Lecture 3 - Exam 4: The Endoplasmic Reticulum & Golgi - Protein Sorting - Exam 4

Question 1

Is the Golgi apparatus a nonpolar organelle?

Answer 1

NO! It is a polarized organelle

Question 2

What are the main roles of the Golgi apparatus?

Answer 2

• It is a 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 3

What is the structure of the Golgi appartus?

Answer 3

It is composed of flattened membrane enclosed sacs (cisternae) and associated vesicles.

Question 4

What does the Golgi apparatus serve as in the plants?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

Describe the cis face. Describe the medical and trans compartments.

Answer 6

Cis: Proteins coming from the ER enter the cis face.
Medial & Trans: Modifications occur in the medial and trans compartments
Trans: Sorting distribution center: secretion, endosomes, lysosomes.

Question 7

Discuss the transport through the golgi.

Answer 7

This is an area of controversy. There are two different models:
The 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 to trans direction.
- In both models transport vesicles also carry Golgi resident proteins back to earlier compartment for re-use.

Question 8

Who is Camilo Golgi? What did he do?

Answer 8

Golgi performed his “black reaction” using a salt solution which he stained the nerve cells of the cerebellum of an owl. The first visualization of neurons ever!!!
-He noticed thread-like networks inside the cells and named them apparato reticolar interno (internal reticular apparatus)

Question 9

Describe Protein Processing: N-Linked Oligosaccharides in the Golgi. Generic example.

Answer 9

There is orderly progression as the protein moves through the Golgi.
The Glycoproteins have 11 sugar residues as they leave the ER.
-To start with, GlcNac (2) & mannose (9) in the glycoprotein in the cis Golgi.
- In the first processing step, mannose residues are removed (6), GlcNAc is added (1) in the medial Golgi
- Then fucose (1), Galactose (3), GlcNAc (1) are added in the trans and trans-Golgi network
- Sialic acid is added (3) in the trans and trans-Golgi network

Question 10

Discuss a specific example of Protein Processing in the Golgi for proteins directed to the Lysosome.

Answer 10

First step: GlcNAc phosphate is transferred to the mannose residues from UDP-GlcNAc. This phosphorylation is key for the receptor.
-Reaction catalyzed by glycosyltransferases (enzymes).
Second step: the GlcNAc group is removed, leaving a mannose-6-phosphate. After this addition, there is no further modifications.
Mannose-6-phosphate recognized by mannose-6-phosphate receptor in the trans-Golgi network and then shuttled to the lysosome.

Question 11

What is O-linked glycosylation?

Answer 11

Carbohydrates added to side chains of serine and threonine. Done sequentially by addition of single sugar residues.

Question 12

What is an example of a glycosaminoglycan?

Answer 12

The GAG chain

Question 13

What does the processing of proteoglycans involve?

Answer 13

Involves addition of 100 or more carbohydrate chains to a polypeptide that is further modified by addition of sulfate groups.

Question 14

What are proteoglycans?

Answer 14

Secreted proteins necessary for the ECM.

Question 15

Discuss sphingomyelin.

Answer 15

It is found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It has a role as an insulator of nerve fibers.
The degradation of sphingomyelin can produce ceramide which is involved in the apoptotic signaling pathway.
Sphingomyelin hydrolysis and ceramide production signals apoptosis (early in the process).
Structurally, it differs from other lipids because it has serine.

Question 16

Describe glycolipids.

Answer 16

Lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition.
They are found on the surface of many eukaryotic cell membranes, where they extend from the phospholipid bilayer into the extracellular environment (blood types). Can extend from a red blood cell.

Question 17

What are the four main human blood types and how are they determined?

Answer 17

A, B, AB, O. They are determined by the oligosaccharide attached to the specific glycolipid on the surface of RBCs, which acts as an antigen.

Question 18

Where are glycerol phospholipids, cholesterol, and ceramides (a type of waxy lipid) synthesized at>

Answer 18

In the ER!

Question 19

Where are glycolipids and sphingomyelin synthesized, and what are they synthesized from?

Answer 19

They are synthesized from ceramide in the Golgi.

