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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Ste7p and Mkk1p are MEK (MAPK/ERK kinase) family members that function in the mating and cell integrity signal transduction pathways in Saccharomyces cerevisiae. We selected STE7 and MKK1 mutations that stimulated their respective pathways in the absence of an inductive signal. Strikingly, serine-to-proline substitutions at analogous positions in Ste7p (position 368) and Mkk1p (position 386) were recovered by independent genetic screens. Such an outcome suggests that this substitution in other MEKs would exhibit similar properties. The Ste7p-P368 variant has higher basal enzymatic activity than Ste7p but still requires induction to reach full activation. The higher activity associated with Ste7p-P368 allows it to compensate for defects in the cell integrity pathway, but it does so only when it is overproduced or when Ste5p is missing. This behavior suggests that Ste5p, which has been proposed to be a tether for the kinases in the mating pathway, contributes to Ste7p specificity.
Mol Cell Biol 1995 Dec
PMID:Yeast MEK-dependent signal transduction: response thresholds and parameters affecting fidelity. 852 19

The Raf-1 gene product is activated in response to cellular stimulation by a variety of growth factors and hormones. Raf-1 activity has been implicated in both cellular differentiation and proliferation. We have examined the regulation of the Raf-1/MEK/MAP kinase (MAPK) pathway during embryonic development in the frog Xenopus laevis. We report that Raf-1, MEK, and MAPK activities are turned off following fertilization and remain undetectable up until blastula stages (stage 8), some 4 h later. Tight regulation of the Raf-1/MEK/MAPK pathway following fertilization is crucial for embryonic cell cycle progression. Inappropriate reactivation of MAPK activity by microinjection of oncogenic Raf-1 RNA results in metaphase cell cycle arrest and, consequently, embryonic lethality. Our findings demonstrate an absolute requirement, in vivo, for inactivation of the MAPK signaling pathway to allow normal cell cycle progression during the period of synchronous cell divisions which occur following fertilization. Further, we show that cytostatic factor effects are mediated through MEK and MAPK.
Mol Cell Biol 1995 Dec
PMID:Regulation of Raf-1-dependent signaling during early Xenopus development. 852 33

The HST7 gene of Candida albicans encodes a protein with structural similarity to MAP kinase kinases. Expression of this gene in Saccharomyces cerevisiae complements disruption of the Ste7 MAP kinase kinase required for both mating in haploid cells and pseudohyphal growth in diploids. However, Hst7 expression does not complement loss of either the Pbs2 (Hog4) MAP kinase kinase required for response to high osmolarity, or loss of the Mkk1 and Mkk2 MAP kinase kinases required for proper cell wall biosynthesis. Intriguingly, HST7 acts as a hyperactive allele of STE7; expression of Hst7 activates the mating pathway even in the absence of upstream signaling components including the Ste7 regulator Ste11, elevates the basal level of the pheromone-inducible FUS1 gene, and amplifies the pseudohyphal growth response in diploid cells. Thus Hst7 appears to be at least partially independent of upstream activators or regulators, but selective in its activity on downstream target MAP kinases. Creation of Hst7/Ste7 hybrid proteins revealed that the C-terminal two-thirds of Hst7, which contains the protein kinase domain, is sufficient to confer this partial independence of upstream activators.
Mol Gen Genet 1995 Dec 20
PMID:Constitutive activation of the Saccharomyces cerevisiae mating response pathway by a MAP kinase kinase from Candida albicans. 854 26

