Transcript Chapter 14

14
The Plasma Membrane
14 The Plasma Membrane
• Structure of the Plasma Membrane
• Transport of Small Molecules
• Endocytosis
Introduction
All cells are surrounded by a plasma
membrane. The plasma membrane:
• Defines the cell boundary and
separates it from the environment.
• Is a selective barrier, and determines
the composition of the cytoplasm.
• Mediates interactions between the cell
and its environment.
Structure of the Plasma Membrane
The fundamental structure of the
membrane is the phospholipid bilayer.
Proteins embedded in the bilayer carry
out specific functions, including
selective transport of molecules and
cell-cell recognition.
Structure of the Plasma Membrane
Mammalian red blood cells
(erythrocytes) have been useful as a
model for studies of membrane
structure.
These cells have no nuclei or internal
membranes, making it easy to isolate
pure plasma membranes.
Structure of the Plasma Membrane
The bilayer structure can be seen in
electron micrographs.
The polar head groups appear as dark
lines because they bind the electrondense metal stains.
The hydrophobic fatty acid chains in the
center are lightly stained.
Figure 14.1 Bilayer structure of the plasma membrane
Structure of the Plasma Membrane
Animal cell plasma membranes have
five major phospholipids:
• Outer leaflet—phosphatidylcholine
and sphingomyelin.
• Inner leaflet—
phosphatidylethanolamine,
phosphatidylserine,
phosphatidylinositol
Table 14.1 Lipid Composition of the Plasma Membrane
Figure 14.2 Lipid components of the plasma membrane
Structure of the Plasma Membrane
Animal cell plasma membranes also
contain:
Glycolipids—only in outer leaflet, with
carbohydrate portions exposed on the
cell surface.
Cholesterol—present in about the same
molar amounts as phospholipids.
Structure of the Plasma Membrane
Phospholipid structure is responsible for
the basic function of membranes—
separating aqueous compartments.
The bilayer interior consists of
hydrophobic fatty acid chains, so it is
impermeable to water-soluble
molecules—ions and most biological
molecules.
Structure of the Plasma Membrane
Bilayers are viscous fluids, not solid.
The fatty acids have one or more double
bonds, which make kinks in the chain
and keep them from packing together.
Lipids and proteins are free to diffuse
laterally within the membrane.
Structure of the Plasma Membrane
Cholesterol affects membrane fluidity
and is involved in formation of
functional domains in the membrane.
Cholesterol and the sphingolipids
(sphingomyelin and glycolipids) tend to
cluster in small semisolid patches
called lipid rafts.
Figure 14.3 Lipid rafts
Structure of the Plasma Membrane
Most plasma membranes are about 50%
lipid and 50% protein by weight.
Since proteins are much larger than
lipids, this corresponds to about one
protein per 50–100 molecules of lipid.
Structure of the Plasma Membrane
The fluid mosaic model of membrane
structure was proposed by Singer and
Nicolson in 1972:
Membranes are two-dimensional fluids
with proteins inserted into lipid bilayers.
Both proteins and lipids are able to
diffuse laterally through the membrane.
Figure 14.4 Fluid mosaic model of the plasma membrane
Structure of the Plasma Membrane
Lateral movement of proteins and lipids
was first demonstrated in 1970.
Human and mouse cells were fused in
culture, then analyzed for membrane
proteins using fluorescent antibodies.
Within 40 minutes, the mouse and
human proteins became intermixed
over the surface of hybrid cells.
Figure 14.5 Mobility of membrane proteins
Structure of the Plasma Membrane
Peripheral membrane proteins:
associated with membranes through
protein-protein interactions; often ionic
bonds.
The bonds can be disrupted by polar
reagents (salts or extreme pH).
Many are part of the cortical cytoskeleton:
spectrin, actin, band 4.1, etc.
Structure of the Plasma Membrane
Integral membrane proteins: inserted
into the lipid bilayer; they can be
dissociated only by reagents that
disrupt hydrophobic interactions.
