Week 1 Intracellular Processes

Question 1

What are the three mechanisms that proteins use to target specific locations within a cell?

Answer 1

1. Nuclear pores (selective gate for nuclear proteins)
2. Protein translocators (for proteins moving from cytosol into ER, mitochondria, peroxisomes)
3. Transport vesicles (for proteins moving from ER onwards)

Question 2

What is the destination of protein DNA polymerase?

Answer 2

Nucleus

Question 3

What is the destination of EGF ?

Answer 3

Extracellular

Question 4

What is the destination of keratin?

Answer 4

Cytosol

Question 5

What is the destination of melanin?

Answer 5

Melanosomes

Question 6

Describe the specific “address labels” contained in amino acid sequence. (how ribosome goes to ER)

Answer 6

1. SRP in cytosol binds to ribosome at signal peptide
2. Complex binds to SRP receptor in ER membrane
3. The binding allows binding to translocon channel, allowing translocating peptide chain to enter ER
4. Signal peptide is secreted from ER via vesicle
5. Peptide chain is secreted out of ER via exocytosis to golgi for further modification.

Question 7

Describe the “post office” function of the golgi in directing proteins to specific subcellular locations.

Answer 7

1. transport vesicles containing protein fuse to become cis cisterna
2. they move through golgi stack and undergo modifications for specific cell destinations
   (i. e. proteins labelled with M6P bind to specific vesicle –> endosome to lysosome)

Question 8

What are some post-translational modifications to proteins?

Answer 8

1. Glycosylation: addition of sugar residues
2. Acetylation: occurs to histones
3. Farnesylation: target proteins to cytoplasmic face of plasma membrane
4. Ubiquitination: target for degradation by proteasomes
5. Phosphorylation: alters activity (hyperphosphorylation of protein Tau –> neurodegenerative)

Question 9

What are the two mechanisms by which proteins are degraded?

Answer 9

Lysosome and proteasomal degradation

Question 10

Of lysosomal and proteasomal degradation, which has longer half life?

Answer 10

Lysosomal degradation

Question 11

How does lysosomal degradation work?

Answer 11

• Membrane proteins are brought into cell via endocytosis.
• Extracellular proteins are brought into cell via receptor-mediated endocytosis
• Pathogenic proteins are brought into cell via phagocytosis

Question 12

Where does proteasomal degradation occur?

Answer 12

In cytosol at proteasomes, ATP dependent, for key metabolic enzymes & defective proteins , proteins covalently tagged with ubiquitin in 3-step pathway

Question 13

What is the mechanism of proteasomal degradation?

Answer 13

1. Shuttling proteins bring ubiquitinated proteins to proteasome
2. Tagged proteins are recognised, unfolded, translocated
3. Proteasome degrades proteins –> peptides
4. Peptides extended and digested by cytosolic peptidase