Chapter 12 (Cell Biology)
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What is the purpose of vesicular transport in cells?
It moves proteins and lipids between compartments in the endomembrane system, ensuring proper delivery to the ER, Golgi, lysosomes, plasma membrane, or for secretion.
What are the two main transport directions in vesicular transport?
Outward (secretory) from ER → Golgi → plasma membrane or lysosome, and inward (endocytic) from plasma membrane → lysosome.
What are the two types of proteins transported by vesicles?
Soluble lumen proteins and membrane proteins.
What ensures each vesicle fuses with the correct target membrane?
Specific marker proteins and receptors on vesicle and target membranes.
What drives vesicle budding?
The assembly of protein coats that shape the membrane into a bud and select the correct cargo.
What are coated vesicles?
Vesicles covered in distinctive protein coats that assist in cargo selection and vesicle formation.
What are the main types of coated vesicles?
Clathrin-coated (endocytosis and Golgi → endosome), COPI-coated (Golgi → ER or within Golgi), and COPII-coated (ER → Golgi).
What are the three main functions of coat proteins?
(1) Shape the membrane into a bud, (2) Concentrate cargo, (3) Capture the right molecules for transport.
What is the structure of clathrin?
A trimer made of three heavy and three light chains, forming a triskelion structure.
How does clathrin assemble into vesicles?
Triskelions form a cage-like lattice that shapes the membrane into a vesicle.
What is an example of clathrin-mediated budding?
Formation of clathrin-coated pits at the plasma membrane that bud off into vesicles.
What role does clathrin play in cargo selection?
Clathrin itself does not select cargo; adaptins and cargo receptors control specificity.
What is the role of adaptins?
Adaptins link clathrin to cargo receptors and select the correct proteins for transport.
What is a cargo receptor?
A transmembrane protein that recognizes and binds specific cargo molecules inside the lumen.
What triggers clathrin assembly?
Adaptin-cargo receptor complexes recruit clathrin triskelions, promoting pit formation.
What is dynamin, and what does it do?
A GTP-binding protein that forms a ring around the neck of budding vesicles and pinches them off via GTP hydrolysis.
What happens after vesicle budding?
The coat disassembles, releasing clathrin back to the cytosol and forming a naked vesicle ready for transport.
What is COPI used for?
Retrograde transport from Golgi back to the ER and between Golgi cisternae.
What is COPII used for?
Transport from the ER to the Golgi apparatus.
What provides the directionality of vesicle movement?
Motor proteins that move along cytoskeletal tracks (mainly microtubules).
What are the main cytoskeletal tracks for vesicle transport?
Microtubules composed of tubulin dimers.
What is the basic building block of a microtubule?
A tubulin heterodimer made of α- and β-tubulin.
How many protofilaments make up a microtubule?
13 protofilaments form a hollow tube.
What gives microtubules polarity?
GTP-bound β-tubulin forms the + end, α-tubulin the – end.