Ch 15 Textbook Material (vesicular transport)

What is usually the first step on the pathway to another cellular destination?

the ER

What organelle is typically the next stop after the ER?

the golgi apparatus

What happens to proteins and lipids in the Golgi apparatus?

they are modified and sorted for shipment to other destinations

How is transport between the ER and Golgi apparatus carried out?

by continual budding and fusion of transport vesicles

What two main directions does vesicular transport move in?

• outward (exocytosis)

• inward (endocytosis)

What does the outward, or secretory, pathway do?

moves material from the ER → Golgi apparatus → plasma membrane → outside the cell

What side branch leaves the Golgi in the outward pathway?

a route through endosomes to lysosomes

What does the inward, or endocytic, pathway do?

brings extracellular molecules from the plasma membrane → endosomes → lysosomes for degradation

Why must vesicles fuse only with the correct target membrane?

to ensure cargo delivery is accurate and each organelle maintains its distinct identity

What must each transport vesicle carry to function correctly?

only the proteins appropriate to its destination

What ensures proper vesicle recognition and targeting?

proteins displayed on the surface of transport vesicles

What defines different types of transport vesicles?

each type carries a distinct set of cargo molecules and surface proteins

What are vesicles with a distinctive protein coat on their cytosolic surface called?

coated vesicles

What happens to the coat after the vesicle buds from its parent organelle?

the vesicle sheds its coat, allowing membrane fusion with the target organelle

What are the two main functions of a vesicle coat?

• shapes the membrane into a bud

• captures molecules for transport

From where do clathrin-coated vesicles bud?

from the Golgi apparatus (outward pathway) and the plasma membrane (inward endocytic pathway)

What are clathrin-coated pits?

sites on the plasma membrane where clathrin molecules assemble and begin shaping the vesicle

How do clathrin molecules shape the vesicle?

by assembling into a basketlike network on the cytosolic side of the membrane

What protein helps pinch off the vesicle from its parent membrane?

dynamin, a GTP-binding protein

What does dynamin do during vesicle formation?

forms a ring around the neck of the budding vesicle and constricts it to complete vesicle release

Which proteins are responsible for selecting cargo molecules?

adaptins

What are the two functions of adaptins?

• ancor the clathrin coat to the vesicle membrane

• help select and capture cargo molecules

How do adaptins help capture cargo?

by trapping cargo receptors that are bound to specific cargo molecules

What do cargo molecules carry that ensures proper selection?

specific transport signals recognized by cargo receptors

What is the function of COP-coated vesicles?

transport molecules between the ER and Golgi apparatus and within the Golgi itself

After a transport vesicle buds from a membrane, how does it reach its correct destination?

it is often actively transported by motor proteins along cytoskeletal fibers

What ensures the specificity of vesicular transport?

molecular markers on the vesicle surface that identify its origin and cargo

What family of proteins provides the main identification system for vesicles?

Rab proteins, a diverse family of monomeric GTPases

What are the two types of SNAREs?

• v-SNAREs on vesicles

• t-SNAREs on target membranes

What happens when tethering proteins capture a vesicle?

the v-SNARE interacts with the t-SNARE, firmly docking the vesicle

Besides docking, what other role do SNAREs play?

they catalyze membrane fusion between the vesicle and target membrane

What happens during vesicle fusion?

the vesicles contents are delivered into the target organelle (or outside the cell), and the vesicle membrane joins the target membrane

When might vesicle fusion require a special stimulatory signal?

in processes ike synaptic vesicle fusion, which releases neurotransmitters

How do SNARE proteins catalyze membrane fusion?

v-SNAREs and t-SNAREs wrap tightly around each other, pulling the two membranes close together like a molecular winch

What is the overall role of SNAREs in vesicular transport?

they ensure specific docking and catalyze the fusion of vesicles with the correct target membrane