The Golgi Complex and the Endomembrane System

Question 1

the golgi complex is closely related to the ER how?

Answer 1

in proximity and in function

Question 2

what does the golgi complex consist of?

Answer 2

consists of a stack of flattened vesicles know as cisternae

Question 3

functions of the golgi complex?

Answer 3

-plays an important role in processing and packaging secretory proteins, and in complex polysaccharide synthesis

Question 4

what does the golgi accept?

Answer 4

vesicles that bud off the ER

Question 5

• what happens to the contents of the vesicles from the ER after reaching the Golgi?
• what happens after that?

Answer 5

• they are modified and processed in the golgi complex

- the processed substances are then moved to other locations in the cell through vesicles that bud off the golgi complex

Question 6

How is the golgi complex functionally and physically linked to the ER?

Answer 6

• glycoproteins and membrane lipids from the ER undergo further processing and are sorted and packaged for transport at the golgi
• golgi complex plays a central role in membrane and protein trafficking in euk cells.

Question 7

The Golgi Commplex Consists of a Series of Membrane Bound Cisternae:
what is a golgi stack?
golgi stacks and cells?

Answer 7

• a golgi stack is a series of cisterna, usually 3-8

- some cells have one large stack, and other, especially secretory cells, have hundreds or thousands of stacks

Question 8

Golgi complex and transport vesicles

• What do these transport vesicles surround? what do they do?
• the golgi complex lumen is part of the..?

Answer 8

• Both ER and the Golgi complex are surrounded by numerous transport vesicles that carry lipids and proteins from the ER to the Golgi complex and then to various destinations in the cell
• the golgi complex lumen (incisternal space) is part of the endomembrane system

Question 9

Each golgi stack has two distinct..? called?

Answer 9

sides, or faces , called the cis and the trans faces

Question 10

The cis face:
oriented towards?
golgi compartment on this side is called the?

Answer 10

• oriented towards the ER

- golgi compartment on this side is called the cis-golgi network (CGN)

Question 11

The trans face:
which side?
golgi compartment called?

Answer 11

opposite side of the cis face

golgi compartment call the trans-golgi network (TGN)

Question 12

The Golgi Network:

• proteins and lipids leave the golgi in?
• between the TGN and CGN are? what occurs here?
• what does each compartment contain?

Answer 12

• leave the golgi in transport vesicles that continuously bud off the tips of the TGN
• between TGN and CGN are medial cisternae, much of the processsing of proteins occurs here
• each compartment shows biochemical polarity, containing specific proteins unique to each portion of the network

Question 13

Two Models Depict the Flow of Lipids and Proteins Through the Golgi Complex:
-similarities of models?
-what suggests that these models are not mutually exclusive?
what are the two models called?

Answer 13

• both models involve the formation of transport vesicles containing sorted cargo targeted for various destinations
• experimental evidence (tracking the movement of substances between the compartments and time-lapse fluorescent microscopy)
• the stationary cisternae model and the cisternal maturation model

Question 14

Two Models Depict the Flow of Lipids and Proteins Through the Golgi Complex: The stationary cisternae model:

• what does is say about the cisternae?
• what is transport of materials done by?
• how does this happen?

Answer 14

• each cisternae in the golgi stack is a stable structure
• transport of materials from one cisternae to another is mediated by shuttle vesicles
• these bud off from one cisternae and fuse with the next cisternae in a cis- to trans- sequence

Question 15

Two Models Depict the Flow of Lipids and Proteins Through the Golgi Complex: The cisternal maturation model:

• what does is say about cisternae?
• what happens to them?
• what happens to enzymes not needed in later compartments?

Answer 15

• golgi cisternae are transient compartments
• these gradually change from CGN through medial cisternae to TGN
• enzymes not needed in later compartments are returned to earlier compartments in vesicles

Question 16

Anterograde and Retrograde Transport:

• what is anterograde transport?
• how does it work?
• this flow of lipids towards the plasma membrane must be?

Answer 16

• movement of material from the ER through the Golgi complex toward the plasma membrane
• every time a secretory granule fuses with the plasma membrane and discharges its contents by exocytosis, a bit of the membrane from the ER becomes part of the plasma membrane
• this flow of lipids towards the plasma membrane must be balanced (ie retrograde transport)

Question 17

Anterograde and Retrograde Transport:

• what is retrograde transport?
• what does it allow?
• what does it ensure?

Answer 17

• the flow of vesicles from the golgi cisternae back to the ER
• cell recycles lipids and proteins no longer needed during the late stages of anterograde, which balances the flow of lipids towards the plasma membrane and ensures a supply of materials for forming new vesicles

Question 18

Roles of the ER and Golgi in Protein Glycosylation:

• how is glycosylation related to the ER and Golgi?
• what is glycosylation
• what happens after glycosylation?

Answer 18

• protein processing carried out in the ER and golgi involves glycosylation
• the addtion of carbohydrate side chains to proteins to produce glycoproteins
• subsequent enzyme-catalyzed reactions then modify the oligosaccharide side chains that was attached to the protein

Question 19

Roles of the ER and Golgi in Protein Glycosylation:

- what are the two general kinrds of glycosylation?

Answer 19

• N-linked glycosylation

- O-linked glycosylation

Question 20

Roles of the ER and Golgi in Protein Glycosylation:
N-linked glycosylation:
-what is it?

