Chapter 5 Review Slides

What is the endomembrane system?

A system of membranes including the ER and Golgi apparatus that work toegher to synthesize, modify, and transport proteins and lipids.

Why is the lumen of a vesicle important?

The lumen of a vesicle is equivalent to the lumen of other membrane-enclosed compartments or the extracellular space, allowing cargo to stay oriented correctly.

What is vesicle transport?

The movement of cargo (like proteins) via transport vesicles that bud from one compartment and fuse with another

Why is vesicular traffic highly organized?

Because vesicles must bud from the correct compartment, fuse with the correct target, and carry only appropriate cargo.

What is the secretory pathway?

Transport from ER → Golgi → Extracellular space

What is the endocytic pathway?

Transport from the plasma membrane inward toward endosomes and lysosomes.

What is glycosylation?

A post-translational modification where sugars are added to proteins.

Where does glycosylation occur?

In both the ER and Golgi, with specific modifications at each location

Why is glycosylation important?

It helps proteins become functional, fold properly, be directed to the correct desination

What are coated vesicles?

Vesicles surrounded by a protein coat on the cytosolic side.

Why is the coat removed before fusion?

Because membranes must directly contact each other to fuse

What are the two main functions of vesicle coats?

Concentrate cargo proteins & Bend the membrane to form vesicles.

What are the three types of vesicle coats?

COPII: ER → Golgi

GOPI: Golgi → ER

Clathrin: Golgi ←> Plasma membrane

What are GTPases?

Enzymes that bind and hydrolyze GTP, acting as molecular switches

When is a GTPase on?

WHen bound to GTP

When is a GTPase Off?

WHen bound to GDP

What does a GEF do?

Exchanges GDP for GTP → Turns GTPase on

What does GAP do?

Stimulated GTP hydrolysis → turns GTPase Off.

Which GTPase regulates COPII vesicles?

Sar1

What activates Sar1?

A Sar1 GEF located in the ER membrane

What happens when Sar1 binds GTP?

It changes shape, It exposes an amphipathic helix, inserts into the ER membrane, recruits COPII coat proteins

How is the COPII coat removed?

SAR1 hydrolyzes GTP to GDP, causing coat disassembly.

Step 1 of vehicle transport?

Membrane deformation using BAR-domain membrane-bending proteins

Step 2 of vehicle transport: HOw does vesicle scission occur?

Dynamin, a GTPase, forms a ring around the vesicle neck

GTP hydrolysis pinches off the vesicle

How do vesicles move?

Along microtubules

Using motor proteins

What determines vesicle targeting specificity?

Rab GTPases

What happens to inactive Rab?

Bound to GDP, held in the cytosol by GDI

How does Rab become active?

GEF echanges GDP for GTP

Rab associated with target membrane

What proteins drive vesicle fusion?

SNARE proteins

Difference between t-SNARE and v-SNARE?

t-SNARE: target membrane (3 proteins)

v-SNARE: vesicle membrane (1 protein)

why is SNARE fusion energetically demanding?

Because membranes must come within 1.5nm to allow lipid mixing

What vesicles transport proteins ER → Golgi Apparatus

COPII

What is homotypic fusion?

Fusion between vesicles from the same origin, still requiring matching SNAREs

What are vesicular tubular clusters?

Fused COPII vesicles that travel to the Golgi.

What coat is used for Golgi → ER transport?

COPI

What kind of signal directs soluble proteins back to the ER?

KDEL sequence

How are KDEL proteins retrieved?

KDEL receptor binds KDEL

Packaged into COPI vesicles

What do lysosomes do?

Digest macromolecules using acid hydrolases

How are lysosomal enzymes targeted?

Via mannose-6-phosphate (M6P) tags

Where is M6P added?

In the cis golgi network

What recognizes M6P?

M6P receptors in the trans golgi network

Why does M6P release enzymes in endosomes?

Endosome pH = 6

M6P receptor releases cargo at lower pH

How are M6P receptors recycled?

Via retromer-coated vesicles back to the TGN