Role of protein phosphorylation in the maturation-induced activation of a myelin basic protein kinase from sea star oocytes

Phospho-regulation pathways during egg activation in Drosophila melanogaster

Egg activation is the series of events that transition a mature oocyte to an egg capable of supporting embryogenesis. Increasing evidence points toward phosphorylation as a critical regulator of these events. We used Drosophila melanogaster to investigate the relationship between known egg activation genes and phosphorylation changes that occur upon egg activation. Using the phosphorylation states of four proteins-Giant Nuclei, Young Arrest, Spindly, and Vap-33-1-as molecular markers, we showed that the egg activation genes sarah, CanB2, and cortex are required for the phospho-regulation of multiple proteins. We show that an additional egg activation gene, prage, regulates the phosphorylation state of a subset of these proteins. Finally, we show that Sarah and calcineurin are required for the Anaphase Promoting Complex/Cyclosome (APC/C)-dependent degradation of Cortex following egg activation. From these data, we present a model in which Sarah, through the activation of calcineurin, positively regulates the APC/C at the time of egg activation, which leads to a change in phosphorylation state of numerous downstream proteins.

Identification of a 115kDa MAP-kinase activated by freezing and anoxic stresses in the marine periwinkle, Littorina littorea

The mitogen-activated protein kinase (MAPK) cascade regulates changes in gene transcription by transmitting extracellular stimuli from the plasma membrane to the cell nucleus and has an important role to play in organismal responses to environmental stresses. The activities of MAPKs were investigated in the marine gastropod mollusk, Littorina littorea, a species that tolerates both extracellular freezing and long term oxygen deprivation. In-gel kinase assays revealed the presence of two MAPKs in foot muscle and hepatopancreas, a 42 and a 115kDa protein. Immunoblot analysis showed that both were MAPK proteins and that one was the periwinkle homologue of p42(ERK2). Size exclusion chromatography confirmed the 115kDa size of the novel snail MAPK and its role as the dominant MAPK activity in foot muscle. In-gel kinase assays, immunoblotting with phospho-specific ERK antibody, as well as kinase activity profiles from hydroxyapatite chromatography demonstrated that p115 MAPK kinase activity was increased in foot muscle in response to in vivo freezing or anoxia exposures. The results suggest a role for this novel kinase in environmental stress response.

Evidence for MAP kinase activation during mitotic division

MAP kinases have been implicated in the control of a broad spectrum of cellular events in many types of cells. In somatic cells, MAP kinase activation seems to be triggered after exit from a quiescent state (in G0 or G2) only and then inactivated by entry into a proliferative state. In oocytes of various species, a one-time activation of MAP kinase that is apparently not repeated during the succeeding mitotic cycles occurs after meiotic activation. However, several reports suggest that a myelin basic protein (MBP) kinase activity, unrelated to that of maturation promoting factor, can sometimes be detected during mitotic divisions in various types of cells and oocytes. We have reinvestigated this problem in order to determine the origin and the role of MBP kinase that is stimulated at time of mitosis in the fertilized eggs of the sea urchin Paracentrotus lividus. We used anti-ERK1 antibodies or substrates specific for different MAP kinases, and performed in-gel phosphorylation experiments. Our results suggest that an ERK1-like protein was responsible for part of the MBP kinase activity that is stimulated during the first mitotic divisions. Furthermore, we observed that wortmannin, an inhibitor of PI 3-kinase that arrests the fertilized sea urchin eggs at the prometaphase stage, inhibited the inactivation of MAP kinase normally observed when the eggs divide, suggesting a role for PI 3-kinase in the deactivation process of MAP kinase. We also discuss how the activities of MPF and MAP kinase may be interconnected to regulate the first mitotic divisions of the early sea urchin embryo.

