Schizosaccharomyces pombe RanGAP homolog, SpRna1, is required for centromeric silencing and chromosome segregation

Association of RanGAP to nuclear pore complex component, RanBP2/Nup358, is required for pupal development in Drosophila

Ran's GTPase-activating protein (RanGAP) is tethered to the nuclear envelope (NE) in multicellular organisms. We investigated the consequences of RanGAP localization in human tissue culture cells and Drosophila. In tissue culture cells, disruption of RanGAP1 NE localization surprisingly has neither obvious impacts on viability nor nucleocytoplasmic transport of a model substrate. In Drosophila, we identified a region within nucleoporin dmRanBP2 required for direct tethering of dmRanGAP to the NE. A dmRanBP2 mutant lacking this region shows no apparent growth defects during larval stages but arrests at the early pupal stage. A direct fusion of dmRanGAP to the dmRanBP2 mutant rescues this arrest, indicating that dmRanGAP recruitment to dmRanBP2 per se is necessary for the pupal ecdysis sequence. Our results indicate that while the NE localization of RanGAP is widely conserved in multicellular organisms, the targeting mechanisms are not. Further, we find a requirement for this localization during pupal development.

RanGAP is required for post-meiotic mitosis in female gametophyte development in Arabidopsis thaliana

RanGAP is the GTPase-activating protein of the small GTPase Ran and is involved in nucleocytoplasmic transport in yeast and animals via the Ran cycle and in mitotic cell division. Arabidopsis thaliana has two copies of RanGAP, RanGAP1 and RanGAP2. To investigate the function of plant RanGAP, T-DNA insertional mutants were analysed. Arabidopsis plants with a null mutant of either RanGAP1 or RanGAP2 had no observable phenotype. Analysis of segregating progeny showed that double mutants in RanGAP1 and RanGAP2 are female gametophyte defective. Ovule clearing with differential interference contrast optics showed that mutant female gametophytes were arrested at interphase, predominantly after the first mitotic division following meiosis. In contrast, mutant pollen developed and functioned normally. These results show that the two RanGAPs are redundant and indispensable for female gametophyte development in Arabidopsis but dispensable for pollen development. Nuclear division arrest during a mitotic stage suggests a role for plant RanGAP in mitotic cell cycle progression during female gametophyte development.

Schizosaccharomyces pombe Snf2SR, a novel SNF2 family protein, interacts with Ran GTPase and modulates both RanGEF and RanGAP activities

Snf2SR, a suppressor of rna1(ts), which is a temperature-sensitive mutation in Schizosaccharomyces pombe RanGAP (GTPase activating protein), possesses both the SNF2 and the helicase domains conserved in the chromatin remodeling SNF2 ATPase/helicase protein family. We have now clarified a function of Snf2SR. Snf2SR indeed showed DNA-stimulated ATPase activity, proving that it is a member of the SNF2 ATPase/helicase family. Consistent with this role, Snf2SR was localized in the nucleus and cell fractionation analysis revealed that Snf2SR was tightly associated with the nuclear matrix. The disruption of snf2SR(+) was detrimental for a cell proliferation of S. pombe. Snf2SR that did not enhance RanGAP activity by itself, but abolished histone-H3-mediated RanGAP inhibition, as previously reported for the histone H3 methyltransferase, Clr4, another rna1(ts) suppressor. In contrast to Clr4, Snf2SR directly bound to the GDP-bound form of the S. pombe Ran homologue Spi1 and enhanced the nucleotide exchange activity of Pim1, the S. pombe RanGEF (guanine nucleotide exchange factor). Over-expression of Spi1-G18V, a Ran GTPase mutant fixed in the GTP-bound form, was lethal to S. pombe Deltasnf2SR. Together, our results indicate that Snf2SR is involved in the Ran GTPase cycle in vivo.

Mutations in Ran system affected telomere silencing in Saccharomyces cerevisiae

The Ran GTPase system regulates the direction and timing of several cellular events, such as nuclear-cytosolic transport, centrosome formation, and nuclear envelope assembly in telophase. To gain insight into the Ran system's involvement in chromatin formation, we investigated gene silencing at the telomere in several mutants of the budding yeast Saccharomyces cerevisiae, which had defects in genes involved in the Ran system. A mutation of the RanGAP gene, rna1-1, caused reduced silencing at the telomere, and partial disruption of the nuclear Ran binding factor, yrb2-delta2, increased this silencing. The reduced telomere silencing in rna1-1 cells was suppressed by a high dosage of the SIR3 gene or the SIT4 gene. Furthermore, hyperphosphorylated Sir3 protein accumulated in the rna1-1 mutant. These results suggest that RanGAP is required for the heterochromatin structure at the telomere in budding yeast.

