Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 178

recruitment and polymerisation of cytosolic coat proteins. Coat proteins (with the
help of auxiliary proteins) (1) help to incorporate the right set of cargo proteins, and
at the same time (2) they physically curve the donor membrane into a vesicle bud,
(3) help to pinch off the nascent transport vesicle from its donor membrane, and
(4) finally must detach from the released vesicle membrane to allow the transport
vesicle to fuse with its target membrane.
The small GTP-binding proteins of the Arf family play a core role in most types
of transport vesicles. Although the small GTP-binding protein Arf was initially
identified as a cofactor in the post-translational cholera toxin-mediated
ADP-ribosylation of the G s α subunit of heterotrimeric G proteins (Kahn and
Gilman 1984 ), the major biological function of Arf proteins is in intracellular
vesicular transport. Formation of several transport carriers in the secretory and
endocytic pathways directly depends on small GTP-binding proteins of the Arf
family. In the formation of such carriers the Arf proteins operate as molecular
switches (Bourne et al. 1990 , 1991 ), which cycle between a GDP-bound, 'inactive',
mainly cytosolic and a GTP-bound, 'active', exclusively membrane-associated
conformation. Guanine nucleotide exchange factors (GEFs) and GTPase activating
proteins (GAPs) regulate these switches. 'Activated' small GTP-binding proteins
then recruit coat protein and help to mediate coat polymerisation, whereas GTP
hydrolysis leads to their release from the vesicle membrane, prerequisite for
shedding off the coat proteins to prepare the vesicle for docking to and fusion
with its target membrane (see Jackson 2014 ).
Mammalian cells possess several Arf proteins (Tsuchiya et al. 1991 ). Based on
sequence homology Arf proteins can be divided into three classes: class I (Arf1,
Arf2, and Arf3), class II (Arf4 and Arf5), and class III (with Arf6 is the sole
member. Saccharomyces cerevisiae has only three Arf proteins: Arf1p and Arf2p
belong to class I, and Arf3p belongs to class III. Arfs of class I and II are localised to
membranes of the early secretory pathway (e.g. the ERGIC and the Golgi appara-
tus), but can be also found on compartments of the late secretory/endocytic pathway
(e.g. endosomes). In contrast Arf6 is primarily localised at the plasma membrane
(see Jackson 2014 ).
Several coat proteins are well defined that characterise various types of coated
vesicles. The best-characterised vesicular transport carriers are COPI and COPII
vesicles, which serve trafficking routes in the early secretory pathway, as well as
clathrin-coated vesicles (CCVs), which operate with various clathrin adaptor pro-
teins in the late secretory pathway.
Arf1 is the central regulator in the formation of COPI vesicles (Serafini
et al. 1991a ). Most recently Arf1, Arf4, and Arf5 but not Arf3 and Arf6 were
found on COPI-coated vesicles generated form Golgi-enriched membranes in vitro
(Popoff et al. 2011b ). In addition, Arf1 is involved in the formation of CCVs that
operate with the tetrameric adaptor protein complexes AP1 (Stamnes and Rothman
1993 ; Traub et al. 1993 ), AP3 (Ooi et al. 1998 ), (Boehm et al. 2001 ) or the
monomeric adaptor protein complexes GGA1-3 (Boman et al. 2000 ; Dell'Angelica
et al. 2000 ; Puertollano et al. 2001 ), as well as in membrane recruitment of the
clathrin-independent adaptor protein complex AP4 (Boehm et al. 2001 ). The small