hGrb10␣ (previously named Grb-IR) is a Src-homology 2 domain-containing protein that binds with high affinity to the tyrosine-phosphorylated insulin receptor and insulin-like growth factor-1 receptor. At least two isoforms of human Grb10, (hGrb10␣ and hGrb10), which differ in the pleckstrin homology (PH) domain and the N-terminal sequence, have previously been identified in insulin target tissues such as human skeletal muscle and fat cells. Here we report the cloning of the third isoform of the hGrb10 family (hGrb10␥) from human skeletal muscle and its localization to human chromosome 7. We have also determined the human chromosome localization of Grb7 to 17q21-q22 and Grb14 to chromosome 2. hGrb10␥ contains an intact PH domain and an N-terminal sequence that is present in hGrb10␣ but absent in hGrb10. RNase protection assays and Western blot analysis showed that hGrb10␣ and hGrb10␥ are differentially expressed in insulin target cells including skeletal muscle, liver, and adipocyte cells. hGrb10␥ is also expressed in HeLa cells and various breast cancer cell lines. The protein bound with high affinity to the insulin receptor in cells, and the interaction was dependent on the tyrosine phosphorylation of the receptor. hGrb10␥ also underwent insulin-stimulated membrane translocation and serine phosphorylation. hGrb10␥ phosphorylation was inhibited by PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, and wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase. Taken together, our data suggest that hGrb10 isoforms are potential downstream signaling components of the insulin receptor tyrosine kinase and that the PH domain may play an important role in the involvement of these isoforms in signal transduction pathways initiated by insulin and other growth factors.Insulin stimulates and regulates cell growth and metabolism by binding to its receptor on the cell membrane. The binding of insulin to the insulin receptor (IR) 1 results in receptor autophosphorylation and receptor tyrosine kinase activation, followed by tyrosine phosphorylation of various cellular substrates including a 185-kDa protein called the IR substrate 1 or IRS-1 (1, 2). Tyrosine phosphorylation of IRS-1 creates docking sites for multiple downstream signaling molecules with specific sequence motifs. One of these functional motifs found in signaling proteins is the Src homology 2 (SH2) domain. The SH2 domain is a sequence of approximately 100 amino acids that binds with high affinity to phosphotyrosine-containing proteins (3). Another functional domain is the pleckstrin homology (PH) domain that has been suggested to play important roles in protein-protein and protein-lipid interactions (4 -6).Although numerous studies have shown that IRS-1 is critical in IR signal transduction, evidence does exist that other proteins may also be involved to transduce a signal from the IR to downstream targets (2). In a search for signaling molecules involved in IR signaling, we used the yeast two-hybrid system to find proteins that i...
Cyclin-dependent kinase 9 (Cdk9) of fission yeast is an essential ortholog of metazoan positive transcription elongation factor b (P-TEFb), which is proposed to coordinate capping and elongation of RNA polymerase II (Pol II) transcripts. Here we show that Cdk9 is activated to phosphorylate Pol II and the elongation factor Spt5 by Csk1, one of two fission yeast CDK-activating kinases (CAKs). Activation depends on Cdk9 T-loop residue Thr-212. The other CAK-Mcs6, the kinase component of transcription factor IIH (TFIIH)-cannot activate Cdk9. Consistent with the specificities of the two CAKs in vitro, the kinase activity of Cdk9 is reduced ϳ10-fold by csk1 deletion, and Cdk9 complexes from csk1⌬ but not csk1 ؉ cells can be activated by Csk1 in vitro. A cdk9 T212A mutant is viable but phenocopies conditional growth defects of csk1⌬ strains, indicating a role for Csk1-dependent activation of Cdk9 in vivo. A cdk9 T212A mcs6 S165A strain, in which neither Cdk9 nor Mcs6 can be activated by CAK, has a synthetic growth defect, implying functional overlap between the two CDKs, which have distinct but overlapping substrate specificities. Cdk9 forms complexes in vivo with the essential cyclin Pch1 and with Pcm1, the mRNA cap methyltransferase. The carboxyl-terminal region of Cdk9, through which it interacts with another capping enzyme, the RNA triphosphatase Pct1, is essential. Together, the data support a proposed model whereby Cdk9/Pch1-the third essential CDK-cyclin complex described in fission yeasthelps to target the capping apparatus to the transcriptional elongation complex.
