M.a. de Matteis and a. godi, nat. Cell Biol. 6:487–492, 2004. with permission from Macmillan publishers ltd.)
Figure 13–10 Phosphatidylinositol (PI) and phosphoinositides (PIPs).
(a, b) The structure of pI shows the  free hydroxyl groups in the inositol  sugar that can in principle be modified.
(c) phosphorylation of one, two, or three of the hydroxyl groups on pI by pI and pIp kinases produces a variety of pIp species. They are named according to the ring position (in parentheses) and the number of phosphate groups (subscript) added to pI. pI(3,4)p2 is shown. (d) animal cells have several pI and pIp kinases and a similar number of pIp phosphatases, which are localized to different organelles, where they are regulated to catalyze the production of particular pIps. The red and green arrows show the kinase and phosphatase reactions, respectively. (e, f) phosphoinositide head groups are recognized by protein domains that discriminate between the different forms. In this way, select groups of proteins containing such domains are recruited to regions of membrane in which these phosphoinositides are present. pI(3)p and pI(4,5)p2 are shown. (d, modified from
M.a. de Matteis and a. godi, nat. Cell Biol. 6:487–492, 2004. with permission from Macmillan publishers ltd.) constitutive exocytosis regulated exocytosis endocytosis phagocytosis KEY: PI(3)P PI(4)P PI(4,5)P2 PI(3,5)P2 PI(3,4,5)P3 702 Chapter 13:  Intracellular Membrane Traffic
BAR domain dimer Figure 13–12 The structure of BAR domains. bar-domain proteins are diverse and enable many membrane-bending processes in the cell. bar domains are built from coiled coils that dimerize into modules with a positively charged inner + surface, which preferentially interacts with negatively charged lipid head groups to bend membranes. local membrane deformations caused by bar-domain proteins facilitate the binding of additional bar-domain proteins, thereby generating a positive feedback cycle for curvature propagation. Individual bar-domain
The pinching-off process brings the two noncytosolic leaflets of the membrane proteins contain a distinctive curvature and into close proximity and fuses them, sealing off the forming vesicle (see Figure often have additional features that adapt them to their specific tasks: some have 13–2). To perform this task, dynamin recruits other proteins to the neck of the bud. Together with dynamin, they help bend the patch of membrane—by directly distorting the bilayer structure, or by changing its lipid composition through the others are flanked by pIp-binding domains recruitment of lipid-modifying enzymes, or by both mechanisms. that direct them to membranes enriched in Once released from the membrane, the vesicle rapidly loses its clathrin coat. A cognate phosphoinositides. PIP phosphatase that is co-packaged into clathrin-coated vesicles depletes PI(4,5) P2 from the membrane, which weakens the binding of the adaptor proteins. In addition, an hsp70 chaperone protein (see Figure 6–80) functions as an uncoating ATPase, using the energy of ATP hydrolysis to peel off the clathrin coat. Auxilin, another vesicle protein, is thought to activate the ATPase. The release of the coat, however, must not happen prematurely, so additional control mechanisms must somehow prevent the clathrin from being removed before it has formed a complete vesicle (discussed below).
GTPase domain of dynamin