Tyrosine kinase growth factor receptors activate MAP kinase by a complex mechanism involving the SH2/3 protein Grb2, the exchange protein Sos, and Ras. The GTP-bound Ras protein binds to the Raf kinase and initiates a protein kinase cascade that leads to MAP kinase activation. Three MAP kinase kinase kinases have been described--c-Raf, c-Mos, and Mekk--that phosphorylate and activate Mek, the MAP kinase kinase. Activated Mek phosphorylates and activates MAP kinase. Subsequently, the activated MAP kinase translocates into the nucleus where many of the physiological targets of the MAP kinase signal transduction pathway are located. These substrates include transcription factors that are regulated by MAP kinase phosphorylation (e.g., Elk-1, c-Myc, c-Jun, c-Fos, and C/EBP beta). Thus the MAP kinase pathway represents a significant mechanism of signal transduction by growth factor receptors from the cell surface to the nucleus that results in the regulation of gene expression. Three MAP kinase homologs have been identified in the rat: Erk1, Erk2, and Erk3. Human MAP kinases that are similar to the rat Erk kinases have also been identified by molecular cloning. The human Erk1 protein kinase has been shown to be widely expressed as a 44-kDa protein in many tissues. The human Erk2 protein kinase is a 41-kDa protein that is expressed ubiquitously. In contrast, a human Erk3-related protein kinase has been found to be expressed at a high level only in heart muscle and brain. The loci of these MAP kinase genes are widely distributed within the human genome: erk2 at 22q11.2; erk1 at 16p11.2; and ek3-related at 18q12-21. In the yeast Saccharomyces cerevisiae, five MAP kinase gene homologs have been described: smkl, mpk1, hog1, fus3, and kss1. Together, these kinases are a more diverse group than the human erks that have been identified. Thus the erks are likely to represent only one subgroup of a larger human MAP kinase gene family. A candidate for this extended family of MAP kinases is the c-Jun NH2-terminal kinase (Jnk), which binds to and phosphorylates the transcription factor c-Jun at the activating sites Ser-63 and Ser-73. Evidence is presented here to demonstrate that Jnk is a distant relative of the MAP kinase group that is activated by dual phosphorylation at Tyr and Thr.
Mol Reprod Dev 1995 Dec
PMID:Transcriptional regulation by MAP kinases. 860 77

In the budding yeast, Saccharomyces cerevisiae, four separate but structurally related mitogen-activated protein kinase (MAPK) activation pathways are known. The best understood of these regulates mating. Pheromone binding to receptor informs cells of the proximity of a mating partner and induces differentiation to a mating competent state. The MAPK activation cascade mediating this signal is made up of Ste11 (a MEK kinase [MEKK]), Ste7 (a MAPK/ERK kinase [MEK]), and the redundant MAPK-related Fus3 and Kss1 enzymes. Another MAPK activation pathway is important for cell integrity and regulates cell wall construction. This cascade consists of Bck1 (a MEKK), the redundant Mkk1 and Mkk2 enzymes (MEKs), and Mpk1 (a MAPK). We exploited these two pathways to learn about the coordination and signal transmission fidelity of MAPK activation cascades. Two lines of evidence suggest that the activities of the mating and cell integrity pathways are coordinated during mating differentiation. First, cells deficient in Mpk1 are susceptible to lysis when they make a mating projection in response to pheromone. Second, Mpk1 activation during pheromone induction coincides with projection formation. The mechanism underlying this coordination is still unknown to us. Our working model is that projection formation generates a mobile second messenger for activation of the cell integrity pathway. Analysis of a STE7 mutation gave us some unanticipated but important insights into parameters important for fidelity of signal transmission. The Ste7 variant has a serine to proline substitution at position 368. Ste7-P368 has higher basal activity than the wild-type enzyme but still requires Ste11 for its function. Additionally, the proline substitution enables the variant to transmit the signal from mammalian Raf expressed in yeast. This novel activity suggests that Ste7-P368 is inherently more permissive than Ste7 in its interactions with MEKKs. Yet, Ste7-P368 cross function in the cell integrity pathway occurs only when it is highly overproduced or when Ste5 is missing. This behavior suggests that Ste5, which has been proposed to be a tether for the kinases in the mating pathway, contributes to Ste7 specificity and fidelity of signal transmission.
Mol Reprod Dev 1995 Dec
PMID:Dynamics and organization of MAP kinase signal pathways. 860 79

