Year: 2002
Type: review
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 potential roles in vesicle trafficking (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; Batoko et al., 2000; McElver et al., 2000; Lu et al., 2001). For example, expression of a tomato Rab11 antisense RNA caused defects in the secretion of cell wall–degrading enzymes in ripening fruits (Lu et al., 2001). Some surprising observations have been reported regarding the function of plant homologs of Rab11, which may be involved in endocytosis or exocytosis in mammalian cells (Sano et al., 1994; Bischoff et al., 1999; Kang et al., 2001; Lu et al., 2001). The aforementioned tomato antisense rab11 plants also showed pleiotropic phenotypes associated with altered hormone accumulation or responses, for example, reduced apical dominance and ectopic shoots on leaves (Lu et al., 2001). On the other hand, overexpression of a tobacco Rab11 caused an elevated cytokinin level and pleiotropic phenotypes similar to the antisense rab11 phenotypes (Sano et al., 1994). Although these observations could be explained by pleiotropic effects of trafficking defects, further studies should determine whether Rab11 plays any direct roles in the regulation of hormone accumulation or responses. Interestingly, a pea Rab11 (PRA2) has been shown to regulate light-mediated brassinosteroid biosynthesis (Kang et al., 2001). GTP-bound PRA2 directly binds the CPC p450 cytochrome (Kang et al., 2001), suggesting a direct role for PRA2 in signaling. This finding raises an intriguing possibility that plants may have adapted to use some of the “traffic control” proteins for the transmission of extracellular signals. Ras and Rho are bona fide signaling proteins known to transmit extracellular in and Although Ras has a role in cellular signaling in and various eukaryotes Arabidopsis no Ras homologs 2000). Rho GTPases shown to regulate the of the and cell in eukaryotes 1999; and 2000; and Yang, 2000; and Yang, 2001; et al., Takai et al., 2001). Rho signaling many processes, including gene cell cell cycle and cell in various eukaryotic 2000; 2001). Functional diversity of Rho GTPases is by the of distinct Rho GTPases in different 2000; and Yang, The Rho family is of conserved and and members or to specific 2000; and Yang, and Rho are present in and most whereas is several Rho GTPases that not into these as 2000). plants not contain for any of these and GTPases. a of GTPases have been found in plants their in pea in and 1993; et al., 2000; and Yang, of the plant GTPases that they all to a GTPase from that apparently from the Rho, or GTPases et al., and Yang, V. Vernoud, A. Horton, Z. Yang, and E. Nielson, unpublished data). the of GTPases is in the The Arabidopsis proteins Arabidopsis and to et al., 2000). The also these GTPases or et al., 1999; et al., 2001). from the use of the same for different by different for example, used by the is but was used by et for et al., 2000; et al., et al., 2001). For and in I use for various members of the of Rho small GTPases. of and et al., et al., et al., 1999; et al., of and et al., et al., et al., 1999; et al., of the Rho GTPases and or a used for functional active of the Rho GTPases and or a used for functional active plants only a of signaling small intriguing is that as a molecular switch in of and Rho as well as Ras in the regulation of and cell as well as for the transmission of extracellular et al., et al., 2000; and Yang, have a of the and are the of of CA and DN for Functional The DN of GTPase is in GDP-bound or and the CA is in GTP-bound of DN mutants by whereas CA mutants are to the action of and activate shown are those used for the of the function of Arabidopsis et al., 1999; et al., 1999; et al., 2001; et al., 2001). indicates the of the between and GTP by the The of the roles of in plants with the finding that pea proteins to the apical of the in et al., This finding was by a requiring of to the and their with the apical are essential role for in was using of an into pea and Yang, and in Arabidopsis to be in et al., et al., 1999). Functional for and was by the of by expression of or et al., 1999; et al., 1999). and through to and at the of phenotypes by or in different cells et al., 1999; et al., et al., are shown at and localization or its is shown at (A) localization to the apical in pea as shown by et al., (B) The localization of in tobacco the localization of GTP-bound active to the apical et al., 2001). to The localization of to the in the cell the of to the of and the of et al., localization of to the of in cells et al., All shown using activate the signaling and which factors control the localization of to the apical in are and A study by et the for an regulation a signaling et al., 1999; and Yang, A level of is activated by an the of more proteins to the of a of of the activated signaling to from the of as in the of caused by (Figure the regulation of by and the signaling to the This can various including the of to the apical by and more by expression (Figure et al., 1999; and Yang, the localization of Rho GTPases to the of and to the of cells in various and