Lecture 17: Vesicle Formation and Transport in Cells

Vesicle

A small membrane-bound sphere important for transporting materials in the cell.

Endomembrane system

Once something enters the endomembrane system, it stays within the endomembrane system forever.

Types of vesicles

There are different types with specific coats and functions.

Coated vesicles

The 3 main types are COPII, COPI, and Clathrin-coated vesicles.

COPII-coated vesicles

Move material from the ER forward to the Golgi.

COPI-coated vesicles

Move material backward from the Golgi to the ER (retrograde transport).

Clathrin-coated vesicles

Post-Golgi; move materials to plasma membrane, endosomes, lysosomes, or vacuoles.

COPII vesicle formation steps

The 3 main steps are: 1) Denote vesicle location, 2) Bind the cargo, 3) Bud from membrane.

SAR1 activation

SAR1-GEF exchanges GDP for GTP on SAR1.

SAR1 after GTP binding

It embeds into the ER membrane, initiating membrane bending.

Proteins binding to SAR1

Sec23 and Sec24 bind to SAR1 to begin cargo loading.

Sec24 function

Sec24 binds the transmembrane cargo receptor.

COPII vesicle release

COPII proteins form a scaffold; SAR1 hydrolyzes GTP and coat falls off.

COPI vesicle transport

Performs retrograde transport (Golgi -> ER).

KDEL tag

Signals proteins to return to the ER via COPI vesicles.

KDEL tag mechanism

Recognized by a receptor involved in COPI vesicle formation.

Protein modification in Golgi

Glycosylation - sugars are added to proteins.

Protein destinations after Golgi

From cis-Golgi to trans-Golgi, then final destinations.

Clathrin structure

It forms a triskelion (three-legged structure).

Clathrin vesicle formation initiation

ARF-GDP is converted to ARF-GTP by ARF-GEF.

AP2 function

Binds ARF-GTP and links cargo receptors to clathrin.

Clathrin coat assembly

Clathrin binds to AP2, forming the coat and loading cargo.

Vesicle targeting steps

The 3 key steps are: 1) Targeting, 2) Tethering, 3) Docking.

Vesicle travel in cell

Along microtubules using motor proteins.

Tethering proteins

RAB proteins (small GTPases) mediate tethering.

RAB protein mechanism

RAB-GDP is activated to RAB-GTP, binds targets, and recruits motors.

Vesicle docking and fusion

Mediated by SNARE proteins.

Types of SNAREs

v-SNAREs (vesicles) and t-SNAREs (targets).

SNARE interaction result

Vesicle fusion occurs - a 'love connection'.

COPII Vesicle Formation Steps

1) SAR1-GDP to ER, 2) SAR1-GTP via GEF, 3) Membrane bending, 4) Sec23/24 bind, cargo loaded, coat formed.

Clathrin Vesicle Formation Steps

1) ARF-GDP -> ARF-GTP, 2) AP2 binds ARF-GTP, 3) Cargo receptor binds AP2, 4) Clathrin binds AP2, vesicle buds.