Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

TRANSPORTERS AND ACTIVE MEMBRANE TRANSPORT
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important that the cell maintains a steep Ca 2+ gradient across its plasma membrane.
Ca 2+ transporters that actively pump Ca 2+ out of the cell help maintain the
gradient. One of these is a P-type Ca 2+ ATPase; the other is an antiporter (called
a Na + –Ca 2+ exchanger) that is driven by the Na + electrochemical gradient (discussed
in Chapter 15).
The Ca 2+ pump, or Ca 2+ ATPase, in the sarcoplasmic reticulum (SR) membrane
of skeletal muscle cells is a well-understood P-type transport ATPase. The
SR is a specialized type of endoplasmic reticulum that forms a network of tubular
sacs in the muscle cell cytoplasm, and it serves as an intracellular store of Ca 2+ .
When an action potential depolarizes the muscle cell plasma membrane, Ca 2+ is
released into the cytosol from the SR through Ca 2+ -release channels, stimulating
the muscle to contract (discussed in Chapters 15 and 16). The Ca 2+ pump, which
accounts for about 90% of the membrane protein of the SR, moves Ca 2+ from the
cytosol back into the SR. The endoplasmic reticulum of nonmuscle cells contains
a similar Ca 2+ pump, but in smaller quantities.
Enzymatic studies and analyses of the three-dimensional structures of transport
intermediates of the SR Ca 2+ pump and related pumps have revealed the
molecular mechanism of P-type transport ATPases in great detail. They all have
similar structures, containing 10 transmembrane α helices connected to three
cytosolic domains (Figure 11–13). In the Ca 2+ pump, amino acid side chains protruding
from the transmembrane helices form two centrally positioned binding
sites for Ca 2+ . As shown in Figure 11–14, in the pump’s ATP-bound nonphosphorylated
state, these binding sites are accessible only from the cytosolic side of the
SR membrane. Ca 2+ binding triggers a series of conformational changes that close
the passageway to the cytosol and activate a phosphotransfer reaction in which
the terminal phosphate of the ATP is transferred to an aspartate that is highly conserved
among all P-type ATPases. The ADP then dissociates and is replaced with
a fresh ATP, causing another conformational change that opens a passageway to
the SR lumen through which the two Ca 2+ ions exit. They are replaced by two H +
ions and a water molecule that stabilize the empty Ca 2+ -binding sites and close
the passageway to the SR lumen. Hydrolysis of the labile phosphoryl-aspartate
bond returns the pump to the initial conformation, and the cycle starts again. The
transient self-phosphorylation of the pump during its cycle is an essential characteristic
of all P-type pumps.
The Plasma Membrane Na + -K + Pump Establishes Na + and K +
Gradients Across the Plasma Membrane
The concentration of K + is typically 10–30 times higher inside cells than outside,
whereas the reverse is true of Na + (see Table 11–1, p. 598). A Na + -K + pump, or Na + -
K + ATPase, found in the plasma membrane of virtually all animal cells maintains
ATP
phosphate
2Ca 2+
CYTOSOL
SR membrane
LUMEN OF
SARCOPLASMIC
RETICULUM
nucleotidebinding
domain
phosphorylation
domain
calcium-binding
cavity
phosphorylated
aspartic acid
ATP
P
activator
domain
Figure 11–13 The structure of the
sarcoplasmic reticulum Ca 2+ pump.
The ribbon model (left), derived from x-ray
crystallographic analyses, shows the pump
in its phosphorylated, ATP-bound state.
The three globular cytosolic domains of
the pump—the nucleotide-binding domain
(dark green), the activator domain (blue),
and the phosphorylation domain (red), also
shown schematically on the right—change
conformation dramatically during the
pumping cycle. These changes in turn alter
the arrangement of the transmembrane
helices, which allows the Ca 2+ to be
released from its binding cavity into the SR
lumen (Movie 11.3). (PDB code: 3B9B.)