Membranes
Structure of Membrane Proteins
The Manifold Roles of Membranes
• The border of the cell/organelle
– Barrier to toxic molecules
– Helps accumulate & retain
nutrients
– Carries out energy transduction
– Modulate signal transduction
– Mediate cell-cell interactions
– export of proteins
• Facilitates cell motion
• Assists in reproduction
Spontaneously formed lipid structures
Polar Head Group
• Like snowflakes, lipids usually travel in packs
• Hydrophobic interactions are the key
Hydrophobic Tail
Monolayers arrange their hydrophobic tails in the air
air
water
Spontaneously formed lipid structures con’t
Micelles bury the nonpolar tails in the
center of a spherical structure
nonpolar solvent
Micelles reverse in nonpolar solvents
water
Still more spontaneously formed lipid structures
Lipid bilayers:
The basis for biological
membranes
unilamellar vesicles
(liposomes)
multilamellar vesicles
The Fluid Mosaic Model of Singer & Nicholson
•
•
•
•
The phospholipid bilayer is a fluid matrix
The bilayer is a two-dimensional solvent
Lipids and proteins can undergo rotational and lateral movement
Two classes of proteins:
– peripheral proteins (extrinsic proteins)
– integral proteins (intrinsic proteins)
Motion in the bilayer
• Lipid chains can bend,
tilt and rotate
• Lipids and proteins can
migrate ("diffuse") in
the bilayer
• Frye and Edidin proved
this (for proteins), using
fluorescent-labelled
antibodies against
membrane proteins
(figure)
• Lipid diffusion has been
demonstrated as well
Membranes are Asymmetric
• Lateral Asymmetry of Proteins:
– Proteins can associate and
cluster in the plane of the
membrane - they are not
uniformly distributed in many
cases eg neurons
• Lateral Asymmetry of Lipids:
– Lipids can cluster in the plane of
the membrane - they are not
uniformly distributed
– certain types may cluster around
particular proteins in the
membrane (a lipid entourage)
– Can also induce asymmetry eg
with calcium ion treatment
Transverse Asymmetry of Membranes
•Functions of membrane
proteins depends on their
orientation within the
membrane
• Membrane proteins are not
tossed into the membrane
randomly but have a specific
topology
eg Glycophorin
Outside
Inside
Transverse asymmetry of membrane lipids
In most cell membranes, the lipid composition of the outer
monolayer differs from that of the inner monolayer:
phosphatidylcholine
phosphatidyethanolamine
phosphatidylserine
sphingomyelin
total
percentage of phospholipid
How to establish & maintain lipid asymmetry?
The role of flippases
• Lipids can be moved from
one monolayer to the other
by flippase proteins
• Some flippases operate
passively and do not
require an energy source
• Other flippases appear to
operate actively and
require the energy of
hydrolysis of ATP
Membrane Phase Transitions: The "melting" of membrane lipids
• Below a certain transition temperature, membrane lipids are rigid & tightly
packed (Gel Phase)
• Above the transition temperature, lipids are more flexible and mobile
(Liquid crystal phase)
• The transition temperature is characteristic of the lipids in the membrane
• Only pure lipid systems give sharp, well-defined transition temperatures
Gel
Liquid crystal
Observing Membrane Phase
Transitions by Calorimetry
Heat absorbed
Transition Temperature
Gel
Liquid
crystal
Anti conformation
Gauche conformations
Temperature
Structure of Membrane Proteins
•Integral (intrinsic) proteins
•Peripheral (extrinsic) proteins
•Lipid-anchored proteins
Peripheral Membrane Proteins
• Not strongly bound to the membrane
• Can be dissociated with mild
detergent treatment or with high salt
concentrations
• What holds them there in the first
place?
Integral Membrane Proteins (IMPs)
• Strongly imbedded in the bilayer
• Can only be removed by
disrupting the membrane (eg
detergents)
• Often span the membrane
(transmembrane)
• “Inside out” compared to other
globular proteins..meaning?
Roles of IMPs (will expand on next term & beyond)
•Identification of cell type (‘face’)
•Structural eg adhesion proteins
•Signalling: eg mediate cell
growth & differentiation
•Pumps & Channels: import &
export control
Bacteriorhodopsin: a classic example of a
serpentine IMP
• Function: a light-driven proton pump
• Consists of 7 transmembrane helical
segments with short loops that
interconnent the helices
• Binds a light-senstive cofactor
(retinal) in the hydrophobic core
• Found in purple patches of
Halobacterium halobium
Porins: Classic example of a “b-basket” IMP
• Function: act as selective pores
for various small molecules
• Structure: ~ membranespanning 18-strand b barrel
forming a hollow cylinder
• Interior of cylinder lined with
hydrophilic residues
• Found in outer membrane of
Gm- bacteria & mitochondria
eg maltoporin
Lipid-Anchored Proteins
• Relatively new class of membrane
proteins
• Four types have been found:
– Amide-linked myristoyl anchors
– Thioester-linked fatty acyl anchors In
– Thioether-linked prenyl anchors
– Glycosyl phosphatidylinositol anchors Out
Critical for proper protein function: location, location, location!
Often found on proteins involved in signal transduction
Amide-Linked Myristoyl Anchors
O
C
R
NH
CH3
• Anchor is myristic acid (what is the abbreviated name?)
• Myristic acid forms an amide linkage with the protein at its
amino terminus
• N-terminal residue is always glycine
• Examples:
 a subunits of G proteins
signal transduction
endothelial nitric oxide synthase
Ester-linked Acyl Anchors
O
C
R
CH3
S
O
C
R
O
CH3
• Broad specificity for lipids - myristate, palmitate, stearate,
oleate all found
• Broad specificity for amino acid links - Cys, Ser, Thr all found
• e.g G-protein-coupled receptors, Transferrin receptor
!
N-myristoylation
S-palmitoylation
Thioether-linked Prenyl Anchors
RS
• Prenylation refers to linking of "isoprene"-based groups
• Consensus sequence CAAX (C=Cys, A=Aliphatic, X= any)
• Isoprene groups include farnesyl (15-C) & geranylgeranyl
(20 C) groups
• e.g yeast mating factors, intracellular signalling proteins
What’s the Point?
• Lipid Anchors are Signaling
Devices
• Anchors are transient
• Reversible anchoring and deanchoring can control signalling
pathways
• (Similar to phosphorylation/
dephosphorylation, substrate
binding/ dissociation, proteolytic
cleavage triggers and signals)
eg farnesylation
Glycosyl Phosphatidylinositol
Anchors (GPI anchors)
• Anchors protein lying outside the cell
• Always attached to a C-terminal residue
• Ethanolamine linked to a phosphate
linked to an oligosaccharide linked in
turn to inositol of phosphatidyl inositol
(embedded in the membrane)
• Examples: surface antigens, adhesion
molecules, cell surface hydrolases
Protein
CO
C
O
HNCH2CH2OPO-
O
(Man)3
GN
PI
Man = mannose
GN = glucosamine
PI = phosphatidyl inositol