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 157

Another distinction between Arf proteins and those of the Rab and Rho families
is that no guanosine diphosphate dissociation inhibitors (GDI) have been identified.
All Arfs are tightly membrane bound in their active GTP-bound conformation
because of nucleotide regulation of the position of the N-terminal amphipathic
helix, and simply hydrolysing the GTP on Arf proteins is sufficient to render them
soluble in vitro. Indeed, Arf1 and Arf3 appear to be released from membranes into
the cytosol upon GTP hydrolysis in cells. However, Arf6 remains bound to mem-
branes in cells in its GDP-bound conformation. There is also evidence that Arf4-
GDP and Arf5-GDP remain membrane bound in cells, in the latter case, to the ER-
Golgi intermediate compartment (ERGIC) (Chun et al. 2008 ; Duijsings et al. 2009 ).
This membrane association is likely due to interaction of the GDP-bound forms
with membrane-associated proteins. For Arf6, members of the Kalirin family of
Rho GEFs have been shown to bind specifically to the GDP-bound form through
their spectrin-like repeat domain (Koo et al. 2007 ). Arf6-GDP recruits Kalirin to the
membrane where it subsequently activates Rac and RhoG to regulate actin dynam-
ics (Koo et al. 2007 ). Arf6-GDP binds several TBC (Tre-2/Bub/Cdc16) domain-
containing proteins, which have Rab GAP activity (Haas et al. 2007 ), including
TBC1D24, a protein mutated in familial infantile myoclonic epilepsy (Falace
et al. 2010 ), and the TRE17 oncogene (Martinu et al. 2004 ). Hence interactions
with the GDP-bound form of a G protein could provide a mechanism for a single
Arf protein to trigger alternative signalling pathways depending on the nucleotide
bound, which could have important implications in human disease (Donaldson and
Jackson 2011 ).
Following activation on membranes, GTP-bound Arfs recruit coat proteins,
lipid-modifying enzymes, tethers, and other effector molecules that modulate the
properties of membranes and mediate vesicle trafficking (Table 8.1 ). The first
function of the Arf proteins to be identified was their ability to recruit cytosolic
coat proteins to membranes. In the early secretory pathway, Arf1 recruits coatomer
complex I (COPI), which sorts cargo proteins into COPI-coated vesicles as it curves
the membrane to form the vesicle (Beck et al. 2009 ). Arf1 at the trans -Golgi
network (TGN) also recruits the heterotetrameric clathrin adaptor proteins (AP),
AP-1, AP-3, and AP-4 and the three monomeric Golgi-localized ʳ -ear-containing,
ADP-ribosylation factor-binding proteins (GGAs 1-3) (Bonifacino and Lippincott-
Schwartz 2003 ). These various coat proteins specifically bind to cargo proteins and
incorporate them into forming vesicles for sorting and transport to their correct
destination.
Arf proteins can also recruit and activate enzymes that alter membrane lipid
composition. The first of these enzymes to be identified was phospholipase D
(PLD), which hydrolyses phosphatidylcholine to generate phosphatidic acid
(Brown et al. 1993 ; Cockcroft et al. 1994 ). PLD is activated by all Arf proteins
and also by Arl1 (Hong et al. 1998 ). PLD activation by Arf6 is involved in a number
of processes at the cell periphery, including regulated endocytosis and cell migra-
tion (D'Souza-Schorey and Chavrier 2006 ).
Another major function of Arf proteins is regulation of phosphoinositide levels
in cells. All Arf proteins can both recruit to membranes and stimulate the activity of