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Williams TD, Paschke PI, Kay RR. Function of small GTPases in Dictyostelium macropinocytosis. Philos Trans R Soc Lond B Biol Sci 2019; 374:20180150. [PMID: 30967009 PMCID: PMC6304742 DOI: 10.1098/rstb.2018.0150] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/31/2018] [Indexed: 12/17/2022] Open
Abstract
Macropinocytosis-the large-scale, non-specific uptake of fluid by cells-is used by Dictyostelium discoideum amoebae to obtain nutrients. These cells form circular ruffles around regions of membrane defined by a patch of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and the activated forms of the small G-proteins Ras and Rac. When this ruffle closes, a vesicle of the medium is delivered to the cell interior for further processing. It is accepted that PIP3 is required for efficient macropinocytosis. Here, we assess the roles of Ras and Rac in Dictyostelium macropinocytosis. Gain-of-function experiments show that macropinocytosis is stimulated by persistent Ras activation and genetic analysis suggests that RasG and RasS are the key Ras proteins involved. Among the activating guanine exchange factors (GEFs), GefF is implicated in macropinocytosis by an insertional mutant. The individual roles of Rho family proteins are little understood but activation of at least some may be independent of PIP3. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.
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Affiliation(s)
| | | | - Robert R. Kay
- MRC-Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK
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Veeranki S, Kim B, Kim L. The GPI-anchored superoxide dismutase SodC is essential for regulating basal Ras activity and for chemotaxis of Dictyostelium discoideum. J Cell Sci 2008; 121:3099-108. [PMID: 18768936 DOI: 10.1242/jcs.030056] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
A genetic screen for Dictyostelium mutant displaying high level of constitutive phosphatidylinositol (3,4,5)-trisphosphate led to the finding that the glycosylphosphatidylinositol (GPI)-anchored superoxide dismutase SodC regulates small GTPase Ras. Cells that lack SodC exhibited constitutively high levels of active Ras, more membrane localization of GFP-PHcrac, and defects in chemoattractant sensing, cell polarization and motility. These defects of SodC-lacking cells were partially restored by expression of wild-type SodC but not by the catalytically inactive mutant SodC (H245R, H247Q). Furthermore, an inhibition of PI3K activity in SodC-deficient cells by LY294002 only partially restored chemoattractant sensing and cell polarization, consistent with the fact that SodC-deficient cells have aberrantly high level of active Ras, which functions upstream of PI3K. A higher level of active GFP-RasG was observed in SodC-deficient cells, which significantly decreased upon incubation of SodC-deficient cells with the superoxide scavenger XTT. Having constitutively high levels of active Ras proteins and more membrane localization of GFP-PHcrac, SodC-deficient cells exhibited severe defects in chemoattractant sensing, cell polarization and motility.
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Affiliation(s)
- Sudhakar Veeranki
- Department of Biological Sciences, Florida International University, Miami, FL 33199, USA
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Chen Y, McQuade KJ, Guan XJ, Thomason PA, Wert MS, Stock JB, Cox EC. Isoprenylcysteine carboxy methylation is essential for development in Dictyostelium discoideum. Mol Biol Cell 2007; 18:4106-18. [PMID: 17699599 PMCID: PMC1995708 DOI: 10.1091/mbc.e06-11-1006] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Members of the Ras superfamily of small GTPases and the heterotrimeric G protein gamma subunit are methylated on their carboxy-terminal cysteine residues by isoprenylcysteine methyltransferase. In Dictyostelium discoideum, small GTPase methylation occurs seconds after stimulation of starving cells by cAMP and returns quickly to basal levels, suggesting an important role in cAMP-dependent signaling. Deleting the isoprenylcysteine methyltransferase-encoding gene causes dramatic defects. Starving mutant cells do not propagate cAMP waves in a sustained manner, and they do not aggregate. Motility is rescued when cells are pulsed with exogenous cAMP, or coplated with wild-type cells, but the rescued cells exhibit altered polarity. cAMP-pulsed methyltransferase-deficient cells that have aggregated fail to differentiate, but mutant cells plated in a wild-type background are able to do so. Localization of and signaling by RasG is altered in the mutant. Localization of the heterotrimeric Ggamma protein subunit was normal, but signaling was altered in mutant cells. These data indicate that isoprenylcysteine methylation is required for intercellular signaling and development in Dictyostelium.