Question 20

How is sphingomyelin synthesized?

Answer 20

By transfer of a phosphorylcholine to ceramide (which are both imported from the ER).

Question 21

How are glycolipids synthesized?
Where do glycolipids end up?

Answer 21

By the addition of carbohydrates to ceramide (hydroxyl group).
End up on the exterior half of the plasma membrane.

Question 22

The glycolipids end up on the exterior half of the plasma membrane, so where are they synthesized? The inside or outside of the Golgi membrane?

Answer 22

The first glycogen added to it happens on the outside, and then it is flipped to the inside of the Golgi membrane.

Question 23

What happens in the trans-Golgi network?

Answer 23

Molecules are sorted and packaged into transport vesicles.

Question 24

Export from the Golgi Complex:
Transport from the Golgi to the cell surface can occur by three routes. What are they?

Answer 24

• Direct transport to the plasma membrane.
• Recycling endosomes : use of endosomes as an intermediate compartment.
• Regulated secretory pathways: granules store their contents until signals direct their fusion with the plasma membrane (e.g. hormones, digestive enzymes, etc.)
  (there’s also a lysosome pathway but will be discussed later)

Question 25

What is the path of the secretory pathway (for vesicular transport)?

Answer 25

Rough ER –> Golgi –> secretory vesicles –> Cell exterior

Question 26

What are KDEL receptors?

Answer 26

Initiate the mechanism by which proteins are transported from the Golgi to the ER. KDEL is an amino acid sequence.

Question 27

Describe the fusion of a transport vesicle with its target.

Answer 27

• The vesicle must recognize the correct target molecule
• Vesicle and target membrane must fuse, delivering the contents to the target organelle or outside of the cell.

Question 28

The plasma membranes of polarized epithelial cells are divided into…?

Answer 28

Apical and basolateral domains

Question 29

The plasma membranes of polarized epithelial cells are divided into apical and basolateral domains. Describe the transport to the plasma membrane of polarized cells.

Answer 29

Specific proteins are targeted to the apical or basolateral membranes either in the the trans-Golgi network or in a recycling endosome.
Tight junctions between neighboring cells maintain the identity of the apical and basolateral membranes by preventing diffusion of proteins between these domains.

Question 30

The Mechanism of Vesicular Transport:
Transport vesicles from the ER/Golgi are coated with…? What are these vesicles called?

Answer 30

Are coated with cytosolic coat proteins and these vesicles are called coated vesicles.

Question 31

The Mechanism of Vesicular Transport:
Transport vesicles from the ER/Golgi are coated with cytosolic coat proteins and these vesicles are called coated vesicles.
What does the assembly of the coats do?

Answer 31

The assembly of the coats drives vesicle budding.

Question 32

The Mechanism of Vesicular Transport:
Transport vesicles from the ER/Golgi are coated with cytosolic coat proteins and these vesicles are called coated vesicles. After the coat is assembled, what happens?

Answer 32

The vesicles travel along cytoskeletal filaments, with the help of motor proteins.

Question 33

The Mechanism of Vesicular Transport:
Transport vesicles from the ER/Golgi are coated with cytosolic coat proteins and these vesicles are called coated vesicles. After the coat is assembled, the vesicles travel along cytoskeletal filaments, with the help of motor proteins. What happens next?

Answer 33

Coats are removed in the cytosol as the vesicles start fusing.
The vesicles fuse with the target membrane, empty their cargo, and insert their membrane proteins AND LIPIDS into the target membrane.

Question 34

Transport by Coated Vesicles:
What are the three families of vesicle coat proteins?

Answer 34

COPII-coated vesicles:
Carry proteins from the ER to the ERGIC and on the Golgi apparatus.
COPI-coated vesicles:
Bud from the ERGIC or Golgi and carry their cargo back, returning proteins to earlier compartments.
(COP = coated protein)
Clathrin-coated vesicles:
Transport in both directions between the trans Golgi network, endosomes, lysosomes, and plasma membrane.

Question 35

What are the three main functions of the Golgi complex?

Answer 35

Protein glycosylation, synthesis of lipids and glycolipids, and protein sorting and export.