Topic 4C - Lysosomes and Endo/exocytosis Flashcards
What is true of orientation of a membrane-bound protein when it undergoes vesicular transport?
It’s orientation is maintained: outside will face outside
What are 3 key characteristics of healthy vesicular transport?
- Organized - they know where to go
- Balanced
- Selective
In a phosphatidyl inositol molecule, which positions on the inositol ring can be phosphorylated?
3, 4 and 5
The primary role of phosphatidylinositols and phosphoinositides are…
Acting as membrane markers
What is the difference between phosphatidylinositiols and phosphoinositides?
Phosphoinositides are phosphorylated
Remember that sugars can only exist on the outside of a membrane, What is special about inositol head of phosphoinositides?
It appears to be a sugar but is actually a sugar alcohol, therefore it can be on the inside of membranes
What side of the membrane do phosphoinositides appear on?
The intracellular membrane
How do PIPs inform specificity of vesicle content?
PIPs serve as docking sites for molecules, depending on which positions of the inositol are phosphorylated, different molecules will bind to them, so they are helpful for selectivity when loading vesicles
What are the 3 key coat proteins discussed in this course?
Clathrin
COPI
COPII
What are the 2 key functions of the vesicle coat?
- Concentrate specific membrane proteins
2. Help to form vesicles by initiating bending of the membrane
What is the name of the “3-legged” structure of clathrin?
Triskelion
Describe the number of chains within a clathrin molecule
3 heavy chains and 3 light chains
What molecule acts as an ATPase to uncoat vesicles?
Hsp70
Do clathrin coated vesicles always pack the same molecules?
No
Does clathrin bind directly to the vesicle membrane?
No - binds to adaptor protein
What causes the deformation of the membrane during vesicle formation?
Integration of cargo receptors and adaptor proteins into the membrane creates ELECTROSTATIC INTERACTIONS where the membrane is attracted to the coat proteins
What are Sec23 and Sec 24?
COPII adaptor coat proteins which form the inner coat
What causes Sar1-GDP to bind to the pre-vesicle membrane?
Interacts with a Sar1-GEF which converts it to Sar1-GTP which exposes its amphipathic helix to associate with the membrane
What is the monomeric GTPase associated with COPII coat formation?
Sar1
What is the monomeric GTPase associated with COPI coat assembly?
Arf1
What is the monomeric GTPase associated with clathrin coat assembly?
Arf1
What activates the GAP for Arf1 (which will cause the coat to disassemble)
The curvature of the membrane
Does the affinity to the membrane of monomeric GTPases associated with vesicle coats when there is a polar substitution of a residue in the amphipathic helix?
Yes
What are the 3 functions of adaptor proteins in clathrin coats?
- Bind cargo receptors
- Collect specific proteins
- Help to bend the membrane
Describe the adaptor proteins of COPI vesicles
There are no adaptor proteins, adaptor proteins are exclusive to clathrin-coated vesicles
The GTPase responsible for transport into vesicles, vesicle formation, and finding the correct vesicle target is…
Rab
Rab-GTP makes initial contact with the target membrane by interacting with…
Rab effector
How many polypeptide chains make up v-SNARE?
One
How many polypeptide chains make up t-SNARE?
2 or 3
What does it mean for the fusion of membranes to be heterotypic?
Two different kinds of membrane fuse together (e.g. ER and Golgi membranes fusing)
Together, v-SNARE and t-SNARE complexes form…
The trans snare complex
Bacterial tetanus targets this protein
SNARE proteins
When two membranes are interacting and about to fuse, which membranes fuse first?
Cytosolic membranes fuse first
What can be a point of control in vesicle docking?
Formation of the SNARE complex and its accessory proteins, if it is not formed then vesicles cannot dock
Are all proteins in the ER packed into vesicles?
No, some stay in the ER
What kind of coat forms around vesicles going from the ER to the Golgi?
COPII
In order for an ER protein to bind to a cargo receptor, it must have…
An ER exit signal
Why is it necessary to have different kinds of coats such as COPI and COPII?
They bind different “adaptors” so that different cargo can be loaded, has different affinity for different cargo
Describe the affinity for the KDEL receptor for COPI and COPII
KDEL wants to stay in ER
COPII = away from ER
COPI = back to ER
Needs high affinity to COPI cargo receptors and low affinity for COPII receptors
What determines the fusion of a vesicle to its correct membrane target? (2)
- SNARE complex
2. Rab proteins
What is the “cis” face of the golgi?
The side where ER vesicles enter the golgi
What is the “trans” face of the golgi?
The side of the Golgi sending contents away from ER to lysosome, PM, etc.
Describe the distribution of enzymes in the Golgi
Enzymes are area-specific, concentrated into a zone
What is the major biochemical modification to proteins that occurs in the ER?
Modification of sugar groups
What is a complex oligosaccharide?
An oligosaccharide chain which was trimmed and then had sugars added to it
What is a high-mannose oligosaccharide?
An oligosaccharide which was trimmed so the terminal branches all end in mannose sugars
When an untrimmed glycoprotein has its saccharide group in an inaccessible place (i.e. the inside of a protein) the chain will be…
A high-mannose oligosaccharide
When an untrimmed glycoprotein has its oligosaccharide group in an accessible place, it is likely that the chain will be…
A complex oligosaccharide
Can a protein have more than one oligosaccharide chain?
Yes
What enzyme is responsible for adding sugars to glycoproteins in the golgi?
Glycosyl transferases
What are 4 roles of glycosylation?
- Promotes correct folding of a protein
- Helps keep proteolytic enzymes away (scaffold)
- Cell-cell recognition
- Sugars can be functional (e.g. mucous in stomach)
What is the lysosome equivalent in plants?
Vacuole
Lysosomal membrane proteins are mostly this kind of transmembrane protein
Transporters
What kind of enzymes are commonly contained in lysosomes?
Proteolytic enzymes, hydrolytic enzymes, acid hydrolases
What activates an enzyme in the lysosome?
Proteolytic cleavage - this explains why the proteins were able to be in the ER/Golgi without destroying anything
How is the inside of the lysosome protected against autodigestion?
Glycosylated proteins keep proteases away
In order to acidify lysosomes, this kind of pump can be used
H+ pump, V-type ATPases
Organelles are degraded through a process called…
Autophagy
What organelle is responsible for autophagy?
Lysosomes
What is transcytosis?
Delivery of a cellular protein from the apical to basal sides, typically in an asymmetrical cell like a neuron
What are the typical 3 destinations for cells leaving the trans golgi network?
Lysosomes
Constitutive secretory pathway (normal secretion)
Signal-mediated diversion to secretory vesicles (regulated secretion)
What 2 biochemical events occur in vesicles?
- Cargo concentration
2. Proteolytic processing
Describe the primed state of synaptic vesicles ready for fusion
Vesicular pool: Snare bundles ready for docking and fusion but complexin blocks full fusion
What are 4 cases where the plasma membrane might need to be enlarged?
- Cytokinesis
- Phagocytosis
- PM repair
- Cellularization
An example of a tissue that undergoes cellularization is…
Fly embryonic tissues