Detergents are amphipathic molecules
with hydrophobic and hydrophilic
groups that can solubilize these
proteins.
Figure 14.6 Solubilization of integral membrane proteins by detergents
Structure of the Plasma Membrane
Transmembrane proteins: integral
proteins that span the lipid bilayer with
portions exposed on both sides.
They can be seen in electron
micrographs of plasma membranes
prepared by freeze-fracture technique.
Figure 14.7 Freeze-fracture electron micrograph of human red blood cell membranes
Structure of the Plasma Membrane
The membrane-spanning portions are
usually α helices of hydrophobic amino
acids; they are inserted into the ER
membrane during synthesis.
Carbohydrate groups are added in the
ER and Golgi; most are glycoproteins
with oligosaccharides exposed on the
cell surface.
Structure of the Plasma Membrane
Glycophorin and band 3 illustrate
transmembrane protein structure.
Glycophorin has a single
transmembrane α helix.
Band 3 is the transporter for HCO3 – and
Cl– ions, with 14 transmembrane α
helices.
Figure 14.8 Integral membrane proteins of red blood cells
Structure of the Plasma Membrane
The first transmembrane protein to be
analyzed by X-ray crystallography was
the photosynthetic reaction center of
the bacterium Rhodopseudomonas
viridis.
Figure 14.9 A bacterial photosynthetic reaction center
Structure of the Plasma Membrane
Some proteins are anchored in the
plasma membrane by covalently
attached lipids or glycolipids.
Glycosylphosphatidylinositol (GPI)
anchors are added to the C terminus
of some proteins in the ER.
These proteins are glycosylated and
exposed on the cell surface.
Figure 14.10 Examples of proteins anchored in the plasma membrane by lipids and glycolipids
Structure of the Plasma Membrane
Other proteins are anchored in the inner
leaflet by covalently attached lipids.
They are translated on free ribosomes
and modified by myristic acid, prenyl
groups, or palmitic acid.
Many of these proteins (including Src
and Ras) play roles in signal
transmission.
Structure of the Plasma Membrane
Glycocalyx: carbohydrate coat formed
by the oligosaccharides of glycolipids
and glycoproteins.
Protects the cell surface from ionic and
mechanical stress and forms a barrier
to invading microorganisms.
Oligosaccharides of the glycocalyx
participate in a variety of cell–cell
interactions.
Figure 14.11 The glycocalyx
Structure of the Plasma Membrane
Ammendments to the fluid mosaic
model:
• Mobility of many plasma membrane
proteins is restricted.
• Membranes are composed of
distinct domains that have different
structural and functional roles.
Structure of the Plasma Membrane
Many epithelial cells are polarized;
plasma membranes are divided into
apical and basolateral domains.
In the small intestine, the apical surface
is covered by microvilli that increase
surface area for absorption.
The basolateral surface mediates
transfer of absorbed nutrients to the
blood.
Figure 14.12 A polarized intestinal epithelial cell
Structure of the Plasma Membrane
To maintain these functions, mobility of
plasma membrane proteins must be
restricted to appropriate domains.
Tight junctions separate the apical and
basolateral domains.
Membrane proteins can move within
each domain but can’t cross from one
to the other.
Structure of the Plasma Membrane
Mobility of many plasma membrane
proteins is restricted by association
with the cytoskeleton or specialized
lipid domains.
Transmembrane proteins anchored to
the cytoskeleton have restricted
mobility and may also act as barriers
that limit mobility of other membrane
proteins.
Figure 14.13 Updated fluid mosaic model
Structure of the Plasma Membrane
Lipid rafts are transient structures in
which specific proteins can be
concentrated to facilitate interactions.
They are enriched in GPI-anchored
proteins and transmembrane proteins
involved in a variety of functions,
including cell signaling, cell movement,
and endocytosis.
Figure 14.14 Super-resolution microscopy of lipid rafts
Structure of the Plasma Membrane
Caveolae are small lipid rafts that start
as invaginations of the plasma
membrane, organized by caveolin.
