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Alberts • Johnson • Lewis • Raff • Roberts • Walter
Molecular Biology of the Cell
Fifth Edition
Chapter 10
Membrane Structure
Copyright © Garland Science 2008
Cell membranes are composed of a lipid bilayer with proteins held
by non-covalent interactions
The lipid bilayer is composed of amphiphilic lipids
Figure 10-1 Molecular Biology of the Cell (© Garland Science 2008)
Lipid molecules constitute about 50% of the mass of most animal
cell membranes (remainder being proteins)
5x106 lipid molecules in 1mm x 1mm area
Most abundant lipids are the phospholipids
They have a polar head group and a hydrophobic hydrocarbon tail
If the tail has double bonds = not saturated – makes kinks
If the tail has only single bonds = saturated
Differences in length and saturation of the fatty acid tails influence
how phospholipid molecules pack against one another, thereby
affecting fluidity of the membrane.
PHOSPHOGLYCERIDES, THE MAIN
PHOSPHOLIPIDS IN ANIMAL CELLS
Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008)
PHOSPHOGLYCERIDES, and SPHINGOSINE
-Have three carbon glycerol as a backbone with two long chains of
fatty acids, 3rd Carbon is attached to a phosphate group that is
linked to one of several different types of head group.
-There are different types of phosphoglycerides, main ones are:
Phosphatidyl ethanolamine
Phosphatidyl serine
Phosphatidyl choline.
- Sphingomyelin has sphingosine as a backbone – because of the
amino and hydroxyl groups on it it contributes to polarity and to
forming H-bonds with other proteins or water or lipid molecules.
PHOSPHOGLYCERIDES, and SPHINGOSINE
STRUCTURES
Figure 10-3 Molecular Biology of the Cell (© Garland Science 2008)
Cholesterol structure:
A sterol ring structure and polar OH head
Figure 10-4 Molecular Biology of the Cell (© Garland Science 2008)
ORIENTATION OF CHOLESTEROL IN THE LIPID BILAYER
Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008)
EFFECTS OF CHOLESTEROL ON THE LIPID BILAYER
• Enhances the permeability barrier; makes the
membrane less permeable to small water soluble
molecules, because of the cholesterol stiffened region of
the phospholipids.
• Tightens the packing of the lipids but does not make the
membrane less fluid.
• Prevents the hydrocarbon chains from coming together
and crystallizing.
Hydrophobic and hydrophilic molecules interact
differently with water
Insoluble molecule
in water: Ice-like
packing of water
Higher Energy
Figure 10-6 Molecular Biology of the Cell (© Garland Science 2008)
Lipids organize together in the lowest Energy structure
Figure 10-7 Molecular Biology of the Cell (© Garland Science 2008)
Membranes have a self-healing property; free edges are
not energetically favorable
Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008)
THE LIPID BILAYER IS A FLUID
-In 1970 researchers recognized that individual lipid molecules are
able to diffuse freely within lipid bilayers:
This came from studies on synthetic lipid bilayers:
1. Bilayers made in the form of spherical liposomes (size 25nm1mm)
2. Black membranes: planar bilayers formed across a hole in
partition between two aqueous compartments
Liposomes can be made artificially in the lab to mimic
membranes and study their properties
Figure 10-9a Molecular Biology of the Cell (© Garland Science 2008)
“Black membranes” are made to study diffusion and
transport of material across a membrane
Figure 10-10 Molecular Biology of the Cell (© Garland Science 2008)
Computer simulation of lipid molecules to show that they
are disordered, with irregular surface
Figure 10-11 Molecular Biology of the Cell (© Garland Science 2008)
Studies that showed the mobility of lipid molecules
• Constructing a lipid molecule with a fluorescent dye attached to
the polar head group and follow the diffusion of individual
molecules in the membrane.
 Lipid molecules rarely move from one leaflet to the one on the other side;
flip-flop – phospholipid translocators catalyse the rapid flip-flop.
 Lipid molecules have high lateral diffusion 2mm/sec
• Modify a lipid head group to carry a “spin label” such as nitroxyl
group with unpaired electrons; if it spins it generates a signal
detected by electron spin resonance ESR spectroscopy.
 Lipid molecules do rotate about their long axis.
