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 148

8.1 Introduction

Arf proteins are low molecular weight GTP-binding (G) proteins that are regulated
through a cycle of GTP binding and hydrolysis, in which binding of GTP activates
and GTP hydrolysis inactivates the G protein (Donaldson and Jackson 2011 ;
Gillingham and Munro 2007b ) (Fig. 8.1 ). In their active GTP-bound form, Arf
proteins are tightly associated with the membrane surface. Hence they bring their
effectors, proteins that bind specifically to the GTP-bound form, into close contact
with the lipid bilayer. These Arf effector proteins include coat complexes that
deform membranes and promote cargo sorting, enzymes such as the phosphatidy-
linositol kinases that alter membrane lipid composition, and actin cytoskeletal
components (Table 8.1 ). Arf1 is the founding member of the family, and was
originally identified as a protein factor required for the ADP-ribosylation of the
adenylate cyclase activator Gs

by cholera toxin (Schleifer et al. 1982 ). Although
subsequent studies led to the discovery that the major cellular function of Arf1 is
regulation of membrane trafficking, its name comes from this initial finding. The
original discovery that Arf1 has an essential function in the secretory pathway at the
level of the Golgi came from studies in yeast (Stearns et al. 1990a , b ). Soon
thereafter, the role of Arf1 in recruiting the COPI coat complex to membranes of
the early secretory pathway to mediate COPI vesicle budding was demonstrated
both in vitro and in cells. The reconstitution in vitro of COPI vesicle budding
provided important mechanistic insights (Rothman and Wieland 1996 ), and the
function of Arf1 in cells was greatly aided by use of the specific inhibitor of Arf1
activation, brefeldin A (Klausner et al. 1992 ).
Mammalian Arf proteins can be divided into three classes based on sequence
homology: Class I (Arfs1-3), Class II (Arfs 4-5), and Class III (Arf6). Class I Arfs
are highly conserved and are present in all eukaryotes, whereas the Class II Arfs
arose during animal cell evolution, diverging from the Class I Arfs in the animal
lineage after fungi separated, but before choanoflagellates did (Manolea et al. 2010 ;
Schlacht et al. 2013 ). Consequently, in all invertebrates, such as D. melanogaster
and C. elegans , there is one member of each of the three Arf classes. In certain
lineages, some Arf classes have undergone expansion. For example, vertebrates
have multiple members of Class I and II Arf proteins (Li et al. 2004 ). Although
yeast lack Class II Arfs, they have two highly similar Class I Arf proteins and a
single Class III member. Plants have numerous Class I Arfs (six in Arabidopsis) that
are all more closely related to each other than to Class I Arfs of other eukaryotic
supergroups, as well as highly diverged Arfs with some characteristics of mamma-
lian Class III Arf6 (Gebbie et al. 2005 ). The Arf proteins are part of a larger family
that also includes the Arf-like (Arl) proteins. There are more than 20 Arl proteins,
with a wide range of functions including membrane trafficking, targeting of pro-
teins to cilia, microtubule regulation, and lysosome function (Donaldson and
Jackson 2011 ; Gillingham and Munro 2007b ). The most divergent Arl protein is
Sar1, an evolutionarily ancient small G protein that shares a highly related function

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