Secretion and Endocytosis-Cell Physiology UA

What membrane does budding refer to?

Donor membrane

What membrane does fusion refer to?

Target membrane

What are V-SNARES?

Proteins that mediate docking, priming and fusion with target membranes

What are t-SNARES?

Receptor on target membrane for incoming transport vesicles

What is the role of v-SNARES in membrane trafficking?

Vesicles uncoats in cytosol to bind with target membrane

What is the role of t-SNARES in membrane trafficking?

Senses incoming fusion

What is required  before SNARE pairs form?

Assembly by GDP to GTP change, conformational change by disassembly

How do GTPases assist in the assembly and disassembly of vesicle coats? Which one uses energy?

Step 1: Sar1-GDP binds to ER integral membrane

protein Sec12

• Sec12 has Guanine Exchange Factor (GEF)

activity, exchanges GDP for GTP

• Sar1-GTP integrates into the ER membrane

cytosolic face

• Step 2: Sar1-GTP recruits the COPII coat protein

complex (Sec23/Sec24, v-SNARES, others), which

curves and pinches off ER membrane region as

COPII-coated vesicle

• Steps 3 and 4: Sar1 hydrolyzes GTP, coat proteins

and Sar1-GDP disassemble, leaving uncoated

cargo vesicle and v-SNARES for target membrane

fusion

• disassembly uses energy

COPII coated vesicles pathways

Step 1: COPII coat assembles on ER

membrane to form vesicles containing:

• v-SNARE

• Membrane proteins with cytoplasmic

signal sequences

• Soluble cargo proteins bound to

membrane receptors

• Step 2-3: COPII pinches off coated

vesicle, disassembly exposes v-SNARE for

fusion targeting at cis-Golgi membrane t-

SNARE

Why is it important to have the sorting signal in the cytosol when considering coat protein formation?

Coat proteins cannot recognize the signal if it is internal

How and what type of cargo is delivered by COPII vesicles?

ER to cis-golgi, secreted and membrane proteins, golgi enzymes

How and what type of cargo is delivered by COPI vesicles?

Retrieves escaped ER proteins, recycles, moves Britain’s backwards, golgi to ER

What types of proteins have a KDEL sorting sequence, and when is this sequence utilized?

Chaperones and enzymes, return escaped ER proteins via COPI

The secretory pathway begins with rough ER proteins being trafficked to the cis-Golgi via COPII vesicles. From there, proteins travel to the trans-Golgi network where they can meet one of 5 fates. What are they?

Regulatory secretory vesicles, constitutive secretory vesicles, endosomes, lysosomes, plasma membrane/cell surface

how do clathrin-coated vesicles differ from COPII/COPI vesicles in their formation and function?

Clathrin uses adaptor proteins while COPII/COPI bind directly

What are the steps of endocytic entry of LDL?

Step 1: Cell-surface LDL Receptor binds LDL, LDL Receptor (LDLR) has an NPXY Sorting, Signal at the cytosolic tail, interacts with AP2 complex

Step 2: Clathrin-coated pit forms, containing LDL-LDLR complexes, Pinched off as clathrin-coated vesicle by dynamin-mediated mechanism (requires GTP hydrolysis!)

Step 3: Vesicle fuses with endosome, acidic pH releases LDL particle from receptor

Step 4: Late endosome fuses with lysosome, LDL particle broken downvby lysosomalbenzymes

Step 5: LDLR is recycled to the cell surface