The Secretory Pathway Flashcards
what is constitutive exocytosis?
continuous secretion
give an example of regulated secretion?
e.g. just had a meal and get an elevation of blood glucose and gets insulin secretion
what happens in the ER?
- post translational modiciations
- specific proteolytic cleavage
- glycosylation
- disulphide bond formation
- folding and assembly of multiple subunit proteins
what is glycosylation?
covalent addition and processing of oligosaccharides (carbohydrates)
what is involved in the folding and assembly of multiple subunit proteins?
- requires chaperones
- disulphide bond in a protein will be extracellular or secreted protein
- happens in the lumen of the ER
what is correct folding?
- entry is in unfolded form when it enters the ER
- in the ER proteins generally reach their corect confirmation in minutes
- mediated by chaperones
what are chaperones?
- undergo cycles of binding and release of polypeptide substrate
- domains move powered by ATP hydrolysis
- binding and release helps substrate polypeptide adopt correct confirmation
what is disulphide bond formation?
- part of the correct folding process
- the lumen is oxidising whereas the cytoplasm is reducing
- get sulphur cross-linked (uses protein duslphide isomerase (PDI))
what does prolyl isomerase do?
proline isomerisation
what configuration do most peptide bonds need to be in?
trans configuration - more favourable
what configuration is proline?
can be in the cis or in the trans state
what is proline isomerisation?
interconversion can be accelerated by peptidyl propyl isomerase (PPI) enzymes
- helps transformation from trans to cis
- can shif conformation
- involved in protein folding and regulation
what is N linked glycosylation?
- Asparagine = Asn = N
- N glycosylation is occurring on asparagine residues
what is glycosylation?
- have a protein in with a lipid chain
- on the cytolasmic side it pick up 2-N-acetyl-glucosamine residues and 9 mannose residues
- it then flips across the membrane
- in the ER lumen gets 3 glucose added
how are glucose residues are involved in quality control?
- help determine if a protein is folded correctly
- unfolded proteins retain a glucose that binds to ER chaperones, calnuxin and calreticulin, retaining them in the ER
- glucose group is added by glucosyl transferase
- folded proteins have glucose removed and allowed exit from the ER
how do prtoeins exit the ER?
use Sec61
what happens if folding fails?
proteins are retro-translocated into the cytosol and degraded by the proteasome
when is UPR triggered?
triggered by ER chaperones when unfolded proteins accumulate in the ER
what is the cause UPR?
the expression of a mutant protein causing disease
how does UPR work?
- triggers the upregulation of chaperone expression
- increased from the ER and proliferation of the ER
- prolonged UPR leads to cell death
how do the proteins leave the ER?
leave the ER in COPII coated transport vesicles
what do the soluble cargo proteins do?
- has an exit signal
- acitvely packaged into COPII
- pinch off ER to form a vesicle
what is the role of the exit signal?
- it binds a receptor
- receptor than binds to cytosolic proteins
- concentrates receptor into budding vesicle
what is the assembly of the coat initiated by?
- Sec12 binding to Sar1
- Sar1 exchanges GDP for GTP
what is the process of coat assembly?
- Sec12 binding to Sar1
- Sar1 exchanges GDP for GTP
- GTP bound Sar1 binds to the ER and acts as a nucleus for Sec23/24 binding
- Sec24 binds transmembrane cargo
- coat assembly is completed by the binding of Sec13/31 to the Sar-GTP, Sec23/24 complex
what does the COPII coat allow?
allows membrane curvature and budding
what can get caught up in vesicle budding?
resident ER proteins
what do vesicles move along?
microtubules
how do vesicles move from the ER to the Golgi?
- move from the ER to the cis Golgi
- have Golgi intermediate compartments
- COPII coat going in
- COPI coat retrieves material back to the ER (retrograde traffic)
why is it important that material is retrieved back to the ER?
- dont want to lose the ER enzymes if they move forward to the Golgi
what is ER retrieval and retrograde transport?
- ER proteins retried
- contain a KDEl sequence
- KDEL receptors bind cargo with KDEl sequences
- KDEL receptors have a ER retrieval signal which allows them to be packaged into COPI coats
how do you get from the cis Golgi to the trans Golgi?
- get a lot of vesicle budding from one cisternae to the next
- fusion requires SNAREs regulated by Rabs
- each modification is specific to each cisternae (can be additions and removals)
what happens at the trans Golgi?
- get stored
- constitutive secretion
- regulated secretion
what are the function roles of glycan?
- folding in the ER
- stability/protease resistance of proteins
- golgi wont do exactly the same reactions to every protein
- molecular interactions/recognition
- blood groups are based off carbohydrates
- immune response
- signalling
- architecture of the extracellular matric and cell to matrix attachment
- glycan structures are altered in diseases
what happens constituvely in the Golgi?
- bulk flow
- vesicles transported to the plasma membrane
- get secreted
what are the key points of targeting towards the lysosome?
- phosphoryaltion of mannose
- 2. specific M6P receptor cycling between late endosomes and the TGN - Acidic pH of late endosome releases M6P from the receptor
what can defects in the delivery of lysosomes enzzymes cause?
- defects in lysosomal degradation
- substrates that are normally degraded start to accumulate
- have mostly neurological symptoms
