Protein sorting & trafficking

Question 1

What is the endomembrane system?

Answer 1

Series of compartments that work together to package, label, and ship proteins and molecules

Question 2

What is the endomembrane system made up of?

Answer 2

Endoplasmic reticulum

Golgi apparatus

Question 3

Why is important for proteins to be sorted into the right compartment?

Answer 3

Would result in chemical chaos

- degradative enzymes in lysosome needed separating from cytoplasm

Question 4

After protein synthesis in the cytosol, where do proteins go?

Answer 4

> stay in cytosol
> nucleus
> ER
> mitochondrion 
> peroxisomes
> chloroplasts

Question 5

What is the difference between targeting + trafficking?

Answer 5

Targeting = how proteins get to 1st destination

Trafficking = how proteins get to final destination

Question 6

What are sorting signals?

Answer 6

Specific stretches of amino acids in proteins

- different organelles have diff sorting signals + are recognised by diff machineries

Question 7

Which amino acids are important for nuclear targeting?

Where are they located?

Answer 7

Positively charged amino acids
= Lysine or Arginine

When protein is folded
–> stretches of amino acids situated on surface

Question 8

How has the importance of +vely charged amino acids for nuclear targeting been proven?

Answer 8

Using T-antigen wild type and w/ an amino acid removed

Wild type efficiently sorted into nucleus
Mutated protein not targeted to nucleus
–> no longer contains nuclear import signal

Question 9

What is special about nuclear membranes?

Answer 9

They’re contiguous w/ the endoplasmic reticulum

via continuous double membrane

Question 10

What are nuclear pores composed of?

Answer 10

Large no. of distinct protein subunits

Fibrils protrude from both sides of complex

Question 11

Define homomeric + heteromeric complexes

Answer 11

Homo = single type of subunit

Hetero = 2+ types of subunit

Question 12

How do small + large molecules enter the nucleus?

Answer 12

Small = diffusion

Large = active transport through nuclear pores

Question 13

What are NLS’s?

Answer 13

Nuclear localisation sequences

Question 14

How are proteins imported through the nuclear pore complex?

Answer 14

1. Nuclear import receptors recognise NLS’s on nuclear protein
2. receptor-protein complex guided to pore by fibrils
3. nucelar protein binds to pore –> opens
4. active transport into nucleus w/ receptor using GTP

Question 15

Describe the proteins imported into nuclei

Answer 15

Fully synthesised

Fully folded

Question 16

How are mitochondrial targeting sequences different to nuclear ones?

Answer 16

> always at N-terminus

> forms amphipathic alpha-helix
–> +ve charge lies on 1 face of helix

Question 17

Briefly, how do proteins get into the mitochondrial matrix?

Answer 17

1. Receptors on outer membrane recognise targeting sequences

2. Conducting channels allow proteins to cross membranes

Question 18

What are the 2 conducting channels?

Answer 18

TOM = translocator of outer membrane

TIM = translocator of inner membrane

Question 19

In what form must proteins be in order to be imported into mitochondria?

Answer 19

Unfolded

Question 20

How are mitochondrial proteins prevented from folding?

Answer 20

By cytoplasmic chaperone proteins

Question 21

Why is mitochondrial protein targeting complex?

Answer 21

> Proteins can be outer membrane, inner membrane, inter-membrane space or matrix
have their own genome + protein synthesis machinery

Question 22

How do nuclear import receptors exit the nucleus?

Answer 22

Ran-GTP binds to receptors inside nucleus

Ran-GDP + Pi dissociate in cytosol

Question 23

What is required for the translocation of mitochondrial proteins across the inner membrane?

Answer 23

Membrane potential (-ve inside)

Energy from ATP hydrolyse