Lecture 4

Question 1

How did the cell nucleus evolve in ancient prokaryotic cells?

Answer 1

Nucleus initially resulted from invagination and internalization of the plasma membrane. The internalization of the plasma membrane contains another branch which forms the ER membrane which is continuous with the ER.

Question 2

Where did the golgi come from in eukaryotic cells?

Answer 2

Golgi arose from the endoplasmic reticulum.

Question 3

What are modern membrane-bound structures of the cell that originally arose from the plasma membrane invagination?

Answer 3

Nuclear Membrane

ER

Golgo

Endosomes

Lysosomes

*These structures are part of what is known as the endomembrane system.

Question 4

Where did mitochondria and plastids arise from?

Answer 4

Endosymbiosis as a result of consuming ancient bacteria.

Question 5

What are the features of membrane-bound structures that arose from endosymbiosis?

Answer 5

Have their own DNA

Are enclosed by 2 membranes

They are isolated from extensive protein transport in the endomembrane system.

Question 6

How is the mitochondria regulated in a cell?

Answer 6

Functionality of mitochondria relies on genes from the nucleus.

Question 7

What is the function of the ER?

Answer 7

ER is the entry point for proteins destined for other compartments of the endomembrane system. This includes exocytosis from the cell.

Question 8

How does cargo travel between membrane bound compartments?

Answer 8

Membrane bound compartments of the endomembrane system transport cargo via transport vesicles.

Question 9

What is the advantage of the endomembrane system used in prokaryotic cells?

Answer 9

Cargo only needs to cross the ER membrane once. After that it is transported between structures of the endomembrane system via vesicles which internalise the cargo. This allows the system to be highly regulated.

Question 10

Where does sorting of cargo and targeting of cargo occur?

Answer 10

Trans Golgi Network (TGN)

Question 11

How are vesicles targeted to the correct location?

Answer 11

Each transport vesicle that buds from a compartment must be loaded with appropriate proteins for its destination.

Question 12

Why is there a great variability in vesicle coats?

Answer 12

Different classes of transport vesicle shuttle between the various organelles

Question 13

How are membranes deformed to form buds?

Answer 13

Vesicle budding is driven by the assembly of a protein coat on cytosolic surface of the membrane.

Question 14

What happens after coat proteins are distinct from the original membrane?

Answer 14

Vesicle coats need to be removed for the vesicle to interact with the next compartment.

Question 15

What is the function of the vesicle beside pinching out the membrane?

Answer 15

Coats are also important for capturing cargo. Only specific cargo can enter the vesicle.

Question 16

What is the usual function of COP1?

Answer 16

Transport from Golgi to ER

Question 17

What is the usual function of COP2?

Answer 17

Transport from ER to golgi

Question 18

What is the usual function of clathrin?

Answer 18

Transport from golgi to plasma membrane

Question 19

What happens to misfolded proteins during vesicle formation?

Answer 19

Only properly folded proteins are allowed to exit the ER. Misfolded proteins are retained by chaperones

Question 20

How are cargo transported from the cis to the trans golgi?

Answer 20

2 theories as to how cargo is transported between membranes:

Vesicular transport model: Vesicles are made between layers.

Cisternal maturation model: Cisternae migrates from cis to trans.

Question 21

What is the function of COP1?

Answer 21

Sometimes resident ER molecules leak out and move to the golgi with transport vesicles. These resident ER proteins contain a KDEL sequence which can bind to a KDEL receptor which returns the resident protein back to the ER. Return is carried out by COP1.
The binding (in the golgi) and dissociation (in the ER) of KDEL to its receptor is mediated by the differing pH.

ER membrane can ‘run out’ and so COP1 transport is essential to maintain membrane homeostasis.

Question 22

What is the amino acid sequence for KDEL?

Answer 22

-Lys-Asp-Glu-Leu- at the C-terminus

Question 23

Where are KDEL receptors found?

Answer 23

Within all cysternae of the golgi where the pH is slightly acidic..

Question 24

How are ER resident proteins released into the ER after the vesicles fuse with the ER?

Answer 24

pH in the ER is more than the pH of the golgi and so the pH causes conformational change in the KDEL receptor.

Question 25

What are the potential pathways of secretion and how are they different from each other?

Answer 25

Direct pathway is constitutive secretion.

Regulated pathway contains many intermediate organelles prior to reaching secretion.