Chapter 7B: The Clathrin and COP coats

Function of a Coat

To form a vesicle, a donor membrane compartment binds and changes shape before scission or release.

Clathrin

-Clathrin is a triskelion-shaped protein, meaning it forms a three-legged structure.

-The shapes of hexagons and pentagons form a basket-like structure called a clathrin-coated pit.

-Different sizes of Clathrin represent different stages of bud formation.

Adaptor Protein Complex (AP)

• Clathrin has no ability to bind to membranes on its own and uses AP.

• Ap is a perhipheral membrane protein that can asscioate directly witht he lipid bilayer and can holld on the the Clathrin at the same time.

• AP makes sure the Clathrin coat only forms on the surface of the right donor membrane compartment

• AP is critical for determining where the Clathrin coat can for on the Golgi or the ER.

• AP selects the cargo that goes into the vesicle

4 Stages of Clathrin Coat Assembly

1) Coat assembly and Cargo selection: Adaptor proteins bind to membrane, recruit clathrin and select cargo for transport

2) Bud formation: The clathrin lattice triggers bud formation on the cytoplasmic side of the membrane. It wraps around the vesicle to shape it

3) Vesicle formation: As the bud matures, it separates from the membrane, forming a clathrin-coated vesicle containing selected cargo molecules. Dynamin, a GTPase protein, assists in the final separation of the vesicle from the membrane.

4) Uncoating: After vesicle formation, uncoating removes the clathrin coat, allowing the vesicle to move freely within the cell for.

2 Proteins that control coat assembly

1) Phosphoinositides→Control clathrin coat assembly

2) Specific GTPases →Control COP coat assembly

Phosphoinositides

Phosphoinositides is modified with 2 phosphates to get Phosphoinositidylinositol-4,5-bisphosphate(PIP2). Specifically Phosphoinositidylinositol-4,5-bisphosphate(PIP2) controls clathrin coat assembly

• Highly enriched and found on the inner leaflet of the plasma membrane

• Allows AP2 adaptor complex, dynamin and other proteins to bind

• Signals to start assembling a clathrin coat

• PIP2 is abundant at Bud formation during vesicle formation

• PIP2 is removed at Uncoating during vesicle formation

• Each phosphoinositide species binds to a specific set of proteins

PIP2 controls “opening” of AP2

In clathrin-mediated endocytosis, PIP2 interacts with AP2 to activate it, enabling it to bind to cargo molecules and form clathrin-coated vesicles for the internalization of specific substances into the cell.

The COPII coat

• COPII is involved in the formation of transport vesicles that carry proteins and lipids from the ER, where they are synthesized, to the Golgi apparatus, where they undergo further processing and sorting.

• COPII recognizes specific cargo molecules, marking them for transport. These cargo molecules are packaged into vesicles for delivery to the Golgi.

• The COPII coat is regulated by Sar

5 STEPS of Assembly of COPII Coats on the ER membrane

1) Activation of Sar1:Sar1, a small GTPase protein, is initially inactive in the cytoplasm. The process begins with the activation of Sar1 through the exchange of GDP for GTP.

2)Association with ER Membrane:Activated Sar1-GTP associates with the ER membrane. The hydrophobic tail of Sar1 inserts into the ER membrane, anchoring Sar1 to the lipid bilayer.

3)Recruitment of Sec23/24: Sar1-GTP recruits the Sec23/24 complex to the ER membrane. This complex acts as an adaptor and cargo-binding module, facilitating the selection and packaging of cargo molecules into the forming vesicle.

4)COPII Coat Formation:The Sar1/Sec23/24 complex forms the inner coat of the COPII vesicle. Additional Sec13/31 complexes are recruited to complete the outer coat, resulting in the assembly of a fully formed COPII coat around the budding vesicle.

5)Budding of Vesicle:The COPII-coated vesicle begins to bud from the ER membrane. This budding process involves the deformation of the membrane, creating a vesicle that encapsulates selected cargo molecules.