Over the past few years, the view that little or no cell turnover or replacement takes place within

TheADP-ribosylation factor proteins (ARFs) are ~20-kDa guanine-nucleotidebinding proteins found in all eukaryotic cells, which are a critical part of the minimum machinery required for vesicle formation at the Golgi and other membranes (1 –3 ). The six mammalian ARFs have been grouped into three classes (4 ) based on molecular size, amino-acid sequence, and gene structure: class I (ARF1-3), class II (ARF4, 5), and class III (ARF6). Proteins closely related to ARFs include ARF-like proteins (ARLs), ARF-related protein (ARP), and ARF-domain protein 1 (ARD1) (5 ). ARFs function as activators of choleratoxin ADP-ribosyltransferase and phospholipase D (PLD) (6 ,7 ), and have been recently implicated in the activation of phosphatidylinositol 4-phosphate 5-kinase (8 ). ARF, like other regulatory GTPases, is active when GTP is bound. The inactivation of ARF-GTP requires its interaction with a GTPase-activating protein (GAP), which accelerates GTP hydrolysis. Activation of the inactive GDP-bound ARF requires the action of a guanine nucleotide-exchange protein or GEP, which accelerates the release of bound nucleotide via stabilization of the guanine nucleotide-free protein (9 ).