Hou, M. C., Wiley, D. J., Verde, F. & McCollum, D. Mob2p interacts with the protein kinase Orb6p to promote coordination of cell polarity with cell cycle progression. J. Cell Sci. 116, 125-135

Department of Biochemistry and Molecular Biology, University of Miami, كورال غيبلز، فلوريدا, Florida, United States
Journal of Cell Science (Impact Factor: 5.43). 02/2003; 116(Pt 1):125-35. DOI: 10.1242/jcs.00206
Source: PubMed


The molecular mechanisms that temporally and spatially coordinate cell morphogenesis with the cell cycle remain poorly understood. Here we describe the characterization of fission yeast Mob2p, a novel protein required for regulating cell polarity and cell cycle control. Deletion of mob2 is lethal and causes cells to become spherical, with depolarized actin and microtubule cytoskeletons. A decrease in Mob2p protein level results in a defect in the activation of bipolar growth. This phenotype is identical to that of mutants defective in the orb6 protein kinase gene, and we find that Mob2p physically interacts with Orb6p. In addition, overexpression of Mob2p, like that of Orb6p, results in a delay in the onset of mitosis. Mob2p localizes to the cell periphery and cytoplasm throughout the cell cycle and to the division site during late anaphase and telophase. Mob2p is unable to localize to the cell middle in mutants defective in actomyosin ring and septum formation. Our results suggest that Mob2p, along with Orb6p, is required for coordinating polarized cell growth during interphase with the onset of mitosis.