Enhanced interaction between tubulin and microtubule-associated protein 2 via inhibition of MAP kinase and CDC2 kinase by paclitaxel

Paclitaxel, an anti-mitotic anti-cancer agent, is active against solid tumors. The inhibition of depolymerization and promotion of microtubular assembly are essential for the anti-tumor activity of paclitaxel. Microtubule-associated proteins (MAPs) co-polymerize with tubulin and play some roles in microtubular dynamics. We examined the effect of paclitaxel on the interaction between tubulin and MAPs. Human lung-cancer cells, PC-14, were synchronized to G1/S border by the thymidine-double-block technique. After release from exposure to thymidine, the cells were treated briefly with 2 nM paclitaxel and the levels of alpha and beta tubulins and MAPs were examined after various times. Immunoblot analysis of paclitaxel-treated cells showed no changes in the overall expression of alpha and beta tubulins, microtubule-associated protein 2 (MAP2) or MAPs in comparison with controls. The samples were immunoprecipitated with anti-alpha- and anti-beta-tubulin antibodies and reblotted with an anti-MAP2 antibody, which showed that the amount of co-immuno-precipitated MAP2 in the synchronized cells, were increased by the brief paclitaxel treatment. These results suggest that paclitaxel treatment enhances the interaction between alpha and beta tubulins and MAP2. Since the phosphorylation state of MAP2 regulates the affinity of MAP2 for tubulins, and mitogen-activated protein (MAP) kinase is considered to be one of the kinases responsible for MAP2 phosphorylation, the effect of paclitaxel treatment on the MAP-kinase activity of synchronized PC-14 cells was examined. Two bands with molecular masses of 42 and 44 kDa were detected by an "intra-gel" MAP-kinase assay using myelin basic protein as the substrate. Paclitaxel treatment inhibited the MAP-kinase activity of PC-14 cells and inhibition was maximal at the G2/M phase of the cell cycle. Similar, concentration-dependent inhibition by paclitaxel of cellular MAP kinase of human synchronized small-cell lung carcinoma, H69, was observed. No inhibition of the MAP kinase of the paclitaxel-resistant sub-line H69/Txl by paclitaxel was observed, suggesting that some change of the MAP-kinase cascade had occurred in these cells. No direct inhibition of MAP-kinase activity by paclitaxel was observed in the cell-free assay (in vitro), suggesting that paclitaxel did not inhibit MAP kinase directly. Since it has been speculated that p34cdc2 kinase is also a kinase that phosphorylates MAP2, the effect of paclitaxel treatment on the p34cdc2-kinase activity of synchronized PC-14 and PC-9 cells was examined. Paclitaxel inhibited p34cdc2-kinase activation at the G2/M phase. These results suggest that paclitaxel inhibited MAP kinase and p34cdc2 kinase in vivo indirectly. These actions of paclitaxel may be responsible for the increased affinity between MAP2 and tubulins that it induces.

The MAP kinase cascade: its role in Xenopus oocytes, eggs and embryos

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine kinase that is activated by mitogens. Now MAPK and its activator, MAPK kinase (MAPKK), are thought to function in a wide variety of intracellular signalling pathways from yeast to vertebrate. We describe here a brief summary of the dissection of the MAPK cascade and its possible functions, especially in Xenopus oocytes and embryos.

Networking with mitogen-activated protein kinases

Mitogen activated protein (MAP) kinases and their target ribosomal protein S6 (RSK) kinases have been recognized as shared components in the intracellular signaling pathways of many diverse cytokines. Recent studies have extended this protein kinase cascade by identifying the major activator of vertebrate MAP kinases as a serine/threonine/tyrosine-protein kinase called MEK, which is related to yeast mating factor-regulated protein kinases encoded by the STE7 and byr1 genes. MEK, in turn, may be activated following its phosphorylation on serine by either of the kinases encoded by proto-oncogenes raf1 or mos, as well as by p78mekk, which is related to the yeast STE11 and byr2 gene products. Isoforms of all of these protein kinases may specifically combine to assemble distinct modules for intracellular signal transmission. However, the fundamental architecture of these protein kinase cascades has been highly conserved during eukaryotic evolution.

p42 mitogen-activated protein kinase in brain: prominent localization in neuronal cell bodies and dendrites

