There are three well-characterized types of coated vesicles, distinguished by their major coat proteins: clathrin-coated, COPI-coated, and COPII-coated (Figure 13–4). Each type is used for different transport steps. Clathrin-coated vesicles, for example, mediate transport from the Golgi apparatus and from the plasma membrane, whereas COPIand COPII-coated vesicles most commonly mediate transport from the ER and from the Golgi cisternae (Figure 13–5). There is, however, much more variety in coated vesicles and their functions than this short list suggests. As we discuss below, there are several types of clathrin-coated vesicles, each specialized for a different transport step, and the COPIand COPII-coated vesicles may be similarly diverse.
The assembly of a clathrin coat drives vesicle formation
Clathrin-coated vesicles, the first coated vesicles to be identified, transport material from the plasma membrane and between endosomal and Golgi compartments. COPI-coated vesicles and COPII-coated vesicles transport material early in the secretory pathway: COPI-coated vesicles bud from Golgi compartments, and COPII-coated vesicles bud from the ER (see Figure 13–5). We discuss clathrin-coated vesicles first, as they provide a good example of how vesicles form.
The major protein component of clathrin-coated vesicles is clathrin itself, which forms the outer layer of the coat. Each clathrin subunit consists of three large and three small polypeptide chains that together form a three-legged structure called a triskelion (Figure 13–6A,B). Clathrin triskelions assemble into a basketlike framework of hexagons and pentagons to form coated pits (buds) on the cytosolic surface of membranes (Figure 13–7). Under appropriate conditions, isolated triskelions spontaneously self-assemble into typical polyhedral cages in a test tube, even in the absence of the membrane vesicles that these baskets normally enclose (Figure 13–6C,D). Thus, the clathrin triskelions determine the geometry of the clathrin cage (Figure 13–6E).
Adaptor proteins, another major coat component in clathrin-coated vesicles, form a discrete inner layer of the coat, positioned between the clathrin cage and the membrane. They bind the clathrin coat to the membrane and trap various transmembrane proteins, including transmembrane receptors that capture soluble cargo molecules inside the vesicle—so-called cargo receptors. In this way, the adaptor proteins select a specific set of transmembrane proteins, together with the soluble proteins that interact with them, and package them into each newly formed clathrin-coated transport vesicle (Figure 13–8).
KEY:
Figure 13–4 Electron micrographs of clathrin-coated, COPI-coated, and COPII-coated vesicles. all are shown in electron micrographs at the same scale. (a clathrin-coated vesicles. (b) copIcoated vesicles and golgi cisternae (red arrows) from a cell-free system in which copI-coated vesicles bud in the test tube.
copII-coated vesicles. (a and b, from l.
orci, b. glick and J. rothman, Cell 46:171–184, 1986. with permission from elsevier; c, courtesy of charles barlowe and lelio orci.)