may a for Rho GTPase control of polar and or in more is to the signaling regulate that active the generation of and the of of which are critical for et al., 1999; et al., 1999; and Yang, et al., 2001; and Yang, 2001). is that activate downstream pathways to and accumulation at the et al., 2001). The may to the of whereas the may regulate vesicle with the apical This is by of distinct putative targets in the control of polar in and et al., 2001). The potential of downstream pathways is to control of polar in including of for cell and et al., 2001). and have no to proteins of known and regulate and accumulation a the study of function in a signaling that cell in plants. studies a role for in various of et al., 2001; et al., complex of with from a the of similar to is at the of the to form a using an and that to the of Arabidopsis as in (Figure et al., 2001; et al., expression of CA mutants of or caused or in Arabidopsis et al., 2001; et al., whereas expression et al., Interestingly, several including et al., These studies that in as well as in As in control in apparently downstream pathways and et al., 2000; and Yang, 2001; et al., 2001; et al., from also control the of and the of et al., This from other including of and of from a of from a and of et al., The localization of to the of and the in as by expression (Figure or using et al., 2001; et al., is with these The is to of whereas the of is regulated by et al., 1999). these processes of through distinct mechanisms different from its control of Although and in the same may be in to only has been shown to be in cells and in et al., the localization for and various phenotypes suggest that a may control various of cell in et al., This is in to the control of cell in distinct G a GTPase or heterotrimeric G and control polar and polar 1999). These observations are with the that as the of signaling small GTPases in plants, have adapted various functions that be by distinct of GTPases in and and Yang, 2001). and which by cells are to by et al., to the cell is by the of and et al., studies have shown that signaling also plays a general role in the regulation of cell in Arabidopsis et al., 2001; et al., On the of the that mutants caused of and et that signaling regulates the of et al., 2001). polar cell in an and or and a only et al., 2001; et al., A of of cell in Arabidopsis plants and to the that regulates the of polar cell (Figure et al., expression a form of that is associated with polar in the of whereas caused of the cell et al., The of to be a the control of polar cell in different cell including and cells and Yang, 2001; et al., et al., from specific mechanisms are required for cell in different cell For example, to regulate the of a in which has not been found in The of mechanisms by which cell and polar in different cell the of specific and targets and regulators involved in function is regulation of in function with the for a plant of of the of using an 1994; et al., In et identified several GTPases that to and showed that expression of and mutants, activated and by et al., 1999). Interestingly, was by the inhibitor mutants of a or in Arabidopsis or et al., 1999; et al., 2000). A role for signaling in the regulation of has also been in Arabidopsis to which and in Arabidopsis to the of and gene expression et al., The of was also by of the that is to in its of a Functional homologs of the of the are present in plants, no plant homologs of other and have been and 2001). plant proteins as of This is in with the of between the insert to to of Arabidopsis and the insert known to with studies should determine whether as an or regulates through other has as an in plant as in the regulation of and cell as well as acid and auxin signaling and 1997; et al., 1999; et al., 2000; et al., 2001). regulation of may to be a signaling in plants. of the most signaling is the of its in the regulation of responses. A in the and of by and expression in Arabidopsis et al., 2001). not whether is a direct of an or a of a that responses. A role for in the regulation of was more shown in cells by et of in Arabidopsis in plants, whereas expression caused in the and the in the of This study that one or more which apparently downstream of the protein to through the of in cells et al., 2001). be to whether the is or is also involved in other responses, including those by and of mutants have and as more general regulators of and Z. Yang, unpublished data). A or a showed weak of of and mutations in these Because and contain a putative C-terminal motif (see one or of these could be of the of protein known to be involved in the regulation of in cell and et al., 1996; et al., These studies an intriguing the regulation of by a distinct or a of The altered caused by expression of or mutants et al., 2001; et al., could be explained by their with the function of and and could regulate distinct In with the is the that and are distinct from and and that expression not with the function of and in the control of cell and processes and Z. Yang, unpublished data). of various and mutants should to the role of GTPases in signaling. regulation of