Activation of mitogen-activated protein kinase kinase by v-Raf in NIH 3T3 cells and in vitro

Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-1 serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-1 in progesterone-induced meiotic maturation of Xenopus laevis oocytes. Raf-1 enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-1 activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone H1 phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-1 was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone H1 kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-1 activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mosxe activity.

Signal transduction via the MAP kinases: proceed at your own RSK

An explosion of new information linking activation of cell surface signal initiators to changes in gene expression has recently emerged. The focus of much of this information has centered around the agonist-dependent activation of the mitogen-activated protein (MAP) kinases. Although this intracellular signal transduction pathway is extremely complex, conservation of many of its components has been observed in yeast, nematodes, Drosophila, and mammals. Thus, these signaling proteins may participate in the regulation of a variety of cellular processes.

Raf-1 and p21v-ras cooperate in the activation of mitogen-activated protein kinase

Mitogen-activated protein (MAP) kinases Raf-1, pp60src, and p21ras all play important roles in the transfer of signals from the cell surface to the nucleus. We have used the baculovirus/Sf9 insect cell system to elucidate the regulatory relationships between pp60v-src, p21v-ras, MAP kinase (p44erk1/mapk), and Raf-1. In Sf9 cells, p44erk1/mapk is activated by coexpression with either v-Raf or a constitutively activated form of Raf-1 (Raf22W). In contrast, p44erk1/mapk is activated to only a limited extent by coexpression with either Raf-1 or p21v-ras alone. This activation of p44erk1/mapk is greatly enhanced by coexpression with both p21v-ras and Raf-1. Since we have previously shown that p21v-ras stimulates Raf-1 activity, the activation of p44erk1/mapk by p21v-ras may occur exclusively via a Raf-1-dependent pathway. However, a dominant-inhibitory mutant of Raf-1 (Raf301) does not block the activation of p44erk1/mapk by p21-v-ras. Further, pp60v-src, which activates Raf-1 at least as effectively as p21v-ras, fails to enhance p44erk1/mapk activity greatly when coexpressed with Raf-1. These data suggest that activation of p44erk1/mapk by p21v-ras may occur via both Raf-1-dependent and Raf-1-independent pathways.

Complexes of Ras.GTP with Raf-1 and mitogen-activated protein kinase kinase

The guanosine triphosphate (GTP)-binding protein Ras functions in regulating growth and differentiation; however, little is known about the protein interactions that bring about its biological activity. Wild-type Ras or mutant forms of Ras were covalently attached to an insoluble matrix and then used to examine the interaction of signaling proteins with Ras. Forms of Ras activated either by mutation (Gly12Val) or by binding of the GTP analog, guanylyl-imidodiphosphate (GMP-PNP) interacted specifically with Raf-1 whereas an effector domain mutant, Ile36Ala, failed to interact with Raf-1. Mitogen-activated protein kinase (MAP kinase) activity was only associated with activated forms of Ras. The specific interaction of activated Ras with active MAP kinase kinase (MAPKK) was confirmed by direct assays. Thus the forming of complexes containing MAPKK activity and Raf-1 protein are dependent upon the activity of Ras.

Metabolic labeling of mitogen-activated protein kinase kinase in A431 cells demonstrates phosphorylation on serine and threonine residues

Mitogen-activated protein (MAP) kinase kinase is an enzyme that activates the growth factor-regulated MAP kinase in vitro by a mechanism that involves direct phosphorylation of MAP kinase on tyrosine and threonine residues. MAP kinase kinase is stimulated by growth factor treatment of cells and has been shown to be inactivated with protein phosphatases, suggesting that it is regulated by protein phosphorylation. Analysis of two epidermal growth factor-stimulated forms of MAP kinase kinase, purified from 32P-labeled A431 cells, shows that the kinase is phosphorylated on serine and threonine residues and that treatment with protein phosphatases leads to serine dephosphorylation. Under conditions that lead to complete inactivation, only partial dephosphorylation of MAP kinase kinase is observed. Consistent with this finding, inactive forms of MAP kinase kinase, which separate from active forms during the course of purification, are also observed to be phosphorylated in intact cells.

