pore and with the Ca 2+ -binding
sites situated on the convex side (Liemann and Huber, 1997) (Figure 1).
Aso-called “calcium bridging” mechanism (Swairjo et al., 1995), where
Ca 2+ is jointly co-ordinated by the annexin Ca 2+ -binding site and phospho-
lipid head groups, was identified as the mechanism that enables all annex-
ins to bind to negatively-charged phospholipids, such as those enriched
in the inner leaflet of the plasma membrane (Liemann and Huber, 1997).
Ca 2+ affinity constants for membrane-binding of annexins therefore always
depend on the phospholipid composition of the membrane investigated,
and are meaningless without that information (Raynal and Pollard, 1994).
The highest Ca 2+ affinity of annexins is invariably observed in the presence
of liposomes or artificial membranes containing acidic phospholipids such
as phosphatidylserine (PtdS), phosphatidylethanolamine (PtdE), phospha-
tidic acid (PtdA) and phosphatidylinositol (PtdIns) which are preferentially
localized on the cytoplasmic face of plasma membranes and secretory
granule membranes. Membrane binding leads to modest conformational
changes which result in the transformation of the slightly curved annexin
molecule into a planar disc (Bitto and Cho, 1998), and may also lead to for-
mation of a second phospholipid binding site on the concave side of the
molecule, possibly involving the N-terminus (de la Fuente and Ossa, 1997).
In contrast to the highly conserved C-terminal protein core, the N-
terminus (or tail) displays high variability among annexins and ranges in
length from 11 to 19 (short tail annexins, e.g. III, IV, V, VIII) to more than
100 residues (annexins VII and XI) (Gerke and Moss, 1997). The N-
terminal, which is located at the concave side of short tail annexins, is
thought to confer functional individuality to annexins. Some annexins
appear to have specific protein ligands, e.g. members of the S100 family of
EF-hand Ca 2+ -binding proteins, to which they bind via the N-terminus. In
the annexin II 2 /p11 2 -heterotetramer a dimer of the S100 protein p11
(S100A10) links two annexin II molecules. Note that p11 is unique within
the S100 protein family in that it contains no functional Ca 2+ -binding sites.
It's interaction with annexin II is thus Ca 2+ -independent. This is in contrast
to the complexes formed between annexin I-S100C and annexin XI-S100A6

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