other hormone functions was by of plants or plants many phenotypes that or plants, whereas plants many phenotypes that or or mutants et al., 2001). expression of whereas expression expression the of of by whereas These and phenotypes could be explained by the that different are involved in the regulation of the accumulation of and auxin et al., 2001). the phenotypes could be the of the of signaling on the regulation of one hormone and a between and and expression also other including defects in embryo and and Yang, et al., 2001). These phenotypes apparently be explained by the of these mutants on hormone functions and cell and Yang, et al., 2001). proteins have been shown to to and are in the and et al., 2001; et al., 2001). studies are to signaling pathways in plants. the of GTPases into four their functional As and could be targets that negatively regulate in expression reduced to but not phenotypes by expression and Z. Yang, unpublished data). mutants for have been shown to but no phenotypes reported for plants these mutants et al., 2001). In expression caused pleiotropic and cell phenotypes and Yang, 2001; et al., 2001; et al., et al., and and expression also caused cell phenotypes et al., 2001). and all to 2). distinct phenotypes in Arabidopsis plants a from et al., these suggest that to participate in responses, including and whereas to regulate cell and cell through et al., 1999; et al., 1999; and Yang, 2001; et al., et al., 2001; et al., 2001; et al., members may regulate distinct processes in different cell and by distinct on their expression and the expression of their functional For example, Arabidopsis and are in and to be in whereas and to control and cell in et al., regulation of may differ from that of cell in as for in the is of the for different with a of different may control distinct signaling GTPases with various upstream regulators and downstream as Rho GTPases in and The diversity of may for the functional of as a molecular switch in cell signaling. As expected, some of these are conserved proteins involved in Rho GTPase signaling. Interestingly, many are novel or to plants, with the that to a plant-specific of the Rho family of small GTPases et al., and Yang, other G membrane is a for the of GTPases or their interaction of In some regulated to a specific membrane is for the function of (see Figure with their roles in regulation of cellular processes et al., 1999; et al., 1999; and Yang, 2001; et al., et al., 2001; et al., 2001; et al., As with most small membrane of is by including prenylation of a in the terminus et al., 1999; et al., 1999). All members of and contain a motif by protein geranylgeranyltransferase I (GGTase and Yang, et al., 2001). prenylation for are more with a putative motif for GGTase et al., Arabidopsis a motif and and a prenylation motif and Yang, et al., 2001). Interestingly, a in the motif of not their localization to the et al., and Arabidopsis can be to the in a and Z. Yang, unpublished data). This raises the possibility that a different of for example, may be for membrane of these (see The of Rho GTPases are by by at the and 1999). The to membrane of proteins. is present in plants, suggesting that similar in GTPases et al., et al., 2000; and 2001). are required for GTPase to specific Several localization have been for different a the and and specific domains of the and and the of or more pea et al., 1999; et al., 1999; Bischoff et al., 2000; et al., 2001; et al., As shown for some Rho for specific localization apparently also in the C-terminal to the (Bischoff et al., 2000). In mammalian for localization of GTPases are a domain or of the C-terminal et al., contain one or that may be and to the et al., 2000). contain a domain in the C-terminal and the members studied to all between the and the et al., 1999; et al., as mammalian and et al., a may a regulation of to the of action (Figure et al., 1999; et al., 1999; et al., et al., The in the regulation of localization between these of with their distinct cellular functions as cellular factors GTPases to specific domains and control their between the and Interestingly, the localization of to the was by brefeldin A suggesting that vesicle trafficking or a protein is involved in to these et al., 2001). that active the of to the apical in and Z. Yang, unpublished data). is its to the A plays an role in the regulation of to the (Bischoff et al., 2000; and Z. Yang, unpublished overexpression of Arabidopsis from the and caused by V. Vernoud, and Z. Yang, unpublished data). In have been identified (Bischoff et al., 2000). A could also be involved in the regulation of localization to specific domains et al., et al., is to be whether different have effects on the localization of different and Rho GTPases are directly activated by Rho These Rho contain a domain, the domain for the to GTP exchange in Rho GTPases and plants proteins homologous to the domain, for other small GTPases as have been identified in plants (Steinmann et al., 1999). can the of homologs in plants. A novel GEF could have for with the of to the of Rho in and a novel could be involved in For example, could