Calcium Biochemistry
159
proteins of the human erythrocytes), but
it is ubiquitous in the plasma membranes
of all cells. Calmodulin (CaM) activates
the PMCA by direct interaction, thereby
lowering the
K
M
for Ca
2
+
by one order
of magnitude and increasing the
V
Max
two- to threefold; that is, as a result, the
Ca
2
+
afFnity and its transport rate are
increased. This calcium-dependent afFn-
ity of the Ca
2
+
pump to CaM was also
exploited to isolate the enzyme in pure
form from detergent-solubilized erythro-
cyte membranes by applying calmodulin
afFnity chromatography. Next to the stim-
ulation by CaM, the plasma membrane
Ca
2
+
pump can also be activated by
alternative treatments, that is, by acidic
phospholipids, fatty acids, phosphoryla-
tion by different protein kinases (PKA
or PKC, see below), oligomerization, or
controlled proteolytic treatment. The lat-
ter procedure helped identify a number
of functional domains of the enzyme
(see ±ig. 4). A signiFcant difference in
the properties between PMCA and the
SERCA pump concerns the Ca
2
+
/ATP
stoichiometry, which is 1 for PMCA
b
u
t2f
o
rS
E
R
C
A
.I
nt
h
i
sr
e
s
p
e
c
t
,i
t
is of interest that most polar residues
that have been predicted (and conFrmed
through the structure) as participating in
Ca
2
+
binding and transport through the
TN90, 85, 81
Phosphorylation
site
ATP (FITC)
binding site
TN76
TC81, 76
TC90
TC85
Hinge
calp, 2
calp, 1
PKC(T)
N
C
Fig. 4
Model of the topography of the plasma
membrane Ca pump, illustrating the
arrangement of the different functional domains
assigned according to the primary structure of
the enzyme and on the basis of secondary
structure predictions. (A similar model of the
SERCA pump was recently validated by solving
the crystal structure, see text). The Fgure also
includes different proteolytic cleavage sites (TN,
TC), producing a number of enzymatically active
fragments of different size (not discussed in the
text); calp, calpain cleavage sites; PKC(T),
threonine phosphorylatable by protein kinase C.
(Reprinted with permission from Carafoli, 1992,
J. Biol. Chem.
267
, 2115–2118.)
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