Last Modified: 2011-12-02
- Lipids are not covalently anchored to other lipids
- Non-covalent interactions among lipids allows individual lipids to migrate.
- Membranes are highly flexible.
- Flip-flop diffusion of a molecule, when a lipid molecule from one leaflet of the bilayer is transferred to the other. This occurs very rarely.
- Lateral diffusion of a lipid molecule occurs more commonly.
- Flip-flop diffusion requires the polar head group to move through the hydrophobic interior of the bilayer. It has a large, positive free-energy change. ~ 40-50 kcal/mol
- Flip-flop diffusion is necessary because some lipids are synthesized on one face of the membrane but are only functional on the other side of the membrane.
- Flippases catalyze the translocation of aminophospholipids: phosphatidylethanolamine and phosphatidylserine from the extracellular to the cytosolic leaflet of the lipid membrane.
- Requires ATP
- Floppases catalyze the translocation of phospholipids from the cytosolic leaflet to the outer leaflet of a membrane.
- Requires ATP
- Actin and Spectrin are filamentous cytoskeletal proteins which tether proteins.
- Proteins are tethered to spectrin and restrict lateral-diffusion of lipid molecules.
- Glycophorin, chloride-bicarbonate exchanger both are immobile and are barriers to lateral diffusion.
- Actin and Spectrin form junctional complexes.
- Ankyrin is attached along Spectrin.
- Cl- - HCO3- exchangers are stabilized by ankyrin.
- Glycophorin is anchored by interactions directly with spectrin.
- Cholesterol-sphingolipid microdomains are on the outer monolayer of the membrane.
- Rafts are composed mainly of sphingolipids which have longer, more saturated acyl groups, and form more stable interactions with Cholesterol.
- Membrane rafts are slightly thicker and more ordered (less fluid) than neighbouring micro domains.
- Membrane rafts are more difficult to dissolve with nonionic detergents.
- Enriched in 2 classes of integral membrane proteins: proteins with two lipid anchored saturated fatty acids, and GPI-anchored proteins
- Some membrane proteins require there membrane proteins for functionality.
- Membrane rafts contain certain subsets of proteins.
- This allows for more collision of membrane proteins for functionality.
- Caveolin, an integral membrane protein which has two globular domains, connected by hydrophobic domain. This domain helps attach it to the membrane.
- 3 Palmitoyl groups (on the carboxyl terminal of the globular domain) anchor it to the plasma membrane.
- Caveolin binds cholesterol and forces inward curvature of a membrane.
- Caveolin is strongly associated because of lipid anchors.
- Cholesterol interacts strongly with Caveolin, which allows for membrane rigidity even when the membrane is curved.
- Caveolin is a dimer it has an alpha helix with a proline residue which turns it.
- Amphiphilic - there are both polar and non-polar regions.
- Integrins are heterodimers: two-unlike subunits, α and β subunits.
- Combine to form a specific binding site for extracellular proteins such as collagen and fibronectin. [They contain a common determinant of integrin binding (Arg-Gy-Asp)]
- α and β subunits are anchored to the membrane via transmembrane helices.
- Surface adhesion proteins
- Carry signals in both directions across plasma membranes.
- Two membranes must recognize one another
- Membrane surfaces requires removal of water molecules normally associated with polar head groups, to bring them close together.
- Bilayer structures must become locally disrupted, fusion of outer leaflets.
- Bilayers must fuse to form one continuous bilayer
- Process must be triggered at the appropriate time
- v-SNAREs (vesicle cytosolic proteins) bind t-SNAREs (target membrane integral membrane protein)
- Structural change produces long thin rods produced from helices from both SNAREs and 2 helices from SNAP25. (NSF is also a required protein)
- SNAREs interact at their ends, then zip up into the bundle of helices, this pulls the membranes together.
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