Membrane Structure Flashcards
Membrane surface area
Cytoskeleton > internal mem. > PM
Makes sense b/c cytoskeleton is scaffold for whole cell.
Fx. of carbs on membrane?
Immunological responses, binding of viruses/toxins, proper protein folding.
They are on extracellular side.
3 classes membrane lipids
- ) Phospholipids (phosphatidylethanolamine PE, phosphatidylcholine PC, phosphatidylserine PS, phosphatidylinositol PI).
- ) Sphingolipids
- ) Cholesterol
Properties of mem. lipids
All amphipathic. All synthesized in ER.
All derived from glycerol except sphingomyelin (type of sphingolipid).
Cholesterol
Polar hyrdoxyl group, rigid steroid ring group, hydrocarbon tail.
Intercalated in mem pshopholipids.
Decreases mem fluidity, straightens lipids (thickness of mem. depends on cholesterol)
Cholesterol amounts in various mems.
PM has most (26%), then golgi, then ER.
PM is thickest.
Membrane asymmetry
Extraplasmic: PC, sphingomyelin, glycolipids
Intraplasmic: neg charged. PS/PE/PI.
Cholesterol equal.
Cholesterol sources
- ) ingestion/uptake
2. ) synthesis by liver
Cholesterol regulation
negative feedback (enough in diet decrease synthesis etc.) LDLR.
Cholesterol synthesis
sensed by ER membrane protein. Low cholesterol, SREBP regulates synthesis/uptake. Trx factor is cleaved from SREBP and thus becomes activated, goes to nuclease. Proteases do the cleaving (they are in Golgi).
SREBP held in ER until low cholesterol, SREBP then moves to golgi and gets cleaved.
Cholesterol synthesis detailed
Insig binds SCAP and blocks signaling motif that allows formation of vesicles from ER to golgi (COPII interaction).
Low cholesterol? Insig drops SCAP and SCAP/SREBP complex get sent in vesicles to golgi where cleaved…
Cleavage is in TM domain (regulated intramembrane poteolysis). S2P cleavage within mem, S1P luminal.
How do mem proteins associate with PM?
Extend across bilayer as: 1.) Single alpha helix 2.) Multiple alpha helixes 3.) Rolled up beta sheet (beta barrel) Some single and multipass proteins have covalently attached FA chain inserted in cytosolic lipid monolayer.
- ) Some exposed only at one side of mem
- ) Anchored to cytosolic surface by amphipathic alpha helix that paritions into cytosolic monolayer of lipid bilayer through hydrophobic face of the helix
- ) Attached via covalently attached lipid chain
- ) Via oligosaccharide linker to PI
- ) Only via noncovalent interactions w/ mem proteins.
GPI linkage
Glycosylphosphatidylinositol.
Complex anchor to mem. Only on extracellular face.
Variety of proteins anchored this way.
Bacterias
Staph aureus, listeria secrete soluble toxins that form pores and rupture membranes.
Aeromonas hydrophilia secretes toxin that binds GPI, inserts into mem, forms pore and kills cell.
Sphingolipid composition
Sphingosine (long acyl unit) with one of 2 polar head groups, and attached FA chain.
Sphingomyelin: phosphocholine head group.