 Cholesterol does flip-flop easily
The fluidity of the membrane depends on its composition
and on the temperature
Figure 10-12 Molecular Biology of the Cell (© Garland Science 2008)
The fluidity of the membrane depends on its composition
and on the temperature
• If hydrocarbon chains are short or with double bonds : this
decreases the freezing temperature (becomes more difficult to
freeze).
• In addition, lipid molecules with double bonds are more spread
and so the lipid bilayer becomes thinner than bilayers of saturated
lipids. (idea of lipid domains, dicussed in the next slides)
• Cholesterol affects the freezing temperature of lipid bilayers; so
it lowers the freezing point. ( do not forget the other roles
cholesterol play in the lipid bilayer- discussed earlier)
Despite their fluidity lipid bilayers can form domains of
different compositions
• With certain lipid mixtures, different lipids can come together
transiently creating a dynamic patchwork of different domains
1:1:1 phosphatidylcholine:sphingomelin +/- : cholesterol
Without Cholesterol
Figure 10-13 Molecular Biology of the Cell (© Garland Science 2008)
With Cholesterol
Domains called lipid rafts – a specialized region of the
plasma membrane
Enriched in sphingolipids and cholesterol
Figure 10-14a Molecular Biology of the Cell (© Garland Science 2008)
Domains called lipid rafts – a specialized region of the
plasma membrane –thicker due to lipid composition
• Specific proteins that assemble there help to stabilize these rafts
and also rafts help concentrate specific proteins in them to do a
specific function.
Figure 10-14b Molecular Biology of the Cell (© Garland Science 2008)
Lipid droplets are surrounded by a phospholipid
monolayer
• Lipid droplets function : to store fat (Adipocyte = fat cells) :
Food source
To liberate fatty acids on demand
Used to synthesize triacylglycerides and cholesterol esters,
from enzymes in ER membrane
Used to make lipids.
Figure 10-15 Molecular Biology of the Cell (© Garland Science 2008)
Lipid bilayers are asymmetrical
Example: Red Blood cell
• Phosphatidylcholine and Sphingomyelin are in outer monolayer
•Phosphatidylserine in inner leaflet is – charged, and so there this
results in a charge difference
ASYMMETRY IS FUNCTIONALLY IMPORTANT
Figure 10-16 Molecular Biology of the Cell (© Garland Science 2008)
Example 1: a lipid kinase (PI-3K) can add phosphate onto
Phosphatidylinositol creating a binding site for docking of
specific proteins.
Figure 10-17a Molecular Biology of the Cell (© Garland Science 2008)
Example 2: PLC is activated by an extracellular signal to
cleave specific phospholipids molecules generating an
intracellular mediator
Figure 10-17b Molecular Biology of the Cell (© Garland Science 2008)
Phospholipids asymmetry is exploited by animal cells
When animal cell undergoes apoptosis phosphatidylserine,
normally present in cytosolic leaflet only rapidly moves to
extracellular monolayer
This on the cell surface signals other cells, like macrophages o
come to phagocytose (eat) the dead cell and digest it.
How this occurs?
1. Translocator that moves this lipid from noncytosolic to
cytosolic layer gets inactivated
2. A scramblase that transfers phospholipids randomly gets
activated
Glycolipids (Sugar containing lipid molecules) – found
only in non-cytosolic monolayer
Made from
Sphingosine
Most complex
Figure 10-18 Molecular Biology of the Cell (© Garland Science 2008)
GLYCOLIPIDS
• Sugars get added in the lumen of the ER and the golgi
• Gangliosides are the most complex : contain oligosaccharides with
one or more sialic acid residue – net negative charge – most
abundant in nerve cells
FUNCTION:
• May help protect the membrane from harsh conditions (pH and
enzymes) (on Apical surface of intestinal epithelium).