MAP kinase (MAPK) and its activator, MAP kinase kinase (MAPKK), are commonly activated by a variety of extracellular stimuli in mammalian cells and in the process of Xenopus oocyte maturation. In order to investigate the function of the MAPK cascade in oocyte maturation, we produced an anti-Xenopus MAPKK which specifically reacts with MAPKK in vitro. When this antibody was microinjected into immature oocytes, MAPK activation induced by progesterone was prevented. Surprisingly, H1 kinase activation and germinal vesicle breakdown were also inhibited in the oocytes injected with this antibody. These results suggest that the MAPK cascade plays an important role in the maturation promoting factor (MPF) activation during the oocyte maturation process. When this antibody together with Mos was microinjected into Xenopus two-cell embryos, the Mos-induced metaphase arrest (CSF arrest) was prevented. Thus, the MAPK cascade may mediate CSF arrest. During Xenopus early embryogenesis, a low but significant level of MAPK remains active. Injection of mRNA encoding a constitutively active MAPKK resulted in mesoderm induction in animal cap explants. In addition, fibroblast growth-factor (FGF)-induced mesoderm induction was inhibited by expressing CL100 (a MAP kinase phosphatase) in animal cap explants. Thus the MAPK cascade may be involved in the mesoderm induction of Xenopus embryos. The activation pathways and roles of the MAPKK/MAPK cascade in various signaling processes will be discussed.
Mol Reprod Dev 1995 Dec
PMID:Activation mechanism and function of the MAP kinase cascade. 860 80

Raf-1 is a key protein involved in the transmission of developmental and proliferative signals generated by receptor and nonreceptor tyrosine kinases. Biochemical and genetic studies have demonstrated that Raf-1 functions downstream of activated tyrosine kinases and Ras and upstream of mitogen-activated protein kinase (MAPK) and MAPK kinase (MKK or MEK) in many signaling pathways. A major objective of our laboratory has been to determine how Raf-1 becomes activated in response to signaling events. Using mammalian, baculovirus, and Xenopus systems, we have examined the roles that phosphorylation and protein-protein interactions play in regulating the biological and biochemical activity of Raf-1. Our studies have provided evidence that the activity of Raf-1 can be modulated by both Ras-dependent and Ras-independent pathways. Recently, we reported that Arg89 of Raf-1 is a residue required for the association of Raf-1 and Ras. Mutation of this residue disrupted interaction with Ras and prevented Ras-mediated, but not protein kinase C-or tyrosine kinase-mediated, enzymatic activation of Raf-1 in the baculovirus expression system. Further analysis of this mutant demonstrated that kinase-defective Raf-1 proteins interfere with the propagation of proliferative and developmental signals by binding to Ras and blocking Ras function. Our findings have also shown that phosphorylation events play a role in regulating Raf-1. We have identified sites of in vivo phosphorylation that positively and negatively alter the biological and enzymatic activity of Raf-1. In addition, we have found that some of these phosphorylation sites are involved in mediating the interaction of Raf-1 with potential activators (Fyn and Src) and with other cellular proteins (14-3-3). Results from our work suggest that Raf-1 is regulated at multiple levels by several distinct mechanisms.
Mol Reprod Dev 1995 Dec
PMID:Mechanisms regulating Raf-1 activity in signal transduction pathways. 860 83

The Ras-GTPase-activating protein (RasGAP) is an important modulator of p21ras - dependent signal transduction in Xenopus oocytes and in mammalian cells. We investigated the role of the RasGAP SH3 domain in signal transduction with a monoclonal antibody against the SH3 domain of RasGaP. This antibody prevented the activation of the maturation-promoting factor complex (cyclin B-p34cdc2) by oncogenic Ras. The antibody appears to be specific because as little as 5 ng injected per oocyte reduced the level of Cdc2 activation by 50% whereas 100 ng of nonspecific immunoglobulin G did not affect Cdc2 activation. The antibody blocked the Cdc2 activation induced by oncogenic Ras but not that induced by progesterone, which acts independently of Ras. A peptide corresponding to positions 317 to 326 of a sequence in the SH3 domain of human RasGAP blocked Cdc2 activation, whereas a peptide corresponding to positions 273 to 305 of a sequence in the N-terminal moiety of the SH3 domain of RasGAP had no effect. The antibody did not block the mitogen-activated protein (MAP) kinase cascade (activation of MAPK/ERK kinase [MEK], MAP kinase, and S6 kinase p90rsk). Surprisingly, injection of the negative MAP kinase mutant protein ERK2 K52R (containing a K-to-R mutation at position 52) blocked the Cdc2 activation induced by oncogenic Ras as well as blocking the activation of MAP kinase. Thus, MAP kinase is also implicated in the regulation of Cdc2 activity. In this study, we further investigated the regulation of the synthesis of the c-mos oncogene product, which is necessary for the activation of Cdc2. We report that the synthesis of the c-mos oncogene product, which is necessary for the activation antibody to the SH3 domain of RasGAP and by injecting the negative MAP kinase mutant protein ERK2 K52R. These results suggest that oncogenic Ras activates two signaling mechanisms: the MAP kinase cascade and a signaling pathway implicating the SH3 domain of RasGAP. These mechanisms might control Mos protein expression implicated in Cdc2 activation.
Mol Cell Biol 1996 Jun
PMID:The Ras-GTPase-activating protein SH3 domain is required for Cdc2 activation and mos induction by oncogenic Ras in Xenopus oocytes independently of mitogen-activated protein kinase activation. 864 28

