3.4 The function of Rab proteins in directing traffic
The SNARE proteins are just one component of the vesicle targeting system. Other participants in this process are the Rab family of GTPases, which regulate traffic between different cellular compartments and which are implicated in directing vesicles to their appropriate target compartments. The Rab family is the largest family of GTPases, with more than 30 members. They are distributed in specific organelles where they mediate the assembly of distinctive groups of proteins. Moreover, Rabs act in a combinatorial manner with Rab effector proteins to regulate almost all stages of membrane traffic, hence they are the key to recruitment of the tethering and docking proteins that facilitate membrane fusion. Rabs are selectively distributed to different membranous systems in the cell (Table 4) and attach to membranes via prenyl or palmitoyl groups at the C-terminus. Localisation of Rabs onto their membranes occurs when they are activated by a guanine nucleotide exchange factor (GEF), which allows them to bind Rab effector proteins and to recruit other specific proteins to the cluster (Figure 25).
Table 4 Intracellular localisation of some Rab proteins.
|Type of Rab||Cellular compartment|
|Rab1||ER and Golgi network|
|Rab2||cis Golgi network|
|Rab6||medial and trans Golgi network|
|Rab5C and Rab4||early endosomes|
|Rab8||basolateral secretory vesicles|
|Rab3||synaptic and secretory vesicles|
Although Rab proteins are structurally similar, their C-termini are highly variable, and it is this part of the molecule that determines their membrane specificity. Rab proteins and their effectors collect integral and peripheral membrane proteins into specific domains by regulating protein–protein interactions. In this way they can link appropriate addressing molecules, such as SNAREs, to the vesicles. Although Rabs are fairly similar in structure, the Rab effectors are very diverse. Rabs remain associated with the vesicles as they move and may also be involved in getting the correct motor proteins attached to the vesicle. After all, there is little point in knowing the address of where you are going if you are on the wrong road to get there.
Once a vesicle has fused with its target membrane, Rab proteins hydrolyse their bound GTP and the inactive Rab–GDP protein returns to the cytoplasm.
Rabs are small GTPases associated with vesicles that shuttle between compartments. What other small GTPases have you encountered that are involved in this process?
The proteins ARF and Sar1, which initiate assembly of the coats, and go through a similar cycle of activation, protein recruitment, transfer on vesicles and GTP hydrolysis and dissociation at the target membrane.
Despite the partially unresolved mystery about some functions of Rabs, such as their ability to link membranes to molecular motors, at least two facets are clear: (i) Rabs contribute to organelle identity and (ii) they catalyse the selective recruitment of proteins onto membranes to promote specific vesicle formation, tethering and fusion. As such, Rab GTPases are thought to be ‘master regulators’ of membrane trafficking in eukaryotic cells.
Our attention has been focused very much on proteins, but numerous aspects of membrane fluidity, specificity and permeability as well as fusion are regulated by lipids. Thus, lipids should not be forgotten because they are inevitably involved in vesicle trafficking. Membrane phospholipids and phosphoinositides in particular regulate many aspects of vesicular trafficking by binding to proteins implicated in tethering and in mixing the membrane bilayers.