Lecture 13 Flashcards
Biological Membranes
• Must maintain H+ and ion gradients
• Must be flexible - adopts various shapes and is self-sealing
• Must accommodate expansions and contractions - cell growth, cell movement, vesicle formation and fusion
• Must accommodate proteins of various types and shapes.
• Extrinsic, Trans-membrane, and Integral membrane proteins and still maintain a barrier
• Must be adaptable to varying cellular conditions and functions
(eg. pH, temperature, ionic strength, etc)
• Must be fluid to allow for diffusion (both Lipids and Proteins)
- but it must also allow for complex organization and architectures
• Must allow for transmission of signals and cellular communication
fluid mosaic model
proteins are embedded in a phospholipid bilayer and are free to move in the plane of the membrane
- The Lipid Bilayer – a thin two-dimensional fluid
- Membrane is assembled by non-covalent interactions
- Membrane is Asymmetric
- Lipids, Peripheral and Integral Proteins, Glycosylation
In general, Fatty Acyl chains tend to be extended but can adopt multiple conformations
peripheral membrane proteins
associated w 1 side of the bilayer and can be separated from the membrane w/o disruptig the bilayer.
intergral membrane proteins
deeply embedded int he bilayer and can only be extracted under conditions that distrupt membrane structure.- many extend through bilayer
transition temperature
depends on lipid comp
lipids w longer saturated tails tend to increase the transition temperature
those w more cis double bonds a/o shorter tails will reduce the transition temperature
translocon
facilitates the insertion of integral membrane proteins into the membrane bilayer
membrane rafts
rich in cholesterol, shingolipids, and GPI- linked proteins
the bilayer is thicker in the raft domains than in the surrounding membrane
Fluorescence Recovery After
Photobleaching (FRAP)
- Lipids labeled with fluorescent molecules
- Bleach fluorescence in a defined area with a laser
- Monitor recovery of fluorescence with the bleached area
Movement within phospholipid membranes is Temperature dependent
Lipid composition also influences membrane properties
lower temp= less gel like
higher temp= more fluild, disordered, liquid-like
s shaped graph
Movement within phospholipid membranes is dependent on Temperature and Lipid Composition
Changes in membrane fluidity mirror properties of
component fatty acids
Shorter chains, Unsaturated
Chains - ^ fluidity v Tm
Longer chains, Saturated
Chains - v fluidity ^ Tm
biological importantance?
Bacteria Adjust Membrane Fatty Acid Composition with Temperature
Cholesterol “plasticizes” phospholipid membranes
• Cholesterol has a rigid, bulky structure
• Found only in Eukaryotic Membranes
• Impacts membrane fluidity – less variation with temperature
1. Hinders phospholipid movement at higher temperatures
2. Hinders packing and solidification at low temperatures
Cholesterol plays a role in membrane organization
Cholesterol can help packing of Glycosphingolipids
- Cholesterol doesn’t readily form a bilayer
- Tends to associate with longer, saturated fatty-acyl tails of other lipids
- Sphingolipids tend to have longer hydrophobic tails
- Can also help in packing between large glycolipids
Lipid Rafts
Cholesterol and sphingolipids combine to form a microdomain in the plasma membrane
Lipid rafts and shingolipids
tend to have longer, saturated chains
- larger head groups
Lipid rafts and GlycoSphingolipids
cluster in the outer leaflet of the plasma membrane
Enriched in cholesterol and sphingolipids
- more ordered, thicker membrane
• Specifically modified proteins preferentially associate with lipid rafts
• One principle of membrane organization
• Platforms for cellular signaling, membrane budding
• Viruses (influenza, measles, Ebola, HIV) appear to localize to lipid rafts
Lipid composition varies with Eukaryotic membrane
Different membranes have different functions
Membrane Bilayers are Asymmetric
Distribution of Phospholipids in the Membrane is not at Equilibrium
Cerebrosides and gangliosides are generally found in the outer leaflet
Presence of PS in the outer leaflet can be a signal for cell death
Protein Transporters in the membrane maintain Asymmetry
Fluid Mosaic Model –Transverse Diffusion is very slow
Protein groove accommodates a phospholipid head group.
Acyl tails stay in the hydrophobic part of the membrane bilayer
Maintaining Asymmetry requires energy (ATP)
How do you move a polar head group across the hydrophobic core of the membrane?