Neurotransmitters and growth factors can trigger activation of a newly described family of mitogen-activated protein kinases. To help define the role of this kinase family in signal transduction in the nervous system, we have conducted immunohistochemical studies to localize p42 mitogen-activated protein kinase in rat brain sections. Light-microscopic studies revealed staining in neuronal cell bodies and dendrites that is particularly prominent in superficial layers of the neocortex, the hippocampal CA3 region and dentate gyrus, as well as cerebellar Purkinje cells. Discrete staining of oligodendrocytes was also apparent in fiber tracts, indicating expression of p42 mitogen-activated protein kinase in both neuronal and glial cell types. Electron-microscopic studies demonstrated that staining in dendrites is closely associated with microtubules. In the cell bodies, prominent staining was associated with the Golgi apparatus. In contrast, immunolabeling of synaptic terminals was not detected. Previous studies have demonstrated that p42 mitogen-activated protein kinase responds to neuronal stimulation. Immunohistochemical studies presented in this paper demonstrate prominent staining for this kinase in neuronal cell bodies and dendrites. Therefore, this kinase is likely to play a key role in postsynaptic signal transduction. As both p42 mitogen-activated protein kinase and microtubule-associated protein 2, an in vitro substrate of p42 mitogen-activated kinase, are associated with dendritic microtubules, this kinase may mediate effects of growth factors or neurotransmitters on the dendritic cytoskeleton.

FAR1 links the signal transduction pathway to the cell cycle machinery in yeast

Alpha factor induces arrest of yeast a cells in G1 and transcription of genes involved in mating. Prior work indicates that FUS3, a member of the MAP kinase family, and FAR1, whose molecular activity is unknown, contribute to cell cycle arrest by inhibiting G1 cyclins. Here we show that FAR1 is a substrate for FUS3 and that this phosphorylation regulates association of FAR1 with CDC28-CLN2 kinase. We show also that FAR1 is phosphorylated in vitro by the CDC28-CLN2 complex and in vivo in a CDC28-dependent manner. Mutational analysis of FAR1 reveals a correlation between its ability to associate with CDC28-CLN2 and to arrest the cell cycle. These results suggest that FAR1 protein is the link between the signaling pathway and the cell cycle machinery.

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.

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.

Application of synthetic phospho- and unphospho- peptides to identify phosphorylation sites in a subregion of the tau molecule, which is modified in Alzheimer's disease

Phospho- and unphospho- peptides were used to define the essential sequence for a tau epitope, which is recognized by Tau-1 antibody and phosphorylated in Alzheimer's disease (AD). The epitope was mapped within the amino acid residues 192-199 of tau and was phosphorylated by the p34cdc2/p58cyclin A proline directed kinase (PDPK), but not by purified mitogen activated protein kinase (p42mapk). Addition of phosphate to the last serine of the epitope was the most effective in abolishing the reactivity of the epitope to Tau-1 antibody. Our results suggest that one and possibly more members of the PDPK family may play a role in the pathogenesis of AD.

Identification of p42 mitogen-activated protein kinase as a tyrosine kinase substrate activated by maximal electroconvulsive shock in hippocampus

Recent studies have demonstrated that administration of an electroconvulsive shock produces a rapid and transient increase in tyrosyl phosphorylation of a approximately 40-kDa protein in rat brain. Initial characterization of this protein's chromatographic properties indicated that it might be a member of a recently identified family of kinases, referred to as mitogen-activated protein (MAP) kinases, that are activated by tyrosyl phosphorylation. In the present study, we have used MAP kinase antisera to assess the identity of this protein. We have found that the approximately 40-kDa phosphotyrosine-containing protein comigrates with p42 MAP kinase (p42mapk) and not with two other 44-kDa MAP kinase family members detected by these antisera. Western blots of proteins immunoprecipitated with MAP kinase antibodies confirm that p42mapk displays increased tyrosyl phosphorylation after an electroconvulsive stimulus. Chromatographic separation of hippocampal extracts indicates that MAP kinase activity elutes in parallel with p42mapk. Accordingly, these studies identify p42mapk as a tyrosyl kinase substrate that is activated by this stimulus and suggest that this form of MAP kinase may be selectively regulated by neuronal stimulation.