• Charged glycolipids may be important to change electric field
across the membrane (ex. Ca++ at membrane surface)
• Cell-cell recognition processes (sperm and egg cell-cell adhesion)
• Provide entry point for certain bacteria toxins (cholera toxin binds
to GM1 ganglioside) [causes too much Na+ and water to enter
intestine]
MEMBRANE PROTEINS
• Perform most of the membrane’s specific task
• Amount and types of proteins in a membrane are
highly variable
• If a membrane is 50% protein by mass it will have
more lipid molecules than protein molecules (as lipids
are smaller than proteins) so about 50 lipids for one
protein
Different ways in which a membrane protein can associate
with the membrane
Associated with
outer monolayer
(GPI anchor)
Transmembrane
proteins (integral
membrane proteins)
amphiphilic
Associated with
inner cyosolic
monolayer
Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008)
Peripheral
membrane
proteins
Different ways proteins attach to membranes
• Transmembrane proteins / integral membrane proteins –
amphiphilic
• Some have covalent attachment of fatty acid chains – more
hydrophobic (1)
• Some associate only with inner leaflet (by alpha helix or lipid
chain) (4)(5)
Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008)
Different ways proteins attach to membranes
• Some are exposed only to the outside attached my specific oligosaccharides
(covalent) attached to a lipid PI. Such as GPI (glycophosphatidylinositol) (in the
ER protein is cleaved and GPI added) PI specific PLC cuts these to be released.
(6)
• Some attach by non-covalent interactions with other membrane proteins – can
be removed with gentle extraction with solution of high or low ionic strength or
extreme pH. = peripheral membrane proteins.
Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008)
Different ways proteins attach to membranes:
Attachment by fatty acid chains
• These function on one side of the membrane (ex. Intracellular
signaling proteins)
Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008)
In transmembrane proteins, they have hydrophobic
domains (alpha helices that span the membrane)
Can be Single pass (as example
shown here) or multipass
(transmembrane protein crosses
multiple times).
Domains that are inside the
bilayer are hydrophobic, they are
composed of amino acids that are
hydrophobic.
Figure 10-21 Molecular Biology of the Cell (© Garland Science 2008)
THE AMINO ACIDS AND THEIR PROPERTIES
REQUIREMENTS FOR A TRANSMEMBRANE PROTEIN
 Segments of 20-30 amino acids with high degree of
hydrophobicity are long enough to span a lipid bilayer as an alphahelix (an alpha helix has maximum hydrogen-bonds between
amino acids).
QUESTION
Monomeric single pass transmembrane proteins span the
membrane with a single a-helix that has specific
characteristic chemical properties in the region of the
bilayer. Which of the three 20-amino acid sequences below
is the most likely candidate for such transmembrane
segment?
A
I
T
L
I
Y
F
G
V
M
A
G
V
I
G
T
I
L
L
I
S
B
I
T
P
I
Y
F
G
P
M
A
G
V
I
G
T
P
L
L
I
S
C
I
T
E
I
Y
F
G
R
M
A
G
V
I
G
T
D
L
L
I
S
Hydropathy plot used to predict from the amino acid
sequence the stretch that is hydrophobic (TM domain)
20% of an organisms total protein are TM proteins
Figure 10-22 Molecular Biology of the Cell (© Garland Science 2008)
Multi-spanning proteins
Structure determined by X-RAY crystalography
Figure 10-23 Molecular Biology of the Cell (© Garland Science 2008)
• TM (transmembrane) domains of single pass proteins do
not contribute to the folding of cytoplasmic or
extracellular parts; so they can be produced each
independently in the cells to see there function as soluble
proteins.
• Sometimes TM domains of different proteins or within
the same protein interact together. This is a property of the
TM domain alone, as if it is cut it can still associate with
the other TM domain.
TM domains associate together even if they are separated
Done by engineering genes encoding separate pieces of a multipass
protein in living cells.
Figure 10-24 Molecular Biology of the Cell (© Garland Science 2008)
TM domains enter the membrane with the help of protein
“translocators” to insert in bilayer first, then they
associate with other TM domains
Figure 10-25 Molecular Biology of the Cell (© Garland Science 2008)
Beta barrel proteins tend to be more rigid:
Some are pore forming (water channels) polar amino acids
located on the inside, nonpolar outside
Some b-barrel proteins are receptors or enzymes
Figure 10-26 Molecular Biology of the Cell (© Garland Science 2008)
Beta barrel proteins
• Restricted to the outer membrane of bacteria, mitochondria and
chloroplast.
• In Eukaryotes and bacteria; most multipass TM proteins have
alpha helices.
• Alpha helices can slide past each other to open/close channels
• Beta barrels are more rigid
In animal cells most TM proteins are glycosylated
 Oligosaccharides are present on noncytosolic side (remember sugars added in
lumen of ER/golgi)
 the cytosol has a reducing
environment, so S-S (disulfide) bonds
form only in the extracellular space.
