Vesicle Transport

Readings

pp. 462 - 466. Vesicular Transport

Learning Objectives

• Understand how vesicles are formed and targeted.
• To appreciate the role of coat proteins in the formation of vesicles and in the concentration of cargo proteins
• To appreciate the role of snares and the fusion protein complex in vesicle targeting and docking.

Vesicle Transport

Once the proteins are in the ER, They are separated from the cytosol compartment by membrane. These proteins are carried by vesicles throughout their life in the cell. Vesicles carrying proteins bud from cisternae of the endoplasmic reticulum or the Golgi apparatus and fuse with target membranes including those of other organelles (such as Golgi, endosomes or lysosomes), or the plasma membrane.

The following diagram summarizes the major vesicle transport pathways in eukaryotic cells.

Diagram of flow of materials within the ER - Golgi - Lysosome system

Although there are many different sources of vesicles and many target membranes, there are sufficient common features to warrant a general discussion of vesicle transport and targeting. We will later use this information in discussions of protein processing, secretion, and intracellular digestion.

As with individual protein molecules that are targeted to specific destinations, vesicle must take the correct cargo to proper target. This process has two components:

• selection and concentration of the vesicle content (as aspect of the vesicle formation process), and
• targeting of the vesicle to the correct destination membrane.

The key to selection and concentration of vesicle contents lies in the vesicle coats.

Each bud has a distinctive coat protein on the cytosol surface. Micrograph of coated vesicles forming.

There are several types of coats (COPI, COPII and clathrin):

• COP-coated vesicles involved in transport of vesicles from ER to Golgi and within Golgi,  and
• clathrin coated vesicles that carry proteins from the Golgi to the cell surface or from the cell surface to endosomes.

All of the coat proteins have two functions:

• to shape the membrane into a bud
• to capture molecules for onward transport.

The coat protein must shape the membrane into a bud.

The bud must capture the correct molecules for outward transport. The coat proteins in integral to this process.

Vesicle formation and cargo selection:

These processes are common to all vesicle types.

Figure 14-19. Formation of clathrin coated vesicles. See animation. See Micrograph of coated pits.

This process requires the interaction of several components: cargo receptor, adaptin, coat protein (COP or clathrin) and dynamin. The essential point is that the coat protein brings about both formation of the bud and concentration of cargo protein.

• The coat protein forms the curved bud membrane configuration. This comes about as a result of the interaction of the coat protein molecules with each other to form a curved cage. Micrograph of clathrin coated pits.
• The cargo molecules are picked up by the cargo receptor, which is a transmembrane protein. The cargo binding site is on the lumen side of the protein.
• The cargo receptor/cargo complex is recognized by adaptin which combines with the cytosolic side of the adaptin molecule.
• The cargo/ cargo receptor/ adaptin complex then combines with the coat protein on the cytosolic surface. This is very important for selection and concentration of cargo proteins. Animation.  Each coat protein unit has binding sites for several adaptin molecules, and thus cargo receptors and their cargo. This results in concentration of cargo molecules in the area where the bud is forming.
• Dynamin constricts the neck of the bud (vesicle), which then pinches off. This is an ATP driven process.
• Uncoating then occurs as the coat protein and adaptin are released and recycled.

The vesicle is now ready for transport.

• After budding, the protein coat is lost.
• Bud can now interact with target - target interaction signals are now exposed.

Transport of vesicles:

• over short distance vesicles move by diffusion;
• over longer distances vesicles move along tracts of microtubules and are moved my motor proteins (kinesins or dynein).

Targeting of vesicles:

Docking must be specific (don't completely understand how it works). One of the elements involved in vesicle targeting is a class of protein molecules known as snares. Snares result is specific attachment of vesicles to their target membranes.

Figure 14-20. Snares and specificity of vesicle transport.

Snares are of two types, vesicle snares (or v-snares) and target snares (t-snares).

Snares occur as complementary pairs of proteins. The vesicle-snare (v-snare) is incorporated into the vesicle membrane, and the target-snare is incorporated into the target membrane.

Docking occurs by interaction of the v-snare and t-snare proteins. This binding is very specific.

Once the vesicle and the target membranes are docked, several other proteins join to form a 'fusion complex' that results in the fusion of the vesicle with the target membrane. Fig 4-21.

Figure 14-21. Transport vesicle fusion. Following the docking of a transport vesicle at its target membrane, a complex of membrane fusion proteins assembles at the docking site and catalyses the fusion of the vesicle with the target membrane.

Thought question: How does the targeting of vesicles compare with that of single protein molecules?