16 - Intracellular Lipid Transport Flashcards
List changes associated with low cholesterol in a cell
- SREBP present on SRE
- HMGR gene upregulated
- LDLR gene upregulated
- Decreased expression of ABCA1
- Low activity of ACAT (esterifying enzyme)
cAMP (via glucagon) activates protein phosphatase inhibitor to dephosphorylate HMG-CoA reductase (activating it)
List cellular changes associated with high levels of cholesterol
- SREBP NOT present on SRE
- HMGR gene downregulated, destroyed and inactivated
- LDLR gene downregulated
- Increased expression of ABCA1
- High activity of ACAT (esterifying enzyme)
AMP-activated kinase phosphorylates HMG-CoA reductase (inactivating it)
Summarize cellular cholesterol homeostasis methods (4)
Synthesis: (HMG-CoA reductase) is regulated on transcriptional level, by phosphorylation/dephosphorylation and by degradation. Decrease in cellular cholesterol leads to increased cholesterol synthesis
Uptake: LDL-receptor expression is upregulated when cholesterol levels are low
Efflux: ABCA1 is upregulated when cellular cholesterol increases, the cell can efflux cholesterol to an external acceptor such as high density lipoprotein (reverse cholesterol transport)
Esterification: ACAT (Acyl CoA acyltransferase) activity is upregulated when cellular cholesterol increases. This converts cholesterol into its storage form of cholesteryl esters.
Describe the inner/outer leaflet distribution of the following:
- phospholipid
- sphingomyelin
- phosphatidylcholine
- phosphatidylethanolamine
- phosphatidylserine
inner/outer
- phospholipid: 50/50
- sphingomyelin: outer
- phosphatidylcholine: outer
- phosphatidylethanolamine: inner
- phosphatidylserine: inner (only)
How does the size of the headgroup/tail of phospholipids determine membrane curvature?
Cone shape/negative curvature: small headgroup, large tail
- eg. inner surface of vesicle (no fusion)
Inverted cone/positive curvature: large headgroup, small tail
- Eg. inner leaflet of vesicle during fusion
How is plasma membrane asymmetry maintained?
ATP-dependent phospholipid translocases
Keeps phosphatidylcholines and sphingomyelin on outer leaf and phosphatidylserine and phosphatidylethanolamine on inner leaf.
Name an organellar membrane with not asymmetry
ER
Lipids are synthesized and inserted into the inner leaflet. Flippase catalyzes the transverse flip flop of phospholipids between leaflets (scrambles the lipid distribution, flippase does not require energy and is just through facilitated diffusion)
List three main types of lipid transport in a cell
Monomeric transport with carrier proteins
- Lipid transfer proteins are found extracellularly and intracellularly. They have a hydrophobic lipid binding pocket and a hydrophilic surface.
Vesicular transport
- Some along cytoskeleton
- Cargo proteins and proteins mediating vesicle transport cluster in a membrane area that will become the transport vesicle
- ATP dependent
Flipping from one membrane into the other at apposition sites
Give steps of vesicle formation, priming and assembly, budding and uncoating
Formation
- Clustering of cargo proteins
- Adapter proteins select cargo
- Coat proteins (eg. clathrin) polymerase around cytosolic side of donor membrane, binding to adapter proteins.
- Coat induces/stabilizes membrane curvature
- Dyamin induces fission (cuts vesicle off the donor membrane)
Priming and assembly
- Activation of ARF proteins (on/off switch)
- Recruitment of adaptor protein (AP) for coincidence detection
- AP binds to cargo/cargo receptors and assembles coat (eg. clathrin triskelions)
Budding off
- Dynamin GTPase binds
- Hydrolysis of dynamin GTP to GDP pinches off the budding vesicle
- Inactivation of ARF-GTP to ARF-GDP
Uncoating
- Release of coat protein
- Release of adaptor protein
What s the protein which controls the rate of vesicle formation?
ADP-ribosylation factor (ARF)
GTPase switch that controls the rate of vesicle formation. It is soluble in its inactive form, binds to membrane and coat protein when active by amphipathic helix.
Describe proteins involved in vesicle fusion
- SNARE proteins: R-SNARE, Q-SNARE, V-SNARE, t-SNARE. SNARE on donor and target membrane recognize and bind each other and tether the vesicle to the membrane
- Rab-GRPases, small G proteins, on/off switch for fusion, similar to ARF in mechanism
- NSF and SNAP proteins: dissassemble the SNARE complex. SNAP (soluble NSF attachment protein) binds to SNAREs, recruits NSF, which disassembles the SNARE complex.
- Calcium sensors to regulate fusion.
Describe vesicle targeting with v-SNAREs and t SNAREs
Vesicle: v-SNARE
Target: t-SNARE