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Affiliation(s)
- Ying Chen
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
| | - Kyle J. McQuade
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
- Department of Biology, Mesa State College, Grand Junction, CO 81501
| | - Xiao-Juan Guan
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
| | - Peter A. Thomason
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
| | - Michael S. Wert
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
| | - Jeffry B. Stock
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
| | - Edward C. Cox
- *Department of Molecular Biology, Princeton University, Princeton, NJ 08544; and
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Lim CJ, Spiegelman GB, Weeks G. Cytoskeletal regulation by Dictyostelium Ras subfamily proteins. J Muscle Res Cell Motil 2003; 23:729-36. [PMID: 12952071 DOI: 10.1023/a:1024471527153] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The Ras subfamily proteins are monomeric GTPases that function as molecular switches in cellular signal transduction. The roles of six of these proteins in regulating actin cytoskeletal functions in Dictyostelium discoideum are discussed in this review.
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Affiliation(s)
- Chinten James Lim
- Department of Microbiology and Immunology, University of British Columbia, 300-6174 University Boulevard, Vancouver, British Columbia, Canada V6T 1Z3
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Secko DM, Khosla M, Gaudet P, Tsang A, Spiegelman GB, Weeks G. RasG regulates discoidin gene expression during Dictyostelium growth. Exp Cell Res 2001; 266:135-41. [PMID: 11339832 DOI: 10.1006/excr.2001.5216] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Activated rasG, rasG(G12T), was expressed in Dictyostelium cells under the control of the folate-repressible discoidin promoter (pVEII-rasG(G12T)) and found to have a unique pattern of expression when cells were transferred to folate-deficient media: an initial increase of RasG(G12T) resulting from the removal of folate, followed by a rapid decline while cells were still in the early exponential phase of growth. Discoidin levels were considerably lower and declined more rapidly in the pVEII-rasG(G12T) transformant than they did in the wild type, suggesting that RasG(G12T) represses discoidin expression. This was independently confirmed by placing the rasG(G12T) gene under the control of the ribonucleotide reductase (rnrB) promoter. Exposure of cells to 10 mM methyl methanesulfonate (MMS) rapidly generated RasG(G12T) and this was accompanied by an equally rapid decrease in discoidin mRNA levels. rasG null cells also contained decreased levels of discoidin under all conditions tested, indicating that RasG is essential for optimum discoidin expression. However, rasG null cells showed normal regulation of discoidin expression in response to PSF, CMF, folate, bacteria, and axenic media, indicating that RasG is not necessary for any of these responses. These results reveal a role for RasG in regulating discoidin gene expression and add a further level of complexity to the regulation of the discoidin promoter.
