Related proteins are phosphorylated at tyrosine in response to mitogenic stimuli and at meiosis

Conservation of MAP kinase activity and MSP genes in parthenogenetic nematodes

Background

MAP (mitogen-activated protein) kinase activation is a prerequisite for oocyte maturation, ovulation and fertilisation in many animals. In the hermaphroditic nematode Caenorhabditis elegans, an MSP (major sperm protein) dependent pathway is utilised for MAP kinase activation and successive oocyte maturation with extracellular MSP released from sperm acting as activator. How oocyte-to-embryo transition is triggered in parthenogenetic nematode species that lack sperm, is not known.

Results

We investigated two key elements of oocyte-to-embryo transition, MSP expression and MAP kinase signaling, in two parthenogenetic nematodes and their close hermaphroditic relatives. While activated MAP kinase is present in all analysed nematodes irrespective of the reproductive mode, MSP expression differs. In contrast to hermaphroditic or bisexual species, we do not find MSP expression at the protein level in parthenogenetic nematodes. However, genomic sequence analysis indicates that functional MSP genes are present in several parthenogenetic species.

Conclusions

We present three alternative interpretations to explain our findings. (1) MSP has lost its function as a trigger of MAP kinase activation and is not expressed in parthenogenetic nematodes. Activation of the MAP kinase pathway is achieved by another, unknown mechanism. Functional MSP genes are required for occasionally emerging males found in some parthenogenetic species. (2) Because of long-term disadvantages, parthenogenesis is of recent origin. MSP genes remained intact during this short intervall although they are useless. As in the first scenario, an unknown mechanism is responsible for MAP kinase activation. (3) The molecular machinery regulating oocyte-to-embryo transition in parthenogenetic nematodes is conserved with respect to C. elegans, thus requiring intact MSP genes. However, MSP expression has been shifted to non-sperm cells and is reduced below the detection limits, but is still sufficient to trigger MAP kinase activation and embryogenesis.

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.

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.

Direct stimulation by tyrosine phosphorylation of microtubule-associated protein (MAP) kinase activity by granulocyte-macrophage colony-stimulating factor in human neutrophils

Human polymorphonuclear neutrophils exhibit a low level of the microtubule-associated protein kinase (MAPK) activity. This enzymic activity is enhanced up to 3-fold upon cell stimulation with the human haematopoietic hormone granulocyte-macrophage colony-stimulating factor (GM-CSF). This is demonstrated both in whole-cell lysates and in DEAE-anion-exchange semi-purified fractions prepared from GM-CSF-stimulated neutrophils, by assaying the kinase activity against either myelin basic protein or a phosphoacceptor peptide that bears the specific phosphorylation site of the MAPK natural substrate. Similarly, phosphorylation of MAPK in tyrosine residues, as found in immunoblots using anti-phosphotyrosine antibodies, follows similar time- and dose-response curves as the kinase activation. Pretreatment of the cells with the tyrosine kinase inhibitor genistein abrogates the above-mentioned effect, whereas the phosphatase inhibitor okadaic acid enhances both the basal and the GM-CSF-stimulated kinase activities. Likewise, MAPK tyrosine phosphorylation is diminished in genistein-treated neutrophils, and enhanced in okadaic acid-treated cells. We conclude that MAPK activity is present in human neutrophils, and that it is stimulated by GM-CSF. This stimulation of the activity is most likely due to the phosphorylation of MAPK in tyrosine residues triggered upon binding of GM-CSF to its receptors.

Biochemical and cytological changes associated with expression of deregulated pp60src in Xenopus oocytes

We have examined the effects of Xenopus pp60c-src with constitutive kinase activity on the morphology and maturation of Xenopus laevis oocytes. When RNA encoding this deregulated variant was injected into stage VI oocytes, we observed a gross alteration in the cortex of the oocyte. This alteration involved aggregation of pigment and invagination of the cortex in a large area proximal to the site of injection. This phenomenon was not seen in oocytes injected with RNA encoding wild-type pp60c-src. We have correlated this phenomenon with the tyrosine phosphorylation of 84- and 100-kDa proteins. These phosphorylated proteins colocalized with the alteration in the oocyte cortex when assayed by both biochemical and immunocytochemical methods. Neither the pigment aggregation nor phosphorylation of the 84- and 100-kDa proteins was observed in oocytes expressing a nonmyristoylated version of the deregulated pp60c-src. Expression of deregulated Xenopus fyn, a src-family member, resulted in a phenotype similar to that seen with deregulated src. However, in the fyn-injected oocytes, many more proteins were phosphorylated on tyrosine than in the src-injected oocytes. Progesterone stimulation of oocytes expressing deregulated pp60c-src resulted in an increase in the number of tyrosine-phosphorylated proteins. This change may represent the response of pp60src to the resumption of the cell cycle in maturing oocytes. These data suggest that the oocyte may be a particularly useful system for investigating the role of pp60c-src in the regulation of cytoskeletal structure and in the regulation of events associated with the cell cycle.

