Rab Proteins and the Organization of Organelle Membrane Domains - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 26

2.3.6 Membrane-Deforming Proteins
The sorting of protein and lipid cargo requires the formation of membrane-enclosed
transport carriers, either vesicle or tubules. The formation of such transport carriers
has to be coordinated in space and time to ensure correct cargo selection and
targeting. Transport carrier formation requires the recruitment and assembly of
coats and membrane-deforming proteins. Rab GTPases interact indirectly with coat
complexes through interaction with tethers. Indeed, at the yeast cis -Golgi, Ypt1p
binds to the coiled coil tether Uso1p (p115 in mammals), which recognizes incom-
ing COPII-coated vesicles from the ER (Cao et al. 1998 ). Thereby it contributes to
correct vesicle targeting in the exocytic system. Ypt1p is also required for retro-
grade Golgi trafficking by binding the multi-subunit tethering complex COG, which
tethers COPI-coated vesicles (Suvorova et al. 2002 ).
Recycling of cargo from endosomes to the trans -Golgi network depends on the
formation of cargo-containing membrane tubules at endosomes. This process relies
on the Retromer, which consists of the tripartite complex of Vps26, Vps29, and
Vps35, responsible for cargo recognition, and a Sorting nexin (SNX) dimer (Sea-
man 2012 ; Pfeffer 2013a ). Members of the SNX family contain a BAR domain that
binds to curved membranes (Carlton et al. 2004 ), and a PX domain which binds to
PI(3)P. The binding of SNX to PI(3)P is essential for retromer function. Interest-
ingly, synthesis of this pool of PI(3)P likely depends on the activity of Rab5 (Rojas
et al. 2008 ). Furthermore, membrane recruitment of retromer depends on the direct
binding of the Vps subcomplex to GTP-Rab7 (Rojas et al. 2008 ), a process in which
the late endosomal Rab9 could play a similar or complementary role (Carroll
et al. 2001 ; Dong et al. 2013 ). Thus, the sequential activity of Rab5 (PI(3)P
synthesis) and Rab7 (Vps complex binding) is required for retromer function.
There are other examples of Rab GTPases recruiting membrane-deforming
proteins. At early endosomes, Rab35 forms a tripartite complex with MICAL-L1
and ACAP2 to serve as a scaffold for recruitment of EHD1 to endosomal recycling
tubules (Kobayashi and Fukuda 2013 ; Kouranti et al. 2006 ). EHD1 belongs to the
Dynamin-like EHD family and contributes to the scission of the tubule. Interest-
ingly, ACAP2 also functions as a GEF for the GTPase Arf6. In return, Arf6 binds to
the Rab35 GAP TBC1D10 (Chesneau et al. 2012 ), generating cross talk between
Arf and Rab GTPase families.
Above, we provide an overview of some of the possible interactions that Rab
GTPases have with membrane-defining factors (Fig. 2.2 ). Through the consecutive
action of Rab GTPases, GEFs, PI kinases and tethers, membrane domain formation
is initiated (Fig. 2.3 ). Membrane domains function as a binding platform for further
downstream effector proteins. This concept of membrane domain formation is
mainly based on research on the well-studied Rab5 and its effectors. Nonetheless,
due to the conservation of Rab GTPase and effectors, it is conceivable that similar
molecular principles apply to the membrane domains formed of other Rab
GTPases.