Members of the Arf family of GTP-binding (G) proteins, including the Arf-like (Arl) proteins and Sar1, regulate membrane traffic and organelle structure through the membrane recruitment of cargo-sorting coat proteins, modulation of membrane lipid composition, and interaction with regulators of other G proteins. New roles of Arf and Arl proteins are emerging, including novel functions at the Golgi complex and in cilia formation. Their function is under tight spatial control, mediated by guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP exchange and hydrolysis, respectively. Important advances are being gained in our understanding of how these GEFs and GAPs are tightly regulated, and the functional networks that can be formed not only by the GEFs and GAPs themselves, but also inactive forms of the Arf proteins.
The Arf family of low molecular weight GTP-binding (G) proteins controls membrane traffic and organelle structure, and is regulated through a cycle of GTP-binding to activate and GTP hydrolysis to inactivate the G protein1, 2. Arfs have several important functions, including the recruitment of coat proteins that promote sorting of cargo into vesicles, the recruitment and activation of enzymes such as the phosphatidylinositol kinases that alter membrane lipid composition, and interaction with cytoskeletal factors (Table 1). There are 6 mammalian Arf proteins that can be divided into three classes based on sequence homology: Class I (Arfs1-3), Class II (Arfs 4-5) and Class III (Arf6) (Figure 1). The Class II Arfs arose late in animal cell evolution, possibly in metazoans, but Class I Arfs are highly conserved and are present in all eukaryotes examined to date. Hence in D. melanogaster and C. elegans, each class has a single Arf orthologue, but yeast lack Class II Arfs. Plants have Class I Arfs, and potentially also Class III Arfs (see Box 1).
In addition, there are over 20 Arf-like (Arl) proteins, which seem to have broader roles than Arf proteins. Some Arl proteins, including Arl1, Arl2 and Arf-related protein 1 (Arfrp1), are ancient with members in plants, yeast and metazoans, whereas others such as Arl11 arose later in evolution and are present only in vertebrates2. Sar1, present in all eukaryotes examined to date, is also considered to be a member of the Arf family due to the presence of an amino terminal amphipathic helix and its functional similarity to Arf1 in recruiting a coat complex during vesicle budding.
The study of Arf protein function was aided greatly by the early discovery of the regulators of Arf GTP binding and GTP hydrolysis. The Arf guanine nucleotide exchange factors (GEFs) all contain a conserved Sec7 domain that catalyses GDP release from, and GTP binding to, their substrate Arf. The GTPase activating proteins (GAPs) catalyse the hydrolysis of GTP-bound Arf and are critically important since Arfs have negligible intrinsic GTP hydrolysis activity. Arf GAPs all contain a conserved Zinc-finger GAP catalytic domain. The conserved, signature catalytic domains in the GEFs and GAPs are what facilitated the identification of these Arf regulators in all organisms from yeast to man. On the other hand, the Arf substrate specificity for these GEFs and GAPs remains unclear, and whether they also work on Arl proteins has yet to be determined.
In the past, G protein activity was viewed as a linear signaling pathway, with the GDP-bound form being inactive and the GTP-bound form initiating effector functions until it returned to the inactive, GDP-bound state. The GEFs and GAPs were thought of as ‘activators’ and ‘inactivators’, respectively, that controlled this on/off switch (Figure 1C). However, work on Arf and Arl proteins over the past decade has revealed that their signaling is more complex and that GEFs and GAPs can initiate their own physiological responses. We see evidence of Arf proteins acting in pairs or in series in the ER-Golgi system and at the PM. In this review, we emphasize how Arf proteins function as a network in which the Arf regulators participate in, and integrate Arf activities with other G protein signaling networks, in addition to initiating their own distinct signalling pathways. We highlight new Arf and Arl activities, discuss how GEFs and GAPs can act as scaffolds both as effectors and in initiating signalling responses, and describe how they participate in development and disease. The reader is referred to two excellent prior reviews that deal comprehensively with Arf1 and Arf6 function 1 and more broadly with Arf/Arl, GEF and GAP proteins 2.