Getting to the cell surface

Question 1

Describe organelles present in the secretory pathway

Answer 1

Endoplasmic reticulum
Gogli apparatus
Endosomes
Vesicles

Question 2

Are all proteins trafficked through the secretory pathway exported?

Answer 2

No

Question 3

What proteins are exported?

Answer 3

Hormones
Enzymes
Extracellular matrix proteins

Question 4

What proteins aren’t exported?

Answer 4

Incorporated into the membrane
Channels and transporters
Receptors
Cell adhesion molecules

Question 5

Where are proteins synthesised?

Answer 5

Ribosomes of the rough endoplasmic reticulum

Question 6

What are signal peptides?

Answer 6

Trigger transfer
15-20 amino acids long
At the N-terminal end
Hydrophobic
Cleaved upon transit across the membrane

Question 7

Where are signal peptides for multi-pass membrane spanning proteins?

Answer 7

Internal

Not cleaved

Question 8

How do proteins cross the endoplasmic reticulum membrane?

Answer 8

Signal recognition particle recognises the signal peptide
Signal recognition particle binds to the signal peptide, pausing translation
Signal recognition particle binds to receptor in the membrane
Signal peptide is inserted into the translator
Translation continues and polypeptide chain is through the membrane
Signal recognition particle and receptor dissociate
The signal peptide is cleaved and protein is released into the ER lumen

Question 9

What cleaves the signal peptide?

Answer 9

Signal peptidase

Question 10

What are the main events that happen in the endoplasmic reticulum?

Answer 10

Signal peptide is cleaved
Protein is glycosylated
Protein is folded
Disulfide bonds form
Multimerisation can occur

Question 11

What is N-glycosylation?

Answer 11

Most soluble and membrane spanning proteins are glycosylated

A preformed oligosaccharide (14 sugars) is transferred from dolichol to the new protein

Question 12

What catalyses N-glycosylation?

Answer 12

Oligosaccharyl transferase

Question 13

How do proteins leave the endoplasmic reticulum?

Answer 13

Vesicles containing soluble/membrane bound proteins bud from the ER and transit to the cis face of the Golgi

Question 14

Describe the Golgi body

Answer 14

Comprises of flattened membrane compartments called cisternae
Each Golgi stack has two faces, the cis face and trans face
Two networks: the cis Golgi network and the trans Golgi network
Proteins arrive in vesicles at the cis face and leave in vesicles from the trans face

Question 15

What are the two types of N-linked oligosaccharides?

Answer 15

Complex oligosaccharides

High mannose oligosaccharides

Question 16

What are complex oligosaccharides?

Answer 16

Generated when the original N-linked oligosaccharide is trimmed and further sugars added

Question 17

What are high mannose oligosaccharides?

Answer 17

Original N-linked oligosaccharide is trimmed and further mannose residues added

Question 18

What is O-glycosylation?

Answer 18

Occurs when sugar residues are added to serine or threonine

Added by glycosyl transferases

Question 19

What are proteoglycans?

Answer 19

Contain glycosaminoglycan chains
Unbranched polysaccharide chains composed of repeating disaccharide units
Special link tetrasaccharide is added to serine

Question 20

What is the constitutive secretory pathway?

Answer 20

Operates continuously
Default pathway
Supplies the plasma membrane with newly synthesised membrane spanning proteins
Soluble proteins are continually secreted from the cell via this pathway

Question 21

What is the regulated secretory pathway?

Answer 21

Some proteins are sorted into secretory granules where they are stored until required
Release is triggered by an extracellular signal
Vesicle fuses with plasma membrane and releases its content

Question 22

What is the purpose of protein glycosylation?

Answer 22

Promotes protein folding
Aid cell recognition processess
Important in inflammatory response
Protect the cell

Question 23

How are enzymes transported to lysosomes?

Answer 23

Lysosomal enzymes enter the secretory pathway and transported to lysosomes in vesicles
Their sorting signal is mannose-6-phosphate

Question 24

How are proteins transported to the nucleus?

Answer 24

Nuclear localisation signals direct proteins to the nucleus

Runs of 5 or more basic amino acids anywhere in the protein sequence

Question 25

How are proteins transported to the mitochondria?

Answer 25

Mitochondrial signal sequences direct proteins to mitochondria
These are amphipathic helices at the N-terminus of mitochondrial proteins

Question 26

Which proteins do not enter the secretory pathway and why?

Answer 26

Nuclear and mitochondrial proteins

They are synthesised in cytoplasmic ribosomes