They have been implicated in
endocytosis, cell signaling, regulation
of lipid transport, and protection of the
plasma membrane against mechanical
stress.
Figure 14.15 Caveolae
Transport of Small Molecules
Plasma membranes are selectively
permeable to small molecules.
Specific transport and channel proteins
mediate passage of glucose, amino
acids, and other small molecules and
ions.
Transport of Small Molecules
Facilitated diffusion: Direction of
movement determined by concentration
gradients; no energy required.
Transport is mediated by proteins, which
allow polar and charged molecules to
cross the plasma membrane
(carbohydrates, amino acids, ions,
nucleosides).
Transport of Small Molecules
Carrier proteins bind molecules on one
side of the membrane, then undergo
conformational changes that allow the
molecule to pass through and be
released on the other side.
Transport of Small Molecules
Channel proteins form open pores
through the membrane, allowing free
diffusion of any molecule of the
appropriate size and charge.
Transport of Small Molecules
Carrier proteins allow facilitated diffusion
of sugars, amino acids, and
nucleosides.
The glucose transporter has 12 α-helical
transmembrane segments (typical of
many carrier proteins).
Transport of Small Molecules
Glucose transporters function by
alternating between two conformational
states.
A glucose-binding site is alternately
exposed on the outside and the inside
of the cell.
Figure 14.16 Facilitated diffusion of glucose (Part 1)
Figure 14.16 Facilitated diffusion of glucose (Part 2)
Transport of Small Molecules
Glucose is rapidly metabolized in the
cell, so intracellular glucose
concentrations remain low and glucose
is transported into the cell.
Glucose transport can also be reversed,
(e.g. in liver cells when glucose is
synthesized and released into the
circulation).
Transport of Small Molecules
Channel proteins, such as porins, form
open pores in the membrane that allow
molecules to pass freely.
Aquaporins allow water molecules to
cross the membrane rapidly.
They are impermeable to charged ions,
allowing passage of water without
affecting electrochemical gradients.
Figure 14.17 Structure of an aquaporin
Transport of Small Molecules
Ion channels are well studied in nerve
and muscle cells, where their opening
and closing is responsible for
transmission of electric signals.
Transport through ion channels is
extremely rapid: more than a million
ions per second.
Transport of Small Molecules
Ion channels are highly selective;
specific channel proteins allow passage
of Na+, K+, Ca2+, and Cl–.
Most have “gates” that open only in
response to specific stimuli.
Figure 14.18 Model of an ion channel
Transport of Small Molecules
Ligand-gated channels open in
response to binding of
neurotransmitters or other signaling
molecules.
Voltage-gated channels open in
response to changes in electric
potential across the plasma membrane.
Transport of Small Molecules
The role of ion channels in transmitting
electric impulses was first shown using
giant squid axons by Hodgkin and
Huxley in 1952.
Electrodes inserted in the axon
measured changes in membrane
potential resulting from opening and
closing of Na+ and K+ channels.
Transport of Small Molecules
Ion pumps use energy from ATP
hydrolysis to actively transport ions
across the plasma membrane to
maintain concentration gradients.
Thus, the ionic composition of the
cytoplasm is substantially different from
that of extracellular fluids.
Table 14.2 Intracellular and Extracellular Ion Concentrations
Transport of Small Molecules
Because ions are electrically charged,
pumping results in electric gradients
across the plasma membrane.
In resting squid axons, there is an
electric potential of about 60 mV; the
inside of the cell is negative with
respect to the outside.
Figure 14.19 Ion gradients and resting membrane potential of the giant squid axon
Transport of Small Molecules
Na+ is pumped out of the cell while K+ is
pumped in.
The plasma membrane also contains
open K+ channels, so the flow of K+
makes the largest contribution to
resting membrane potential.
