Rab Proteins and the Organization of Organelle Membrane Domains - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 24

complex organization to the CORVET complex, we can assume that Vps3p and
Vps8p are located on opposing ends of the complex and therefore ideally positioned
for tethering Vps21p-positive membranes (Br ¨ cker et al. 2012 ).
Other examples of Rab proteins binding to multi-subunit tethering complexes
are Rab1/Ypt1p binding to TRAPP, Dsl, and COG complexes at the Golgi, Rab6
binding to GARP complex at the trans -Golgi network (Br ¨ cker et al. 2010 ), and
yeast Sec4p to its effector Sec15p, a subunit of the plasma membrane tethering
Exocyst complex (Guo et al. 1999 ). Interestingly, Sec15p binds to Sec2p, the GEF
for Sec4p, creating another elegant feed-forward system for Rab membrane domain
generation (Stalder et al. 2013 ). Therefore, the general principle of the recruitment
of tethers to Rab domains is conserved throughout evolution. Importantly, tethers
are not just merely recruited, but a critical layer in the establishment of Rab
membrane domains through interactions with GEFs, SNAREs, and PIs (Figs. 2.2
and 2.3 ).
2.3.4 Rab GTPases Regulate SNAREs Through Tethers
and SM Proteins
Membrane fusion within the endocytic and exocytic pathways critically depends on
the family of SNARE proteins. SNAREs on the donor and target membranes form a
trans -SNARE complex that leads to membrane fusion. After fusion, the SNARE
complex is disassembled with the help of NSF and Soluble NSF attachment protein
(SNAP) so the SNAREs can be reutilized for new fusion events (S¨dhof and
Rothman 2009 ). As SNARE pairing is rather promiscuous (Brandhorst
et al. 2006 ), tight control of SNARE membrane localization and the fusogenic
activity is essential. Rab GTPases are ideal candidates to perform such a function as
they interact through their tethering effectors with SNAREs and Sec1/Munc18
(SM) priming factors (Fig. 2.2 ). The human genome encodes four classes of SM
type of proteins, each restricted to a specific intracellular trafficking domain. Sly1
functions predominantly at the ER and Golgi, Munc18 at the plasma membrane,
Vps45 at early endosomes, and Vps33 at the late endosomes and lysosomes (Rizo
and S¨dhof 2012 ).
Vps45 is recruited to early endosomes through interaction with the Rab5 effector
and PI(3)P-interacting protein Rabenosyn-5 (Nielsen et al. 2000 ) (Fig. 2.3e ).
Although the underlying molecular mechanism is not well understood, both
Rabenosyn-5 and Vps45 are indispensable for endosomal fusion (Nielsen
et al. 2000 ; Ohya et al. 2009 ). Most likely Vps45 controls the open/closed confor-
mational state of endosomal SNAREs. The SM family member Vps33 is a subunit
of both the CORVET and HOPS tethering complexes and binds Rab5 and Rab7,
respectively. In the context of the CORVET complex, Vps33 could regulate
endosomal SNARE activity. Although this has not been shown experimentally,
this notion is supported by the fact that as a part of the HOPS complex, Vps33p