Small GTPases: Versatile Signaling Switches in Plants

Abstract

Conserved Structure and Regulation of Small GTPases. (A) Conserved structure of small GTPase. All small GTPases contain four conserved domains for guanine nucleotide binding and GTPase activities (I through IV) and an effector domain (E). Residues in red have been used for the generation of CA mutants, and those in purple for DN mutants (see Figure 2). The Rho insert (10 to 12 amino acids) is only found in Rho GTPases. Rab, Ras, and Rho GTPases also contain a C-terminal motif for prenylation (P). The motif can be CAAL (C, cysteine; A, aliphatic amino acid; L, leucine) for geranylgeranylation by geranylgeranyltransferase I (GGTase I), CC/CXC for geranylgeranylation by GGTase II, or CAAX (X indicates any amino acid except for leucine and phenyalanine) for farnesylation. Arf is myristoylated at the N terminus, but Ran has no known modification. (B) A general scheme for the regulation and action of small GTPases. GAP, GTPase-activating protein; GDI, guanine nucleotide dissociation inhibitor; GEF, guanine nucleotide exchange factor. Bent arrowhead indicates a lipid moiety that becomes attached to membranes. Small GTPases differ from heterotrimeric G proteins in the mechanisms by which they are regulated by upstream factors as well as those by which they activate downstream targets (Figure 1B). Upon stimulation by an upstream signal, a guanine nucleotide exchange factor (GEF) converts the GDP-bound inactive form into the GTP-bound active form through GDP/GTP replacement. Through its effector domain, the GTP form interacts with one or more specific downstream effector proteins. The GTP form exhibits a weak intrinsic GTPase activity for GTP hydrolysis, requiring a GTPase-activating protein (GAP) for efficient deactivation. In addition, most small GTPases cycle between membrane-bound and cytosolic forms. Because only membrane-associated GTPases can be activated by GEF, their removal by a cytosolic factor called guanine nucleotide dissociation inhibitor (GDI) negatively regulates these GTPases (Figure 1B). These complex modes of regulation and action for small GTPases are apparently conserved in all organisms, including plants. In spite of their small size, the interaction of small GTPases with various regulators and effectors generates functional diversity and creates novel functions in different phyla. Functional innovation of a conserved GTPase together with birth of new members of the small GTPase superfamily endows plants with the capacity to use small GTPases as a key molecular switch for the modulation of many plant-specific signaling pathways and functions. The small GTPase superfamily is divided into at least five families, including Ras, Rho, Rab, Arf, and Ran (Bischoff et al., 1999; Takai et al., 2001). The Rab, Arf, and Ran families are conserved in eukaryotes and directly participate in the regulation of eukaryotic hallmark cellular processes (Takai et al., 2001). Arf is required for vesicle budding in the secretory system, whereas different Rabs control transport and docking of specific vesicles. Ran regulates trafficking of RNA and proteins through the nuclear pore. Because of their essential functions in these fundamental processes, they are generally not considered as signaling proteins that transmit extracellular signals. As expected, Rab, Arf, and Ran are all found in plants and are assumed to regulate the same general processes in plant cells (d'Enfert et al., 1992; Palme et al., 1992; Cheon et al., 1993; Regad et al., 1993; Yoshida et al., 1993; Merke et al., 1996; Haizel et al., 1997; Moore et al., 1997; Bischoff et al., 1999; Steinmann et al., 1999; McElver et al., 2000; Lu et al., 2001; V. Vernoud, A. Horton, Z. Yang, and E. Nielson, unpublished data). The physiological functions of several of these GTPases or their associated proteins have been examined in plants (Bischoff et al., 1999; V. Vernoud, A. Horton, Z. Yang, and E. Nielson, unpublished data). The roles of Ran have not been studied in plants, but the finding that the N terminus of RanGAPs is homologous to nuclear matrix attachment proteins is intriguing (Meier, 2000). Most of our knowledge of Arf function in plants was based on the study of the GNOM gene identified from genetic screen for mutations affecting embryo patterning in Arabidopsis (Steinmann et al., 1999; Geldner et al., 2001). GNOM encodes an Arf GEF (Steinmann et al., 1999). Experiments using gnom mutations, brefeldin A (an Arf GEF inhibitor), and auxin transport inhibitors suggest that an Arf or Arf GEF is critical for polar localization of the PIN1 putative auxin efflux carrier by modulating general protein trafficking between the plasma membrane (PM) and the cytoplasm (Geldner et al., 2001). Rab is the largest family of small GTPases; 57 distinct Rab GTPases are present in Arabidopsis (Bischoff et al., 1999; Takai et al., 2001; V. Vernoud, A. Horton, Z. Yang, and E. Nielson, unpublished data). Several functional studies of plant Rab homologs seem to conform to their po