160
Calcium Biochemistry
membrane within the SERCA pump (see
below) have also been conserved in the
PMCA pump. Consistent with the idea
that the two pumps differ in the stoi-
chiometry of Ca
2
+
/ATP was the Fnding
that mutating Met882 of PMCA (M6) that
corresponds to Thr799 (M6) in SERCA,
and is known to be part of Ca
2
+
site
II, had no influence on the activity of
the PMCA pump. However, replacing
Ala854 in PMCA (homologous to Glu771
in SERCA in M5) by a charged residue
made the PMCA pump more similar to
the SERCA pump with respect to the
Ca
2
+
/ATP stoichiometry.
As typical for the P-type ion pumps,
the Ca
2
+
pump of plasma membranes
consists of a single polypeptide (ca. 1200
amino acids
=
135 kDa, depending on the
isoform) spanning the membrane an even
number of times (see ±ig. 4); that is, the
N- and the C-terminus are located on
the same side of the membrane. This
model of the PMCA pump gained validity
owing to the recently solved structures
of the Ca
2
+
-bound and the Ca
2
+
-free
forms of the SERCA pump by Toyoshima
et al., as will be discussed below. As
demonstrated by this model, most of the
pump mass protrudes into the cytosol
with very short loops connecting the
proposed transmembrane domains on the
extracellular side. The cytosolic part of
the enzyme can be divided into three
different units.
1. Transduction or actuator (A) domain:
It
comprises
the
part
of
the
pump
between transmembrane domain 2 and
3, and, according to the structure of the
SERCA pump, it should also contain
the N-terminal sequence upstream of
transmembrane domain 1. It is proposed
to couple ATP hydrolysis to ion transport
as in other ion pumps. An interesting part
of this domain is the phospholipid binding
domain that is unique to the plasma
membrane Ca
2
+
pump as compared to
other ion pumps, and that might play a role
in speciFc tissue expression of isoforms
since it was identiFed as one of the two
alternative splicing zones of this protein
(see below).
2. Catalytic domain:
This unit spans the
cytosolic part between the fourth and
the Ffth transmembrane domain (see
±ig. 4) and, in analogy to the SERCA
pump, contains the phosphorylation (P)
and the nucleotide binding (N) domains.
The P-domain contains the aspartyl phos-
phatesite
,theN-domaintheATPbinding
site, and the so-called ‘‘hinge’’ region, a
highly conserved amino acid sequence
among ion-pumping ATPases, which is
involved in bringing the phosphorylation
site near to the bound ATP. The cat-
alytic unit further contains a receptor
for the ‘‘autoinhibitory’’ calmodulin bind-
ing domain, a property that seems to
be typical of CaM-dependent enzymes,
and which differs PMCA from SERCA.
This receptor is composed of two con-
tact sites as identiFed by cross-linking
experiments, one located between the P-
and the N-domain, the other N-terminal
to
the
phospholipid
binding
domain,
thereby ‘‘bridging’’ the transduction and
the catalytic domain and thus limiting
the access of the substrate Ca
2
+
to the
enzyme.
3. Regulatory domain:
Thesequencep
ro
-
truding into the cytosol after the last trans-
membrane domain until the C-terminus
comprises several sites important for the
regulation of the Ca
2
+
pump: the CaM
binding domain and consensus sequences
of two protein kinases, PKA and PKC (see
±ig. 4). The PKA site is located C-terminal
to the CaM binding domain, whereas the
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