Identification of novel suppressors for Mog1 implies its involvement in RNA metabolism, lipid metabolism and signal transduction

Mog1 is conserved from yeast to mammal, but its function is obscure. We isolated yeast genes that rescued a temperature-sensitive death of S. cerevisiae Scmog1Delta, and of S. pombe Spmog1(ts). Scmog1Delta was rescued by Opi3p, a phospholipid N-methyltransferase, in addition to S. cerevisiae Ran-homologue Gsp1p, and a RanGDP binding protein Ntf2p. On the other hand, Spmog1(ts) was rescued by Cid13 that is a poly (A) polymerase specific for suc22(+) mRNA encoding a subunit of ribonucleotide reductase, Ssp1 that is a protein kinase involved in stress response pathway, and Crp79 that is required for mRNA export, in addition to Spi1, S. pombe Ran-homologue, and Nxt2, S. pombe homologue of Ntf2p. Consistent with the identification of those suppressors, lack of ScMog1p dislocates Opi3p from the nuclear membrane and all of Spmog1(ts) showed the nuclear accumulation of mRNA. Furthermore, SpMog1 was co-precipitated with Nxt2 and Cid13.

The karyopherin Kap95 regulates nuclear pore complex assembly into intact nuclear envelopes in vivo

Nuclear pore complex (NPC) assembly in interphase cells requires that new NPCs insert into an intact nuclear envelope (NE). Our previous work identified the Ran GTPase as an essential component in this process. We proposed that Ran is required for targeting assembly factors to the cytoplasmic NE face via a novel, vesicular intermediate. Although the molecular target was not identified, Ran is known to function by modulating protein interactions for karyopherin (Kap) beta family members. Here we characterize loss-of-function Saccharomyces cerevisiae mutants in KAP95 with blocks in NPC assembly. Similar to defects in Ran cycle mutants, nuclear pore proteins are no longer localized properly to the NE in kap95 mutants. Also like Ran cycle mutants, the kap95-E126K mutant displayed enhanced lethality with nic96 and nup170 mutants. Thus, Kap95 and Ran are likely functioning at the same stage in assembly. However, although Ran cycle mutants accumulate small cytoplasmic vesicles, cells depleted of Kap95 accumulated long stretches of cytoplasmic membranes and had highly distorted NEs. We conclude that Kap95 serves as a key regulator of NPC assembly into intact NEs. Furthermore, both Kap95 and Ran may provide spatial cues necessary for targeting of vesicular intermediates in de novo NPC assembly.

Temperature-sensitive defects of the GSP1gene, yeast Ran homologue, activate the Tel1-dependent pathway

RanGTPase is involved in many cellular processes. It functions in nuclear-cytosolic transport and centrosome formation. Ran also localizes to chromatin as RCC1 does, its guanine nucleotide exchange factor, but Ran's function on chromatin is not known. We found that gsp1, a temperature-sensitive mutant of GSP1, a Saccharomyces cerevisiae Ran homologue, suppressed the hydroxyurea (HU) and ultra violet (UV) sensitivities of the mec1 mutant. In UV-irradiated mec1 gsp1 cells, Rad53 was phosphorylated despite the lack of Mec1. This suppression depended on the TEL1 gene, given that the triple mutant, mec1 gsp1 tel1, was unable to grow. The gsp1 mutations also suppressed the HU sensitivity of the rad9 mutant in a Tel1-dependent manner, but not the HU sensitivity of the rad53 mutant. These results indicated that Rad53 was activated by the Tel1 pathway in mec1 gsp1 cells, suggesting that Gsp1 helps regulate the role switching the ATM family kinases Mec1 and Tel1.