MKK1 and MKK2, which encode Saccharomyces cerevisiae mitogen-activated protein kinase-kinase homologs, function in the pathway mediated by protein kinase C

The PKC1 gene of Saccharomyces cerevisiae encodes a homolog of mammalian protein kinase C that is required for normal growth and division of yeast cells. We report here the isolation of the yeast MKK1 and MKK2 (for mitogen-activated protein [MAP] kinase-kinase) genes which, when overexpressed, suppress the cell lysis defect of a temperature-sensitive pkc1 mutant. The MKK genes encode protein kinases most similar to the STE7 product of S. cerevisiae, the byr1 product of Schizosaccharomyces pombe, and vertebrate MAP kinase-kinases. Deletion of either MKK gene alone did not cause any apparent phenotypic defects, but deletion of both MKK1 and MKK2 resulted in a temperature-sensitive cell lysis defect that was suppressed by osmotic stabilizers. This phenotypic defect is similar to that associated with deletion of the BCK1 gene, which is thought to function in the pathway mediated by PCK1. The BCK1 gene also encodes a predicted protein kinase. Overexpression of MKK1 suppressed the growth defect caused by deletion of BCK1, whereas an activated allele of BCK1 (BCK1-20) did not suppress the defect of the mkk1 mkk2 double disruption. Furthermore, overexpression of MPK1, which encodes a protein kinase closely related to vertebrate MAP kinases, suppressed the defect of the mkk1 mkk2 double mutant. These results suggest that MKK1 and MKK2 function in a signal transduction pathway involving the protein kinases encoded by PKC1, BCK1, and MPK1. Genetic epistasis experiments indicated that the site of action for MKK1 and MKK2 is between BCK1 and MPK1.

A yeast mitogen-activated protein kinase homolog (Mpk1p) mediates signalling by protein kinase C

Mitogen-activated protein (MAP) kinases are activated in response to a variety of stimuli through a protein kinase cascade that results in their phosphorylation on tyrosine and threonine residues. The molecular nature of this cascade is just beginning to emerge. Here we report the isolation of a Saccharomyces cerevisiae gene encoding a functional analog of mammalian MAP kinases, designated MPK1 (for MAP kinase). The MPK1 gene was isolated as a dosage-dependent suppressor of the cell lysis defect associated with deletion of the BCK1 gene. The BCK1 gene is also predicted to encode a protein kinase which has been proposed to function downstream of the protein kinase C isozyme encoded by PKC1. The MPK1 gene possesses a 1.5-kb uninterrupted open reading frame predicted to encode a 53-kDa protein. The predicted Mpk1 protein (Mpk1p) shares 48 to 50% sequence identity with Xenopus MAP kinase and with the yeast mating pheromone response pathway components, Fus3p and Kss1p. Deletion of MPK1 resulted in a temperature-dependent cell lysis defect that was virtually indistinguishable from that resulting from deletion of BCK1, suggesting that the protein kinases encoded by these genes function in a common pathway. Expression of Xenopus MAP kinase suppressed the defect associated with loss of MPK1 but not the mating-related defects associated with loss of FUS3 or KSS1, indicating functional conservation between the former two protein kinases. Mutation of the presumptive phosphorylated tyrosine and threonine residues of Mpk1p individually to phenylalanine and alanine, respectively, severely impaired Mpk1p function. Additional epistasis experiments, and the overall architectural similarity between the PKC1-mediated pathway and the pheromone response pathway, suggest that Pkc1p regulates a protein kinase cascade in which Bck1p activates a pair of protein kinases, designated Mkk1p and Mkk2p (for MAP kinase-kinase), which in turn activate Mpk1p.

