Rab Proteins and the Organization of Organelle Membrane Domains - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 25

controls vacuolar fusion by priming the vacuolar SNAREs Vam3p, Vti1p, and
Vam7p (Stroupe et al. 2006 ).
An additional way for Rab proteins to influence SNARE pairing is through
interactions with molecular tethers. For example, the Rab6/Ypt6p-binding tethering
complex GARP interacts with a variety of SNAREs, including the Golgi t-SNARE
Tlg1p (Conibear et al. 2003 ; Siniossoglou and Pelham 2002 ), an evolutionary
conserved interaction (P ´ rez-Victoria et al. 2010 ). Similarly, in yeast Sec4p inter-
acts with Sro7p, an Lgl family member shown to regulate exocytic SNARE
function (Grosshans et al. 2006a ). At endosomes, EEA1 interacts directly with
the endosomal SNAREs Syntaxin 6 and Syntaxin 13 (McBride et al. 1999 ;
Simonsen et al. 1999 ) (Fig. 2.3e ). Importantly, Syntaxin 13 is a part of a high
molecular weight oligomer, whereby formation depends on Rab5 and EEA1
(McBride et al. 1999 ). This clustering is required for SNARE-driven fusion activ-
ity. These data argue that SNAREs alone have only limited membrane fusion
activity. However, through enrichment and clustering of SNAREs, SNARE regu-
lators, and tethers in Rab membrane domains, increased efficiency and specificity
of membrane fusion can be achieved.
2.3.5 Motors Proteins Directly and Indirectly Interact
with Rab GTPases
Both exocytic and endocytic membrane traffic depend on the transport of cargo-
containing vesicles by molecular motors. Rab domains present ideal regulators of
motor recruitment and activity as they define the identity and thereby the destina-
tion of the vesicle. A wide variety of actin and microtubule-dependent motors have
been localized to early endosomes (Hunt and Stephens 2011 ). Although early
endosome motility depends on Rab5 (Nielsen et al. 1999 ), no direct interaction
between Rab5 and an endosomal molecular motor is currently known. However, the
endosomal Kinesin Kif16B contains a PX domain via which it interacts with
endosomal PI(3)P (Hoepfner et al. 2005 ). Also, Kif16B was recently shown to
interact with the Rab4-related Rab14 (Ueno et al. 2011 ). Another example of a
direct Rab/motor interaction is the binding of Rab4 to Kif3, important for the
transport of GLUT4 vesicles (Imamura et al. 2003 ). Rab11 regulates plasma
membrane recycling through a direct interaction with MyosinVb (Lapierre
et al. 2001 ) and indirectly via the linker Rab11-FIP2 (Hales et al. 2002 ). In yeast,
the Golgi-localized Rab Ypt31p/Ypt32p facilitates the recruitment of the Myosin V
type motor Myo2p to exocytic vesicles, whereas the downstream GTPase Sec4p
binds directly to Myo2p to coordinate transport of exocytic vesicles along the actin
cytoskeleton (Jin et al. 2011 ; Lipatova et al. 2008 ). Interestingly, these processes
are regulated by PI(4)P (Santiago-Tirado et al. 2011 ). In conclusion, Rab membrane
domains function as a landmark for the binding of downstream effectors like motor
proteins necessary to control the specificity of organelle and vesicular transport.