 Long polysaccharides chains linked to
a protein core in the extracellular matrix
make up the proteoglycans (they can be
attached to the PM as well).
Figure 10-27 Molecular Biology of the Cell (© Garland Science 2008)
Glycocalyx = sugar coat
Glycocalyx is seen by specific stain ruthenium red, or with
fluorescent lectins (a carbohydrate-binding protein).
Figure 10-28a Molecular Biology of the Cell (© Garland Science 2008)
Different types of sugar attachment
• Proteoglycans also are found in the extracellular matrix
Figure 10-28b Molecular Biology of the Cell (© Garland Science 2008)
Table 2-1 Molecular Biology of the Cell (© Garland Science 2008)
Membrane proteins can be solubilized and purified by
detergents
Agents that disrupt the hydrophobic
interactions and destroy the lipid
bilayer can solubilize TM proteins:
•Detergents are amphiphilic much
more soluble in water than lipids
• Ionic charged (SDS)
• Uncharged nonionic (TX-100, and bocty).
• At [low] are monomer, at [high ] they
form micelles
• They depend on Temp, pH and [salt]
Figure 10-29a Molecular Biology of the Cell (© Garland Science 2008)
Detergent micelles have irregular shapes
Figure 10-29c Molecular Biology of the Cell (© Garland Science 2008)
When mixed with membranes – hydrophobic ends of
detergent bind to hydrophobic membrane parts – displace
lipid molecules  soluble detergent-protein complexes
Figure 10-30 Molecular Biology of the Cell (© Garland Science 2008)
Figure 10-31 Molecular Biology of the Cell (© Garland Science 2008)
Bacteriorhodopsin: first membrane protein with known
structure
Figure 10-32 Molecular Biology of the Cell (© Garland Science 2008)
Bacteriorhodopsin: first membrane protein with known
structure
• On membrane of the archaean Halobacterium Salinarum
• Lives in seawater, exposed to light
• It is a light activated proton pump
• Each contains a light absorbing chromophore called retinal
(vitamin A)
• Light causes a change in conformation of the protein and H+ goes
from inside to outside of the cell
• In bright light each molecule pumps several hundred protons per
second
• The H+ gradient drives ATP production by other protein in the
plasma membrane.
• other proteins in same family is rhodopsin, similar structure with
different function in vertebrate retina (this is a signal transducer and
not a transporter; functions to activate G-protein inside the cell).
Bacteriorhodopsin: 7 TM alpha helices
Figure 10-33 Molecular Biology of the Cell (© Garland Science 2008)
Membrane proteins can be quite complex in structure:
multicomponent
Figure 10-34 Molecular Biology of the Cell (© Garland Science 2008)
Membrane proteins diffuse in plane of PM: heterokaryons
of human and mouse cells
Figure 10-35 Molecular Biology of the Cell (© Garland Science 2008)
Diffusion rate measure: fluorescent recovery after
photobleaching
Figure 10-36a Molecular Biology of the Cell (© Garland Science 2008)
Diffusion rate measure: fluorescent loss in photobleaching
Figure 10-36b Molecular Biology of the Cell (© Garland Science 2008)
FRAP and FLIP
• Diffusion coefficient is measured.
• Monitor movement of large proportion of molecules, in a
relatively large area of the cell (single particle tracking techniques
have overcome this problem).
•Found that the viscosity of membrane similar to olive oil (without
proteins in it).
Cells can confine proteins and lipids to specific domains within a
membrane: tight junctions
Figure 10-37 Molecular Biology of the Cell (© Garland Science 2008)
Sperm cells have specific membrane domains : not known how they
are maintained
Figure 10-38 Molecular Biology of the Cell (© Garland Science 2008)
Common ways to restrict mobility of proteins
Figure 10-39 Molecular Biology of the Cell (© Garland Science 2008)
Membrane of RBCs maintained by a network of cytoskeletal protein
complexes
Figure 10-40 Molecular Biology of the Cell (© Garland Science 2008)
Membrane of RBCs maintained by a network of cortical cytoskeletal
protein complexes
Figure 10-41a Molecular Biology of the Cell (© Garland Science 2008)
Figure 10-41b Molecular Biology of the Cell (© Garland Science 2008)
The cortical barriers act as a mechanical barriers for membrane
proteins; movement measured by high speed single particle tracking
Figure 10-42 Molecular Biology of the Cell (© Garland Science 2008)