The ras signaling pathway mimics insulin action on glucose transporter translocation

Recent observations suggest that insulin increases cellular levels of activated, GTP-bound Ras protein. We tested whether the acute actions of insulin on hexose uptake and glucose-transporter redistribution to the cell surface are mimicked by activated Ras. 3T3-L1 fibroblasts expressing an activated mutant (Lys-61) N-Ras protein exhibited a 3-fold increase in 2-deoxyglucose uptake rates compared with non-transfected cells. Insulin stimulated hexose uptake by approximately 2-fold in parental fibroblasts but did not stimulate hexose uptake in the N-Ras61K-expressing fibroblasts. Overexpression of N-Ras61K also mimicked the large effect of insulin on 2-deoxyglucose transport in 3T3-L1 adipocytes, and again the effects of the two agents were not additive. Total glucose transporter protein (GLUT) 1 was similar between parental and N-Ras61K-expressing 3T3-L1 fibroblasts or adipocytes, whereas total GLUT-4 protein was actually lower in the N-Ras61K-expressing compared with parental adipocytes. However, expression of N-Ras61K in 3T3-L1 adipocytes markedly elevated both GLUT-1 and GLUT-4 in plasma membranes relative to intracellular membranes, and insulin had no further effect. These modulations of glucose transporters by N-Ras61K expression are not due to upstream regulation of insulin receptors because receptor tyrosine phosphorylation and association of phosphatidylinositol 3-kinase with tyrosine-phosphorylated proteins were unaffected. These results show that activated Ras mimics the actions of insulin on membrane trafficking of glucose transporters, consistent with the concept that Ras proteins function as intermediates in this insulin signaling pathway.

Raf-1 kinase is essential for early Xenopus development and mediates the induction of mesoderm by FGF

Animal cap explants from Xenopus embryos injected with a dominant negative Raf-1 mutant, termed NAF (not a functional Raf), demonstrated a complete block to basic fibroblast growth factor (FGF)-stimulated mesoderm induction. Activin induction of mesoderm was normal in embryos that expressed NAF. Injection of NAF RNA into 2-cell stage embryos blocked normal development during neurula stages and caused severe posterior truncations in tadpoles. The phenotype induced by NAF could be rescued by coinjection of wild-type raf-1 RNA. The NAF mutant functioned by specifically blocking the activation of endogenous Raf kinase activity. These findings suggest that Raf-1 mediates FGF, but not activin, receptor signaling during mesoderm induction and implicate Raf-1 as a key signaling molecule in the development of posterior structure.

Activation of the mitogen-activated protein kinase pathway in Triton X-100 disrupted NIH-3T3 cells by p21 ras and in vitro by plasma membranes from NIH 3T3 cells

We describe a novel Triton-disrupted mammalian cell system wherein the pathways for activation of mitogen-activated protein (MAP) kinases (MAPKs) are capable of direct biochemical manipulation in vitro. MAPKs p42mapk and p44mapk are activated in signal transduction cascade(s) initiated by occupancy of plasma membrane receptors for peptide growth factors, hormones, and neurotransmitters. One likely activation pathway for MAPKs consists of sequential activations of c-ras, c-raf-1, and a protein-tyrosine/threonine kinase, MAP kinase kinase. Triton-disrupted cells retained capacity for activation of the pathway by both peptide growth factors and by addition of GTP-loaded p21 rasVal12. Incubation of disrupted cells with an antibody that neutralized the function of c-ras (Y13-259) abolished receptor-mediated stimulation of MAPK as did acute addition of 200 microM azatyrosine. Activation of the pathway was reconstituted in a cell-free system using high-speed supernatants generated from Triton-disrupted cells together with purified plasma membranes from parental cells and as a heterogeneous system using purified plasma membranes from v-ras-transformed cells. These systems will allow biochemical dissection in vitro of the interaction(s) between c-ras and the MAPK pathway in mammalian cells.

MKK1 and MKK2, which encode Saccharomyces cerevisiae mitogen-activated protein kinase-kinase homologs, function in the pathway mediated by protein kinase C

