Lecture 8: Protein transport

Question 1

How do proteins ensure they are delivered to the correct location?

Answer 1

Part of the amino acid sequence of a protein may contain a signal sequence (or sorting sequence) which provides the information as to the correct location of that protein.

Question 2

What happens to proteins without a signal sequence?

Answer 2

They remain in the cytosol

Question 3

What happens to proteins synthesised by ribosomes in the cytosol?

Answer 3

Either remain in the cytosol or get transported into mitochondria, peroxisomes or the nucleus.

Question 4

What happens to proteins synthesised by ribosomes at the endoplasmic reticulum?

Answer 4

Proteins are destined for secretion so are sorted by the Golgi apparatus.

Question 5

How does transport across different membranes?

Answer 5

Proteins transported through pores or by vesicles normally remain folded, whereas proteins transported across or into membranes must first be unfolded;

Question 6

Why does protein transport require energy?

Answer 6

Some proteins needed to unfolded and refolded in order to be transported across membranes, etc.

Question 7

What is the signal sequence that tells proteins to migrate to the nucleus?

Answer 7

Nuclear Localisation Signal (NLS)

Question 8

What molecule aids in the transport of proteins into the nucleus?

Answer 8

Importin

Question 9

Describe the state of proteins entering the nucleus

Answer 9

Proteins are translocated in their full folded conformation.

Question 10

What is the role of Ran in the transport of proteins into the nucleus?

Answer 10

Ran interacts with nucleus and causes the release of protein in nucleus and the return of importin.

Question 11

How are proteins transported into the nucleus?

Answer 11

1. Protein nuclear localisation signal (NLS) is recognised by importin which then bind to proteins within the cytosolic fibrils.
2. Importin carries the protein into the nucleus by disrupting the nuclear pores. Proteins are translocated in their full folded conformation.
3. Nucleus interaction with Ran, coupled to GTP, causes release of the protein and returns importin back to the cytosolic side of the pore ready to start again.

Question 12

How are proteins transported into the mitochondria?

Answer 12

1. Protein signal sequence is recognized by an import receptor on outer membrane of the mitochondria.
2. Protein translocators pass the protein through the membranes
3. Receptor and translocator migrate to area where membranes come into close proximity to engage second translocator.
4. Once inside mitochondria, protein folds and the signal peptide is cleaved off.

Question 13

What happens once proteins enter the mitochondria?

Answer 13

Protein refolds ad signal peptide is cleaved off.

Question 14

Describe the state of proteins entering the mitochondria

Answer 14

Unfolded proteins

Question 15

How are proteins transported into the endoplasmic reticulum?

Answer 15

1. Signal recognition particle (SRP) binds to the signal sequence and the ribosome
2. SRP then binds to a receptor on the ER membrane at which point it is released, allowing the signal sequence on ribosome to bind to protein translocator on ER.
3. Polypeptide is threaded across ER lipid bilayer as a loop and signal peptide is removed by signal peptidase.
4. Molecular chaperone, BiP, then helps to refold the protein within the ER. Protein translocator closes.

Question 16

What molecule aids in the movement of proteins into the ER?

Answer 16

Signal recognition particle (SRP)

Question 17

How do molecular chaperones aid proteins that have entered the ER?

Answer 17

Molecular chaperones help protein to refold.

Question 18

Describe the state of proteins entering the ER

Answer 18

Unfolded proteins

Question 19

How does SRP help proteins enter the ER?

Answer 19

SRP brings together the protein at the ribosome and the ER. This brings the ribosome closer to the protein translocator on the ER.

Question 20

How are single-pass transmembrane proteins inserted into the membrane?

Answer 20

N-terminal ER signal sequence initiates the transfer, which is then halted by a stop-transfer sequence. Once inserted, the N-terminal signal sequence is removed by a signal peptidase.

Question 21

How are double-pass transmembrane proteins inserted into the membrane?

Answer 21

Internal sequence acts as a start-transfer signal and also anchors the protein in the membrane. When a stop-transfer sequence enters the protein translocator, the translocator discharges both sequences into the lipid bilayer.

Question 22

What is glycosylation?

Answer 22

A form of post-translational modification (PTM) whereby asparagine (Asn) is glycosylated by the covalent addition of a branched oligosaccharide side chain.

Question 23

What happens when glycosylation goes wrong?

Answer 23

Changes and errors to glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis.

Question 24

What happens when there is an excessive amount of protein misfolding?

Answer 24

Excess protein misfolding in the ER triggers the unfolded protein response (UPR)

Question 25

What happens during the unfolded protein response (UPR)?

Answer 25

There is an expansion of the ER, an increase in the number of chaperones and a reduction in the amount of new proteins entering the ER.

Question 26

What happens during exocytosis?

Answer 26

Vesicles fuse with the plasma membrane releasing their contents outside the cell.

Question 27

What are the 2 pathways of exocytosis?

Answer 27

1. Constitutive pathway

2. Regulated pathway

Question 28

What happens during the constitutive exocytosis pathway?

Answer 28

Continual secretion of soluble proteins from the cell; also replaces proteins and lipids in the membrane.

Question 29

What happens during the regulated exocytosis pathway?

Answer 29

Secretory vesicles store proteins until an extracellular signal stimulates their secretion.

Question 30

What happens during endocytosis?

Answer 30

Vesicles fuse with the plasma membrane allowing for uptake of material from outside the cell.

Question 31

What happens to receptor proteins after endocytosis?

Answer 31

Degraded by lysosomes, recycled or transcytosis to a different part of the plasma membrane.

Question 32

What is the role of coat proteins?

Answer 32

Coat proteins shape the membrane into a bud and directly or indirectly capture the cargo proteins for transport.

Question 33

Give an example of a coat protein

Answer 33

Clathrin; involved in shaping the budding membrane and other proteins called adaptins select the cargo molecules.

Question 34

How are vesicles recognised by target membranes?

Answer 34

Rab proteins, tethering proteins, and SNAREs.

Question 35

How do vesicles dock to their target membrane?

Answer 35

Tethering protein binds to Rab on vesicles as it docks and v-SNARE binds to a t-SNARE. v-SNAREs and t-SNAREs bring the lipid bilayers close together and allow for fusion

Question 36

What happens during autophagy?

Answer 36

Breakdown of the cells own proteins and organelles by enclosing them in a membrane that fuses with the lysosome.