Figure 13–5 Use of different coats for different steps in vesicle traffic. different coat proteins select different cargo and shape the transport vesicles that mediate the various steps in the secretory and endocytic pathways. when the same coats function in different places in the cell, they usually incorporate different coat protein subunits that modify their properties (not shown). Many differentiated cells have additional pathways beside those shown here, including a sorting pathway from the trans golgi network to the apical surface of epithelial cells and a specialized recycling pathway for proteins of synaptic vesicles in the nerve terminals of neurons (see figure 11–36). The arrows are colored as in figure 13–3.
Figure 13–6 The structure of a clathrin coat. (a) electron micrograph of a clathrin triskelion shadowed with platinum.
each triskelion is composed of three clathrin heavy chains and three clathrin light chains, as shown in the diagram. (c and a cryoelectron micrograph taken of a clathrin coat composed of 36 triskelions organized in a network of 12 pentagons and 6 hexagons, with some heavy chains (c) and light chains (d) highlighted (Movie 13.1). The light chains link to the actin cytoskeleton, which helps generate force for membrane budding and vesicle movement, and their phosphorylation regulates clathrin coat assembly. The interwoven legs of the clathrin triskelions form an outer shell from which the n-terminal domains of the triskelions protrude inward. These domains bind to the adaptor proteins shown in figure 13–8. The coat shown was assembled biochemically from pure clathrin triskelions and is too small to enclose a membrane vesicle. (e) Images of clathrincoated vesicles isolated from bovine brain. The clathrin coats are constructed in a similar but less regular way, from pentagons, a larger number of hexagons, and sometime heptagons, resembling the architecture of deformed soccer balls. The structures were determined by cryoelectron microscopy and tomographic reconstruction. (a, from e. ungewickell and d. branton, nature 289:420–422, 1981; c and d, from a. fotin et al., nature 432:573–579, 2004. all with permission from Macmillan publishers ltd; e, from y. cheng et al., J. Mol. Biol. 365:892–899, 2007. with permission from elsevier.)
There are several types of adaptor proteins. The best characterized have four different protein subunits; others are single-chain proteins. Each type of adaptor protein is specific for a different set of cargo receptors. Clathrin-coated vesicles budding from different membranes use different adaptor proteins and thus package different receptors and cargo molecules.
The assembly of adaptor proteins on the membrane is tightly controlled, in part by the cooperative interaction of the adaptor proteins with other components of the coat. The adaptor protein AP2 serves as a well-understood example. When it binds to a specific phosphorylated phosphatidylinositol lipid (a phosphoinositide), it alters its conformation, exposing binding sites for cargo receptors in the membrane. The simultaneous binding to the cargo receptors and lipid head groups greatly enhances the binding of AP2 to the membrane (Figure 13–9).
Because several requirements must be met simultaneously to stably bind AP2 proteins to a membrane, the proteins act as coincidence detectors that only assemble at the right time and place. Upon binding, they induce membrane curvature, which makes the binding of additional AP2 proteins in its proximity more likely. The cooperative assembly of the AP2 coat layer then is further amplified by clathrin binding, which leads to the formation and budding of a transport vesicle.