MGD - protein secretory pathways

Question 1

what is the 1st point of call for secreted or membranous proteins or those destined for certain organelles e.g. lysosomes?

Answer 1

endoplasmic reticulum

RER–> SER –> golgi

Question 2

what does RER allow for? examples?

Answer 2

protein folding and modification

e.g. hydroxylation, disulphide bridge formation (PDI), N-linked glycosylation

Question 3

What is the function of golgi body?

Answer 3

transport vesicles are received at the cis end and released at the trans end
folded proteins are modified, labelled, concentrated and packaged into secretory vesicles (or desetined for lysosomes - breakdown)

Question 4

what is continuous secretion? examples?

Answer 4

mRNA is translated, modified in ER, concentrated in golgi body, packaged into secretory vesicles and released from cell continuously
e.g. COLLAGEN & serum albumin!!
proteins (exocytosis), fibroblasts (fibre), osteoblasts (bones), chondrocytes (cartilage)

Question 5

what is regulated secretion? examples?

Answer 5

mRNA is translated, modified in ER, concentrated in golgi body, packaged into secretory vesicles where they are STORED
EXOCYTOSIS occurs only when signalled to do so
e.g. ß-cells of pancreas (insulin), goblet cells (secrete mucus)

Question 6

what is the importance of glycosylation?

Answer 6

for correct protein folding and function

Question 7

what is N-linked glycosylation

Answer 7

addition of oligosaccharides to an asparagine residue within a protein
occurs in RER

Question 8

what is O-linked glycosylation

Answer 8

addition of oligosaccharides to the HYDROXYL group of serine, tyrosine or threonine
occurs in golgi body

Question 9

explain the process of protein targeting to the ER lumen

Answer 9

1. protein synthesis initiated on free ribosomes
2. signal sequence recognised by SRP (N-terminus)
3. using GTP, SRP bind to signal sequence ceasing translation in the cytoplasm
4. SRP takes the whole complex to SRP-R on surface of ER (protein unfolded)
5. SRP released and recycled (protein synthesis continues)
6. ribosome anchors complex in place
7. protein secreted through peptide translocation complex (pore in ER membrane)
8. ribosome dissociates and is recycled
9. signal peptidase cleaves signal sequence
10. protein folds inside ER lumen (through disulphide bonds) - increase stability

Question 10

what is an example of a peptide secreted to ER lumen?

Answer 10

preproinsulin –> proinsulin –> insulin

Question 11

Explain secretion of protein to nucleus

Answer 11

large molecules can’t pass through nuclear membrane into nucleus
so bind to cargo protein and importin (carrier protein)
1. carrier protein (importin) recognises NLS (nuclear localisation signal of protein) in order to bind and transport
2. once the importin with NLS enter the nucleus, Ran-GTP recognises the complex and bind to it causing a conformational change of importin
3. the cargo is released inside the nucleus
4. the importin and Ran-GDP complex will now exit the nucleus via nuclear pore complex
5. in the cytoplasm, interact with Ran-GTP activator protein which hydrolysis the GDP back to GTP to repeat the process
protein folded

Question 12

explain how proteins are secreted to the mitochondria

Answer 12

amphipathic signal
chaperones are used to keep protein in unfolded state until secreted into mitochondria
1. ATP used to associate chaperone protein - 2 chaperone proteins used so 2 ATP
2. signal sequence = matrix targeting sequence
3. protein guided by chaperone travel through TOM and TIM pore (protein translocating complexes)
TOM = translocate outer membrane
TIM = translocate inner membrane
4. once inside the lumen, another chaperone to precent immature folding
5. matrix processing protease cleave matrix targeting sequence (signal sequence)
6. protein folded as active protein inside matrix of mitochondria

Question 13

explain how proteins are secreted into the lysosome

Answer 13

1. post translational addition of mannose-6-phosphate to protein
2. protein from cis taken to trans of golgi
3. at trans surface, protein bind to M6P receptor (clathrin coat present)
4. receptor dependent transport takes protein out via transport vesicle
5. transport vesicle taken to lysosome where the acidic pH causes dissociation between the receptor and the protein complex
6. receptor is recycled by bringing back as a transport vesicle to the trans membrane of golgi
7. protein complex has phosphate removed via phosphatase to become mature lysosomal hydrolase
   (protein folded and require ATP)

Question 14

explain how proteins are left in retention in the ER

Answer 14

ER retension signal = KDEL (containing protein), KDEL receptor constantly circling

1. KDEL + KDEL receptor in the RER membrane
2. the KDEL and & receptor taken via transport vesicle from ER to golgi
3. at the cis golgi, there is alower pH, so KDEL receptor has higher affinity with KDEL containing protein, so capturing it
4. the protein is then secreted to cis-golgi
5. the KDEL receptor is then transported back to the RER for the process to repeat again