The PKC1 gene of Saccharomyces cerevisiae encodes a homolog of mammalian protein kinase C that is required for normal growth and division of yeast cells. We report here the isolation of the yeast MKK1 and MKK2 (for mitogen-activated protein [MAP] kinase-kinase) genes which, when overexpressed, suppress the cell lysis defect of a temperature-sensitive pkc1 mutant. The MKK genes encode protein kinases most similar to the STE7 product of S. cerevisiae, the byr1 product of Schizosaccharomyces pombe, and vertebrate MAP kinase-kinases. Deletion of either MKK gene alone did not cause any apparent phenotypic defects, but deletion of both MKK1 and MKK2 resulted in a temperature-sensitive cell lysis defect that was suppressed by osmotic stabilizers. This phenotypic defect is similar to that associated with deletion of the BCK1 gene, which is thought to function in the pathway mediated by PCK1. The BCK1 gene also encodes a predicted protein kinase. Overexpression of MKK1 suppressed the growth defect caused by deletion of BCK1, whereas an activated allele of BCK1 (BCK1-20) did not suppress the defect of the mkk1 mkk2 double disruption. Furthermore, overexpression of MPK1, which encodes a protein kinase closely related to vertebrate MAP kinases, suppressed the defect of the mkk1 mkk2 double mutant. These results suggest that MKK1 and MKK2 function in a signal transduction pathway involving the protein kinases encoded by PKC1, BCK1, and MPK1. Genetic epistasis experiments indicated that the site of action for MKK1 and MKK2 is between BCK1 and MPK1.

A yeast mitogen-activated protein kinase homolog (Mpk1p) mediates signalling by protein kinase C

Mitogen-activated protein (MAP) kinases are activated in response to a variety of stimuli through a protein kinase cascade that results in their phosphorylation on tyrosine and threonine residues. The molecular nature of this cascade is just beginning to emerge. Here we report the isolation of a Saccharomyces cerevisiae gene encoding a functional analog of mammalian MAP kinases, designated MPK1 (for MAP kinase). The MPK1 gene was isolated as a dosage-dependent suppressor of the cell lysis defect associated with deletion of the BCK1 gene. The BCK1 gene is also predicted to encode a protein kinase which has been proposed to function downstream of the protein kinase C isozyme encoded by PKC1. The MPK1 gene possesses a 1.5-kb uninterrupted open reading frame predicted to encode a 53-kDa protein. The predicted Mpk1 protein (Mpk1p) shares 48 to 50% sequence identity with Xenopus MAP kinase and with the yeast mating pheromone response pathway components, Fus3p and Kss1p. Deletion of MPK1 resulted in a temperature-dependent cell lysis defect that was virtually indistinguishable from that resulting from deletion of BCK1, suggesting that the protein kinases encoded by these genes function in a common pathway. Expression of Xenopus MAP kinase suppressed the defect associated with loss of MPK1 but not the mating-related defects associated with loss of FUS3 or KSS1, indicating functional conservation between the former two protein kinases. Mutation of the presumptive phosphorylated tyrosine and threonine residues of Mpk1p individually to phenylalanine and alanine, respectively, severely impaired Mpk1p function. Additional epistasis experiments, and the overall architectural similarity between the PKC1-mediated pathway and the pheromone response pathway, suggest that Pkc1p regulates a protein kinase cascade in which Bck1p activates a pair of protein kinases, designated Mkk1p and Mkk2p (for MAP kinase-kinase), which in turn activate Mpk1p.

Stimulation by phospholipids of a protein-tyrosine-phosphatase containing two src homology 2 domains

PTP1C, a protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) containing two src homology 2 domains, is poorly active when assayed with various protein substrates in vitro. Its activity is stimulated > 1000-fold by anionic phospholipids when myelin basic protein or mitogen-activated protein kinase is used as substrate but reduced in the presence of several other substrates. Data are presented to indicate a direct interaction of the enzyme with phospholipids. Enzyme stimulation directed only toward certain specific substrates is interpreted by assuming that these compounds also bind to the phospholipid vesicles where they will be subjected to rapid enzymatic attack. A possible regulation of PTP1C by its translocation to the cell membrane is hypothesized.

The SRF accessory protein Elk-1 contains a growth factor-regulated transcriptional activation domain

The Elk-1 and SRF transcription factors form a ternary complex at the c-fos serum response element (SRE). Growth factor stimulation rapidly induces a reversible change in the electrophoretic mobility of the ternary complex, accompanied by increased phosphorylation of the Elk-1 C-terminal region and by the activation of a 42 kd cellular Elk-1 kinase. Phosphorylation of Elk-1 in vitro by partially purified p42/p44 MAP kinase induces a similar reduction in ternary complex mobility but has little effect on the efficiency of its formation. In vitro, MAP kinase phosphorylates the Elk-1 C-terminal region at multiple sites, which are also phosphorylated following growth factor stimulation in vivo. The Elk-1 C-terminal region functions as a regulated transcriptional activation domain whose activity in vivo is dependent on the integrity of the MAP kinase sites. These findings directly link transcriptional activation by the SRE to the growth factor-regulated phosphorylation of an SRE-binding protein.

Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro

Several protein kinases, including Mos, maturation-promoting factor (MPF), mitogen-activated protein (MAP) kinase, and MAP kinase kinase (MAPKK), are activated when Xenopus oocytes enter meiosis. De novo synthesis of the Mos protein is required for progesterone-induced meiotic maturation. Recently, bacterially synthesized maltose-binding protein (MBP)-Mos fusion protein was shown to be sufficient to initiate meiosis I and MPF activation in fully grown oocytes in the absence of protein synthesis. Here we show that MAP kinase is rapidly phosphorylated and activated following injection of wild-type, but not kinase-inactive mutant, MBP-Mos into fully grown oocytes. MAP kinase activation by MBP-Mos occurs within 20 min, much more rapidly than in progesterone-treated oocytes. The MBP-Mos fusion protein also activates MPF, but MPF activation does not occur until approximately 2 h after injection. Extracts from oocytes injected with wild-type but not kinase-inactive MBP-Mos contain an activity that can phosphorylate MAP kinase, suggesting that Mos directly or indirectly activates a MAPKK. Furthermore, activated MBP-Mos fusion protein is able to phosphorylate and activate a purified, phosphatase-treated, rabbit muscle MAPKK in vitro. Thus, in oocytes, Mos is an upstream activator of MAP kinase which may function through direct phosphorylation of MAPKK.

Activation of myelin basic protein and S6 peptide kinases in phorbol ester- and PAF-treated sheep platelets

The involvement of myelin basic protein (MBP) kinases and ribosomal S6 peptide kinases in sheep platelet signal transduction was investigated. Treatment of platelets with 200 nM 12-O-tetradecanoylphorbol-13-acetate (PMA) led to 5-fold stimulations of cytosolic MBP and S6 peptide kinase activities within 1 min. Immunoblotting analysis of phenyl-Superose-fractionated cytosol from PMA-treated platelets with a panel of mitogen-activated protein (MAP) kinase anti-peptide antibodies revealed that one of the activated MBP kinases was p42mapk. This MAP kinase isoform was also stimulated to a lesser extent (approximately 2-fold) when platelets were exposed to 200 microM platelet-activating factor (PAF) for 3 min. The pathways of PAF-activation of p42mapk also involved a protein kinase C-independent route, since the staurosporin analog compound 3 reduced PAF-induced activation by approximately 30% under conditions in which it inhibited PMA-activation of p42mapk by approximately 80%. Another MAP kinase isoform of 44 kDa, most probably p44erk1, was also detected in platelet cytosol, but it was only marginally modulated in response to PMA or PAF. The predominant PMA- and PAF-activated MBP kinase detected after MonoQ fractionation of platelet cytosol did not appear to correspond to a MAP kinase. MonoQ chromatography of platelet cytosol also resolved two PMA- and PAF-activated S6 peptide kinases, which appeared to coelute on phenyl-Sepharose. Western blotting analysis of the MonoQ fractions with antibodies raised against peptide sequences in the S6 kinases p90rsk and p70S6K revealed immunoreactive proteins of approximately 75 kDa and approximately 95 kDa that coincided with the first S6 peptide kinase peak. These proteins probably corresponded to the 502 and 525 amino-acid-length forms of p70S6K. Only the second peak of S6 peptide kinase activity from MonoQ was appreciably stimulated in response to PAF-treatment of platelets, and this was largely abolished by compound 3. It is more likely that the novel MBP and S6 peptide kinases described here, rather than p42mapk and p70S6K, play a significant role in PAF signal transduction in the platelet.