Transport of Small Molecules
The Nernst equation describes the
relationship between ion concentration
and membrane potential:
RT Co
V
ln
zF Ci
Transport of Small Molecules
RT Co
V
ln
zF Ci
V—equilibrium potential in volts
R—gas constant
T—absolute temperature
Z—charge of the ion
F—Faraday’s constant
Co and Ci —concentrations of the ion
outside and inside the cell
Transport of Small Molecules
As nerve impulses (action potentials)
travel along axons, the membrane
depolarizes.
Membrane potential goes from –60 mV
to +30 mV in less than a millisecond.
This results from rapid sequential
opening and closing of voltage-gated
Na+ and K+ channels.
Figure 14.20 Membrane potential and ion channels during an action potential (Part 1)
Figure 14.20 Membrane potential and ion channels during an action potential (Part 2)
Transport of Small Molecules
Depolarization of adjacent regions of the
plasma membrane allows action
potentials to travel the length of a nerve
cell.
At the nerve end, neurotransmitters are
released into the synapse where they
bind to receptors on another nerve cell
to open ligand-gated ion channels.
Figure 14.21 Signaling by neurotransmitter release at a synapse
Transport of Small Molecules
Nicotinic acetylcholine receptors in
muscle cells are ligand-gated channels:
Binding of acetylcholine opens a channel
that allows rapid influx of Na+, which
depolarizes the cell membrane and
triggers an action potential.
Binding of acetylcholine induces a
conformational change in the receptor.
Figure 14.22 Model of the nicotinic acetylcholine receptor
Transport of Small Molecules
Voltage-gated Na+ and K+ channels are
more selective.
Na+ (0.95 Å) is smaller than K+ (1.33 Å),
and it is thought that the Na+ channel
pore is too narrow for K+ or larger ions.
Figure 14.23 Ion selectivity of Na+ channels
Transport of Small Molecules
The structure of K+ channels was
determined by X-ray crystallography.
Part of the channel is lined with carbonyl
oxygen (C=O) atoms from the
polypeptide backbone. They displace
the water to which K+ is bound, and the
K+ ion passes through.
Na+ is too small to interact and remains
bound to water.
Figure 14.24 Selectivity of K+ channels
Transport of Small Molecules
Voltage-gated Na+, K+, and Ca2+ channels
belong to a family of related proteins.
Ion channels play critical roles in signaling
in all cell types.
Regulated opening and closing of ion
channels is a sensitive and versatile
mechanism for responding to
environmental stimuli.
Figure 14.25 Structures of voltage-gated cation channels (Part 1)
Figure 14.25 Structures of voltage-gated cation channels (Part 2)
Figure 14.25 Structures of voltage-gated cation channels (Part 3)
Transport of Small Molecules
In active transport, molecules are
transported against their concentration
gradients.
Energy is provided by a coupled reaction
(such as ATP hydrolysis).
Transport of Small Molecules
Ion pumps are examples of active
transport.
The Na+-K+ pump (Na+-K+ ATPase)
uses energy from ATP hydrolysis to
transport Na+ and K+ against their
electrochemical gradients.
Figure 14.26 Structure of the Na+–K+ pump
Transport of Small Molecules
The Na+-K+ pump operates by ATPdriven conformational changes.
3 Na+ are transported out of the cell and
2 K+ are transported into the cell for
every ATP used.
Figure 14.27 Model for operation of the Na+–K+ pump (Part 1)
Figure 14.27 Model for operation of the Na+–K+ pump (Part 2)
Transport of Small Molecules
The Na+-K+ pump uses nearly 25% of
the ATP in many animal cells.
The gradients are necessary for
propagation of electric signals in nerve
and muscle cells, to drive active
transport of other molecules, and to
maintain osmotic balance and cell
volume.
Figure 14.28 Ion gradients across the plasma membrane of a typical mammalian cell
Transport of Small Molecules
The differences in ion concentrations
balance the high concentrations of
organic molecules inside cells,
equalizing osmotic pressure and
preventing the net influx of water.
Transport of Small Molecules
The Ca2+ pump is also powered by ATP
hydrolysis.
