Lectures 13&14 - Protein Trafficking Flashcards
What is the anterograde pathway
Forward - ER to Golgi, Golgi to plasma membrane
What is the retrograde pathway
Retrieval - Golgi to ER eg.
How do molecules move from one organelle to the next
1) Vesicle buds from donor compartment
2) Vesicle pinches off and translocates
from donor to acceptor compartment
3) Vesicle docks with acceptor compartment
4) Vesicle fuses with acceptor compartment releasing contents into lumen
How do buds form
Driven by the assembly of protein coats onto membrane
What is a Cathrin Coat
a protein that coats vesicles - made of Cathrin proteins and ‘adaptor’ proteins)
What is the triskelion structure
Three clathrin molecules join at a common hub to form a three-legged “triskelion”, which is the basic building block of the coats.
Animation 15.5 on Cathrin coat formation - can form cages
What are the processes in pinching off
Fission and scission
Scission event is carried out by a protein called dynamic that separates the membrane associated with the vesicle and the rest of the membrane
What pathways are associated with clathrin coats
Budding from the Golgi and from pasta membrane are associated with clathrin coated vesicles
What other types of coat are there
COPI and COPII coats (involved in retrograde trafficking and ER respectively)
What is clathrin coats used for
budding from the plasma membrane (endocytosis) and from the Golgi netowrk
What are COPII coats involved in
Anterograde transport from the ER.
What are COPI coats involved in
Retrograde transport from gold apparatus
In a clathrin-coated vesicle with coat proteins clathrin and adaptin 1 - where is the origin and the destination
O - Golgi apparatus
D - Lysosome (via endosomes)
In clathrin coated vesicles with clathrin and adaptin 2 - where is the origin and where is the destination
O - Plasma Membrane
D - Endosomes
In COP-coated proteins, made from COP proteins - what is the origin(s) and the destination
O1 - ER D1 - Golgi apparatus
O2 - Golgi cisterna D2- Golgi cisterna
O3 - Golgi apparatus D3 - ER
What controls coat formation
Coat recruitment GTPases
How is cargo selected
Active recruitment
Selective exclusion
Passive inclusion
What is bulk sorting
Molecules are passively included in vesicles (bulk sorting)
How does budding from the ER occur
Some proteins are selectively recruited into buds. For example some integral membrane proteins have cytoplasmic domains that will interact with coat proteins. This will concentrate them into buds. Lumenal cargo proteins can also be selectively recruited into buds bey interacting with cargo receptors –that span the ER membrane and interact with coat proteins. The cargo and coat are indirectly linked via protein-protein interactions.
Some proteins are excluded from entering budding vesicles-for example proteins that mis-fold in the ER.
Some proteins are passively included in the budding vesicle. Some of these proteins may need to be returned to the ER.
What is a DXE motif
Diacidic motifs consist of two acidic amino acid residues, separated by any other amino acid
How are vesicles uncoated
Vesicle uncoating is mediated by phosphorylation of adaptor subunits. The phosphatases involved are recruited to the vesicle by chaperone proteins such as Hsc70 and auxilin.
slide 19
Why many some proteins be excluded from budding vesicles
Antibodies do not get packaged into ER transport vesicles until they are correctly assembled. The ER can be considered as a ‘quality control’ station in the secretory pathway.
How are vesicles targeted to a membrane
Rabs and SNAREs help direct transport vesicles to their target membrane through:
Tethering
Docking and
Fusion
(slide 25)
What do SNARE proteins do
Brings two membranes close together and ‘forces’ them to fuse
As 2 membranes are forced together - water is squeezed out. what is produced as a result of this
Fusion of two bilayers to form a fusion pore proceeds through stalk and hemifusion intermediates. In the last picture the lumenal contents of one compartment are in direct contact with the contents of the lumen of the other compartment.
How do v and t SNARES drive fusion
v-SNARE on vesicle interlocks and winds with t-SNARE on target membrane
Transport vesicle docks
Membranes coalesce
Membranes fuse
What do BoTox do
Botulinum Neurotoxin prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromuscular junction, thus causing flaccid paralysis - also used for cosmetic procedures
How are cis-SNARE complexes disassembled
NEM sensitive factor (NSF) uses ATP hydrolysis to disassemble SNARE complexes
v-SNARE can then be recycled to an earlier organelle and re-used in other fusion reactions
What is required for viral membrane fusion
Fusion proteins -
Enveloped viruses need to fuse their membranes with host cell membranes to release viral constituents (nucleic acid, proteins) into host cell. SARS spike protein, Influenza hemagglutinin (HA) protein and HIV gp41 all examples of viral fusion proteins
What are the two models for Golgi maturation
Vesicular transport model
Cisternal maturation model (slide 13 ppt 2)
What are the sections of the golgi apparatus
Cis-golgi network
cis cisterna
medial cisterna
trans cisterna
trans Golgi netowrk
(Golgi stack are the cisterna)
What is the sorting that occurs in the cis golgi network from proteins from ER
Phosphorylation of oligosaccharides on lysosomal proteins
What is the sorting that occurs in the cis cisterna
removal of man
What is the sorting that occurs in the medial cisterna
removal of man
addition of GlcNAc
What is the sorting that occurs in the trans cisterna
addition of GaI
addition of NANA
What is the sorting that occurs in the trans golgi network
Salvation of tyrosines and carbohydrates
What is the mechanism for constitutive secretion pathway
newly synthesised soluble proteins that will be secreted are encased in newly synthesised plasma membrane proteins and lipids from the TGN
newly synthesised proteins and lipids fuse to membrane, releasing soluble proteins from cell (slide 15 ppt 2)
describe the mechanism for the regulatory secretory pathway
Secretory vesicle storing secretory proteins leaves the TGN
When a signal eg. hormone or neurotransmitter is detected, it signals the fusion and release of stored proteins into the extracellular space
What are some examples of regulated secretion
Insulin (secreted from b-cells in response to glucose)
Neurotransmitter release
ACTH (adrenocortitropic hormone)
Trypsinogen (in pancreatic acinar cells)
Glut4 (localised to PM in response to insulin)
How do lysosomal proteins get into the lysosomes
Proteins are modified ion the golgi by mannose 6 phosphate (M6P)
. The N-acetylglucosamine phosphotransferase (cis-Golgi) binds to the acid hydrolase (that is to be trafficked to the lysosome) and UDP-GlcNAc. GlcNAc-posphate is added to mannose residues (of the glycosylated hydrolase). In the trans-Golgi the GlcNAc is cleaved off leaving mannose 6 phosphate. This can now bind to M6PR in the TGN
Describe the mechanism of how proteins are sorted into the lysosome
Addition of P-GlcNAc
Uncovering of M6P signal
Binds to M6P receptor
Receptor dependent transport
Fuses to lysosome and dissociate at acidic pH
Removal of Phosphate
Receptor Recycling
Describe endocytosis and receptor recycling mechandim
Endocytosis occurs
Uncoated
Fused with endosome
Budding off of transport vesicles
Return of LDL receptors to plasma membrane
Phagocytosis (animation 15.9
How do we know about trafficking pathways
Protein secreted
Protein accumulates in ER
Protein accumulates in Golgi
Protein accumulates in transport vesicles
