Fate of Newly Synthesised Proteins: ER and Golgi

Question 1

What is the secretory pathway?

Answer 1

It is the pathway taken by proteins destined for lysosomes, the plasma membrane or secretion.
Proteins are moved from the ER to the cis side of the Golgi complex in transport vesicles. Sorting occurs primarily in the trans side of the Golgi complex.

Question 2

What is the ER?

Answer 2

It is a continuous network of membrane tubules that is continuous with the nuclear envelope.

Question 3

What are some functions of the ER?

Answer 3

• protein synthesis
• glycosylation
• folding and assembly of multi-protein complexes
• lipid synthesis (cholesterol, phospholipids)
• Ca2+ sequestration (storing calcium ions for future use)
• detoxification (by cytochrome P450 enzymes)

Question 4

Describe the co-translation protein targeting to the ER.

Answer 4

1) The targeting pathway begins with the initiation of protein synthesis on free ribosomes.
2) The signal sequence appears early in the synthetic process because it is at the amino terminus, which is synthesised first.
3) As it emerges from the ribosome, the signal sequence - and the ribosome itself - are bound by the large signal recognition particle (SRP). SRP then binds GTP and halts elongation of the polypeptide when it is about 70 a.as long and the signal sequence has completely emerged from the ribosome.
4) The GTP-bound SRP now directs the ribosome (still bound to the mRNA) and the incomplete polypeptide to GTP-bound SRP receptors in the cytosolic face of the ER. The nascent polypeptide is delivered to a peptide translocation complex in the ER, which may interact directly with the ribosome.
5) SRP dissociates from the ribosome, accompanied by the hydrolysis of GTP in both the SRP and SRP receptor.
6) Elongation of the polypeptide now resumes, with the ATP-driven translocation complex feeding the growing polypeptide into the ER lumen until the complete protein has been synthesised.
7) The signal sequence is removed by a signal peptidase within the ER lumen.
8) The ribosome dissociates and is recycled.

Question 5

Describe how mature insulin is made.

Answer 5

Mature Insulin is formed from its large precursor Preproinsulin by proteolytic processing.
Removal of a 23 amino acid segment (signal sequence) at the amino acid terminus of Preproinsulin and the formation of 3 disulphide bonds produces Proinsulin.
Further proteolytic cuts remove the C peptide from Proinsulin to produce Mature Insulin, composed of A and B chains.

Question 6

What are some protein modifications that occur in the ER?

Answer 6

• proteolysis (breakdown of proteins into smaller polypeptides)
• disulfide bond formation
• glycosylation
• deglycosylation
• protein folding and assembly (tertiary and quaternary structure)

Question 7

Describe how the COPII coat is made for ER transport.

Answer 7

Membrane-bound, active Sar1-GTP recruits COPII adaptor proteins to the membrane. They select certain transmembrane proteins and cause the membrane to deform. The adaptor proteins then recruit the outer coat proteins which help form a bud. A subsequent membrane fusion event pinches off the coated vesicle (other coated vesicles are thought to form in a similar way).

Question 8

Describe the structure of the Golgi Apparatus.

Answer 8

The Golgi apparatus is like a set of flattened bags. There are two parts: the surface closer to the nucleus is called the ‘cis’ face, where the vesicles enter the Golgi apparatus and the ‘trans’ which faces the plasma membrane and where vesicles leave the Golgi apparatus.

Question 9

What are some functions of the Golgi Apparatus?

Answer 9

• PROTEIN MODIFICATION:
  glycosidases, sulfatases, proteases, O-linked glycosylation, glycosyltransferases
• LIPID SYNTHESIS:
  sphingomyelin, glucosylceramide
• PROTEIN AND LIPID SORTING TO:
  secretory, granules, plasma membrane, basolateral vs apical membrane, endosomes, lysosomes