Figure 13–9 Lipid-induced conformation switching of AP2. The ap2 adaptor protein complex has four subunits (α, β2, μ2, and σ2). upon interaction with the phosphoinositide pI(4,5)p2 (see figure 13–10) in the cytosolic leaflet of the plasma membrane, ap2 rearranges so that binding sites for cargo receptors become exposed. each ap2 complex binds four pI(4,5)p2 molecules (for clarity, only one is shown). In the open ap2 complex, the μ2 and σ2 subunits bind the cytosolic tails of cargo receptors that display the appropriate endocytosis signals. These signals consist of short amino acid sequence motifs. when ap2 binds tightly to the membrane, it induces curvature, which favors the binding of additional ap2 complexes in the vicinity.
OHOHHOHOHOPOOOOOOCCOO_CH2CH2CH123456123456PIPI(3,4)P2PI(3,5)P2PI(3)PPI(3,4)P2PIPI(4)PPI(5)PPI(4,5)P2PI(3,4,5)P3(A)(C)(E)(F)(B)(D)PPPP
Membrane-bending proteins help deform the Membrane during vesicle formation
The forces generated by clathrin coat assembly alone are not sufficient to shape and pinch off a vesicle from the membrane. Other membrane-bending and force-generating proteins participate at every stage of the process. Membrane-bending proteins that contain crescent-shaped domains, called BAR domains, bind to and impose their shape on the underlying membrane via electrostatic interactions with the lipid head groups (Figure 13–12; also see Figure 10–40). Such BAR-domain proteins are thought to help AP2 nucleate clathrin-mediated endocytosis by shaping the plasma membrane to allow a clathrin-coated bud to form. Some of these proteins also contain amphiphilic helices that induce membrane curvature after being inserted as wedges into the cytoplasmic leaflet of the membrane. Other BAR-domain proteins are important in shaping the neck of a budding vesicle, where stabilization of sharp membrane bends is essential. Finally, the clathrin machinery nucleates the local assembly of actin filaments that introduce tension to help pinch off and propel the forming vesicle away from the membrane.
cytoplasmic proteins regulate the pinching-off and uncoating of coated vesicles
As a clathrin-coated bud grows, soluble cytoplasmic proteins, including dynamin, assemble at the neck of each bud (Figure 13–13). Dynamin contains a PI(4,5) P2-binding domain, which tethers the protein to the membrane, and a GTPase domain, which regulates the rate at which vesicles pinch off from the membrane.
Figure 13–11 The intracellular location of phosphoinositides. different types of pIps are located in different membranes and membrane domains, where they are often associated with specific vesicle transport events. The membrane of secretory vesicles, for example, contains pI(4)p. when the vesicles fuse with the plasma membrane, a pI 5-kinase that is localized  there converts the pI(4)p into pI(4,5)p2. The pI(4,5)p2, in turn, helps recruit adaptor proteins, which initiate the formation of a clathrin-coated pit, as the first step in clathrin-mediated endocytosis. once the clathrin-coated vesicle buds off from the plasma membrane, a pI(5)p phosphatase hydrolyzes  pI(4,5)p2, which weakens the binding of the adaptor proteins, promoting vesicle uncoating. we discuss phagocytosis and the distinction between regulated and constitutive exocytosis later in the chapter. (Modified from