In contrast to the highly conserved structure of Rab proteins, the structures and functions of Rab effectors vary greatly, and the same Rab proteins can often bind to many different effectors. Some Rab effectors are motor proteins that propel vesicles along actin filaments or microtubules to their target membrane. Others are tethering proteins, some of which have long, threadlike domains that serve as “fishing lines” that can extend to link two membranes more than 200 nm apart; other tethering proteins are large protein complexes that link two membranes that are closer together and interact with a wide variety of other proteins that facilitate the membrane fusion step. The tethering complex that docks COPII-coated
Figure 13–16 Tethering of a transport vesicle to a target membrane. rab effector proteins interact with active rab proteins (rab-gTps, yellow) located on the target membrane, vesicle membrane, or both, to establish the first connection between the two membranes that are going to fuse. In the example shown here, the rab effector is a filamentous tethering protein (dark green). next, snare proteins on the two membranes (red and blue) pair, docking the vesicle to the target membrane and catalyzing the fusion of the two apposed lipid bilayers. during docking and fusion, a rab-gap (not shown) induces the rab protein to hydrolyze its bound gTp to gdp, causing the rab to dissociate from the membrane and return to the cytosol as rab-gdp, where it is bound by a gdI protein that keeps the rab soluble and inactive.
vesicles, for example, contains a protein kinase that phosphorylates the coat proteins to complete the uncoating process. Coupling uncoating to vesicle delivery helps to ensure directionality of the transport process and fusion with the proper membrane. Rab effectors can also interact with SNAREs to couple membrane tethering to fusion (see Figure 13–16).
The assembly of Rab proteins and their effectors on a membrane is cooperative and results in the formation of large, specialized membrane patches. Rab5, for example, assembles on endosomes and mediates the capture of endocytic vesicles arriving from the plasma membrane. The experimental depletion of Rab5 causes disappearance of the entire endosomal and lysosomal membrane system, highlighting the crucial role of Rab proteins in organelle biogenesis and maintenance.
A Rab5 domain concentrates tethering proteins that catch incoming vesicles. Its assembly on endosomal membranes begins when a Rab5-GDP/GDI complex encounters a Rab-GEF. GDI is released and Rab5-GDP is converted to Rab5-GTP. Active Rab5-GTP becomes anchored to the membrane and recruits more Rab5GEF to the endosome, thereby stimulating the recruitment of more Rab5 to the same site. In addition, active Rab5 activates a PI 3-kinase, which locally converts PI to PI(3)P, which in turn binds some of the Rab effectors including tethering proteins and stabilizes their local membrane attachment (Figure 13–17). This type of positive feedback greatly amplifies the assembly process and helps to establish functionally distinct membrane domains within a continuous membrane.
The endosomal membrane provides a striking example of how different Rab proteins and their effectors help to create multiple specialized membrane domains, each fulfilling a particular set of functions. Thus, while the Rab5 membrane domain receives incoming endocytic vesicles from the plasma membrane, distinct Rab11 and Rab4 domains in the same membrane organize the budding of recycling vesicles that return proteins from the endosome to the plasma membrane.
rab cascades can change the Identity of an organelle