Telomeres protect chromosome ends from end-to-end fusion and degradation. Loss of telomere function causes cell-cycle arrest or cell death. Autosomal dominant dyskeratosis congenita (AD DC), a rare inherited bone marrow failure syndrome, is caused by mutations in TERC, the RNA component of telomerase. Here, we studied the telomere dynamics over three generations in a 32-member extended family with AD DC due to a TERC gene deletion. Our analysis shows that peripheral blood cells from family members haploinsufficient for TERC have very short telomeres. Telomeres are equally short in all individuals carrying the TERC gene deletion irrespective of their age. Chromosome-specific telomere analysis distinguishing the parental origin of telomeres showed that in gene deletion carriers, paternal and maternal telomeres are similarly short and similar in length to those of the affected parent. In children of affected parents who have normal TERC genes, parental telomeres are again similar in length, but two generations appear to be necessary to fully restore normal telomere length. These results are consistent with a model in which telomerase preferentially acts on the shortest telomeres. When TERC is limiting, this preference leads to the accelerated shortening of longer telomeres. The limited amount of active telomerase in TERC RNA haploinsufficiency may not be able to maintain the minimal length of the increasing number of short telomeres. Thus, the number of cells with excessively short telomeres and the degree of residual telomerase activity may determine the onset of disease in patients with AD DC. dyskeratosis congenita ͉ anticipation ͉ quantitative-FISH
The fission yeast Mcs6 -Mcs2-Pmh1 complex, homologous to metazoan Cdk7-cyclin H-Mat1, has dual functions in cell division and transcription: as a partially redundant cyclin-dependent kinase (CDK)-activating kinase (CAK) that phosphorylates the major cell cycle CDK, Cdc2, on Thr-167; and as the RNA polymerase (Pol) II carboxyl-terminal domain (CTD) kinase associated with transcription factor (TF) IIH. We analyzed conditional mutants of mcs6 and pmh1, which activate Cdc2 normally but cannot complete cell division at restrictive temperature and arrest with decreased CTD phosphorylation. Transcriptional profiling by microarray hybridization revealed only modest effects on global gene expression: a one-third reduction in a severe mcs6 mutant after prolonged incubation at 36°C. In contrast, a small subset of transcripts (ϳ5%) decreased by more than twofold after Mcs6 complex function was compromised. The signature of repressed genes overlapped significantly with those of cell separation mutants sep10 and sep15. Sep10, a component of the Pol II Mediator complex, becomes essential in mcs6 or pmh1 mutant backgrounds. Moreover, transcripts dependent on the forkhead transcription factor Sep1, which are expressed coordinately during mitosis, were repressed in Mcs6 complex mutants, and Mcs6 also interacts genetically with Sep1. Thus, the Mcs6 complex, a direct activator of Cdc2, also influences the cell cycle transcriptional program, possibly through its TFIIH-associated kinase function. INTRODUCTIONCyclin-dependent kinases (CDKs) play a central role in driving cell division in eukaryotic organisms and also perform essential, conserved functions in the transcription cycle of RNA polymerase (Pol) II (reviewed by Morgan, 1997). Whereas most CDKs can be classified as either cell cycle or transcriptional regulators based on a preeminent physiological function, the metazoan Cdk7 complex is essential in both cell division and gene expression, as the CDK-activating kinase (CAK) and as a component of the general transcription factor IIH (TFIIH) (reviewed by Harper and Elledge, 1998). To execute its dual functions in vivo, Cdk7 has evolved distinct substrate specificities for the activation segment (T-loop) of CDKs and the carboxy-terminal domain (CTD) of the Pol II large subunit, and mechanisms to allow their independent regulation (Garrett et al., 2001;Larochelle et al., 2001). Less clear is how (or whether) the Cdk7 complex serves to coordinate cell division with gene expression.To address this question genetically, we turned to the fission yeast Schizosaccharomyces pombe, which also relies, in part, on a dual-function CDK complex to activate CDKs and phosphorylate Pol II (Buck et al., 1995;Damagnez et al., 1995;Hermand et al., 1998;Lee et al., 1999;Saiz and Fisher, 2002). The orthologue of Cdk7 in S. pombe is Mcs6, which associates with the cyclin Mcs2 and the RING-finger protein Pmh1. Both mcs6 and mcs2 were identified in genetic screens for positive regulators of Cdc2 (S. pombe Cdk1), the major cell cycle CDK (Molz et al., 1989...
