A Rab domain can be disassembled and replaced by a different Rab domain, changing the identity of an organelle. Such ordered recruitment of sequentially acting Rab proteins is called a Rab cascade. Over time, for example, Rab5 domains are replaced by Rab7 domains on endosomal membranes. This converts an early endosome, marked by Rab5, into a late endosome, marked by Rab7. Because the set of Rab effectors recruited by Rab7 is different from that recruited by Rab5, this change reprograms the compartment: as we discuss later, it alters the membrane dynamics, including the incoming and outgoing traffic, and repositions the organelle away from the plasma membrane toward the cell interior. All of the cargo contained in the early endosome that has not been recycled to the plasma membrane is now part of a late endosome. This process is also referred to as endosome maturation. The self-amplifying nature of the Rab domains renders the process of endosome maturation unidirectional and irreversible (Figure 13–18).
Figure 13–17 The formation of a Rab5 domain on the endosome membrane.
a rab5-gef on the endosome membrane binds a rab5 protein and induces it to exchange gdp for gTp. gdI is lost and gTp binding alters the conformation of the rab protein, exposing an amphiphilic helix and a covalently attached lipid group, which together anchor the rab5-gTp to the membrane. active rab5 activates pI 3-kinase, which converts pI into pI(3) p. pI(3)p and active rab5 together bind a variety of rab effector proteins that contain pI(3)p-binding sites, including filamentous tethering proteins that catch incoming clathrin-coated endocytic vesicles from the plasma membrane. with the help of another effector, active rab5 also recruits more rab5-gef, further enhancing the assembly of the rab5 domain on the membrane.
controlled cycles of gTp hydrolysis and gdp–gTp exchange dynamically regulate the size and activity of such rab domains. unlike snares, which are integral membrane proteins, the gdp/ gTp cycle, coupled to the membrane/ cytosol translocation cycle, endows the rab machinery with the ability to undergo assembly and disassembly on the membrane. (adapted from M. Zerial and h. Mcbride, nat. rev. Mol. Cell Biol. 2:107–117, 2001. with permission from Macmillan publishers ltd.)
Once a transport vesicle has been tethered to its target membrane, it unloads its cargo by membrane fusion. Membrane fusion requires bringing the lipid bilayers of two membranes to within 1.5 nm of each other so that they can merge. When the membranes are in such close apposition, lipids can flow from one bilayer to the other. For this close approach, water must be displaced from the hydrophilic surface of the membrane—a process that is highly energetically unfavorable and requires specialized fusion proteins that overcome this energy barrier. We have already discussed the role of dynamin in a related task during the pinching-off of clathrin-coated vesicles (see Figure 13–13).