12.3 The Mechanism of Vesicular Transport

Question 1

summarize the process of vesicle transport of proteins to plasma membrane

Answer 1

1. assembly of coat proteins drives the budding of vesicles containing selected cargo proteins from the donor membrane
2. vesicles then travel along cytoskeletal filaments to their targets
3. protein coats are removed at the target membrane, allowing the membranes to fuse
4. vesicles empty their lumenal cargo and insert their membrane proteins into the target membrane

Question 2

how do clathrin-coated vesicles form

Answer 2

1. Arf/GDP is converted to the active Arf/GTP by GEF (guanine nucleotide exchange factor) localized in trans-Golgi network membrane
2. Arf/GTP recruits an adaptor protein that nucleates both cargo selection and coat assembly
3. clathrin assembles, membrane is distorted and bud is initiated
4. budding vesicle is pinched off by dynamin

Question 3

large GTP-binding protein that assembles into helical polymers that constrict (driven by GTP hydrolysis) to promote membrane fission at the necks of budding vesicles

Answer 3

dynamin

Question 4

GTP-binding proteins () recruit adaptors that mediate cargo selection and coat assembly

Answer 4

Arf and Sar GTPases

Question 5

summarize the process of cargo selection

Answer 5

• Arf functions in the formation of COPII- and clathrin-coated vesicles
• Sar functions in the formation of COPI-coated vesicles
• these proteins mediate cargo selection by interacting both with cargo proteins and with vesicle coat proteins

Question 6

what are the 3 vesicle coat protein families

Answer 6

1. clathrin
2. COPI
3. COPII

Question 7

()-coated vesicles are responsible for transport in both direction between the trans-Golgi network, endosomes, lysosomes, and the plasma membrane

Answer 7

clathrin

Question 8

()-coated vesicles are involved in transport of proteins between different compartments of the secretory pathway

Answer 8

COPI (coat protein I)

Question 9

COPI-coated vesicles bud from the (1) or the (2) and carry their cargo backwards

Answer 9

1. ERGIC (ER-Golgi Intermediate Compartment)
2. Golgi

Question 10

COPI-coated vesicles are the only vesicles that can carry their cargo to (…)

Answer 10

earlier compartments of the secretory pathway

Question 11

()-coated vesicles carry proteins from the ER to the ERGIC and onto the Golgi apparatus

Answer 11

COPII (coat protein II)

Question 12

COPII-coated vesicles bud from the () and carry their cargo forward along the secretory pathway

Answer 12

transitional ER

Question 13

vesicle fusion with its target involves 2 types of events:

Answer 13

1. recognition of the correct target membrane by the transport vesicle (docking)
2. fusion of vesicle and target membrane

Question 14

initial interaction between transport vesicles and specific target membranes is targeted by (1) and small GTP-binding proteins (2)

Answer 14

1. tethering factors
2. Rab proteins

Question 15

how do transport vesicles bind to the correct target membrane?

Answer 15

• vesicle Rab/GTP bind membrane-tethering factors, providing the initial bridge between target membrane and transport vesicle
• tethering factors also bind coat proteins, which contributes to their interaction with vesicles

Question 16

how do transport vesicles fuse with target membrane after docking?

Answer 16

• tethering is followed by the formation of SNAREs on the vesicle and target membrane
• the pairing of the vesicle and target SNAREs provides the energy to drive the fusion of the phospholipid bilayers