Maturation hormone induced an increase in the translational activity of starfish oocytes coincident with the phosphorylation of the mRNA cap binding protein, eIF-4E, and the activation of several kinases

The stimulation of translation in starfish oocytes by the maturation hormone, 1-methyladenine (1-MA), requires the activation or mobilization of both initiation factors and mRNAs [Xu and Hille, Cell Regul. 1:1057, 1990]. We identify here the translational initiation complex, eIF-4F, and the guanine nucleotide exchange factor for eIF-2, eIF-2B, as the rate controlling components of protein synthesis in immature oocytes of the starfish, Pisaster orchraceus. Increased phosphorylation of eIF-4E, the cap binding subunit of the eIF-4F complex, is coincident with the initial increase in translational activity during maturation of these oocytes. Significantly, protein kinase C activity increased during oocyte maturation in parallel with the increase in eIF-4E phosphorylation and protein synthesis. An increase in the activities of cdc2 kinase and mitogen-activated myelin basic protein kinase (MBP kinase) similarly coincide with the increase in eIF-4E phosphorylation. However, neither cdc2 kinase nor MBP kinase phosphorylates eIF-4E in vitro. Casein kinase II activity does not change during oocyte maturation, and therefore, cannot be responsible for the activation of translation. Treatment of oocytes with phorbol 12-myristate 13-acetate, an activator of protein kinase C, for 30 min prior to the addition of 1-MA resulted in the inhibition of 1-MA-induced phosphorylation of eIF-4E, translational activation, and germinal vesicle breakdown. Therefore, protein kinase C may phosphorylate eIF-4E, after very early events of maturation. Another possibility is that eIF-4E is phosphorylated by an unknown kinase that is activated by the cascade of reactions stimulated by 1-MA. In conclusion, our results suggest a role for the phosphorylation of eIF-4E in the activation of translation during maturation, similar to translational regulation during the stimulation of growth in mammalian cells.

Regulation of protein serine-threonine phosphatase type-2A by tyrosine phosphorylation

Extracellular signals that promote cell growth activate cascades of protein kinases. The kinases are dephosphorylated and deactivated by a single type-2A protein phosphatase. The catalytic subunit of type-2A protein phosphatase was phosphorylated by tyrosine-specific protein kinases. Phosphorylation was enhanced in the presence of the phosphatase inhibitor okadaic acid, consistent with an autodephosphorylation reaction. More than 90% of the activity of phosphatase 2A was lost when thioadenosine triphosphate was used to produce a thiophosphorylated protein resistant to autodephosphorylation. Phosphorylation in vitro occurred exclusively on Tyr307. Phosphorylation was catalyzed by p60v-src, p56lck, epidermal growth factor receptors, and insulin receptors. Transient deactivation of phosphatase 2A might enhance transmission of cellular signals through kinase cascades within cells.

Assembly and cell cycle dynamics of the nuclear lamina

The nuclear lamina is a karyoskeletal structure composed of intermediate filament type proteins. It underlies the inner nuclear membrane and confers mechanical stability to the nuclear envelope. In addition, it interacts with chromatin and may thereby participate in determining the three-dimensional organization of the interphase nucleus. During mitosis, the nuclear lamina is transiently disassembled, most probably through hyperphosphorylation of lamin proteins by the protein kinase p34cdc2, a key regulator of the eukaryotic cell cycle. Mitotic disassembly of the lamina is necessary but not sufficient for nuclear envelope breakdown. Electron microscopic analyses have begun to provide insights into the principles that govern lamina assembly in vitro, and sequence motifs required for targeting newly synthesized lamins to the nuclear envelope have been identified. Of particular interest, lamins were shown to undergo a type of hydrophobic modification known as isoprenylation. Finally, recent studies addressing the nature of lamin-chromatin interactions may provide the basis for elucidating the role of lamins in organizing the distribution of interphase chromatin.

