Lecture 2 - Exam 4: Protein Sorting and Vesicular Trafficking

Question 1

What is the difference between the rough ER and smooth ER?

Answer 1

Rough ER is covered by ribosomes on the outer surface. The smooth ER has no ribosomes and is involved in lipid metabolism.

Question 2

What are the main roles of the Smooth ER?

Answer 2

Synthesis of phospholipids and synthesis of cholesterol, ceramide detoxifying enzymes.

Question 3

Where do we find a lot of smooth ER?

Answer 3

The liver, because of detoxification function.

Question 4

After synthesis, what happens to lipids?

Answer 4

After synthesis, lipids are transported from the smooth ER to their ultimate destinations in other membranes.

Question 5

Phospholipids are synthesized how? Why are they synthesized this way?

Answer 5

Phospholipids are synthesized in association with already existing cellular membranes due their extreme hydrophobic nature.

Question 6

Most phospholipids are derived from __________?

Answer 6

Glycerol!

Question 7

Most phospholipids are derived from glycerol. What is the exception?

Answer 7

Sphingomyelin has serine in it, not glycerol.

Question 8

Describe the synthesis of phospholipids (membrane lipids).

Answer 8

*Starting material is water soluble.
1. Two fatty acids are transferred from coenzyme A carriers to glycerol-3-phosphate, resulting in a phosphatidic acid (immediately inserted into the membrane).
2. Enzymes on the cytosolic face of the ER convert phosphatic acid to diacylglycerol and catalyze the addition of different polar groups = phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol.

Question 9

What is the orientation of phospholipids in the cell membrane?

Answer 9

Asymmetric.
Phospholipids with negative charges are located on the cytosolic face (inside the cell) (some involved in cell signaling process)

Question 10

All lipid synthesis occurs where?

Answer 10

On the cytosolic face.

Question 11

Are negative phospholipids located on the inside or outside of the cell?

Answer 11

Negative phospholipids are located on the inside (cytosolic face) of the cell.

Question 12

Where do the hydrophobic fatty acids of lipids remain?

Answer 12

Buried in the membrane

Question 13

Newly synthesized lipids are added only to where?

Answer 13

Added only to the cytosolic half of bilayer.
But the need to be on both sides to replace old lipids and/or generate the correct lipid balance!

Question 14

What are Flippases? What do they do?

Answer 14

They are membrane proteins that help pass the polar head group through the membrane. This is no easy task. There are families of these enzymes, and some are specific for particular phospholipids.

Question 15

What is the process of translocation of phospholipids across the ER membrane?

Answer 15

ER membrane
– phospholipid synthesis–
Newly synthesized lipids added only to cytosolic half of bilayer
–lipid transfer facilitated by flippases–
Growth of both halves of phospholipid bilayer.

Question 16

Describe the Smooth ER being abundant in cell types that are particularly active in lipid metabolism.

Answer 16

Steroid hormones are synthesized (from cholesterol) in the ER. Large amount of smooth ER are found in steroid producing cells such as testis and ovaries.
In the liver, smooth ER contains enzymes that can metabolize lipid-soluble compounds.

Question 17

Describe the export of proteins and lipids from the ER.

Answer 17

Both proteins and phospholipids are exported from the ER in vesicles that form in a specialized region of the ER, called the ER exit side (ERES).
The vesicles that exit the ER fuse to form the ER-Golgi intermediate compartment (ERGIC), from which the cargo is transported to the Golgi.
It is significant that the topological orientation of membrane proteins and lipids is maintained as they travel from one membrane-enclosed organelle to another.

Question 17

Describe transmembrane proteins (TM) targeted for export from the ER to the Golgi.

Answer 17

Transmembrane proteins targeted for export from the ER to the Golgi have peptide carbohydrate signals that are used to direct them to the ERES.
Most lumenal proteins to be exported from the ER are bound to TM proteins that are selectively packaged into vesicles and then released when they get to the ERGIC or Golgi.

Question 18

Describe the resident proteins of the ER.

Answer 18

Unmarked resident ER proteins are often packaged into vesicles by a default pathway which does not require specific signals for protein packaging (eg. chaperones like BiP, are important residents of the ER).
ER-Golgi intermediate and Golgi, transport back these proteins in a retrieval pathway. Receptors recognize a targeting sequence (Lys-Asp-Glu-Leu or KDEL) on the C-terminal of these proteins and initiate the retrieval back.

Question 19

What is the most popular view of the function of the ER-Golgi Intermediate Compartment?

Answer 19

The ERGIC clusters are mobile transport complexes that deliver secretory cargo from ER-exit sites to the Golgi.

Question 20

What does the Golgi Apparatus look like?

Answer 20

A flattened stack of membranes (like pita bread).
More precisely, it is composed of flattened membrane-enclosed sacs (cisternae) and associated vesicles.

Question 21

Is the Golgi Apparatus a polarized organelle?

Answer 21

YES!

Question 22

What are the main roles of the Golgi Apparatus?

Answer 22

Factory in which proteins received from the ER are further processed and sorted for transport to their eventual destination: secretion, endosomes, lysosomes.
Synthesis of glycolipids and sphingomyelin.

Question 23

In plants, the Golgi Apparatus serves as?

Answer 23

Serves as the site in which complex polysaccharides of the cell wall are synthesized.

Question 24

What are the four compartments (four functionally regions) of the Golgi apparatus?

Answer 24

The cis, medial, and trans compartments and the trans-Golgi network.

Question 25

What is the difference in cis and trans face?

Answer 25

Cis face: proteins coming from the ER enter the cis face, modifications occur in medial and trans compartments.
Trans face: sorting distribution center: secretion, endosomes, lysosomes.

Question 26

The mechanism of protein movement through the Golgi is an area of controversy. What are the two models and what are they?

Answer 26

Stable cisternae model: Proteins are carried between cisternae in transport vesicles. Active enzymes in each cisterna (sac).
The cisternal maturation model: proteins are carried within the cisternae, which gradually mature and progressively move through the Golgi in the cis and trans direction.
In both models, transport vesicles also carry Golgi resident proteins back to earlier compartment for re-use.