Rab GEFs and GAPs: The Enigma Variations - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 89

may have an N-terminal longin fold (Gerondopoulos et al. 2012 ). Despite this
potential longin region there is no sequence similarity with TRAPP or DENND1,
the GEFs known to contain longin domains from X-ray structure determination.
Initial structural analysis has been performed on the BLOC-3 complex, which
forms a 1:1 heterodimer, but it has not proven possible to obtain a structure possibly
due to the presence of a long unstructured loop in the central region of Hps4 (Kloer
et al. 2010 ). Mon1-Ccz1 and Hps1-Hps4 therefore form the founding members of
the Her mansky Pudlak Syndrome and Mon 1-Ccz1 (HerMon) GEF family acting on
Rab7 and related GTPases.
5.2.7 Ric1-Rgp1 Is a Binary GEF Complex for Rab6
Rab6 family GTPases function in retrograde trafficking at or to the Golgi apparatus
(White et al. 1999 ; Girod et al. 1999 ; Martinez et al. 1994 ). The best understood
family member in terms of its regulation is budding yeast Ypt6, which functions in
recycling of the exocytic SNARE Snc1 from endosomes back to the late Golgi
where it is packaged into secretory vesicles targeted to the bud tip (Siniossoglou
et al. 2000 ). Genetic screens in budding yeast identified a complex formed from
Ric1 and Rgp1 that has Ypt6 GEF activity in vitro (Siniossoglou et al. 2000 ). Both
Ypt6 and Ric1 are needed more generally for recycling of membrane proteins to the
late Golgi in yeast (Bensen et al. 2001 ). In vivo both Ric1 and Rgp1 are required for
correct localisation of Ypt6, consistent with the idea they act as its GEF. In human
cells, the Ric1-Rgp1 complex acts as the GEF for Rab6 at the Golgi, and interest-
ingly it is targeted via interactions with another Golgi Rab GTPase Rab33B
(Pusapati et al. 2012 ). This is reminiscent of the BLOC-3 that interacts with, and
may be targeted by, Rab9 (Kloer et al. 2010 ). It is tempting to speculate that the
properties and dimeric form of Ric1-Rgp1 might indicate some relationship to the
HerMon GEFs. However, Ric1-Rgp1 displays no homology to other Rab GEFs and
lacks any conserved domains. It may therefore represent a unique class of Rab GEF
for which structural data would be valuable.
5.2.8 Structural Themes in Rab Regulators
Based on the available evidence, Rab GEFs fall into one of two classes. Those that
act by inserting an acidic finger residue into the Rab magnesium and phosphate-
binding site, and those that induce restructuring of Rab switch I or II without
insertion into the nucleotide-binding site. In both cases altered coordination of the
P-loop lysine displacing the bound magnesium is likely to be central to the
mechanism of GDP release. Likewise, specificity for the Rab GDP form may in
part be due to the availability of the RabF1 TIGID motif in GDP-bound but not
GTP-bound Rabs. Despite the lack of, or only limited sequence homology,