Getting to the cell surface

Question 1

How are proteins that are synthesised in the cytosol trafficked to the plasma membrane?

Answer 1

Trafficked to the Pm via the secretory pathway

Not all proteins are exported

Question 2

What happens to proteins trafficked throught the secretory pathway?

Answer 2

Some are exported - hormones, enzymes, ECM proteins

Some incorporated into membrane - Channels, transporters, receptors, adhesion molecules

Question 3

How are proteins able to get across ER membrane into ER lumen?

Answer 3

1. Signal recognition particle recognises the signal peptide as it emerges from the ribosome
2. Binding of SRP to the signal peptide causes a pause in translation
3. SRP binds to its receptor in the membrane, and the ribosome-SRP complex is thereby localised to the translocator
4. Signal peptide is inserted into the translocator, translation continues and the growing polypeptide chain is threaded through the membrane (translocation)
5. SRP and SRP receptor dissociate from the ribosome-SRP-SRP receptor complex
6. The signal peptide is cleaved (by signal peptidase) and newly synthesised protein is released into ER lumen.

Question 4

What is the signal peptide?

Answer 4

15-20 hydrophobic amino acid sequence that are cleaved upon transit across the membrane

Question 5

What is the SRP and where is it located?

Answer 5

SRP – in cytoplasm – recognises signal sequence as it emerges from the ribosome

Question 6

What is the SRP Receptor and where is it located?

Answer 6

SRP receptor – in membrane – recognises ribosomeSRP complex and localises it to the membrane and the translocator

Question 7

What is the translocator and where is it located?

Answer 7

Translocator – in membrane – pore through which the growing polypeptide chain is threaded (‘translocation is co-translational’)

Question 8

What does translocation is co-translational mean?

Answer 8

Translocation occurs as the proteins are being produced and translated in the ribosome

Question 9

What are the main events that occur in the ER once a protein is imported into the lumen?

Answer 9

• signal peptide is cleaved (in most cases) - co-translational
• protein is glycosylated – co-translational and posttranslational
• protein is folded into its 3D conformation – co- and posttranslational
• disulfide bonds are formed – co- and post-translational
• multimerisation can occur – post-translational

Question 10

What is N-glycosylation?

Answer 10

addition of a sugar molecule to a protein where:
A preformed oligosaccharide (14 sugars) is transferred from dolichol (a lipid in the membrane) to newly synthesised proteins as they are translocated

Question 11

How does glycosylation occur?

Answer 11

Transfer is catalysed by an oligosaccharyl transferase (active site on luminal side)

Oligosaccharide is trimmed during transit though ER/Golgi (by glycosidases)

Question 12

What is the acceptor sequence for glycosylation?

Answer 12

Acceptor sequence is : Asn-X-Ser/Thr (where X is any amino acid except Pro

Question 13

What happens when proteins exit the ER?

Answer 13

Proteins must be properly folded to exit the ER. Improperly folded proteins are degraded.

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

Question 14

What is the structure of the Golgi Apparatus?

Answer 14

Flattened stacks (cisternae)

Each sac has a cis (entry) face and Trans (exit) face

Question 15

What is the CGN?

Answer 15

Cis Golgi network

Question 16

What is the TGN?

Answer 16

Trans Golgi network

Question 17

How are proteins passed through the Golgi?

Answer 17

Proteins destined for export from the cell arrive (in vesicles) at the CGN, move between the cisternae (in vesicles) and leave (in vesicles) from the TGN

Question 18

What happens to proteins as they pass through the Golgi apparatus?

Answer 18

Proteins are modified

Question 19

What are the classes of N-linked oligosaccharides?

Answer 19

Complex oligosaccharides

High mannose oligosaccharides

Both share a common core region derived from the original N-linked oligosaccharide added in the ER

Question 20

What are Complex oligosaccharides?

Answer 20

generated when the original N-linked oligosaccharide is trimmed (by glycosidases) and further sugars are added onto the core (by glycosyl transferases)

Question 21

What are high mannose oligosaccharides?

Answer 21

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

Question 22

What is the core oligosaccharide region shared between Complex and high mannose?

Answer 22

2x N-acetylglucosamine

3x Mannose

Question 23

What is O-glycosylation?

Answer 23

Where sugar residues are added to selected Serine or threonine residues.

Sugar is added to an oxygen molecule

Question 24

What are proteoglycans?

Answer 24

proteins containing glycosaminoglycan (GAG) chains

Question 25

Give an example of a proteoglycan?

Answer 25

Aggrecan:
very very big
ECM protein

Question 26

What are the 2 types of secretory pathway?

Answer 26

Constitutive

Regulated

Question 27

What is the constitutive secretory pathway?

Answer 27

Default pathway that operates continuously

Supplies the plasma membrane with newly synthesised membrane spanning proteins (and newly synthesised lipids)

Question 28

What is the regulated secretory pathway?

Answer 28

Proteins sorted into secretory granules at TGN and are stored until required.
Secreted in response to a signal (insulin)

Question 29

Which pathway are soluble proteins usually secreted by?

Answer 29

constitutive secretory pathway

Question 30

What does they regulated secretory pathway usually release?

Answer 30

hormones, neurotransmitters, and digestive enzymes

Question 31

What is the purpose of protein glycosylation?

Answer 31

Promotes protein folding by making proteins more soluble and preventing aggregation during synthesis

Question 32

What are the uses of cell surface glycoproteins?

Answer 32

• aid cell recognition processes
• are important in the inflammatory response
• protect the cell from chemical and mechanical damage (glycocalyx)

Question 33

How do lipids get to the cell surface?

Answer 33

lipids are synthesised in the ER and carried to the surface of the cell via vesicles of the secretory pathway

Question 34

How are Lysosomes targeted to organelles?

Answer 34

lysosomal enzymes enter the secretory pathway, are sorted at the TGN and transported to lysosomes in vesicles

Sorting signal is a mannose-6-phosphate sugar