The p21-activated kinase, Shk1, is required for cell viability, establishment and maintenance of cell polarity, and proper mating response in the fission yeast, Schizosaccharomyces pombe. Previous genetic studies suggested that a presumptive protein methyltransferase, Skb1, functions as a positive modulator of Shk1. However, unlike Shk1, Skb1 is not required for viability or mating of S. pombe cells and contributes only modestly to the regulation of cell morphology under normal growth conditions. Here we demonstrate that Skb1 plays a more significant role in regulating cell growth and polarity under conditions of hyperosmotic stress. We provide evidence that the inability of skb1⌬ cells to properly maintain cell polarity in hyperosmotic conditions results from inefficient subcellular targeting of F-actin. We show that Skb1 localizes to cell ends, sites of septation, and nuclei of S. pombe cells. Hyperosmotic shock results in substantial delocalization of Skb1 from cell ends and nuclei, as well as stimulation of Skb1 protein methyltransferase activity. Taken together, our results demonstrate a new role for Skb1 as a mediator of hyperosmotic stress response in fission yeast. We show that the protein methyltransferase activity of the human Skb1 homolog, Skb1Hs, is also stimulated by hyperosmotic stress in fission yeast, providing evidence for evolutionary conservation of a role for Skb1-related proteins as mediators of hyperosmotic stress response, as well as mechanisms involved in regulating this novel class of protein methyltransferases.
The p21-activated kinase (PAK) homolog, Shk1, is a critical component of a multifunctional Ras/Cdc42/ PAK complex required for viability, polarized growth and cell shape, and sexual differentiation in the fission yeast, Schizosaccharomyces pombe. Substrate targets of the Shk1 kinase have not previously been described. Here we show that the S. pombe cell polarity factor, Tea1, is directly phosphorylated by Shk1 in vitro. We demonstrate further that Tea1 is phosphorylated in S. pombe cells and that its level of phosphorylation is significantly reduced in cells defective in Shk1 function. Consistent with a role for Tea1 as a potential downstream effector of Shk1, we show that a tea1 null mutation rescues the Shk1 hyperactivity-induced lethal phenotype caused by loss of function of the essential Shk1 inhibitor, Skb15. All phenotypes associated with Skb15 loss, including defects in actin cytoskeletal organization, chromosome segregation, and cytokinesis, are suppressed by tea1⌬, suggesting that Tea1 is a potential mediator of multiple Shk1 functions. S. pombe cells carrying a weak hypomorphic allele of shk1 together with a tea1⌬ mutation exhibit a cytokinesis defective phenotype that is significantly more severe than that observed in the respective single mutants, providing evidence that Shk1 and Tea1 cooperate to regulate cytokinesis. In addition, we show that S. pombe cells carrying the orb2-34 allele of shk1 exhibit a pattern of monopolar growth similar to that observed in tea1⌬ cells, suggesting that Shk1 and Tea1 may regulate one or more common processes involved in the regulation of polarized cell growth. Taken together, our results strongly implicate Tea1 as a potential substrate-effector of the Shk1 kinase.
The early antecedents of cerebral palsy (CP) are unknown but are suspected to be due to hypoxia-ischemia (H-I). In our rabbit model of CP, the MRI biomarker, ADC on diffusion-weighted imaging, predicted which fetuses will develop postnatal hypertonia. Surviving H-I fetuses experience reperfusion-reoxygenation but a sub-population manifested a continued decline of ADC during early reperfusion-reoxygenation, which possibly represented greater brain injury (RepReOx). We hypothesized that oxidative stress in reperfusion-reoxygenation is a critical trigger for postnatal hypertonia. We investigated if RepReOx predicted postnatal neurobehavior, indicated oxidative stress, and if targeting antioxidants at RepReOx ameliorated motor deficits, which included testing of a new superoxide dismutase mimic (MnTnHex-2-PyP). Rabbit dams, 79% gestation (E25), were subjected to 40-min uterine ischemia. Fetal brain ADC was followed during H-I, immediate reperfusion-reoxygenation, and 4-72 hours after H-I. Endpoints were postnatal neurological outcome at E32, ADC at end of, ADC nadir during H-I and reperfusion-reoxygenation, and area under ADC during first 20-min reperfusion-reoxygenation. Antioxidants targeting RepReOx were administered before and/or after uterine ischemia. The new MRI-ADC biomarker for RepReOx improved prediction of postnatal hypertonia. Greater superoxide production, mitochondrial injury and oligodendroglial loss occurred in fetal brains exhibiting RepReOx than in those without. The antioxidants, MnTnHex-2-PyP, and Ascorbate and Trolox combination, significantly decreased postnatal motor deficits and extent of RepReOx. The etiological link between early injury and later motor deficits can thus be investigated by MRI and allows us to distinguish between critical oxidative stress that causes motor deficits from non-critical oxidative stress that does not.
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