A divergence in the MAP kinase regulatory network defined by MEK kinase and Raf

Mitogen-activated protein kinases (MAPKs) are rapidly phosphorylated and activated in response to various extracellular stimuli in many different cell types. Such regulation of MAPK results from sequential activation of a series of protein kinases. The kinases that phosphorylate MAPKs, the MAP kinase kinases (MEKs) are also activated by phosphorylation. MEKs are related in sequence to the yeast protein kinases Byr1 (from Schizosaccharomyces pombe) and Ste7 (from Saccharomyces cerevisiae), which function in the pheromone-induced signaling pathway that results in mating. Byr1 and Ste7 are in turn regulated by the protein kinases Byr2 and Ste11. The amino acid sequence of the mouse homolog of Byr2 and Ste11, denoted MEKK (MEK kinase), was elucidated from a complementary DNA sequence encoding a protein of 672 amino acid residues (73 kilodaltons). MEKK was expressed in all mouse tissues tested, and it phosphorylated and activated MEK. Phosphorylation and activation of MEK by MEKK was independent of Raf, a growth factor-regulated protein kinase that also phosphorylates MEK. Thus, MEKK and Raf converge at MEK in the protein kinase network mediating the activation of MAPKs by hormones, growth factors, and neurotransmitters.

Isolation of two members of the rat MAP kinase kinase gene family

Mitogen-activated protein (MAP) kinase kinase (MAPKK) is a recently characterized activator of MAP kinase (MAPK), and is considered to be regulated by a protooncogene product c-Raf-1. It is, however, unclear whether the signals originating from c-Raf-1 utilize this phosphorylation cascade to lead to oncogenesis. To clarify this point, we isolated rat MAPKK cDNAs, and identified two distinct cDNAs encoding MAPKK and a highly related kinase, both with molecular weights of approximately 45 kDa (MEK1 and MEK2). Genomic Southern blot analyses suggested that MAPKK may form a large gene family.

Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro

Several protein kinases, including Mos, maturation-promoting factor (MPF), mitogen-activated protein (MAP) kinase, and MAP kinase kinase (MAPKK), are activated when Xenopus oocytes enter meiosis. De novo synthesis of the Mos protein is required for progesterone-induced meiotic maturation. Recently, bacterially synthesized maltose-binding protein (MBP)-Mos fusion protein was shown to be sufficient to initiate meiosis I and MPF activation in fully grown oocytes in the absence of protein synthesis. Here we show that MAP kinase is rapidly phosphorylated and activated following injection of wild-type, but not kinase-inactive mutant, MBP-Mos into fully grown oocytes. MAP kinase activation by MBP-Mos occurs within 20 min, much more rapidly than in progesterone-treated oocytes. The MBP-Mos fusion protein also activates MPF, but MPF activation does not occur until approximately 2 h after injection. Extracts from oocytes injected with wild-type but not kinase-inactive MBP-Mos contain an activity that can phosphorylate MAP kinase, suggesting that Mos directly or indirectly activates a MAPKK. Furthermore, activated MBP-Mos fusion protein is able to phosphorylate and activate a purified, phosphatase-treated, rabbit muscle MAPKK in vitro. Thus, in oocytes, Mos is an upstream activator of MAP kinase which may function through direct phosphorylation of MAPKK.

Molecular cloning and expression of a MAP kinase homologue from pea

The cdc2 kinases are important cell cycle regulators in all eukaryotes. MAP kinases, a closely related family of protein kinases, are involved in cell cycle regulation in yeasts and vertebrates, but previously have not been documented in plants. We used PCR to amplify Brassica napus DNA sequences using primers corresponding to amino sequences that are common to all known protein kinases. One sequence was highly similar to KSS1, a MAP kinase from Saccharomyces cerevisiae. This sequence was used to isolate a full-length MAP kinase-like clone from a pea cDNA library. The pea clone, called D5, shared approximately 50% amino acid identity with MAP kinases from yeasts and vertebrates and about 41% identity with plant cdc2 kinases. An expression protein encoded by D5 was recognized by an antiserum specific to human MAP kinases (ERKs). Messenger RNA corresponding to D5 was present at similar levels in all tissues examined, without regard to whether cell division or elongation were occurring in those tissues.

A conserved kinase cascade for MAP kinase activation in yeast

Mitogen-activated protein kinases are regulators of proliferation and differentiation in many eukaryotes. Studies during the last year have revealed that functionally distinct signal pathways in yeast use related protein kinase cascades for mitogen-activated protein kinase activation. These cascades act as intracellular signaling modules that are likely to be conserved from yeast to mammals.

