Question

(1) _____How are proteins normally resident in the ER retrieved when they escape the ER? (please explain why)

A. They have short amino acid sequences that serve as retrieval sequences.

B. Specific receptors bind to soluble proteins to return them to the ER in COP-I vesicles.

C. Specific receptors bind to soluble proteins to return them to the ER in COP-II vesicles.

D. Both A and B are correct.

(2) What is the major difference between glycosylation of proteins in the ER and glycosylation of proteins in the Golgi complex?

(3) Some proteins are imported into their appropriate organelle after they are created in the cytosol. This is done by different mechanisms for the different organelles. Compare and contrast the mechanisms, noting which ones have similarities and which ones have differences.

Answer 1

1) Endoplasmic reticulum is the cell organelle where all secretory protein synthesis.

Secretory Protein of endoplasmic reticulum lysosome, golgi, plasma membrane are made in rough endoplasmic reticulum than move to its desired organelle through protein sorting signal present in them

Now coming to the question When ER protein escape it is retrived to ER by

Option 1 Have short amino acid sequences that serve as retrieval sequences.

Yes it is correct as ER protein has retrieval signal in them

ER luminal retrieval signal is KDEL at C terminus

ER membranous protein has 2 retrival signal KKXX/KXKK at C terminus & RRXX at N terminus

Hence option 1 is correct

Option 2)Specific receptors bind to soluble proteins to return them to the ER in COP-I vesicles.

Yes it is correct COP1 is coat protein of vesicle involve in retrograde path

From cis golgi to trans golgi vesicle contain coat protein COP1 of 2 type

COP1A present in vesicle from trans golgi to ERGIC

COP1B present in vesicle from ERGIC to ER

Receptor of KDEL is present in golgi which only bind to KDEL sequence at slightly low pH than 7 and return to ER in retrograde pathway in vesicles containing COP 1 coat protein

Hence option 2 is correct

Option 3Specific receptors bind to soluble proteins to return them to the ER in COP-II vesicles.

It is not correct as COP2 vesicles are involved in anograde pathway that is protein move from ER to cis golgi to trans golgi

Hence option 3 is not correct

Hence Correct Answer is option d Both a &b are correct

Question 2 Major difference in glycosylation occurs in golgi and ER is that in Endoplasmic reticulum N - linked glycosylation occurs where oligosaccharide containing 14 sugar attach to asparagine that occurs in the tripeptide sequence, Asn-X-Ser or Asn-X-Thr.

While in Golgi O linked glycosylation occurs where N-acetyl-galactosamine followed by other Oligosaccharide attach to serine or threonine residues

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