The ER and secretory pathway

Question 1

Functions of the ER

Answer 1

• Protein synthesis
• Glycosylation
• Folding and assembly and multi protein complexes
• Lipid synthesis
• Ca2+ sequestration (storage of Ca2+)
• Detoxification by P450 enzymes

Question 2

ER protein synthesis and modification

Answer 2

• AUG is the first codon of which an mRNA is translated

1. Ribosome begins translation to synthesise the nascent protein
2. The amino terminus(first part of the nascent polypeptide) contains a signal sequence.
3. An SRP(signal recognition particle) recognises the signal sequence and binds to the signal sequence as well as the ribosome resulting in protein synthesis in halting
   a. SRP binds to GTP which halts elongation of the polypeptide chain.
4. The GTP-bound SRA directs the ribosome to the GTP-bound SRA receptors and the nascent polypeptide is delivered to a peptide translocation complex which directly interacts with the ribosome
5. SRP dissociates from the ribosome accompanied with the hydrolysis of GTP in both SRP and SRP receptors.
6. The ribosome binds to sec61 translocation channel and the signal sequence is threaded into the channel.
7. Elongation of the polypeptide resumes until the complete protein has been synthesised. The signal sequence is removed by peptidase and the ribosome dissociates and recycled

Question 3

Structure of the golgi apparatus

Answer 3

Cis face is where the proteins enter from

Trans face is where the modified proteins leave

Question 4

Protein export from the ER

Answer 4

• Protein first packaged into COPII-coated transport vesicles
• These vesicles bud from regions called ER exit sites which have no ribosomes

Question 5

Roles of the golgi apparatus(Protein modification)

Answer 5

○ Glycosidase-hydrolysis of glycosidic bond
○ O-linked glycosylation
○ Sulfatases-hydrolysis of sulfate esters
○ Proteases-hydrolysis of peptide bonds

Question 6

Roles of the golgi apparatus(Lipid synthesis)Which lipids are synthesised?

Answer 6

○ Sphingomyelin

○ Glycosyleramide

Question 7

Roles of the golgi apparatus(Protein and lipid sorting to):

Answer 7

○ Secretory granules
○ Plasma membrane 
○ Basolateral versus apical membrane
		§ Apical is the portion of cell exposed to the lumen
		§ Basolateral is the rest of the cell
○ Endosomes
○ Lysosomes