vesicle transport- week 3 Flashcards
what are the functions of the Golgi apparatus (2 points)
- proteins that are destined for secretion are Sorted, modified & dispatched from the Golgi apparatus
- Major site of carbohydrate synthesis- form of glycoproteins & proteoglycans
Explain how cargo proteins are transported from the ER to the Golgi
-almost all proteins in ER lumen are glycosylated (acts as a folding tag)
-2 of the 3 terminal glucose molecules are removed
-calnexin acts as a chaperone- which recognises the single glucose of an incomplete folding of protein- so it prevents its export to Golgi- sent to glucose transferase
-glucose transferase keeps adding another glucose until the protein is fully folded
correctly folded proteins go on to be exported to Golgi
what must happen to a protein before it leaves the ER
protein folding must be completed
what happens to misfolded proteins in the Golgi apparatus
they are chaperoned back to the ER protein translocator and are sent to the cytoplasm for degradation
how are soluble cargo proteins targeted to exit sites in the ER for transport to the Golgi
-it involves interactions with Transmembranous Receptors
-have a signal that allows them to bind to the receptors & form this bud (the exit signal tells them what to do)
-theses receptors also play a role in recruitment of COPII proteins
what do the COPII proteins do
it binds to the receptor which bends the ‘bud’ even more to start making a vesicle - which allows the protein to leave the ER
explain the process of the movement of the transport of vesicles to the golgi
- Once COPII coated vesicles bid from ER they rapidly shed their coat
-then undergo HOMOTYPIC fusion (them joining together) which forms the vesticular tubular cluster (VTC)
-the VTC moves along microtubules to deliver its contents to the Golgi
what happens when the vesicle reaches the Golgi apparatus
the cargo is released from its receptor- which is mediated by a decrease in PH
what does the term cargo mean
its a generic name for proteins that are properly folded
explain how escaped proteins are retrieved
- COPI coated vesicles bud from VTS /Golgi are uncoated & transported back to the ER
-Resident ER membrane proteins contain the Cytosolic sequence -which interacts with COPI directly. this signals them to go back
Recall that oligosaccharides are processed in the Golgi
- 14-sugar oligosaccharide added in the ER can be processed in many ways: there are over 200 modifying enzymes
-Oligosaccharide processing is ordered: sugars are removed & added in turn- each step relies on previous one
Compare the processes of O-linked and N-linked glycosylation
O-linked glycosylation:
-addition of sugars to the Oxygen atoms of serine or threonine
-catalysed by Glycosyl transferase
N-linked glycosylation:
-addition of sugars to Nitrogen atoms of proteins
Describe some of the functions of glycosylation
-as a protein marker (if proteins are folded properly/ how long its been in the ER or targeting transport between ER & Golgi)
-as a protector
-as a cellular marker (cell recognition)
-regulatory roles
what is the function of Vesticular Tubular Structure (VTS)
sorting-phosphorylation of oligosaccharides of those lysosomal proteins
Discuss the role of the coat proteins COPI, COPII and clathrin in vesicle budding
COPI- coats the vesicles which transport molecules from the golgi back to the ER
COPII- coats the vesicles which transport molecules from the ER to the Golgi
Clathrin- coats the vesicles which transport molecules between the Golgi, the Lysosomes and the Plasma membrane
Describe how vesicles are released from their donor membrane by dynamin
- a protein complex, Adaptin, is needed to attach to Clathrin coat to the membrane
-when the coat proteins start to press on the ER membrane surface- more coat proteins bind & create a lolipop shape
-Once it starts to bend up, the protein Dynamin raps itself around the stork of that vesicle bud. - once that has happened the coat is lost & recycled and vesicle is ready for transport
Explain why vesicles have to be particular about what the fuse with
->vesicles have surface markers to identify them
-> complementary receptors are displayed on target membranes
discuss the role
that SNARE proteins play in selection (docking) and fusion
-there are over 20 SNAREs which work as pairs- for every V-SNARE there’s a T-SNARE.
-when these meet they wrap around each other forming a trans-SNARE which locks the membranes together in a process called Docking.
why do the V-SNARE & T-SNARE wrap around each other only when the 2 membranes are very close to each other
to exclude water between them
*the trans-SNARE complex may act as a winch as they force fusion to happen using energy released as the helices wrap to fund water exclusion & membrane fusion
Give examples of the constitutive secretory pathway
Examples:
1)secretion of components of the extracellular matrix
2)delivery of new plasma membrane proteins and lipids to cell surface
what happens in the Golgi if a protein isn’t returned to the ER or retained in the Golgi lumen
it’s directed to the lysosomes or regulated secretion is transported by this route – i.e. it is the default pathway
- Compare the three types of endocytosis (pinocyotsis, receptor mediated endocytosis and
phagocytosis)
*Pinocytosis is non-specific (no receptors involved)
*phagocytosis - targets large structures (e.g. bacteria) and not particularly specific
*receptor-mediated endocytosis is specific for substances recognized by a cell-surface receptor
Give specific examples of regulated exocytosis, naming the cell type, secreted protein
and signal that triggers fusion of the vesicles with the plasma membrane
- Digestive enzymes are secreted in response to food in
the gut.
– In nerve cells secretory vesicles at the synapse to fuse
with the plasma membrane.
– Pancreatic β-cells release insulin in response to raised
blood sugar.
– Histamine secretion by mast cells is stimulated by
antigen.