Must overcome a significant energy barrier!
Membranes can be crowded with proteins
- ~30% of the genome encodes membrane proteins
- Protein content of membranes ranges from 18-76% (by mass)
- Lipids have to interact with a diverse array of membrane proteins.
- Fill holes in protein-lipid interfaces (self-sealing)
- Are there certain characteristics that define membrane proteins?
Integral Membrane Proteins (ppt)
• Extensive contacts with
hydrophobic regions of the
lipid bilayer
• Require strong detergents for release from membranes
• Most span the bilayer
• Have only ONE orientation in the membrane.
Non-polar side chains contact lipids.
- Satisfy polar interactions within hydrophobic bilayer
- Membrane-Spanning Proteins: Hydrophobic “belt” ~26-30 wide
Peripheral Membrane Proteins (ppt)
• Interact with exposed surfaces of integral membrane proteins or phospholipid head groups • Can be dissociated by high salt or change in pH
Properties of membrane-spanning α-Helices
23 residue transmembrane
segment
Glycophorin A (131 Amino Acid Residues)- SEPEITLIIFGVMAGVIGTILLISYGIRRLIKK - need at least 20 residues ~30Å for a helix to span the membrane
Not just the properties of ONE residue,
but also surrounding residues
How do membrane phospholipids interact with integral membrane proteins?
Integral Membrane Proteins – intimately associated with membrane lipids
- Require detergents to remove protein from the membrane
Annular lipids – (next
to the protein)
Acyl chains adapt to knobs and grooves of hydrophobic protein surface
Network of salt-bridges and hydrogen bonds with lipid polar head groups
Provides smooth surface for bulk lipids
Self-Sealing
(Aquaporin:
allows the rapid movement of water across a cell membrane)
Fatty Acylation
Myristate: linked to protein N-terminus
Palmitate: usually a thioester linkage
is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4’-phosphopantetheine moiety.
Prenylation -anchor type
Farnesyl (C15) or
GeranylGeranyl (C20)
Linked to Cys (CaaX motif) near protein C-terminus
is the addition of hydrophobic molecules to a protein or chemical compound. It is usually assumed that prenyl groups (3-methyl-but-2-en-1-yl) facilitate attachment to cell membranes, similar to lipid anchors like the GPI anchor, though direct evidence is missing.
Phosphoethanolamine
(linked to protein C-terminus)
Glycosyl-Phosphatidyl- Inositol (GPI)-Linked
a glycolipid that can be attached to the C-terminus of a protein during posttranslational modification. … The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane
Lipid modification can affect protein localization within a membrane
GPI-Anchored
Proteins - Outer leaflet of Plasma Membrane
Palmitoyl-dependent Raft association of Integral Membrane Proteins
Prenylated Proteins -
• associate with non-raft regions of the membrane
• Inner leaflet
Membrane associated proteins can influence membrane shape and function
Caveolin forms dimers, is palmitoylated, and binds
cholesterol
Promotes membrane curvature : Caveola
Enriched in cholesterol, shingolipids, and GPI-anchored proteins
Caveolin associates with lipid rafts and promotes membrane curvature
Membrane proteins can give the membrane shape
Local Structure - Caveola
Caveola tend to be enriched in sphingolipids and cholesterol- Specific GPI anchored proteins are often found in Caveola
Caveola tend to be enriched in sphingolipids and cholesterol- Specific GPIanchored proteins are often found in Caveola
Membrane cytoskeletal proteins give the cell its shape.
• Biconcave disk
• Increases surface area
allows rapid diffusion of O2 across the membrane to Hb
• Shape is maintained by a network of proteins (cytoskeleton)
• The phospholipid membrane can “flow” over the skeleton.
Can restrict diffusion of membrane proteins
• The erythrocyte can be deformed and then recover its shape (Squeeze through small capillaries)
Summary
Membrane proteins can be loosely divided into Integral and Peripheral Membrane Proteins
Membrane-spanning proteins have to “hide” polar backbone atoms α-helical and β-barrel membrane spanning proteins
Lipid modified proteins can also associate with membranes
Lipid rafts and Lipid modifications help organize proteins in a membrane
Cytoskeleton also gives the membrane shape and function