Identification of an activator of the microtubule-associated protein 2 kinases ERK1 and ERK2 in PC12 cells stimulated with nerve growth factor or bradykinin

Treatment of PC12 pheochromocytoma cells with nerve growth factor (NGF) or bradykinin leads to the activation of extracellular signal-regulated kinases ERK1 and ERK2, two isozymes of microtubule-associated protein 2 (MAP) kinase that are present in numerous cell lines and regulated by diverse extracellular signals. The activation of MAP kinase is associated with its phosphorylation on tyrosine and threonine residues, both of which are required for activity. In the present studies, we have identified a factor in extracts of PC12 cells treated with NGF or bradykinin, named MAP kinase activator, that, when reconstituted with inactive MAP kinase from untreated cells, dramatically increased MAP kinase activity. Activation of MAP kinase in vitro by this factor required MgATP and was associated with the phosphorylation of a 42- (ERK1) and 44-kDa (ERK2) polypeptide. Incorporation of 32P into ERK1 and ERK2 occurred primarily on tyrosine and threonine residues and was associated with a single tryptic peptide, which is identical to one whose phosphorylation is increased by treatment of intact PC12 cells with NGF. Thus, the MAP kinase activator identified in PC12 cells is likely to be a physiologically important intermediate in the signaling pathways activated by NGF and bradykinin. Moreover, stimulation of the activator by NGF and bradykinin suggests that tyrosine kinase receptors and guanine nucleotide-binding protein-coupled receptors are both capable of regulating these pathways.

Identification of epidermal growth factor Thr-669 phosphorylation site peptide kinases as distinct MAP kinases and p34cdc2

A synthetic peptide modeled after the major threonine (T669) phosphorylation site of the epidermal growth factor (EGF) receptor was an efficient substrate (apparent Km approximately 0.45 mM) for phosphorylation by purified p44mpk, a MAP kinase from sea star oocytes. The peptide was also phosphorylated by a related human MAP kinase, which was identified by immunological criteria as p42mapk. Within 5 min of treatment of human cervical carcinoma A431 cells with EGF or phorbol myristate acetate (PMA), a greater than 3-fold activation of p42mapk was measured. However, Mono Q chromatography of A431 cells extracts afforded the resolution of at least three additional T669 peptide kinases, some of which may be new members of the MAP kinase family. One of these (peak I), which weakly adsorbed to Mono Q, phosphorylated myelin basic protein (MBP) and other MAP kinase substrates, immunoreacted as a 42 kDa protein on Western blots with four different MAP kinase antibodies, and behaved as a approximately 45 kDa protein upon Superose 6 gel filtration. Another T669 peptide kinase (peak IV), which bound more tightly to Mono Q than p42mapk (peak II), exhibited a nearly identical substrate specificity profile to that of p42mapk, but it immunoreacted as a 40 kDa protein only with anti-p44mpk antibody on Western blots, and eluted from Superose 6 in a high molecular mass complex of greater than 400 kDa. By immunological criteria, the T669 peptide kinase in Mono Q peak III was tentatively identified as an active form of p34cdc2 associated with cyclin A. The Mono Q peaks III and IV kinases were modestly stimulated following either EGF or PMA treatments of A431 cells, and they exhibited a greater T669 peptide/MBP ratio than p42mapk. These findings indicated that multiple proline-directed kinases may mediate phosphorylation of the EGF receptor.

Mitogen-activated protein kinases: versatile transducers for cell signaling

Mitogen-activated protein (MAP) kinases are proline-directed serine/threonine protein kinases that are activated via phosphorylation of their own tyrosine residues. Highly conserved during eukaryotic evolution, they serve as common signaling components in distinct transduction pathways initiated by many stimuli. They have been implicated in the control of a broad spectrum of cellular events but are particularly known for their possible roles in cell cycle progression and the control of meiosis.

Mitogen-activated protein kinases phosphorylate nuclear lamins and display sequence specificity overlapping that of mitotic protein kinase p34cdc2

Members of the mitogen-activated protein (MAP) kinase family are implicated in mediating entry of cells into the cell cycle, as well as passage through meiotic M phase. These kinases have attracted much interest because their activation involves phosphorylation on both tyrosine and threonine residues, but little is known about their physiological targets. In this study, two distinct members of the MAP kinase family (p44mpk and p42mapk) are shown to phosphorylate chicken lamin B2 at a single site identified as Ser16. Moreover, these MAP kinases cause depolymerization of in-vitro-assembled longitudinal lamin head-to-tail polymers. Ser16 was previously shown to be phosphorylated during mitosis in vivo, and to be a target of the mitotic protein kinase p34cdc2 in vitro. Accordingly, lamins were proposed to be direct in vivo substrates of p34cdc2. This proposal is supported by quantitative analyses indicating that lamin B2, when assayed in vitro, is a substantially better substrate for p34cdc2 than for MAP kinases. Nevertheless, a physiological role of MAP kinases in lamin phosphorylation is not excluded. The observation that members of the MAP kinase family display sequence specificities overlapping that of p34cdc2 raises the possibility that some of the purported substrates of p34cdc2 may actually be physiological substrates of MAP kinases.