Ca2+ is transported out of the cell or into
the ER lumen, so intracellular Ca2+
concentrations are extremely low.
Transient, localized increases in
intracellular Ca2+ are important in cell
signaling (as in muscle contraction).
Figure 14.29 Structure of the Ca2+ pump
Transport of Small Molecules
Ion pumps in bacteria, yeasts, and plant
cells transport H+ out of the cell.
H+ is pumped out of stomach lining cells,
resulting in the acidity of gastric fluids.
Structurally distinct pumps transport H+
into lysosomes and endosomes.
Transport of Small Molecules
ATP synthases of mitochondria and
chloroplasts are another type of H+
pump.
These pumps operate in reverse, with
the movement of ions down the
electrochemical gradient used to drive
ATP synthesis.
Transport of Small Molecules
ABC transporters have highly
conserved ATP-binding domains or
ATP-binding cassettes.
More than 100 members of this family
have been identified in both prokaryotic
and eukaryotic cells.
Transport of Small Molecules
All ABC transporters use energy from
ATP hydrolysis to transport molecules
in one direction.
In prokaryotes, they transport nutrient
molecules into the cell.
In both prokaryotic and eukaryotic cells
they transport toxic substances out of
the cell.
Transport of Small Molecules
ABC transporters have two ATP-binding
domains and two transmembrane
domains.
The substrate binding site alternates
between outward facing and inward
facing, depending on ATP binding and
hydrolysis.
Figure 14.30 Model of active transport by an ABC transporter
Transport of Small Molecules
The first eukaryote ABC transporter was
discovered as a product of the mdr
(multidrug resistance) gene.
MDR transporters remove potentially
toxic foreign compounds from cells.
They are often expressed at high levels
in cancer cells, and can remove a
variety of chemotherapy drugs.
Transport of Small Molecules
In cystic fibrosis, defective Cl– transport
in epithelial cells results in abnormally
thick, sticky mucus which obstructs
respiratory passages.
The cystic fibrosis gene encodes a
protein (CFTR or cystic fibrosis
transmembrane conductance regulator)
in the ABC transporter family.
Transport of Small Molecules
Cystic fibrosis is the result of a mutation
in CFTR that interferes with proper
folding of the protein.
Isolation of the gene allows for potential
genetic screening and gene therapy.
Drugs such as ivacaftor, which increases
Cl– transport, are being developed.
Molecular Medicine, Ch. 14, p. 557
Transport of Small Molecules
Some molecules can be transported
against their concentration gradients
using energy from coupled transport of
another molecule in the energetically
favorable direction.
Gradients established by Na+-K+ and H+
pumps provide a source of energy for
active transport.
Transport of Small Molecules
Glucose transporters in the apical domain
of intestine epithelial cells transport two
Na+ and one glucose into the cell.
Flow of Na+ down its electrochemical
gradient provides the energy that drives
uptake of glucose against its
concentration gradient.
Figure 14.31 Active transport of glucose
Transport of Small Molecules
In the basolateral domain, glucose is
transferred to the underlying connective
tissue and blood capillaries by facilitated
diffusion.
The system is driven by Na+-K+ pumps.
Figure 14.32 Glucose transport by intestinal epithelial cells
Transport of Small Molecules
Uptake of glucose and Na+ is an
example of symport—transport of two
molecules in the same direction.
Facilitated diffusion of glucose is an
example of uniport—transport of a
single molecule.
Transport of Small Molecules
Antiport—two molecules are
transported in opposite directions.
Ca2+ is exported from cells by the Ca2+
pump and by a Na+-Ca2+ antiporter that
transports Na+ in and Ca2+ out.
Na+-H+ antiporter transports Na+ into the
cell and H+ out, preventing acidification
by H+ produced in metabolism.
Figure 14.33 Examples of antiport
Endocytosis
Endocytosis allows cells to take up
macromolecules, fluids, and large
particles such as bacteria.
