ADP-Ribosylation Factors (ARF)

Definition
Adenosine diphosphate-ribosylation factor (ARF) proteins are members of the GTP-binding proteins of the Ras superfamily1. They are major regulators of vesicle biogenesis in intracellular traffic, lipid metabolism, microtubule dynamics, development and other cellular processes2.

Discovery
ARF was originally identified as a cofactor for cholera toxin A catalyzed ADP-ribosylation of the stimulatory GTP-binding component of adenylate cyclase3.

Classification
The mammalian ARFs can be grouped into three classes on the basis of their size and sequence identity. ARF1, ARF2 and ARF3 are grouped under class I, ARF4 and ARF5 under class II and ARF6 under class III4.

Structural Characteristics
ARFs contain consensus amino acid sequences involved in GTP binding and hydrolysis which determine their catalytic activity3. They contain two switch regions, which change relative positions between cycles of GDP/GTP-binding. They are similar to heterotrimeric G protein subunits, these peptides are frequently myristoylated in their N-terminal region, which contributes to their membrane association.

Mode of action
The controlled binding and hydrolysis of GTP is critical to ARF function. ARF proteins cycle between GDP-bound, inactive and GTP-bound, active forms, and the cycling is regulated by specific guanine nucleotide releasing factors (GEPs) and GTPase-activating protein (GAPs). GTPase activating proteins (GAPs) hydrolyze bound GTP to GDP, and guanine nucleotide exchange factors adopt a new GTP molecule in place of a bound GDP. The GTP hydrolysis is required in many secretory pathways like formation and docking of vesicles at various membranes. It affects membrane traffic by recruiting coat proteins, including COPI and clathrin adaptor complexes to membranes.

Functions
ARFs function both constitutively within the secretory pathway and as targets of signal transduction in the cell periphery1. ARF proteins function in the regulation of membrane traffic and the organization of the cytoskeleton that are crucial to fundamental cellular processes, such as intracellular sorting/trafficking of newly synthesized proteins and endocytosis/exocytosis. They act at membrane surfaces to modify lipid composition and to recruit coat proteins for the generation of transport vesicles5. ARF proteins play a key regulatory role in the remodeling of actin cytoskeleton necessary for the formation of membrane ruffles and protrusions in association with phospholipase D and members of the Rho GTPase family. These activities of ARF proteins influence the formation, stability and functional integrity of epithelial junctions6.

References

1. Randazzo PA, Nie Z, Miura K, and Hsu VW, (2000). Molecular Aspects of the Cellular Activities of ADP-Ribosylation Factors. Sci. STKE, 2000 (59)
2. Pasqualato S, Renault L, Cherfils J (2002). Arf, Arl, Arp and Sar proteins: a family of GTP-binding proteins with a structural device for 'front-back' communication. EMBO Rep, 3(11):1035-41.
3. Kahn RA and Gilman AG (1984). Purification of a protein cofactor required for ADP-ribosylation of the stimulatory regulatory component of adenylate cyclase by cholera toxin. J. Biol. Chem, 259, 6228-6234.
4. Moss J and Vaughan M (1995). Structure and Function of ARF Proteins: Activators of Cholera Toxin and Critical Components of Intracellular Vesicular Transport Processes. The American Society for Biochemistry and Molecular Biology, 270(21): 12327-12330.
5. Donaldson JG (2008). Arfs and membrane lipids: sensing, generating and responding to membrane curvature. Biochem J, 414(2):189-94.
6. Hiroi T (2009). Regulation of epithelial junctions by proteins of the ADP-ribosylation factor family. Front Biosci., 14:717-730.