Protein Transport

Question 1

How may proteins which leave the ribosome know their correct location? (2)

Answer 1

1. Through a signal/sorting sequence. This is necessary and sufficient to target protein to correct location.l
2. Proteins without a signal remain in the cytosol.

Question 2

What are 3 ways proteins can be transported intracellularly? (3)

Answer 2

Protein Transport requires energy

1. Transport through Nuclear pores. (Protein remains folded)
2. Transport across membranes (Protein must first be unfolded)
3. Transport by vesicles (Protein remains folded)

Question 3

What type of proteins are transported back across nuclear pores? (2)

Answer 3

Only small molecules with corresponding signal sequence such as transcription factors.

Question 4

Describe the process of proteins being Imported back into the Nucleus (3)

Answer 4

1. IMPORTIN recognises the protein nuclear localisation signal (NLS) on the protein, binding it to other proteins within the cytosolic fibrils.
2. Importin carries the protein into the nucleus through the nuclear pore channel.
3. Inside the nucleus, Ran allows release of protein from Importin; importing returns to cytosolic side of the pore.

Question 5

What proteins make up the Nuclear pores? (1)

Answer 5

A complex of around 30 proteins called Nucleoporins.

Question 6

Describe the process of proteins being transported into the Mitochondrial membranes (5)

Answer 6

1. Mitchondria has an outer and inner membrane.
2. The signal sequence is recognised an IMPORT receptor on the outer membrane.
3. The protein TRANSLOCATOR pass the protein through the membranes.
4. The Import receptor and protein Translocator diffuse to an area wherethe membranes are close together and engages the second translocator.
5. Once inside the mitochondria, the protein folds and the signal sequence is cleaved off.

Question 7

How are Ribosomes directed to the ER? (3)

Answer 7

1. The signal sequence of the Ribosome binds to The signal recgonition Particle (SRP).
2. The SRP then binds to ER receptor, then released.
3. The Signal sequence of the ribosome binds to the Protein translocator of the ER.

Question 8

Describe import of polypeptide in the ER during translation (3)

Answer 8

1. The signal sequence of the protein binds to the protein translocator, threading the polypeptide through the lipid bilayer as a loop.
2. Signal peptide is removed from the polypeptide by SIGNAL PEPTIDASE, with polypeptide left in the membrane to degrade.
3. As the polypeptide passes through the membrane, MOLECULAR CHAPERONE BiP binds ready to help fold protein in the ER. After protein synthesis, soluble protein is released.

Question 9

Proteins that span the membrane more than twice contain additional pairs of start- and stop-transfer sequences, repeating the same process.

Describe the two ways proteins can be inserted into the membrane? (4)

Answer 9

1. A Single Pass Transmembrane Protein: The N-terminal signal sequence (at the end of the protein) initiates the transfer but is stopped by a stop-transfer sequence. Signal peptidase then removes the signal sequence.
2. A double pass transmembrane protein: This has an internal signal sequence (in the middle of the protein) which anchors the protein in the membrane. The stop-transfer sequence enters the protein translocator, which discharges both sequences into the lipid bilayer.

Question 10

How can proteins be modified? How and why does it occur? (4)

Answer 10

1. Glycosylation
2. This occurs during protein transport into ER.
3. When Asparagine (Asn) enters ER, brached oligosacchide chain is it is added by gyclosylation.
4. This can regulate function of protein e.g immune response, apoptosis etc.

Question 11

What occurs when Proteins are misfolded in the ER? (4)

Answer 11

1. The proteins are retained by chaperones.
2. Chaperones hold protein to fold correctly or be removed into cytosol for degradation by proteasome.
3. Excess Protein misfolding in ER triggers UNFOLDED PROTEIN RESPONSE (UPR)
4. UPR activates SENSOR proteins, which stimulate expansion of ER, increase chaperones and inhibit protein synthesis.

Question 12

What is the function of Endosomes? (2)

Answer 12

1. They sort/regulate protein and lipid trafficking in the cell.
2. Early->late endosomes transport proteins from GA or endocyted proteins to the lysosomes.

Question 13

What are the two forms of Exocytosis? (2)

Answer 13

1. Constitutive Pathway - Continual secreion of proteins from cell, replaces protein and lipids in membrane.
2. Regulated Pathway - Proteins are stored in vesicles until extra cellular signal stimulates secretion.

Question 14

What do coat proteins do? What are some examples? (2)

Answer 14

1. They help to shape membrane into a bud and help capture cargo proteins for transport.
2. Examples: Clathrin, adaptin 1 and 2, COPI and COPII proteins

Question 15

How are vesicles from Cell membrane and GA formed? (3)

Answer 15

1. Clathrin coated pits form on the membrane and are concentrated with receptors which shape the bud.
2. Adaptin selects the cargo via cargo receptors.
3. Dynamin (GTP binding protein) pinches off the vesicle.

Question 16

How are vesicles recognised at target membrane? (3)

Answer 16

1. Rab Proteins, tethering proteins and SNAREs help direct vesicle.
2. Tethering protein binds to Rab on vesicle.
3. v-SNARE on vesicle binds to t-SNARE n target membrane and vesicle fuses to membrane.

Question 17

How are receptors from a membrane translocated based on fate? (3)

Answer 17

1. Degradation - to lysosomes, through phagocyte, endosome or autophagy.
2. Recycling - returned to plasma membrane they came from.
3. Transcytosis - Returned to different area of plasma membrane.