Explain the need to recycle membranes by endocytosis
as there is an increase in surface area to the cells when reciveing the vesicles, there is a process called: endocytotic-exocytotic cycle
what are the 3 types of endocytosis
-pinocyotsis,
-receptor mediated endocytosis
-phagocytosis
describe the process of phagocytosis
Specialised white blood cells take up large particles like microorganisms, the cell binds to the receptors of the phagocyte and are recognised by the cell surface receptors-leads to phagocyte engulfing that cell as it stimulates the cell to extend pseudopods (arm like structures)
describe the process of Receptor-mediated endocytosis
binding of a Ligand to a receptor- causes change of shape which stimulates invagination of the membrane
describe the process of pinocytosis
Clathrin-coated pits, they rapidly invaginate(folds in) and forms vesicles which lose their coat and fuse with early endosomes
Describe the role of lysosomes, attributing this to some named acid
hydrolases and their target macromolecules in order to prepare them for transport into the cytosol.
- Hydrolyte enzymes are transported from the ER where they’re synthesised
- macromolecules targeted for digestion come from a variety of sources
-Digestion products (amino acids, sugars…) are transported out of the lysosome by transporters in the membrane
Explain why and how the pH inside the lysosome is maintained at ~ pH 5.
a H+ pump in the lysosomal membrane- uses ATP to pump H+ into the lysosome
Describe the routes taken by structures and macromolecules targeted for degradation; by endocytosis, phagocytosis and autophagy to the lysosomes
1) RECYCLING: membrane & many receptors sent to ‘recycling endosome’- vesicles return to plasma membrane
2)TRANSCYTOSIS: vesicles return to different part of Plasma Membrane; transports materials across cell
3)DEGRADATION: Cargos (& some receptors) sent to late endosomes which mature into lysosomes- macromolecules degraded & there contents used to make new molecules
Describe how acid hydrolases produced into the ER are targeted to the
lysosome.
recognized by mannose-6-phosphate receptor protein in the trans Golgi network that segregates the hydrolases and helps package them with a clathrin coat and transports that vesicle, which delivers their contents to late endosomes
What is the role of acid hydrolases in lysosomes
lysosomal hydrolase provides double protection against uncontrolled digestion of the contents of the cytosol
What are the flattened compartments of the Golgi called?
Cisternae, Cisterna
Name two modifications which can occur to proteins within the Golgi.
(Glycosylation, oligosaccharide processing, O-linked glycosylation, Phosphorylation, Sulphation)
The KDEL receptor binds to its cargo in the
____conditions of the ______ to transport it to the ______ Once there the conditions are _____ and the cargo is released. The empty KDEL receptor is then recycled to the ______ where is collects another cargo protein.
1) acidic
2) Golgi apparatus
3) endoplasmic reticulum
4) Neutral
5) Golgi apparatus
Vesicles which return proteins to the endoplasmic reticulum from the Golgi and coated in ______. The return of proteins to the ER is known as ________ transport.
1) COPI
2)retrograde, retrieval
What fate awaits a protein containing the KDEL sequence when it reaches the Golgi?
Incorrect Response
1)Degradation
2)Retrieval to the endoplasmic reticulum
3)Dispatch to the plasma membrane (exocytosis)
4)Retention within the cis cisterna
5)Transport to the lysosomes
2) Retrieval to the endoplasmic reticulum
Moving from the face nearest the ER to the face from which vesicles leave the Golgi, name each of these three compartments:
1) cis, cis cisterna
2) medial, medial cisterna
3) trans, trans cisterna
What is the three legged structure of clathrin known as?
Triskelion
In vesicular transport what are the surface markers on the recipient compartment called?
1)COPs
2)clathrin
3)dynamin
4)v-SNAREs
5)t-SNAREs
t-SNARES
In vesicular transport what are the surface markers on the transport vesicles called?
1)dynamin
2)COPs
3)clathrin
4)v-SNAREs
5)t-SNAREs
4) V-SNAREs
Give an example of a type of protein which is modified by proteolytic cleavage within the secretory vesicles.
peptide hormone/, /neuropeptide/, /hydrolytic enzyme/, /digestive enzyme
The following is and example of receptor mediated endocytosis. Fill in the blanks…
Cholesterol is taken up by mammalian cells, particularly the hepatocytes of the liver. It is transported in the blood complexed as ________. ________ on the cell surface bind to LDLs, and guide them into pits where an ________ links the receptor to _______. A vesicle is then formed and is transported to the ________ where the ________ is released from its receptor, which can be recycled back to the plasma membrane.
1) low density lipoprotein (LDL)
2) (LDL-receptor, LDL receptor, receptor
3) adaptin
4) clathrin
5) early endosome
6) LDL, low density lipoprotein
On reaching the late endosome, what causes the mannose-6-phosphate on a lysosomal protein to dissociate from its receptor? The environment is more…
acidic
What marker is recognised by the receptors which target acid hydrolases into vesicles which will transport them to the lysosome?
mannose-6-phosphate, mannose 6 phosphate, /phosphate/, /mannose
Which organelles would a lysosomal hydrolase pass through?
Lysosomal hydrolases are synthesized into the rough endoplasmic reticulum and pass through the smooth ER and Golgi before being dispatched in vesicles destined to fuse with the late endosome.
what are the three routes that substrates for intracellular digestion take to the lysosome. Name them:
endocytosis ,phagocytosis
and Autophagy
How does the cell protects itself from its own lysosomal hydrolases?
-By keeping them within a membrane
-They only function at low pH (as in the lysosome)
there are acid hydrolases, which means they only work at pH<7 (actually about pH5-6 is needed for full activity); since the cytoplasm is typically at pH 7.2 these digestive enzymes don’t work even if they manage to escape from the lysosome
A bacterium has been phagocytosed and needs to be digested in the lysosome. Name two classes of enzyme which might contribute to its degradation.
protease/, /nuclease/, /glycosidase/, /lipase/, /phospholipase/, /phosphatase/, /sulphatase
What is the generic name for the type of enzyme found in lysosomes?
acid hydrolase/, /hydrolase
