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ADP-ribosylation factors (ARFs) are 20-kDa guanine nucleotide-binding proteins, members of the Ras GTPase superfamily that were initially recognized and purified because of their ability to stimulate the ADP-ribosyltransferase activity of the cholera toxin A subunit. We now know that they are critical components of several different vesicular trafficking pathways in all eukaryotic cells and activators of specific phospholipase Ds (PLDs). ARF interacts with many proteins and other molecules that regulate its state of activation or are involved in its intracellular function. Arf proteins cycle between GDP-bound, inactive and GTP-bound, active forms, and the cycling is regulated by specific GEPs and GAPs. Members of the Arf GEP family can be grouped into two major subfamilies on the basis of their sequence similarities and functional differences. The high-molecular-weight Arf GEP subfamily includes yeast Sec7, Gea1, and Gea2, and mammalian BIG1/p200, BIG2, and GBF1, which all consist of 1,400-2,000 amino acid residues. Many Arf GAPs are multidomain proteins and have been found to interact with multiple signaling molecules. For instance, ASAP1 has a PH domain, ankyrin (ANK) repeats, proline-rich and Src-homology (SH) 3 domains and has been shown to bind PI, P2, Src, and Crk. PAPa/PAG3, with a similar domain structure, binds phosphoinositides, Src, Pyk, and paxillin. The GITs bind bARK, paxillin and the Rac/Cdc42 exchange factor PIX (also known as Cool). ASAP-related proteins, ARAP1, 2 and 3 have Arf GAP, Rho GAP, Ankyrin repeat, Ras-associating (RA), and five PH domains, and therefore have the potential of integrating four signaling pathways. (This definition may be outdated - see the DesignNote.)
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