Nuclear RanGAP is required for the heterochromatin assembly and is reciprocally regulated by histone H3 and Clr4 histone methyltransferase in Schizosaccharomyces pombe

Although the Ran GTPase-activating protein RanGAP mainly functions in the cytoplasm, several lines of evidence indicate a nuclear function of RanGAP. We found that Schizosaccharomyces pombe RanGAP, SpRna1, bound the core of histone H3 (H3) and enhanced Clr4-mediated H3-lysine 9 (K9) methylation. This enhancement was not observed for methylation of the H3-tail containing K9 and was independent of SpRna1-RanGAP activity, suggesting that SpRna1 itself enhances Clr4-mediated H3-K9 methylation via H3. Although most SpRna1 is in the cytoplasm, some cofractionated with H3. Sprna1(ts) mutations caused decreases in Swi6 localization and H3-K9 methylation at all three heterochromatic regions of S. pombe. Thus, nuclear SpRna1 seems to be involved in heterochromatin assembly. All core histones bound SpRna1 and inhibited SpRna1-RanGAP activity. In contrast, Clr4 abolished the inhibitory effect of H3 on the RanGAP activity of SpRna1 but partially affected the other histones. SpRna1 formed a trimeric complex with H3 and Clr4, suggesting that nuclear SpRna1 is reciprocally regulated by histones, especially H3, and Clr4 on the chromatin to function for higher order chromatin assembly. We also found that SpRna1 formed a stable complex with Xpo1/Crm1 plus Ran-GTP, in the presence of H3.

Loss of RanGEF/Pim1 activity abolishes the orchestration of Ran-mediated mitotic cellular events in S. pombe

RCC1, a conserved chromosomal protein with a seven-bladed propeller is a GDP/GTP nucleotide exchange factor for RanGTPase that mediates various cellular events. We isolated 16 temperature-sensitive (ts) mutants of S. pombeRCC1-homolog, pim1+, by error-prone PCR. Five pim1(ts) mutants had a single mutation. The obtained pim1(ts) mutations and previously reported mutations were localized on similar sites in seven RCC1 repeats. Those mutations resulted in a reduced binding of Pim1 with Spi1. All pim1(ts) mutants showed a defect in nucleocytoplasmic protein transports, whereas the majority of them showed a normal mRNA export. In all pim1(ts) examined, chromosomal DNA replication was completed. However, mitotic spindle formation was abrogated, the septum was formed being uncoupled with nuclear division and abnormally widened, thus resulting in chromosomal DNA mis-segregation and the accumulation of enucleated cells. As a result, a defect of RanGEF/Pim1 abolished the orchestration of sequential mitotic events, spindle formation, septation and cytokinesis that are essential to produce two identical daughter cells.

RNA interference and heterochromatin in the fission yeast Schizosaccharomyces pombe

Fission yeast is a useful model for RNA interference because it has single-copy genes for components of the RNAi pathway such as argonaute, dicer and RNA-dependent RNA polymerase (RdRP). Functions for RNAi revealed in S. pombe, such as heterochromatic silencing and chromosome segregation, are likely to be ancient because they are shared with some other eukaryotes. The underlying mechanisms are being rapidly unraveled.

PIRLs: A Novel Class of Plant Intracellular Leucine-rich Repeat Proteins

Leucine-rich repeat (LRR) proteins feature tandem leucine-rich motifs that form a protein-protein interaction domain. Plants contain diverse classes of LRR proteins, many of which take part in signal transduction. We have identified a novel family of nine Arabidopsis LRR proteins that, based on predicted intracellular location and LRR motif consensus sequence, are related to Ras-binding LRR proteins found in signaling complexes in animals and yeast. This new class has been named plant intracellular Ras group-related LRR proteins (PIRLs). We have characterized PIRL cDNAs, rigorously defined gene and protein annotations, investigated gene family evolution and surveyed mRNA expression. While LRR regions suggested a relationship to Ras group LRR proteins, outside of their LRR domains PIRLs differed from Ras group proteins, exhibiting N- and C-terminal regions containing low complexity stretches and clusters of charged amino acids. PIRL genes grouped into three subfamilies based on sequence relationships and gene structures. Related gene pairs and dispersed chromosomal locations suggested family expansion by ancestral genomic or segmental duplications. Expression surveys revealed that all PIRL mRNAs are actively transcribed, with three expressed differentially in leaves, roots or flowers. These results define PIRLs as a distinct, plant-specific class of intracellular LRR proteins that probably mediate protein interactions, possibly in the context of signal transduction. T-DNA knock-out mutants have been isolated as a starting point for systematic functional analysis of this intriguing family.