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    • "The MOR pathway, also named the RAM (regulation of Ace2 and morphogenesis) pathway, is conserved among different fungi (Maerz and Seiler, 2010). This pathway includes a nuclear Dbf2-related (NDR) kinase termed Cbk1 in Saccharomyces cerevisiae (Racki et al., 2000), Orb6 in Schizosaccharomyces pombe (Verde et al., 1998), and COT1 in Neurospora crassa (Yarden et al., 1992), associated with a regulatory subunit, Mob2 (Hou et al., 2003; Maerz et al., 2009; Nelson et al., 2003). The NDR-Mob2 complex is activated by a germinal center kinase called Kic1 in S. cerevisiae (Nelson et al., 2003), Nak1 in S. pombe (Huang et al., 2003), and Pod6 in N. crassa (Seiler et al., 2006). "
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    ABSTRACT: The MOR (Morphogenesis-related NDR kinase) pathway regulates morphogenesis in fungi. In spite of the high conservation of its components, impairing their functions results in highly divergent cellular responses depending on the fungal species. The reasons for such differences are unclear. Here we propose that the species-specific connections between the cell cycle regulation and the MOR pathway could be in part responsible for these divergences. We based our conclusion on the characterization of the MOR pathway in the fungus Ustilago maydis. Each gene that encodes proteins of this pathway in U. maydis was deleted. All mutants exhibited a constitutive hyperpolarized growth contrasting with the loss of polarity observed in other fungi. Using a conditional allele of the central NDR kinase Ukc1, we found that impairing MOR function resulted in an elongated G2 phase. This cell cycle delay appears to be the consequence of an increase in Cdk1 inhibitory phosphorylation. Strikingly, abrogation of the inhibitory Cdk1 phosphorylation prevents the hyperpolarized growth associated with MOR pathway depletion. We found that enlarged G2 phase resulted in higher levels of expression of crk1, a conserved kinase that promotes polar growth in U. maydis. Deletion of crk1 also abolished the dramatic activation of polar growth in cells lacking MOR pathway. Taken together, our results suggest that Cdk1 inhibitory phosphorylation may act as an integrator of signaling cascades regulating fungal morphogenesis and that the distinct morphological response observed in U. maydis upon impairment of the MOR pathway could be due to a cell cycle deregulation.
    Preview · Article · Jul 2012 · Journal of Cell Science
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    • "Previous reports suggest that activation of Ndr kinases is a multistep process involving binding of Mob proteins and phosphorylation at the conserved sites [13], [14], and the conserved phosphorylation sites have been shown to be important for the function of several Ndr kinases [15], [16], [21]–[23]. The mechanism of Orb6 activation remains unclear, although it was reported that Orb6 interacts with Mob2 [24], and Orb6 kinase activity is dependent on the MOR network components Pmo25, Nak1 and Mor2 [9]. In this paper, we have further investigated the relationship of Nak1 and Orb6, and the role of conserved phosphorylation sites in Orb6 function. "
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    ABSTRACT: The Ndr-related Orb6 kinase is a key regulator of polarized cell growth in fission yeast, however the mechanism of Orb6 activation is unclear. Activation of other Ndr kinases involves both autophosphorylation and phosphorylation by an upstream kinase. Previous reports suggest that the Nak1 kinase functions upstream from Orb6. Supporting this model, we show that HA-Orb6 overexpression partially restored cell polarity in nak1 ts cells. We also demonstrated by coimmunoprecipitation and in vitro binding assays that Nak1 and Orb6 physically interact, and that the Nak1 C-terminal region is required for Nak1/Orb6 complex formation in vivo. However, results from in vitro kinase assays did not show phosphorylation of recombinant Orb6 by HA-Nak1, suggesting that Orb6 activation may not involve direct phosphorylation by Nak1. To investigate the role of Orb6 phosphorylation and activity, we substituted Ala at the ATP-binding and conserved phosphorylation sites. Overexpression of kinase-dead HA-Orb6(K122A) in wild-type cells resulted in a loss of cell polarity, suggesting that it has a dominant-negative effect, and it failed to rescue the polarity defect of nak1 or orb6 ts mutants. Recombinant GST-Orb6(S291A) did not autophosphorylate in vitro suggesting that Ser291 is the primary autophosphorylation site. HA-Orb6(S291A) overexpression only partially rescued the orb6 polarity defect and failed to rescue the nak1 defect, suggesting that autophosphorylation is important for Orb6 function. GST-Orb6(T456A) autophosphorylated in vitro, indicating that the conserved phosphorylation site at Thr456 is not essential for kinase activity. However, HA-Orb6(T456A) overexpression had similar effects as overexpressing kinase-dead HA-Orb6(K122A), suggesting that Thr456 is essential for Orb6 function in vivo. Also, we found that both phosphorylation site mutations impaired the ability of Myc-Nak1 to coimmunoprecipitate with HA-Orb6. Together, our results suggest a model whereby autophosphorylation of Ser291 and phosphorylation of Thr456 by an upstream kinase promote Nak1/Orb6 complex formation and Orb6 activation.
    Preview · Article · May 2012 · PLoS ONE
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    • "It will be of interest to determine whether the steady-state level of phosphorylation of these sites in Sid2p is affected in PP2A mutants. In S. pombe, the NDR-family kinase Mob2p–Orb6p (Hou et al. 2003) is a component of the morphology network (MOR), which regulates polar growth in S. pombe. It has been suggested that increased SIN activity is antagonistic to the MOR (Ray et al. 2010), and that SIN activity in mitosis turns off polar growth in preparation for cytokinesis; reviewed by Gupta and McCollum (2011). "
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    ABSTRACT: The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis. Cdc7p is the first kinase in the core SIN; we have screened genetically for SIN regulators by isolating cold-sensitive suppressors of cdc7-24. Our screen yielded a mutant in SPAC1782.05, one of the two fission yeast orthologs of mammalian phosphotyrosyl phosphatase activator. We have characterized this gene and its ortholog SPAC4F10.04, which we have named ypa2 and ypa1, respectively. We find that Ypa2p is the major form of protein phosphatase type 2A activator in S. pombe. A double ypa1-Δ ypa2-Δ null mutant is inviable, indicating that the two gene products have at least one essential overlapping function. Individually, the ypa1 and ypa2 genes are essential for survival only at low temperatures. The ypa2-Δ mutant divides at a reduced cell size and displays aberrant cell morphology and cytokinesis. Genetic analysis implicates Ypa2p as an inhibitor of the septation initiation network. We also isolated a cold-sensitive allele of ppa2, the major protein phosphatase type 2A catalytic subunit, implicating this enzyme as a regulator of the septation initiation network.
    Preview · Article · Jan 2012 · Genetics
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