Calcium Biochemistry
155
interface for the interaction with possible
t
a
r
g
e
t
so
fwh
i
chanumb
e
ro
fd
i
f
f
e
r
en
t
proteins such as AIP or Alix have been
identiFed, which in Xenopus is involved
in the maturation process of oocytes, pe-
flin, and different annexins. All these
targets interact with ALG-2 in a calcium-
dependent manner indicating that ALG-2,
like calmodulin, may have calcium sens-
ing, that is, signal transduction–regulating
properties. Therefore, the involvement in
apoptosis, which gave the protein its name,
wou
ldbeonebu
tno
tthee
x
c
lu
s
i
ves
ig
-
nal transduction pathway. In addition,
as mentioned before, it has been shown
that ALG-2 is not essential for apoptotic
responses since by using T cells from ALG-
2-deFcient mice, apoptotic stimuli could
be induced by either stimulating T-cell
receptors (TCR), ±as/CD95, or by gluco-
corticoids.
Recent experiments carried out in our
lab indicated that ALG-2 may not only be
involved in apoptotic processes but may
also play a role during cell proliferation.
A signiFcant nuclear concentration of
ALG-2 was found in cells prior to cell
division,
in
addition
to
a
signiFcant
increase in ALG-2 expression in highly
proliferative cells obtained from cancerous
tissues, especially evident for those from
metastatic tissues. ALG-2 may thus be
an important modulator involved in the
cellular decision between cell proliferation
and cell death.
5.2
Non-EF-hand Ca
2
+
Binding Proteins
5.2.1
Annexins
Next
to
the
family
of
the
E±-hand-
containing Ca
2
+
binding proteins, another
intracellular Ca
2
+
binding protein fam-
ily became known in recent years. These
soluble, amphipathic proteins bind to
membranes containing negatively charged
phospholipids in a Ca
2
+
-dependent man-
ner and are therefore called
annexins
.They
are widespread in the animal and plant
kingdom and have been claimed to be in-
volved in a variety of cellular functions
such as interaction with the cytoskeleton,
membrane fusion, anticoagulation, signal
transduction, or phospholipase inhibition.
The primary structure of the annexins
known to date contain 4 or 8 conserved
repeat units of about 75 amino acids
in length, which are separated by inter-
vening sequences of variable length. ±or
some of these proteins, the crystal struc-
ture is known. The most detailed studied
protein, annexin V, is almost entirely
α
-
helical. It consists of Fve
α
-helices bundled
into a right-handed super-helix. On the
b
a
s
i
so
fth
i
ss
t
ru
c
tu
r
e
,i
tw
a
sp
r
op
o
s
ed
that annexin V functions as a calcium
channel, and some experiments using a
reconstituted system seem to support a
voltage-gated mechanism. In contrast to
the E±-hand-containing Ca
2
+
binding pro-
teins, the ligands coordinating calcium in
the annexins are not adjacent in sequence.
Three calcium binding sites seem to ex-
ist in annexins, two invariably in repeats
II and IV, one in either repeat I or III.
As indicated for the E±-hand proteins,
also in the annexins, calcium is hepta-
coordinated with ligands organized in a
pentagonal bipyramid. The ligating oxy-
gens mainly stem from peptide carbonyls
or water molecules.
An interesting feature of annexin I and
II is their phosphorylation by either the
EG± receptor kinase or the
src
-encoded ty-
rosine kinase respectively suggesting their
involvement as coupling factors between
growth factor receptors and their cellu-
lar targets. ±urthermore, annexin II has
been found to form stable heterotetrameric
complexes with p11, a member of the S100
