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

MEMBRANE PROTEINS
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anterior head
posterior head
tail
(B)
Figure 10–35 Three domains in the
plasma membrane of a guinea pig
sperm. (A) A drawing of a guinea
pig sperm. (B–D) In the three pairs of
micrographs, phase-contrast micrographs
are on the left, and the same cell is shown
with cell-surface immunofluorescence
staining on the right. Different monoclonal
antibodies selectively label cell-surface
molecules on (B) the anterior head,
(C) the posterior head, and (D) the tail.
(Micrographs courtesy of Selena Carroll
and Diana Myles.)
(A)
(C)
(D)
20 µm
leaving their domain is not known. Many other cells have similar membrane
fences that confine membrane protein diffusion to certain membrane domains.
The plasma membrane of nerve cells, for example, contains a domain enclosing
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the cell body and dendrites, and another enclosing the axon; it is thought that a
belt of actin filaments tightly associated with the plasma membrane at the cellbody–axon
junction forms part of the barrier.
Figure 10–36 shows four common ways of immobilizing specific membrane
proteins through protein–protein interactions.
The Cortical Cytoskeleton Gives Membranes Mechanical Strength
and Restricts Membrane Protein Diffusion
As shown in Figure 10–36B and C, a common way in which a cell restricts the lateral
mobility of specific membrane proteins is to tether them to macromolecular
assemblies on either side of the membrane. The characteristic biconcave shape of
a red blood cell (Figure 10–37), for example, results from interactions of its plasma
membrane proteins with an underlying cytoskeleton, which consists mainly of a
meshwork of the filamentous protein spectrin. Spectrin is a long, thin, flexible
rod about 100 nm in length. As the principal component of the red cell cytoskeleton,
it maintains the structural integrity and shape of the plasma membrane,
which is the red cell’s only membrane, as the cell has no nucleus or other organelles.
The spectrin cytoskeleton is riveted to the membrane through various membrane
proteins. The final result is a deformable, netlike meshwork that covers
the entire cytosolic surface of the red cell membrane (Figure 10–38). This spectrin-based
cytoskeleton enables the red cell to withstand the stress on its membrane
as it is forced through narrow capillaries. Mice and humans with genetic
abnormalities in spectrin are anemic and have red cells that are spherical (instead
of concave) and fragile; the severity of the anemia increases with the degree of
spectrin deficiency.
(A)
(B)
(C)
(D)
Figure 10–36 Four ways of restricting
the lateral mobility of specific plasma
membrane proteins. (A) The proteins can
self-assemble into large aggregates (as
seen for bacteriorhodopsin in the purple
membrane of Halobacterium salinarum);
they can be tethered by interactions with
assemblies of macromolecules (B) outside
or (C) inside the cell; or (D) they can interact
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with proteins on the surface of another cell.