Biochemical and cytological changes associated with expression of deregulated pp60src in Xenopus oocytes

We have examined the effects of Xenopus pp60c-src with constitutive kinase activity on the morphology and maturation of Xenopus laevis oocytes. When RNA encoding this deregulated variant was injected into stage VI oocytes, we observed a gross alteration in the cortex of the oocyte. This alteration involved aggregation of pigment and invagination of the cortex in a large area proximal to the site of injection. This phenomenon was not seen in oocytes injected with RNA encoding wild-type pp60c-src. We have correlated this phenomenon with the tyrosine phosphorylation of 84- and 100-kDa proteins. These phosphorylated proteins colocalized with the alteration in the oocyte cortex when assayed by both biochemical and immunocytochemical methods. Neither the pigment aggregation nor phosphorylation of the 84- and 100-kDa proteins was observed in oocytes expressing a nonmyristoylated version of the deregulated pp60c-src. Expression of deregulated Xenopus fyn, a src-family member, resulted in a phenotype similar to that seen with deregulated src. However, in the fyn-injected oocytes, many more proteins were phosphorylated on tyrosine than in the src-injected oocytes. Progesterone stimulation of oocytes expressing deregulated pp60c-src resulted in an increase in the number of tyrosine-phosphorylated proteins. This change may represent the response of pp60src to the resumption of the cell cycle in maturing oocytes. These data suggest that the oocyte may be a particularly useful system for investigating the role of pp60c-src in the regulation of cytoskeletal structure and in the regulation of events associated with the cell cycle.

Oncogenic ras triggers the activation of 42-kDa mitogen-activated protein kinase in extracts of quiescent Xenopus oocytes

Quiescent, full-grown Xenopus oocytes, which are arrested at the G2/M border of meiosis, contain an inactive 42-kDa mitogen-activated protein kinase (p42MAPK) that is activated when oocytes are stimulated to resume the meiotic cell cycle. We have made extracts from these oocytes that respond to four cell cycle activators: oncogenic [Val12]Ras protein, clam cyclins A delta 60 and B delta 97, and the phosphatase inhibitor okadaic acid. All four induce the tyrosine phosphorylation and activation of p42MAPK. Both cyclins and okadaic acid, but not [Val12]Ras, also lead to activation of the endogenous cyclin B/cdc2 kinase complexes in extracts of quiescent oocytes. Using extracts prepared from cycloheximide-arrested interphase cells, we show that although p42MAPK activation can occur in response to cyclin-activated cdc2, the Ras-induced activation of p42MAPK occurs without intervening cdc2 activation. Neither the nononcogenic [Gly12]Ras nor [Val12,Arg186]Ras, a mutant that lacks the C-terminal consensus sequence directing prenylation and subsequent membrane association, is an effective activator of p42MAPK in vitro.

Endothelin rapidly stimulates mitogen-activated protein kinase activity in rat mesangial cells

Mitogen-activated protein (MAP) kinases are regarded as switch kinases in the phosphorylation cascade initiated by various agonists. We have investigated whether endothelins (ET), which are constrictor and mitogenic isopeptides, can increase MAP kinase activity in rat mesangial cells, using bovine myelin basic protein (MBP) as a substrate for an in vitro kinase assay. Treatment of quiescent mesangial cells with ET-1 rapidly stimulated a kinase activity which phosphorylated exogenous MBP. This stimulation was dose-dependent, with threshold responses at 1 nM-ET-1. Epidermal growth factor and thrombin also activated this kinase in mesangial cells. We also examined the ET signal transduction pathways leading to activation of MBP kinase. Pertussis toxin had no effect on ET-stimulated MBP kinase activity. Stimulation of protein kinase C by phorbol ester increased MBP kinase activity, and down-regulation of PKC partially inhibited ET-stimulated MBP kinase as well as phorbol ester-stimulated MBP kinase activity. Interestingly, genestein, an inhibitor of protein tyrosine kinases, partially inhibited MBP kinase stimulated by ET but not by phorbol esters. These results suggest that ET stimulates MBP kinase activity in rat mesangial cells via at least two pathways: one which is protein kinase C-dependent and a second one that involves a protein tyrosine kinase. Finally, by raising rabbit antibodies against the two forms of MAP kinase, p44mapk and p42mapk, we demonstrated that both isoforms are expressed in mesangial cells. Antibody alpha 1 Cp42 specifically immunoprecipitated p42mapk and allowed us to demonstrate that ET stimulates MBP kinase activity in the p42mapk immunocomplex. In conclusion, we have provided evidence that, in rat mesangial cells, MAP kinases are rapidly activated by ET-1, a regulatory process that involves at least protein kinase C activation and also a contribution of a tyrosine kinase not yet characterized.

