Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 151

nature of the primordial Arf protein comes from the protozoan parasite
Trypanosoma brucei , which expresses a single Arf protein that has characteristics
of both Class I and Class III mammalian Arfs. TbArf1 is a basic protein with a high
pI similar to that of human Arf6, but contains the Golgi-targeting motif MxxE
(Price et al. 2007 ), found in human Arf1 and Arf3 (Honda et al. 2005 ). TbArf1
localizes to the Golgi, and functional studies indicate that it has roles in both
endocytosis and in Golgi-lysosome trafficking (Price et al. 2007 ).
The spatiotemporal control of Arf protein function is mediated by regulators of
Arf GTP binding and GTP hydrolysis (Fig. 8.1d ). The Arf guanine nucleotide-
exchange factors (GEFs) catalyse GDP release from their substrate Arf, allowing
GTP, which is more abundant in cells, to bind. This nucleotide-exchange activity is
carried out by the Sec7 domain, a highly evolutionarily conserved domain first
identified as a homology domain in the yeast Sec7p protein, and whose function
was first identified in the yeast Gea1p protein (Peyroche et al. 1996 ). A Sec7
domain is present in all Arf GEFs identified to date. The Arf GTPase activating
proteins (GAPs) catalyse the hydrolysis of GTP on their substrate Arf, a function
carried out by a conserved GAP domain, characterized by the presence of a zinc
finger. The Arf GAPs are essential because Arf proteins have negligible intrinsic
GTP hydrolysis activity (Kahn and Gilman 1986 ). The primary sequence homology
of the catalytic domains of the GEFs and GAPs facilitated their identification, but
their Arf substrate specificity has not been fully elucidated. A recent study of the
Arf GAPs has revealed that they have likely co-evolved with their Arf substrates
(Schlacht et al. 2013 ). These results support the conclusion that the Arfs function in
a tightly coordinated manner with their regulators.
There are seven subfamilies of Arf GEFs in eukaryotic cells (Cox et al. 2004 )
(Table 8.2 , Fig. 8.2 ). The GBF/Gea and BIG/Sec7 GEFs are localized to the Golgi,
and use Arf1 as a substrate (Donaldson and Jackson 2011 ). In animal cells and
yeast, these GEFs act sequentially, with GBF/Gea proteins functioning at the early
Golgi, and BIG/Sec7 proteins at the trans- Golgi and trans -Golgi network (TGN)
(Franzusoff et al. 1991 ; Peyroche et al. 2001 ; Zhao et al. 2002 ). The cytohesin/
Arno, EFA6, and IQSEC/BRAG subfamilies function primarily in endosomal-PM
trafficking pathways at the cell periphery, and primarily act on Arf6 (Casanova
2007 ; Cox et al. 2004 ; Gillingham and Munro 2007b ). The EFA6 GEFs regulate
endocytosis, endosomal membrane recycling, and actin cytoskeleton remodelling
(Casanova 2007 ; Franco et al. 1999 ). Yel1p is the orthologue of EFA6 in budding
yeast, Saccharomyces cerevisiae , exhibiting exchange activity in vitro on Arf3p
(the yeast Class III member) (Gillingham and Munro 2007a ), which is involved in
endocytosis (Smaczynska-de Rooij et al. 2008 ). Yel1p has a homology domain in
the C-terminus resembling that of EFA6 family members (Gillingham and Munro
2007a ). Syt1p has a Sec7 domain that most closely resembles that of the IQSEC/
BRAG family in mammalian cells, although it lacks an IQ motif (Cox et al. 2004 ),
and functions with Arl3p and Arl1p at the trans -Golgi in yeast (Chen et al. 2010 ).
The FBXO8 Arf GEFs contain an F-box in addition to the Sec7 domain, and are
present in vertebrates, and at least one invertebrate (Gillingham and Munro 2007b ),
but are not present in yeast, worms, or flies. Little is known of its function, although