Effectors of Rab GTPases: Rab Binding Specificity and Their Role in Coordination of Rab Function and Localization - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 56

3.5 Effectors and the Coordination of Rab Function
3.5.1 Effectors and Rab Cascades
To ensure unidirectionality of intracellular transport, Rab function needs to be
coordinated. Effectors of upstream Rab GTPases with GEF domains for down-
stream Rab proteins play a key role in this process also called Rab cascade
(Fig. 3.6 ). The first cascade to be identified involved the yeast Rab protein Ypt32
that recruits the Sec4p nucleotide exchange factor Sec2p (Ortiz et al. 2002 ). This
cascade has also been demonstrated to exist with the homologous proteins in
mammalian cells. Rabin8, the GEF protein of Rab8, is also a direct effector protein
of Rab11, and binding of Rab11 to Rabin8 even enhances the nucleotide exchange
activity of the GEF protein (Knodler et al. 2010 ). Novick and colleagues then
demonstrated that this cascade is complemented by a GAP cascade (Fig. 3.7 ), in
which an active Rab protein (Ypt32p) recruits an effector protein that contains a
GAP domain for an upstream Rab protein—in this case Gyp1p that deactivates
Ypt1p (Rivera-Molina and Novick 2009 ).
Two other cascades involve protein complexes as GEF effector units. The first
cascade involves the Ric1-Rgp1 complex that is an effector protein of Rab33b and
a nucleotide exchange factor for Rab6 (Pusapati et al. 2012 ). Rab6IP1, an effector
protein of Rab6 and Rab11 (Janoueix-Lerosey et al. 1995 ; Miserey-Lenkei
et al. 2007 ), then likely continues the cascade on a second level as its DENN
domain has been demonstrated to have GEF activity towards Rab39 in vitro
(Yoshimura et al. 2010 ). The second GEF cascade that was discovered in recent
years involves the BLOC-3 complex consisting of Hps1 and Hps4. Hps4 binds to
the GTP-bound form of Rab9 (Kloer et al. 2010 ), and the complex is therefore
recruited to membranes on which active Rab9 is localized. The Hps1-Hps4 com-
plex activates Rab32/38 (Gerondopoulos et al. 2012 ) which then recruits Varp
(Tamura et al. 2009 ) that has a GEF domain to activate Rab21 (Zhang
et al. 2006 ). Surprisingly, Rab9 was also demonstrated to participate in a GAP
cascade to deactivate Rab32 by RUTBC1 (Nottingham et al. 2011 ). It remains to be
determined how these two activities can lead to regulation of Rab9 activity in vivo.
In addition to the classical cascades, Rab proteins also create positive effector
loops to enhance their activation. The best studied example of this is the feedback
loop between Rabex5, Rab5, and Rabaptin5. In this feedback loop, the GEF protein
of Rab5, Rabex5, is recruited by the effector protein Rabaptin5, which in turn is
recruited by the active Rab5 (Stenmark et al. 1995 ; Horiuchi et al. 1997 ). A similar
feedback loop has been demonstrated to exist in yeast where the Sec4p nucleotide
exchange factor Sec2p (Walch-Solimena et al. 1997 ) is recruited by the Sec4p
effector complex Exocyst (Guo et al. 1999 ; Medkova et al. 2006 ).
Effector binding itself can also be part of a feedback loop that involves mem-
brane remodeling. The Rab5 effector hVPS34, for example, catalyzes the synthesis
of phosphatidylinositol 3-phosphate (PI3P) from phosphatidylinositol
(Christoforidis et al. 1999 ). This in turn is important for the membrane binding of