12.2 The Golgi Apparatus

Question 1

there is a distinct () in both the structure and function of the Golgi apparatus

Answer 1

polarity

Question 2

what are the discrete compartments of the Golgi?

Answer 2

1. cis compartment
2. medial compartment
3. trans compartment
4. trans-Golgi network

Question 3

what occurs in the cis compartment

Answer 3

• proteins from the ERGIC enter the Golgi
• site of protein, lipid, and polysaccharide modification

Question 4

what happens in the medial compartment

Answer 4

modification of molecules

Question 5

what happens in the trans compartment

Answer 5

further modification of molecules

Question 6

what happens in the trans-Golgi network

Answer 6

• sorting and distribution of proteins
• direction of molecular traffic to endosomes, lysosomes, the plasma membrane, or the cell exterior

Question 7

protein processing within the Golgi involves extensive modification of the (1) of (2)

Answer 7

1. carbohydrate portions
2. glycoproteins

Question 8

what are the kinds of protein glycosylation that occur in the Golgi?

Answer 8

1. N-linked glycosylation
   1.1. N-linked glycosylation of lysosomal proteins
2. O-linked glycosylation

Question 9

give an overview of the processing of N-linked oligosaccharides in the Golgi

Answer 9

1. in the cis compartment: mannose residues are removed from N-linked oligosaccharides of glycoproteins (from ER); N-acetylglucosamine is added
2. in medial compartment: Fucose, N-acetlyglucosamine, and galactose are added
3. in trans compartment and trans-Golgi network: sialic acid is added

Question 10

glycoproteins destined for the lysosomal membrane are modified by (1) that occurs in the (2) of the Golgi

Answer 10

1. mannose phosphorylation
2. cis compartment

Question 11

give an overview of the N-linked glycosylation of lysosomal proteins

Answer 11

1. N-acetylglucosamine phosphates are transferred from UDP-N-acetylglucosamine to mannose residues (in the #6 position)
2. N-acetylglucosamine group is removed
3. the resulting mannose-6-phosphate residues remain on the N-linked oligosaccharide
   - the phosphorylated mannose residues are specifically recognized by a mannose-6-phosphate receptor in the trans-Golgi network (directs the transport of these proteins to endosomes and on to lysosomes)

Question 12

the addition of carbohydrates to the side chains of acceptor serine and threonine residues

Answer 12

O-linked glycosylation

Question 13

give an example of a product of O-linked glycosylation

Answer 13

proteoglycans

Question 14

brief summary of what happens in O-linked glycosylation

Answer 14

modifications take place in the Golgi apparatus by the sequential addition of single sugar residues

Question 15

the Golgi is involved in the synthesis of (1) and (2) (from (3) synthesized in ER)

Answer 15

1. glycolipids
2. sphingomyelin
3. ceramide

Question 16

summarize the synthesis of sphingomyelin in the Golgi

Answer 16

in the lumenal surface of the Golgi, a phosphorylcholine group is transferred from phosphatidylcholine to ceramide

Question 17

give an overview of glycolipid synthesis in the Golgi

Answer 17

1. glucose is added to cermide on the cytosolic side
2. glucosylceramide is flipped to the lumenal side and additional carbohydrates are added

Question 18

note that a variety of glycolipids can be synthesized by the (…)

Answer 18

addition of one or more sugar residues to ceramide

Question 19

following vesicular transport and fusion to the plasma membrane, the () of the lipids remain exposed on the cell surface

Answer 19

polar head groups

Question 20

() of glycolipids are important surface markers in cell to cell recognition

Answer 20

oligosaccharide portions

Question 21

most proteins, lipids, and polysaccharides are transported from the Golgi to their final destinations through the ()

Answer 21

secretory pathway

Question 22

how are proteins carried to the plasma membrane?

Answer 22

either carried directly to plasma membrane by vesicles or via endosomes (that act as intermediate compartments)

Question 23

what happens to proteins that function within the Golgi?

Answer 23

• these proteins are usually usually transmembrane proteins (function in glycosylation) and are retained within the Golgi by signals within their membrane-spanning and cytoplasmic domains
• signals in some Golgi proteins mediate their retrieval from compartments along the secretory pathway and transport from endosomes back to the trans-Golgi network

Question 24

proteins can also be transported to the plasma membrane via (), which also act to recycle endocytosed proteins to the plasma membrane

Answer 24

recycling endosomes

Question 25

how are regulated proteins secreted in response to environmental signals?

Answer 25

1. proteins are sorted into the regulatory secretory pathway (by trans-Golgi network) and are packaged into immature secretory granules
2. aggregation of proteins into secretory granules is driven by the proteins’ structural characteristics (selective aggregation in the slightly acidic environment of the trans-Golgi network)
3. the proteins are stored in matured secretory granules until they are to be secreted in response to an environmental signal

Question 26

explain how proteins are targeted to plasma membrane domains of polarized cells (e.g. epithelial)

Answer 26

• proteins leaving the trans-Golgi network of these cells must be selectively packaged into vesicles specifically targeted to either the apical or basolateral domain
• selective packaging occurs in either tGn or recycling endosomes
• proteins are targeted by short animo acid sequences

Question 27

() between the plasma membrane domains in epithelial cells prevent the diffusion of proteins between the domains

Answer 27

tight junctions

Question 28

proteins are targeted to lysosomes by ()

Answer 28

mannose-6-phosphate residues