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The Conserved NDR Kinase Orb6 Controls Polarized Cell Growth by Spatial Regulation of the Small GTPase Cdc42

Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL 33101-1015, USA.
Current biology: CB (Impact Factor: 9.92). 08/2009; 19(15):1314-9. DOI: 10.1016/j.cub.2009.06.057
Source: PubMed

ABSTRACT The conserved NDR kinase regulates cell morphogenesis and polarized cell growth in different eukaryotic cells ranging from yeast to neurons. Although studies have unraveled the mechanism of regulation of NDR kinase activity, the mechanism of morphology control by NDR and the effectors that mediate NDR function are unknown. Via a chemical genetic approach, we show that the fission yeast NDR homolog, Orb6 kinase, maintains polarized cell growth at the cell tips by spatially regulating the localization of Cdc42 GTPase, a key morphology regulator. Loss of Orb6 kinase activity leads to the recruitment of Cdc42 GTPase and the Cdc42-dependent formin For3, normally found only at the cell tips, to the cell sides. Furthermore, we show that loss of Orb6 kinase activity leads to ectopic lateral localization of the Cdc42 guanine nucleotide exchange factor (GEF) Gef1, but not of the other Cdc42 GEF, Scd1. Consistent with these observations, gef1 deletion suppresses the increased cell diameter phenotype of orb6 mutants. In contrast, the microtubule cytoskeleton and the localization of the microtubule-dependent polarity markers Tea1 and Tea4 are not altered by loss of Orb6 kinase activity. Our findings indicate that the conserved NDR kinase Orb6 regulates cell polarity by spatially restricting the localization and activity of Cdc42 GTPase.

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    • "In fission yeast, cdc42 is essential and spores that lack the gene cannot form germination tubes, because they cannot polarize growth (Miller and Johnson, 1994). In addition, mutations in the essential kinase Orb6 lead to the mislocalization of Cdc42 to cell sides (Das et al., 2009), and render cells round (Verde et al., 1998). But we have shown here that even when the basic shape of the cell is polarized, the spatial control of Cdc42 affects the width of the growth zone. "
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    ABSTRACT: The fission yeast Schizosaccharomyces pombe is a rod-shaped cell that grows by linear extension at the cell tips, with a nearly constant width throughout the cell cycle. This simple geometry makes it an ideal system for studying the control of cellular dimensions. In this study, we carried out a near-genome-wide screen for mutants wider than wild-type cells. We found 11 deletion mutants that were wider; seven of the deleted genes are implicated in the control of the small GTPase Cdc42, including the Cdc42 guanine nucleotide exchange factor (GEF) Scd1 and the Cdc42 GTPase-activating protein (GAP) Rga4. Deletions of rga4 and scd1 had additive effects on cell width, and the proteins localized independently of one another, with Rga4 located at the cell sides and Scd1 at the cell tips. Activated Cdc42 localization is altered in rga4Δ, scd1Δ, and scd2Δ mutants. Delocalization and ectopic retargeting experiments showed that the localizations of Rga4 and Scd1 are crucial for their roles in determining cell width. We propose that the GAP Rga4 and the GEF Scd1 establish a gradient of activated Cdc42 within the cellular tip plasma membrane, and it is this gradient that determines cell growth-zone size and normal cell width.
    Molecular biology of the cell 08/2011; 22(20):3801-11. DOI:10.1091/mbc.E11-01-0057 · 5.98 Impact Factor
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    • "Recent evidence suggests that Cbk1 regulates polarized growth by controlling Golgi-and Sec2/Sec4- dependent processes (Kurischko et al., 2008). In fission yeast, Orb6 controls polarized cell growth by spatial regulation of the small GT- Pase Cdc42 (Das et al., 2009). "
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    Molecular biology of the cell 05/2011; 22(14):2458-69. DOI:10.1091/mbc.E11-03-0205 · 5.98 Impact Factor
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    • "Future efforts will focus on determining whether the protein kinase constituents of the SIN (SepH, SepL, and SidB) (Bruno et al. 2001; Kim et al. 2006, 2009) interact directly with AnBud3 to control its localization and/or activity. Notably, in S. pombe, Orb6, which like SidB is a member of the NDR kinase family, spatially regulates polarized growth by restricting the localization of the Cdc42 GEF Gef1 (Das et al. 2009). Finally, it should also be noted that our data imply that the Bud3– Rho4 module is not the sole target of the SIN. "
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    ABSTRACT: The ability of fungi to generate polarized cells with a variety of shapes likely reflects precise temporal and spatial control over the formation of polarity axes. The bud site selection system of Saccharomyces cerevisiae represents the best-understood example of such a morphogenetic regulatory system. However, the extent to which this system is conserved in the highly polarized filamentous fungi remains unknown. Here, we describe the functional characterization and localization of the Aspergillus nidulans homolog of the axial bud site marker Bud3. Our results show that AnBud3 is not required for polarized hyphal growth per se, but is involved in septum formation. In particular, our genetic and biochemical evidence implicates AnBud3 as a guanine nucleotide exchange factor for the GTPase Rho4. Additional results suggest that the AnBud3-Rho4 module acts downstream of the septation initiation network to mediate recruitment of the formin SepA to the site of contractile actin ring assembly. Our observations provide new insight into the signaling pathways that regulate septum formation in filamentous fungi.
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