GTP with a and GTP-bound could be as as GDP-bound is to the In the for could be the of specific factor. an Rho with the of with for et al., 2000). Interestingly, a protein directly with the active complex but not with the inactive complex et al., 1999). could be directly regulated by the functional of to be of direct is a in the study of signaling. other small is required for efficient of Rho GTPases. A novel of plant Rho was identified using the et al., 1999; et al., 2000). These contain a domain with the to the mammalian et al., 2000). in showed that Arabidopsis Rho GTP by but only GTP by suggesting that they as the plant Rho are et al., 2000). are with a binding motif upstream of the This present in many their specific binding to active GTPases but has not been found in any or contain a The motif is for of GTP hydrolysis, mutations the motif activity to a level as well as its binding to et al., 2000). Because the domain from binds the of was that domain or the of the GTP et al., 2000). In studies that the motif is critical for the localization of Arabidopsis to the apical in and for the function of as a in signaling to and Z. Yang, unpublished data). in the study of signaling is the of a role for in the regulation of signaling using a et al., This exhibits a level of activity and expression by Interestingly, signaling and a These observations that as a of a switch to activate the of as a and to the accumulation of to a be to whether similar control other are by a family et al., 1999; et al., 2000). Arabidopsis and homologs are present in and and C-terminal of of the conserved and domains are and may functional for different Interestingly, the level of active et al., not phenotypes that are caused by expression et al., 2001). should determine whether different with and mutants for different distinct In and of Rho GTPases a of distinct targets with various from protein through regulators of to 1999). no homologs of Rho targets from and have been found in plants et al., 2001). Because effectors only with the GTP form of small a was used as a in a screen to putative effectors from Arabidopsis et al., 2001). This screen identified a novel protein a which with GTP-bound not GDP-bound A Arabidopsis of the motif are in the Arabidopsis Because the motif is a hallmark structure of many are for distinct GTPase A the Functional of GTPases. signaling many processes and many proteins. may with different regulators and proteins to distinct signaling regulation not only by an and by and but also in specific or membrane In to the as shown in is also found in specific domains and other as to specific could specific or docking proteins. distinct targets including various and other proteins are to control specific downstream effects to functional of is that some many and not use a motif for their interaction with 1999). potential is a that was from tobacco and shown to with in This was shown to et al., 1999). A domain that binds and was to the apical of and its expression as et al., 1999). is not whether directly binds and whether is involved in the regulation of regulation or downstream of in the control of polar in is was shown that but not GDP-bound with a putative of Arabidopsis et al., 2001). This is is known to directly with and activate are to determine whether these have functional Although the study of small GTPases in plants is its the has been in our of signaling pathways by these by the plant-specific of Rho GTPases. The surprising finding of Rab11 regulation of biosynthesis (Kang et al., that that the conserved Rab, Arf, and Ran GTPases control fundamental cellular processes as in and the of these GTPases and their potential function in these fundamental processes present a for their specific roles in signaling. functional in should be to their functions using mutants in with of their and cellular localization and of their functional as a of signaling small GTPases in plants, has as an molecular switch in plant cellular signaling. The use of mutants has a of functional for GTPases (Figure but many signaling the roles of to be using only Arabidopsis have been using A functional using may also to the of many new be to complex functions for for example, and functions. A study cell and genetic in a example, is to determine regulate is at cellular and and interacts with different regulators and The has several regulators and potential targets (Figure factor to cellular and is the molecular Although a protein is known to with the is the functional of is and whether is associated with other control downstream to specific processes also further to be targets that to various downstream but their functions have to be using Because no to known be critical to proteins to control downstream is by a distinct signaling of to specific signaling with cell and genetic of these should the mechanisms for and as well as those the functional diversity and of GTPases in plants. I to and for their and and to for in is by from the the of and the of
Source: The Plant Cell
DOI: 10.1105/tpc.001065
Author: Zhenbiao Yang
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2121253860