Ligation of the B cell Ag receptor (BCR) activates a protein-tyrosine kinase (PTK) and CD45 protein-tyrosine phosphatase (PTPase)-dependent signaling cascade that results in the activation of Ras. This pathway of Ras activation can operate independently of protein kinase C (PKC) activity. Activation of Ras may lead to two distinct Ras-dependent pathways involving either a Raf1/MEK/MAPK module or a MEKK/SEK/SAPK module; however, it is unclear as to how Ras controls the independent activation of either of these pathways. We have used genistein and phenylarsine oxide (PAO) as inhibitors of PTK and PTPase, respectively, to investigate whether they regulate the BCR- and Ca2+/PKC-dependent activation of the Ras/Raf1/MEK/MAPK module. Assays of phosphotransferase activities conducted with Ag (TNP6-OVA)-specific 7.9 murine B lymphoma cells demonstrated that BCR-mediated stimulation of the Raf1/MEK/MAPK module is controlled by PTK and PTPase activities. An elevation in [Ca2+]i was required to optimally activate Raf1 and MEK through the BCR. However, when signaling through the BCR was bypassed by direct stimulation of the Raf1/MEK/MAPK module via a rise in [Ca2+]i and phorbol ester-induced PKC activation, the phosphotransferase activities of Raf1, MEK and MAPK were still regulated in a PTK-dependent manner that was also partially sensitive to the PTPase inhibitor PAO. Thus, at least two alternate routes, i.e. a BCR/PTK/Ras-dependent route and another PKC/Ca(2+)-dependent route, may converge at the level of Raf1 for activation of the Raf1/MEK/MAPK module in B cells.
Mol Immunol 1996 Feb
PMID:Regulation of BCR- and PKC/Ca(2+)-mediated activation of the Raf1/MEK/MAPK pathway by protein-tyrosine kinase and -tyrosine phosphatase activities. 864 50

To elucidate signal transduction pathways leading to neuronal differentiation, we have investigated a conditionally immortalized cell line from rat hippocampal neurons (H19-7) that express a temperature sensitive simian virus 40 large T antigen. Treatment of H19-7 cells with the differentiating agent basic fibroblast growth factor at 39 degrees C, the nonpermissive temperature for T function, resulted in the activation of c-Raf-1, MEK, and mitogen-activated protein (MAP) kinases (ERK1 and -2). To evaluate the role of Raf-1 in neuronal cell differentiation, we stably transfected H19-7 cells with v-raf or an oncogenic human Raf-1-estrogen receptor fusion gene (deltaRaf-1:ER). deltaRaf-1:ER transfectants in the presence of estradiol for 1 to 2 days expressed a differentiation phenotype only at the nonpermissive temperature. However, extended exposure of the deltaRaf-1:ER transfectants to estradiol or stable expression of the v-raf construct yielded cells that extended processes at the permissive as well as the nonpermissive temperature, suggesting that cells expressing the large T antigen are capable of responding to the Raf differentiation signal. deltaRaf-1:ER, MEK, and MAP kinase activities in the deltaRaf-1:ER cells were elevated constitutively for up to 36 h of estradiol treatment at the permissive temperature. At the nonpermissive temperature, MEK and ERKs were activated to a significantly lesser extent, suggesting that prolonged MAP kinase activation may not be sufficient for differentiation. To test this possibility, H19-7 cells were transfected or microinjected with constitutively activated MEK. The results indicate that prolonged activation of MEK or MAP kinases (ERK1 and -2) is not sufficient for differentiation of H19-7 neuronal cells and raise the possibility that an alternative signaling pathway is required for differentiation of H19-7 cells by Raf.
Mol Cell Biol 1996 Apr
PMID:Raf, but not MEK or ERK, is sufficient for differentiation of hippocampal neuronal cells. 865 19


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