The material is surrounded by an area of
plasma membrane, which buds off
inside the cell to form a vesicle
containing the ingested material.
Endocytosis
Phagocytosis (cell eating) occurs in
specialized cell types.
Binding of a particle to receptors on the
cell surface triggers extension of
pseudopodia, which surround the
particle and fuse to form a large vesicle
called a phagosome.
Endocytosis
Phagosomes fuse with lysosomes to
form phagolysosomes, in which the
material is digested by acid hydrolases.
Figure 14.34 Phagocytosis
Endocytosis
Many amoebas use phagocytosis to
capture food particles, such as
bacteria.
In multicellular animals, phagocytosis is
used as a defense against invading
microorganisms, and to eliminate aged
or damaged cells.
Figure 14.35 Examples of phagocytic cells
Endocytosis
In mammals, macrophages and
neutrophils (white blood cells) are the
“professional phagocytes.”
They remove microorganisms from
infected tissues, and macrophages
eliminate aged or dead cells from
tissues throughout the body.
Endocytosis
Macropinocytosis: uptake of
extracellular fluids in large vesicles.
Lamellipodia (sheet-like projections of
the plasma membrane) curve into open
cups, followed by membrane fusion to
form a large intracellular vesicle.
Endocytosis
Clathrin-mediated endocytosis is a
mechanism for selective uptake of
specific macromolecules.
Mechanisms of cargo selection, vesicle
budding, and vesicle fusion are similar
to those involved in vesicular transport
in the secretory pathway.
Endocytosis
Macromolecules bind to cell surface
receptors in specialized regions called
clathrin-coated pits.
The pits bud from the membrane with
the help of dynamin, to form small
clathrin-coated vesicles; these then
fuse with early endosomes.
Figure 14.36 Clathrin-coated vesicle formation (Part 1)
Figure 14.36 Clathrin-coated vesicle formation (Part 2)
Endocytosis
Clathrin-mediated endocytosis was first
studied in patients with familial
hypercholesterolemia (FH).
Cholesterol is transported through the
bloodstream mostly in the form of lowdensity lipoprotein, or LDL particles.
Uptake of LDL requires binding to
specific receptors in clathrin-coated pits.
Figure 14.37 Structure of LDL
Endocytosis
LDL binding sites on normal cells were
determined by adding radiolabeled LDL
to cell cultures.
Cells of FH patients did not bind LDL.
Mutations in the LDL receptors prevent
them from binding LDL, or prevent the
receptors from concentrating in the
coated pits.
Key Experiment, Ch. 14, p. 564 (3)
Endocytosis
Mutations that prevent LDL receptors from
concentrating in coated pits are in the
internalization signal in the cytoplasmic
tail of the receptor.
The signal is a sequence of six amino
acids, including tyrosine.
Similar signals are found in other receptors
taken up via clathrin-coated pits.
Figure 14.38 The LDL receptor
Endocytosis
After internalization, clathrin-coated
vesicles shed their coats and fuse with
early endosomes.
The molecules are sorted, recycled to
the plasma membrane, or remain in the
early endosomes as they mature to late
endosomes and lysosomes for
degradation.
Figure 14.39 Sorting in early endosomes
Endocytosis
Early endosomes have membrane H+
pumps which maintain acidic internal
pH (6.0 to 6.2).
This causes dissociation of many ligands
from their receptors.
Receptors can be returned to the plasma
membrane via transport vesicles.
Ligands such as LDL remain and are
degraded to release cholesterol.
Endocytosis
About 50% of the plasma membrane is
internalized by receptor-mediated
endocytosis every hour and must be
replaced at an equivalent rate by
recycling.
Most of this internalized membrane is
replaced by recycling.
Endocytosis
Clathrin-independent endocytosis
does not involve specific membrane
receptors or coated vesicles.
Macropinocytosis and internalization of
caveolae are examples.
One pathway mediates uptake of GPIanchored plasma membrane proteins
clustered in lipid rafts.