Lecture 12-Protein Sorting: The Golgi Apparatus

List the steps of the secretory pathway

1. Translates from mRNA in the ribosomes in the cytoplasm.

2. Enters the ER lumen

3. Goes from the ER to the Golgi in a vesicle

4. Transits the Golgi

5. Leaves the Golgi in a vesicle

6. The vesicle fuses the cell membrane

7. It is outside

What is a transport vesicle?

Transport vesicles bud off from one compartment and fuse with another. As they do so, they carry material as cargo.

Explain the difference between the secretory and endocytic pathways.

The secretory pathway leads outward from the endoplasmic reticulum (ER) towards the Golgi apparatus and cell surface, while the endocytic pathway leads inward from the plasma membrane.

Most transport vesicles form from specialized, coated regions of membranes. They bug off as ___, which have a distinctive cage of proteins.

coated vesicles

What is the point of the endocytic pathway?

To replenish vesicles in the ER

What are the four well-characterized types of coated vesicles?

clathrin-coated, COPI-coated, COPII-coated, and retromer-coated

Explain where COPI-coated vesicles would be found.

Golgi→ER

Explain where COPII-coated vesicles would be found.

ER-Golgi

What are coats?

Geometrical structures that assemble into vesicle cages

Explain what guides transport vesicles to their target membrane and how they work.

Rab proteins guide transport vesicles to their target membrane. All transport vesicles display surface markers (proteins) that identify them and target membranes display complementary receptors.

Explain what a SNARE protein is and how it works.

SNARE proteins mediate membrane fusion by putting the membranes in close proximity. V-snare and t-snare bind each-other and buckle, pushing the two membranes together until they fuse.

Proteins leave the ER in COPII-coated transport vesicles that form at the ___.

ER exit site (ERES)

How do ER exit sites work

Cargo proteins have exit signals (no glucose). Receptor recognizes this signal. Exit signal outside of ER membrane attaches to adaptor proteins, which recruit COPII proteins.

Cargo receptors inside vesicles ensure they are ___.

loaded

How are large cargoes accommodated?

COPII transport vesicles can accommodate the large cargoes by assembling tubes instead of vesicles.

Who discovered the Golgi Apparatus?

Camillo Golgi

The Golgi consists of a collection of flattened, membrane-enclosed compartments called ___.

cisternae

The Golgi is ___, proteins will be secreted one-way out of the cell, in one direction.

polarized

Glycosylation and phosphorylation at the Golgi are a destination code. Explain this statement.

Sugar coats serve as destination tags. Some sugar coats are transient. Glycosylation steps are compartmentalized in different cisternae.

Where can proteins go after leaving the Golgi?

Lysosome, plasma membrane, and secretory vesicle.

What are the two models for transport through the Golgi Apparatus?

1. Vesicle transport mechanism

2. Cisternal maturation mechanism

Is there vesicle transport involved in the cisternal maturation mechanism?

Yes, vesicular transport must occur from Golgi→ER

What was the original evidence for the vesicular model?

1. Vesicles bud and fuse

2. COPI vesicles surround the Golgi

What was the original evidence for the maturation model?

1. Proto Collagen is bigger than vesicles

2. PC is not detected inside vesicles

3. It localizes to enlarged Golgi cisternae

What is recent evidence for the maturation model?

Fluorescent live imagining using GFP.

Are cisternae always stacked?

No, in some species, they are not stacked.

What brings ER proteins back from the Golgi?

COPI vesicles

ER proteins have the ___ and ____ receptor initiates vesicle formation.

KDEL sequence, KDEL

A ____ secretory pathway transports most other proteins directly to the cell surface.

nonselective constitutive

No specific signal=___

secrete the protein (default pathway)

___ signals are needed to direct secretory proteins into secretory vesicles and lysosomal proteins into different specialized transport vesicles.

Specific

Which proteins don’t pass through the Golgi?

Cytoplasmic and nuclear proteins