3 Trafficking vesicles
In the following sections, we shall describe the sequential steps involved in the movement of vesicles from one membrane to another (see Figure 9). Some of these steps are quite well defined, but for others there are gaps in our knowledge. Although we have emphasised the importance of proteins as cargo, vesicles also transfer membrane lipids between compartments, and so are important in maintaining the lipid composition and relative proportions of membrane lipids between different compartments, a process that is much less well understood.
The first stage of vesicular transport is the production of the transport vesicles. Vesicle formation requires deformation of the lipid bilayer, forming a goblet-shaped invagination of the membrane that will eventually be pinched off to form the vesicle, a process called budding. The induction of membrane curvature required for vesicle formation is an energy-dependent process mediated by proteins such as epsins, which are required specifically for budding. When a vesicle is generated, it carries proteins that were resident in that stretch of membrane as well as soluble molecules. Fusion of the vesicle with a target membrane is essentially a reversal of the process by which it originates. The proteins that direct the targeting of the vesicle to the correct cellular location also mediate fusion, and in some systems regulate the precise time at which fusion occurs.
We emphasised the importance of signal sequences in proteins, which ensure they are packaged in the right vesicles. What other proteins are needed to ensure they reach their correct destination?
To deliver their cargo to the right compartment, the transport vesicles must be able to recognise and fuse with the correct target membrane. This means that they need to carry molecules that address them to the target membrane.
Some vesicles have a coat of proteins surrounding their membrane and are therefore called coated vesicles. The coat is acquired as the vesicle buds from the donor membrane and is shed before the vesicle fuses with the target membrane (see Figure 9). One purpose of the coat is to enable the type of vesicle to be identified, so that is directed to the appropriate target membrane; it may also be involved in the selection of cargo to be transported.
Figure 15 shows in more detail how the vesicular coat is assembled, a process that involves small G proteins. Budding is initiated by recruitment of small G proteins to a region of membrane curvature, which then assemble the complex of coat proteins and adaptor proteins, which link the coat to molecules in the membrane. The G proteins exchange GDP for GTP, converting into the active form, which can then insert spontaneously into lipid bilayers, by means of a hydrophobic tail. It seems that the G protein activity and ability to recycle is controlled by hydrolysis of GTP. The G proteins associate with proteins in the membrane, and there is some evidence that they may interact with receptors in the donor membrane. In other words, the function of these G proteins is to provide the binding sites at which coat proteins can assemble. Examples of such G proteins and the types of coated vesicle they associate with are shown in Table 3.
Endocytic and secretory vesicles have clathrin as the most prominent protein in their coats and they are called clathrin-coated vesicles.
The vesicles involved in transport between Golgi cisternae are called COPI-coated vesicles (COPI is an acronym for COat Protein I (roman one).)
The vesicles involved in trafficking from the ER to the cis Golgi have a different coat (COPII), and are called COPII-coated vesicles.
Table 3 Protein and G protein components of coated vesicles.
|Vesicle||Coat and adaptor proteins||Small G protein||Transport step|
|clathrin||clathrin heavy and light chains, AP2||ARF||plasma membrane → endosome|
|clathrin heavy and light chains, AP1||ARF||Golgi → endosome|
|clathrin heavy and light chains, AP3||ARF||Golgi → lysosome|
|COPI||COP α, β, β′, γ, δ, ε, ζ||ARF||Golgi → ER; between Golgi cisternae|
Sec13/Sec31 complex; Sec1β
|Sar1||ER → Golgi|