The ARF GTPases: Defining roles in membrane traffic and organelle structure
The ARF GTPases: Defining roles in membrane traffic and organelle structure
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DOI：
10.1101/sqb.1995.060.01.026
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发表时间：
1995-01-01
期刊：
影响因子：
--
通讯作者：
Peters, PJ
中科院分区：
文献类型：
--
作者：
Donaldson, JG;Radhakrishna, H;Peters, PJ
Research during the past decade has demonstrated that GTP-binding proteins are involved in a variety of processes carried out in cells, including signal transduction, nuclear transport, membrane traffic in the secretory and endocytotic pathways, cell movement, and transformation. In addition to the classic, receptorcoupled trimeric G proteins, there is an ever-expanding list of Ras-related, low-molecular-weight GTP-binding proteins. These small GTPases are subdivided into families based on sequence relatedness (Bourne et al. 1991; Boguski and McCormick 1993). These Rasrelated families include the Ras, Rab, Rho, Ran, and ARF families. We have been studying the ARF family of GTPases during the past 5 years. The ARF (or ADP-ribosylation factor) proteins were first identified, and thus named, as the cofactor required to reconstitute the cholera toxin-catalyzed ADP-ribosylation of Gsa in in vitro assays (Kahn and Gilman 1986). There are currently five sequenced human ARF proteins (see Fig. 1), and they appear to be expressed in all cell and tissue types (Kahn et al. 1991; Tsuchiya et aL 1991). The initial studies implicating ARF proteins in the secretory pathway came from studies in Saccharomyces cerevisiae. There are two ARF genes in yeast (Stearns et al. 1990a), and deletion of yeast ARF1 results in a defect in secretion. Deletion of both yeast ARFs 1 and 2 is lethal, but can be complemented by mammalian ARF1 (Steams et al. 1990b). ARFs are considered true ARFs if they meet the criteria of (1) having activity in the cholera toxin assay and (2) being able to complement the yeast ARF null phenotype (Boman and Kahn 1995). Other sequences related to ARFs have been found by polymerase chain reaction (PCR) but are referred to as ARLs, for ARF-like proteins, since they are not true ARFs by these criteria (Clark et al. 1993). Like all GTPases, ARF exists in two states depending on the guanine nucleotide bound. ARF-GDP is believed to be in the inactive conformation and is activated by nucleotide exchange whereby GDP is released in exchange for binding of soluble GTP. Active ARF-GTP returns to the inactive GDP form upon hydrolysis of the bound GTP. In biochemical assays, ARFs bind GDP with high affinity (Weiss et al. 1989), and thus activation of ARF by nucleotide exchange is believed to be catalyzed in cells by guanine nucleotide exchange factors; evidence for such factors exists (Donaldson et al. 1992a; Helms and Rothman 1992; Randazzo et al. 1993). Likewise, ARFs have an extremely low rate of GTP hydrolysis, and thus so-called GTPase-activating proteins or GAPs are believed to be required for hydrolysis of GTP bound to ARF. Indeed, there are reports of partial purification of ARF nucleotide exchange proteins (Tsai et al. 1994) and ARF GAP proteins (Randazzo and Kahn 1994; Makler et al. 1995). All ARFs have the consensus sequence (MGXXXS/AfF) at the amino terminus for cotranslational myristoylation (Weiss et al. 1989). This is essential for biological activity of these proteins, since mutations of the second amino acid glycine to an alanine (G2A) of ARF1 protein result in nonmyristoylated ARF1 that is inactive in all in vivo and in vitro assays. What are the cellular functions of the different ARF proteins? Much is known about the ARF1 protein, since it was the first recombinant ARF made in bacteria (Weiss et al. 1989), and accordingly, its effects in many in vitro biochemical assays have been documented. What of the roles of these different ARFs in the cell? How can we discern the functions of each in the cell? In this paper, we review the approach taken in our laboratory to answer this question. It arises out of the extensive …
