Cell membranes
Cell membranes from the functional
point of view
Major functions of a cellular membrane
 A selective barrier
 A receiver and a transducer of extracellular signals
 An exporter/importer of small molecules
 A supporter of cell internal and provider of changes in the
cell size and form
 A mean to move
Barrier functions
 Only small uncharged molecules can
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diffuse freely through phospholipid
bilayers.
Small nonpolar molecules, such as O2 and
CO2, are soluble in the lipid bilayer and
therefore can readily cross cell
membranes.
Small uncharged polar molecules, such as
H2O, also can diffuse through membranes,
but larger uncharged polar molecules,
such as glucose, cannot.
Charged molecules, such as ions, are
unable to diffuse through a phospholipid
bilayer regardless of size; even H+ ions
cannot cross a lipid bilayer by free
diffusion.
Gases, hydrophobic molecules, and small
polar uncharged molecules can diffuse
through phospholipid bilayers
Physical bases of membrane
organization
 The property of the hydrophobic
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moieties of lipids to self-associate
The tendency of the hydrophilic
moieties to interact with aqueous
environment and with each other
Liquid-liquid immiscibility
Sphingolipid-cholesterol selfassembly (lipid rafts)
High concentrations of integral
proteins
The geometrical shape of lipids
determines supramolecular structural
organization of lipid layer in the
solution.
Simons and Sampaio, 2011
Correlation between complexity of lipid
composition and cell architecture and
functions
 PE –
phosphatidylethanola
mine
 PG –
Phosphatidylglycerol
 SPs – sphingolipids
 GP –
glycerophospholipids
Simons and Sampaio, 2011
 SPs and sterols enable vesicular transporting
 Tissue -specific SPs enable specific cell architecture
and fucntions
Membrane fluidity and leaflet
composition
 The lipid bilayer is a flexible two-dimentional fluid (lateral diffusion,
rotation, flexion)
 The fluidity of the membrane depends on the length and saturation of
hydrocarbon tails
 Transition between leaflets is restricted
 Three classes of enzymes
catalyze the transfer betwee
leaflets of the lipid bylayer:
flippases, floppases and
scramblases
Clark. Nature Immunology 12, 373–375 (2011)
Lipid rafts
 Lipid rafts are dynamic ordered sterol and sphingolipid
assemblies enriched with specific proteins
 The lipids in these assemblies are enriched in saturated
and longer hydrocarbon chains and hydroxylated
ceramide backbones.
 Nanoscale rafts can coalesce to form bigger complexes
 Lipid rafts are involved in post-Goldgi trafficking,
endocytosis, signaling and used by some pathogesns as a
cell entry
"Lipid raft organisation scheme" by real name: Artur Jan
Raft-associated proteins
 Rafts are crowded with a
specific set of raftophilic
proteins
 Raft association is
promoted by
palmytoylation, doubly
acylation, GPI-anchoring
and cholesterolanchoring.
 Raft assemblies can be
stabilized by specific
oligomerization of raft
proteins (or lipids) with
little energy input
 Major classes of raftassociated proteins:
Caveolins, hedgehog, the Srcfamily tyrosine kinases,
annexins, EGF-R, MAPK
etc
Simons and Sampaio, 2011
Raft functionalization
 Merging of specific
nanoscale rafts into
larger and more stable
platforms represents the
functionalization of
specific rafts in
membrane trafficking
both in the biosynthetic
and the endocytic
pathways as well as in
signal transduction and
other raft-associated
processes
Rafts in the biosynthetic traffic: cholesterol
content can induce protein sorting
 The length of TMD matches
the thickness of the lipid
bilayer
 TMDs of plasma membrane
proteins are in general
longer than those of the ER
and the Golgi complex
 Cholesterol gradient plays
an important role in
organizing the biosynthetic
pathway. From ER to Golgi
and then to PM.
Rafts in post-Golgi-traffic in epithelial cells
Raft clustering and domain-induced budding.
Trafficking pathways in polarized epithelial cells.
Galectin-glycolipid interactions are responsible for
raft clustering and membrane budding in NTNG
of polarised MDCK and other epithelial cells
Schuck S , and Simons K J Cell Sci 2004;117:5955-5964
Caveolae
 While some rafts are planar
others represent caveolae (“little
caves”), which are flask-like
invaginations of the plasma
membrane.
 Caveolins (cavs) are specific
markers of caveolae
 Caveolae provide a residence for
many regulatory proteins
 Caveolae might be endocytosed.
Particularly, it happens upon
binding Simian virus-40
Patel and Inselm, 2009
Raft employment by viruses
 Rafts provide a
platform for
virus assembly
 Influenza virus
uses rafts for cell
surface transport
and apical sorting
Nayak et al., 2009
Model of HIV-1 assembly and budding through membrane rafts. gp160 trimerizes within th
and, on reaching the TGN, associates with rafts because of its affinity for lipid rafts.
Chazal N , and Gerlier D Microbiol. Mol. Biol. Rev.
Raft-associated diseases: Alzheimer’s disease
The α-secretase is not raft-associated,
while the β- and γ-secretases
predominate in rafts.
What to remember about membranes
 Membrane functions
 Physical basis of membrane functioning
 Rafts: definition and composition
 Calveolae
Membrane proteins
Alberts et al., Essential Cell Biology
Mechanisms of membrane binding
Alberts et al., Essential
Cell Biology
 Integral
membrane
proteins including
 Transmembrane
proteins
 Peripheral
proteins
Transmembrane α-spiral
EGFR
Jura et al.,
2009
Transmembrane proteins with different
topology
Rhomboid protease from
L. monocytogenes
Major integral membrane proteins of
red blood cells
 Glycophorin is a small
glycoprotein of 131 amino acids,
with a molecular weight of about
30,000, half of which is protein
and half carbohydrate.
 The band 3 polypeptide chain is
929 amino acids and is thought to
have 14 membrane-spanning αhelical regions. Band 3, is the
anion transporter responsible for
the passage of bicarbonate
(HCO3-) and chloride (Cl-) ions
across the red blood cell
membrane.
Beta-barrels transmembrane proteins
 OmpA is a small ion channel
[28], OmpT is a protease, NalP
is an autotransporter, FadL is a
long chain fatty acid
transporter, PhoE is a diffusion
pore, ScrY is a sucrose specific
porin, OmPlA is a
phospholipase. FhuA and BtuB
are active transporters for
ferrichrom iron and vitamin
B12 uptake, respectively. OMPs
of mitochondria are predicted
to form similar TM β-barrels.
Examples are the VDAC
channels, out of which more
than a dozen have been
sequenced [29].
 Kleinschmidt, J.H. 2005
Peripheral proteins: lipid-anchored
Peripheral proteins: protein-protein
associations, electrostatic forces
 Peripheral membrane proteins that
attach to integral membrane proteins
play regulatory or adaptory roles
The cytoplasmic side of plasma
membranes, the outer leaflet of outer
bacterial membranes and mitochondrial
membranes are negatively charged.
electrostatic interactions play an
important role in membrane targeting
of electron carriers such as cytochrome
c, cationic toxins such as charybdotoxin,
and specific membrane-targeting
What to remember about membrane
proteins
 Functional classes
 Structural classes
 Receptors
 Mechanisms of peripheral membrane protein binding