week 7: Intracellular compartments

Question 1

Where are proteins made?

Answer 1

Made in the cytoplasm then transported to the organelles

Question 2

What are the 3 mechanisms for protein transportation

Answer 2

1. through nuclear pores
2. transport across membranes
3. transport via vesicles through the endomembrane system

Question 3

what is a signal sequence

Answer 3

a specific sequence of amino acids which tells the cell where that protein belongs

Question 4

what is the transport of proteins through nuclear pores

Answer 4

• nuclear membrane has many pores
• proteins are trafficked between the nucleus and the cytoplasm through nuclear pores
• aided by receptor proteins that trap the proteins & delivers them to the pore complex

Question 5

Do nuclear pores transport fully folded proteins?

Answer 5

Yes

Question 6

How does the transport of protein into the nucleus work?

Answer 6

Transport receptors in the cytosol bind to the nuclear signal sequences on the proteins.

receptors interact with the nuclear pore fibrils and direct the protein transport

Question 7

Does the transport of protein into the nucleus via transport receptors require energy

Answer 7

yes

Question 8

what is the transport of proteins across membranes into organelles?

Answer 8

A signal sequence at the N-terminus of the protein allows the protein to enter the mitochondrion

The protein is transported across both the inner and outer membranes, and it is transported in the unfolded state

Chaperone proteins aid this process & help the refolding of the protein once inside the mitochondrion

The signal sequence is removed once the protein is inside the mitochondrion

Question 9

what are chaperone proteins?

Answer 9

they aid in the process of transport of proteins across membranes and help the refolding of the protein once inside the mitochondrion

Question 10

What is the transport of proteins through the endomembrane system?

Answer 10

Proteins travel through vesicles

Question 11

what is the endoplasmic reticulum

Answer 11

serves as an entry point for proteins destined for other parts of the cell.

proteins enter the ER and will move through the cell in membrane-bound vesicles never going into the cytosol

Question 12

What are the two proteins made in the ER?

Answer 12

1. water-soluble - stay in ER lumen

2. Membrane-bound - these stay within ER membrane

Question 13

where does the translation of all proteins begin?

Answer 13

ribosomes in the cytosol except for protein that are destined for the ER have a signal sequence

Question 14

what are soluble proteins

Answer 14

water-soluble ( hydrophilic) and so can enter the watery lumen of ER

Question 15

how are soluble proteins translated

Answer 15

translated through translocator channels in the ER membrane

signal sequence stays bound to translocator & the rest of the protein is threaded through into lumen

once complete, the signal sequence is cleaved off by peptidase enzyme

Question 16

How are membrane-bound proteins translated

Answer 16

translated through translocator channels in the ER membrane

Proteins are destined to stay in the membrane.

have one or more hydrophobic regions that slide into the ER membrane once translated

the hydrophobic region starts and stops the signal

Question 17

what is the transport of proteins through the endomembrane system

Answer 17

proteins are synthesised in the ER and transported through vesicles

Transport vesicles form when a small part of the ER membrane buds off

Question 18

what protein is involved in vesicle budding?

Answer 18

clathrin is involved in forming the vesicle. It coats the outside of the vesicle creating the shape

Question 19

how does clathrin make a vesicle?

Answer 19

Clathrin forms a basket-like network around the vesicle, and this involves other accessory proteins eg. adaptins, dynamin

Question 20

how do clathrin-coated vesicles form?

Answer 20

form from Golgi and also the plasma membrane (endocytosis)

Question 21

what are the Golgi stacks of membrane?

Answer 21

proteins move through the stacks from the cis Golgi network closer to the nucleus, to the trans-Golgi network closer to the plasma membrane

Question 22

what is protein secretion

Answer 22

secretory proteins are released via exocytosis

Question 23

What is constitutive secretion

Answer 23

continually supplies the plasma membrane with newly synthesised lipid and proteins

• continual or unregulated
  e. g albumin, immunoglobins, collagen

Question 24

what are regulated secretion

Answer 24

proteins are stored in vesicles until an extracellular signal stimulates their secretion

requires extracellular signal for secretion

e.g insulin, neurotransmitters

Question 25

what is a chaperone protein

Answer 25

hold the protein in the ER until proper folding occurrs

misfolded protein is directed to cytoplasm instead of Golgi and degraded

Question 26

What is cystic fibrosis

Answer 26

inherited disease involving secretory glands of mucus and sweat
Thick mucus in lungs

Question 27

What is the CFTR gene?

Answer 27

Encodes for chloride ion channel in airway epithelium cells

Chloride ions are transported into the airway lumen, water follows due to osmosis, and this normally keeps mucus fluid

Question 28

CFTR protein is a slightly misfolded chloride ion

Answer 28

The misfolded protein is unfortunately recognized by chaperones & degraded, rather than being trafficked to the plasma membrane

Question 29

what is enodcytosis

Answer 29

Uptake of material into cell by invagination of plasma membrane & internalization in a membrane-bound vesicle

Question 30

Phagocytosis

Answer 30

cell eating

• ingestion of large particles via vesicles
• specialised phagocytic cells

Question 31

Pinocytosis

Answer 31

cell drinking
receptor mediated
ingestion of fluid & small molecules via small vesicles

Question 32

Receptor-mediated endocytosis

Answer 32

Macromolecules (e.g. LDL) bind to complementary receptors (proteins that recognize the molecule) on the cell surface

Bound receptors cluster together in a clathrin-coated pit

Clathrin protein allows the vesicle to form & dynamin enzyme helps to it ‘pinch off’ the membrane

Ingested molecules are transferred to endosomes which remove the receptor, and the endosome then delivers the molecules to the lysosome for digestion

Receptors are usually recycled back to the cell surface

Low density lipoproteins (LDL cholesterol) enter cells via this mechanism