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Affiliation(s)
- D M Secko
- Department of Microbiology and Immunology, University of British Columbia, 6174 University Boulevard, Vancouver, British Columbia V6T 1Z3, Canada
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Palmieri SJ, Nebl T, Pope RK, Seastone DJ, Lee E, Hinchcliffe EH, Sluder G, Knecht D, Cardelli J, Luna EJ. Mutant Rac1B expression in Dictyostelium: effects on morphology, growth, endocytosis, development, and the actin cytoskeleton. CELL MOTILITY AND THE CYTOSKELETON 2000; 46:285-304. [PMID: 10962483 DOI: 10.1002/1097-0169(200008)46:4<285::aid-cm6>3.0.co;2-n] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Rac1 is a small G-protein in the Ras superfamily that has been implicated in the control of cell growth, adhesion, and the actin-based cytoskeleton. To investigate the role of Rac1 during motile processes, we have established Dictyostelium cell lines that conditionally overexpress epitope-tagged Dictyostelium discoideum wild-type Rac1B (DdRac1B) or a mutant DdRac1B protein. Expression of endogenous levels of myc- or GFP-tagged wild-type DdRac1B had minimal effect on cellular morphologies and behaviors. By contrast, expression of a constitutively active mutant (G12-->V or Q61-->L) or a dominant negative mutant (T17-->N) generated amoebae with characteristic cellular defects. The morphological appearance of actin-containing structures, intracellular levels of F-actin, and cellular responses to chemoattractant closely paralleled the amount of active DdRac1B, indicating a role in upregulating actin cytoskeletal activities. Expression of any of the three mutants inhibited cell growth and cytokinesis, and delayed multicellular development, suggesting that DdRac1B plays important regulatory role(s) during these processes. No significant effects were observed on binding or internalization of latex beads in suspension or on intracellular membrane trafficking. Cells expressing DdRac1B-G12V exhibited defects in fluid-phase endocytosis and the longest developmental delays; DdRac1B-Q61L produced the strongest cytokinesis defect; and DdRac1B-T17N generated intermediate phenotypes. These conditionally expressed DdRac1B proteins should facilitate the identification and characterization of the Rac1 signaling pathway in an organism that is amenable to both biochemical and molecular genetic manipulations.
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Affiliation(s)
- S J Palmieri
- Department of Cell Biology, University of Massachusetts Medical School, Worcester, UK
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Khosla M, Spiegelman GB, Insall R, Weeks G. Functional overlap of the dictyostelium RasG, RasD and RasB proteins. J Cell Sci 2000; 113 ( Pt 8):1427-34. [PMID: 10725225 DOI: 10.1242/jcs.113.8.1427] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Disruption of the rasG gene in Dictyostelium discoideum results in several distinct phenotypes: a defect in cytokinesis, reduced motility and reduced growth. Reintroduction of the rasG gene restores all of the properties of the rasG(-) cells to those of the wild type. To determine whether the defects are due to impaired interactions with a single or multiple downstream effectors, we tested the ability of the highly related but non identical Dictyostelium ras genes, rasD and rasB, to rescue the defects. Introduction of the rasD gene under the control of the rasG promoter into rasG null (rasG(-)) cells corrected all phenotypes except the motility defect, suggesting that motility is regulated by a RasG mediated pathway that is different to those regulating growth or cytokinesis. Western blot analysis of RasD protein levels revealed that vegetative rasG(-)cells contained considerably more protein than the parental AX-3 cells, suggesting that RasD protein levels are negatively regulated in vegetative cells by RasG. The level of RasD was enhanced when the rasD gene was introduced under the control of the rasG promoter, and this increase in protein is presumably responsible for the reversal of the growth and cytokinesis defects of the rasG(-)cells. Thus, RasD protein levels are controlled by the level of RasG, but not by the level of RasD. Introduction of the rasB gene under the control of the rasG promoter into rasG(-) cells produced a complex phenotype. The transformants were extremely small and mononucleate and exhibited enhanced motility. However, the growth of these cells was considerably slower than the growth of the rasG(-) cells, suggesting the possibility that high levels of RasB inhibit an essential process. This was confirmed by expressing rasB in wild-type cells; the resulting transformants exhibited severely impaired growth. When RasB protein levels were determined by western blot analysis, it was found that levels were higher in the rasG(-)cells than they were in the wild-type parental, suggesting that RasG also negatively regulates rasB expression in vegetative cells. Overexpression of rasB in the rasG(-)cells also reduced the level of RasD protein. In view of the fact that alternate Ras proteins correct some, but not all, of the defects exhibited by the rasG(-) cells, we propose that RasG interacts with more than one downstream effector. In addition, it is clear that the levels of the various Ras proteins are tightly regulated in vegetative cells and that overexpression can be deleterious.
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Affiliation(s)
- M Khosla
- Department of Microbiology and Immunology, University of British Columbia, Vancouver, BC V6T 1Z3, Canada
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