Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE MOLECULAR MECHANISMS OF MEMBRANE TRANSPORT
707
Rab5-GDP
filamentous
tethering proteins
Rab effector
proteins
GDI
covalently
attached lipid
amphiphilic
helix
PI 3-kinase
more Rab5-GEF
recruitment
Rab5-GEF
CYTOSOL
P
+
P P P P
ENDOSOME
LUMEN
active
Rab5-GTP
PI
PI(3)P
Rab5 membrane domain
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).
MBoC6 m13.15/13.16
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 Rab5-
GEF 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.
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.)
Rab Cascades Can Change the Identity of an Organelle
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).