Chapter 13: Vesicle Traffic

What are the 5 steps of vesicle transport?

Budding
Coat assembly
Release
Targeting
Fusion

What does clathrin do?

Shapes vesicles for endocytosis and TGN transport

What do adaptins do?

Link clathrin to cargo receptors

What does dynamin do?

Pinches off vesicle neck

What does COPII do?

ER → Golgi transport

What does COPI do?

Golgi → ER retrieval

What GTPase initiates COPII budding?

Sar1-GTP

What is the role of Rab proteins?

Targeting/tethering specificity

What is the role of SNAREs?

Membrane fusion

Difference between v-SNARE and t-SNARE

v = vesicle, t = target membrane

What does NSF do?

Uses ATP to separate SNAREs after fusion

What happens if Rab is wrong?

Vesicle fails to reach correct target

What happens if SNARE pairing is wrong?

Fusion fails

What is KDEL?

Retrieval signal for soluble ER proteins

What is KKXX?

Retrieval signal for ER membrane proteins

What happens if KDEL is missing?

ER protein is secreted

What is the function of the Golgi?

Modify, sort, and ship proteins

What is M6P?

Mannose-6-phosphate tag for lysosomal enzymes

What does the M6P receptor do?

Binds and sorts lysosomal enzymes

What causes M6P cargo to dissociate from receptor?

Acidic pH in endosome

What happens if M6P tag is missing?

Lysosomal enzymes are secreted

What are the 3 TGN pathways?

Default, regulated secretion, lysosomal

What is receptor-mediated endocytosis?

Selective uptake via clathrin-coated pits

What is LDL uptake an example of?

Receptor-mediated endocytosis

Why are lysosomes acidic?

V-type ATPase pumps H+

What happens if lysosome pH increases?

Hydrolases function poorly

What two things must always be correct in vesicle traffic?

Correct cargo packaging and correct target membrane fusion

What is the role of coat recruitment GTPases?

Promote coat assembly at the correct membrane

What determines where Sar1 promotes budding?

The location of the Sar1 GEF

What do adaptor proteins ensure during clathrin budding?

Budding occurs when cargo is present

What are vesicular tubular clusters?

Fused ER-derived transport intermediates that help form the cis-Golgi side

What two major functions do Golgi-to-ER retrieval pathways serve?

Return ER resident proteins and maintain membrane balance

What else is returned to the ER by retrieval pathways besides resident proteins?

Receptors and v-SNAREs

What is the orientation of the Golgi stack?

Cis-Golgi faces ER; trans-Golgi is farther away

What are the three sorting routes from the TGN?

Default pathway, regulated secretory pathway, lysosomal pathway

What is constitutive secretion?

Continuous vesicle traffic from Golgi to plasma membrane

What is regulated secretion?

Vesicles are stored until a signal triggers release

Classic example of regulated secretion from the notes

Neurotransmitter vesicles

How is M6P added to lysosomal proteins?

Enzyme recognizes a signal patch, adds GlcNAc-phosphate to mannose, then GlcNAc is removed leaving phosphate

What is the difference between M6P and the M6P receptor?

M6P is the lysosomal targeting tag; M6P receptor is the sorting receptor

What causes cargo release from the M6P receptor in the endosomal pathway?

Acidic pH / phosphate removal prevents rebinding

What is pinocytosis?

Uptake of extracellular fluid and membrane recycling

What is phagocytosis?

Uptake of large particles

What pathways deliver material to lysosomes?

Endocytosis, phagocytosis, and autophagy

Why do lysosomal hydrolases work best in lysosomes?

They have a low pH optimum

What happens if V-type ATPase is lost?

Lysosome becomes less acidic and hydrolases work poorly