Answer 20

-the addition of a specific oligosaccharide unit to the nitrogen atom on the terminal amino group of certain asparagine residues

Question 21

Roles of the ER and Golgi in Protein Glycosylation:
O-linked glycosylation:
-what is it?

Answer 21

-the addition of an oligosaccharide to the oxygen atom on the hydroxyl group of certain serine or threanine residues

Question 22

Roles of the ER and Golgi in Protein Glycosylation:
The initial steps of N-glycosylation
-where do the initial steps occur?
-where do later steps take palce?
-what do carbohydrate side chain initially have?

Answer 22

-first steps occur on the cytosolic surface of the ER membrane
-later steps take place in the ER lumen
-initially have a common core oligosaccharide:
>two units of N-acetylglycosamine
>nine mannose units
>three glucose units

Question 23

Roles of the ER and Golgi in Protein Glycosylation:
Initial glycosylation occurs in the ER
-what are the steps? (6)

Answer 23

1. glycosylation begins as dolichol phosphate, an oligosaccharide carrier, is inserted into the ER membrane
2. GLcNAc and mannose groups are then added to the phosphate group
3. the growing core oligosaccharide is translocated to the ER lumen by a flippase
4. once inside the lumen, more mannose and glucose are added
5. the completed core oligosaccharide is transferred from dolichol to the asparagine residue of the recipient protein
6. the core oligosachharide is attached to the protein and is trimmed and modified.

Question 24

Roles of the ER and Golgi in Protein Glycosylation:
Initial glycosylation occurs in the ER:
-what happens in cotranslational glycosylation?

Answer 24

-usually the oligosaccharide is added to the recipient protein as the polypeptide is being synthesized; promotes proper protein folding

Question 25

Roles of the ER and Golgi in Protein Glycosylation:
Initial glycosylation occurs in the ER:
Further glycosylation occurs in the Golgi complex:
-when does further processing of N-glycosylated proteins occur?
-what are terminal glycosylations and what do they do?

Answer 25

• further processing of N-glycosylated proteins occurs as the glycoproteins move from the cis face through the trans face
• terminal glycosylations are variable and create diversity. which involves the removal of a few units of the core oligosaccharide and sometimes nothing more

Question 26

The Golgi Complex: Overview of trafficking

• sorting of proteins begins where?
• there are mechanisms to do what..?
• what occurs in the TGN?

Answer 26

• begins in the ER and early compartments of the Golgi
• there are mechanisms to retrieve or retain compartment specific proteins
• final sorting of material that will leave the Golgi complex occurs in the TGN

Question 27

The Golgi Complex: Roles of the ER and Golgi in protein trafficking

• what happens to proteins synthesized in the rough ER?
• what does each protein contain? what does this do?
• what can these “tags” also be used for?

Answer 27

• they must be directed to a variety of locations and kept there
• each protein contains a specific “tag”, targeting it to a specific vesicle that will take it to the correct location
• tags can also be used to exclude material from certain vesicles

Question 28

The Golgi Complex: Roles of the ER and Golgi in protein trafficking:
Protein tags
-depending on the protein and destination, a tag may be an..? (3)

Answer 28

• amino acid sequence, a hydrophobic domain
• oligosaccharide side chain
• some other feature

Question 29

The Golgi Complex: Roles of the ER and Golgi in protein trafficking:
Lipid tags
-are membrane lipids tagged?

Answer 29

• membrane lipids may be tagged to help vesicles reach their destinations
• one or more phosphate groups attached to positions 3,4, and/or 5 of a membrane phosphatidyl inositol

Question 30

The Golgi Complex: Roles of the ER and Golgi in protein trafficking:
ER Specific Proteins Contain Retention and Retrieval Tags
-how is protein composition in the ER maintained?

Answer 30

-protein composition in the ER is maintained by preventing some proteins from escaping the ER and by retrieving others from the Golgi

Question 31

The Golgi Complex: Roles of the ER and Golgi in protein trafficking:
ER Specific Proteins Contain Retention and Retrieval Tags
-some proteins localized to the ER contain the sequence RXR (arg-x-arg)
-what is this sequence?
-example of something that has the RXR tag?
-why do some proteins contain this tag?
-what must happen to allow the assembled complex to leave the ER?

Answer 31

• this is a retention tag, also found in some proteins destined for the plasma membrane
• the N-methyl-D-aspartate (NMDA) receptor, important in mammalian neurotransmission, has the RXR tag
• it is thought that the individual subunits of the receptor must be retained in the ER until assembly is complete
• the RXR tag must be masked

Question 32

The Golgi Complex: Roles of the ER and Golgi in protein trafficking:
ER Specific Proteins Contain Retention and Retrieval Tags
-some proteins returned from the Golgi to the ER contain retrieval tags
-what are those retrieval tags?
-what happens when a protein with this tag binds to a receptor?
-evidence about the importance of retrieval tags comes from?

Answer 32

• these tags are short C-terminal sequences such as KDEL, or KKXX in mammals and HDEL in yeast
• when a protein with this tag binds to a receptor, the receptor-ligand complex is packaged into a transport vesicle for return to the ER
• experiments involving chimeric proteins (fusion proteins)