ras mediates nerve growth factor receptor modulation of three signal-transducing protein kinases: MAP kinase, Raf-1, and RSK

p21c-ras plays a critical role in mediating tyrosine kinase-stimulated cell growth and differentiation. However, the pathways through which p21c-ras propagates these signals remain unknown. We report that in PC12 cells, expression of a dominant inhibitory mutant of ras, c-Ha-ras(Asn-17), antagonizes growth factor- and phorbol ester-induced activation of the erk-encoded family of MAP kinases, the 85-92 kd RSKs, and the kinase(s) responsible for hyperphosphorylation of the proto-oncogene product Raf-1. In addition, we find that expression of the activated ras oncogene is sufficient to stimulate these events. These data indicate that ras mediates nerve growth factor receptor and protein kinase C modulation of MAP kinases, RSKs, and Raf-1.

Tyrosyl phosphorylation and activation of MAP kinases by p56lck

T cell signaling via the CD4 surface antigen is mediated by the associated tyrosyl protein kinase p56lck. The 42-kilodalton mitogen-activated protein (MAP) kinase (p42mapk) was tyrosyl-phosphorylated and activated after treatment of the murine T lymphoma cell line 171CD4+, which expresses CD4, with antibody to CD3. Treatment of the CD4-deficient cell line 171 with the same antibody did not result in phosphorylation or activation of p42mapk. Purified p56lck both tyrosyl-phosphorylated and stimulated the seryl-threonyl phosphotransferase activity of purified p44mpk, a MAP kinase isoform from sea star oocytes. A synthetic peptide modeled after the putative regulatory phosphorylation site in murine p42mapk (Tyr185) was phosphorylated by p56lck with a similar Vmax, but a fivefold lower Michaelis constant (Km) than a peptide containing the Tyr394 autophosphorylation site from p56lck. MAP kinases may participate in protein kinase cascades that link Src family protein-tyrosyl kinases to seryl-threonyl kinases such as those encoded by rsk and raf, which are putative substrates of MAP kinases.

Tyrosyl phosphorylation and activation of the myelin basic protein kinase p44mpk during sea star oocyte maturation

The most prominent tyrosyl-phosphorylated protein in maturing sea star oocytes was identified as the 44 kDa myelin basic protein (MBP) kinase p44mpk. Immunoblotting studies with anti-phosphotyrosine PY-20 antibody and phosphoamino acid analysis of in vivo [32P]phosphate-labelled p44mpk showed that the tyrosyl phosphorylation of the kinase correlated with a greater than 10-fold stimulation of its MBP phosphotransferase activity. The activation of p44mpk was reversed almost completely by purified preparations of the protein-tyrosyl phosphatases CD45 and 1B. Purified p44mpk has previously been shown to undergo autophosphorylation in vitro on seryl residues and this was associated with further enhancement of its MBP phosphorylating activity (Sanghera et al. (1991) J. Biol. Chem. 266, 6700-6707). p44mpk also underwent seryl phosphorylation during oocyte maturation, and the protein-seryl/threonyl phosphatase 2A reversed partially the maturation-associated stimulation of its MBP kinase activity. The properties of p44mpk resemble the murine 42 kDa mitogen-activated protein kinase (p42mapk). While p44mpk may feature the phosphorylatable tyrosyl residue that is critical for activation in p42mapk, it lacks the upstream threonyl phosphorylation site that is also required for p42mapk activity (Payne et al. (1991) EMBO J: 10, 885-892). These findings indicate partial differences in the regulatory mechanisms that govern the activities of these isozymes.