The MAP kinase cascade is essential for diverse signal transduction pathways

Mitogen-activated protein (MAP) kinases are activated by combined tyrosine and threonine phosphorylation catalysed by MAP kinase kinase, a novel class of protein kinases with dual specificity for both tyrosine and serine/threonine. MAP kinase kinase is turned on by serine/threonine phosphorylation catalysed by an immediate upstream kinase. The MAP kinase cascade appears to be conserved during evolution and thus might play an essential role in diverse intracellular signaling processes from yeasts to vertebrates.

MAP kinase and the activation of quiescent cells

Virtually all mitogens lead to the rapid activation of one or more mitogen-activated protein (MAP) kinases. In almost all cases, mitogen-activated surface signaling complexes transmit an essential signal via ras on to a protein kinase cascade that involves the serine/threonine kinase raf. Raf appears to be a MAP kinase kinase kinase, activating MAP kinase kinase which, in turn, activates MAP kinase. Among the targets of MAP kinase are other kinases, nuclear transcription factors and other proteins with roles in cell cycle activation. Both G0-arrested somatic cells and G2-arrested oocytes use many of the same signaling mechanisms to break cell cycle arrest; this is a useful concept in light of newly developed cell-free systems from quiescent oocytes that can be used to study signal transduction in vitro.

Functional expression of a MAP kinase kinase in COS cells and recognition by an anti-STE7/byr1 antibody

Mitogen-activated protein (MAP) kinases p42mapk and p44mapk are activated by dual tyrosine and threonine phosphorylation in vivo. Both MAPKs are phosphorylated and activated in vitro by an activator recently identified as a protein-tyrosine/threonine kinase. We have isolated a putative cDNA for a MAP kinase kinase (MAPKK) and determined its structure [Proc. Natl. Acad. Sci. USA, in press]. The protein encoded by this cDNA shares sequence homology with two yeast protein kinases byr1 and STE7. We now report that stimulation with serum of COS cells expressing this shares sequence homology with two yeast protein kinases byr1 and STE7. We now report that stimulation with serum of COS cells expressing this protein amplifies MAPK activator activity markedly. The increased activity co-migrates during chromatography with the expressed 45 kDa protein, recognized by an anti-STE7/byr1 antibody, and is abrogated by treatment with phosphatase 2A. Thus, this cDNA encodes a functional MAPKK. The anti-STE7/byr1 antibody also recognized a 46 kDa COS cell protein that was resolved from the expressed MAPKK by anion-exchange chromatography. This immunoreactive protein co-eluted with endogenous MAPKK activity, suggesting identification of the immunoreactive band as monkey MAPKK.

Biochemistry of B lymphocyte activation

The activation of B lymphocytes from resting cells proceeds from the events of early activation to clonal proliferation to final differentiation into either an antibody-secreting plasma cell or a memory B cell. This is a complex activation process marked by several alternative pathways, depending on the nature of the initial antigenic stimulus. Over the past 5-10 years, there has been an explosion of studies examining the biochemical nature of various steps in these pathways. Some of that progress is reviewed here. In particular, we have described in detail what is known about the structure and function of the AgR, as this molecule plays a pivotal role in B cell responses of various types. We have also reviewed recent progress in understanding the mechanism of action of contact-dependent T cell help and of the cytokine receptors, particularly the receptors for IL-2, IL-4, and IL-6. Clearly, all of these areas represent active areas of investigation and great progress can be anticipated in the next few years.

Identification of a MAP kinase kinase kinase in phaeochromocytoma (PC12) cells

A MAP kinase kinase kinase (MAPKKK) was identified in phaeochromocytoma (PC12) cells which reactivated homogeneous MAP kinase kinase (MAPKK) from rabbit skeletal muscle that had been inactivated by incubation with protein phosphatase 2A. Reactivation was accompanied by stoichiometric phosphorylation of MAPKK on a serine residue(s). Following stimulation of PC12 cells with nerve growth factor and chromatography of the extracts on Mono Q, MAP kinase and MAPKK were detected as active phosphorylated enzymes, whereas MAPKKK was inactive and only activated after prolonged storage at 4 degrees C. The results suggest that the activation of MAPKKK by growth factors is likely to occur by a non-covalent mechanism.