Inhibition of c-Jun DNA binding by mitogen-activated protein kinase

Here we demonstrate that partially purified Xenopus p42 mitogen-activated protein (MAP) kinase phosphorylates bacterially expressed human c-Jun at a single site, serine 243. Several lines of evidence argue that this phosphorylation is due to p42 MAP kinase itself rather than some contaminating species. Phosphorylation of serine 243 markedly decreases the binding of c-Jun to oligonucleotides containing the 12-O-tetradecanoylphorbol-13-acetate response element. These findings suggest that MAP kinase may play a role in the down-regulation of c-Jun or in the cycle of transcriptional initiation and elongation.

Phosphorylation of conserved serine residues does not regulate the ability of mosxe protein kinase to induce oocyte maturation or function as cytostatic factor

Expression of the mosxe protein kinase is required for the normal meiotic maturation of Xenopus oocytes and overexpression induces maturation in the absence of other stimuli. In addition, mosxe functions as a component of cytostatic factor (CSF), an activity responsible for arrest of the mature egg at metaphase II. After microinjection of Xenopus oocytes with in vitro synthesized RNA encoding either wild-type mosxe or kinase-inactive mosxe(R90), both proteins are phosphorylated exclusively on serine residues and exhibit essentially identical chymotryptic maps. Since the phosphorylated kinase-inactive mosxe(R90) protein was recovered from resting oocytes that have not yet begun to translate endogenous mosxe, this indicates that the major phosphopeptides of mosxe(R90) are phosphorylated by a preexisting protein kinase present in resting oocytes, and are not the result of autophosphorylation. The results presented here also indicate that the mosxe protein does not undergo significant phosphorylation at unique sites during oocyte maturation. If the biological activity of mosxe were regulated by phosphorylation, a site of regulatory phosphorylation would most likely be conserved among mos proteins of different species. Site-directed mutagenesis was used to construct 13 individual serine----alanine mutations at conserved residues (3, 16, 18, 25, 26, 57, 71, 76, 102, 105, 127, 211, and 258). These 13 mutants were analyzed for their abilities to induce oocyte maturation and to function as CSF. Results obtained with the mosxe(A105) mutant revealed that serine-105 is required for both maturation induction and CSF activity, even though serine-105 does not represent a major site of phosphorylation. All of the remaining serine----alanine mosxe mutants induced oocyte maturation and exhibited CSF activity comparable with the wild type. These results demonstrate that none of the conserved serines examined in this study function as regulatory phosphorylation sites for these biological activities. Peptide mapping of the remaining mosxe mutants identified serine-3 as a major phosphorylation site in vivo, which is contained within the chymotryptic peptide MPSPIPVERF.

Extracellular signal-regulated kinases: ERKs in progress

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Temperature-dependent tyrosine phosphorylation of microtubule-associated protein kinase in epidermal growth factor-stimulated human fibroblasts

Treatment of normal human fibroblasts with epidermal growth factor (EGF) results in the rapid (0.5 min) and simultaneous tyrosine phosphorylation of the EGF receptor (EGFr) and several other proteins. An exception to this tyrosine phosphorylation wave was a protein (42 kDa) that became phosphorylated on tyrosine only after a short lag time (5 min). We identified this p42 kDa substrate as the microtubule-associated protein (MAP) kinase using a monoclonal antibody to a peptide corresponding to the C-terminus of the predicted protein (Science 249, 64-67, 1990). EGF treatment of human fibroblasts at 37 degrees C for 5 min resulted in the tyrosine phosphorylation of 60-70% of MAP kinase as determined by the percent that was immunoprecipitated with antiphosphotyrosine antibodies. Like other tyrosine kinase growth factor receptors, the EGFr is activated and phosphorylated at 4 degrees C but is not internalized. Whereas most other substrates were readily tyrosine phosphorylated at 4 degrees C, MAP kinase was not. When cells were first stimulated with EGF at 4 degrees C and then warmed to 37 degrees C without EGF, tyrosine phosphorylation of MAP kinase was again observed. Treatment of cells with the protein kinase C activator phorbol myristate acetate (PMA) also resulted in the tyrosine phosphorylation of MAP kinase, and again only at 37 degrees C. Tryptic phosphopeptide maps demonstrated that EGF and PMA both induced the phosphorylation of the same peptide on tyrosine and threonine. This temperature and PMA sensitivity distinguishes MAP kinase from most other tyrosine kinase substrates in activated human fibroblasts.

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.

Cell cycle tyrosine phosphorylation of p34cdc2 and a microtubule-associated protein kinase homolog in Xenopus oocytes and eggs

We have examined the time course of protein tyrosine phosphorylation in the meiotic cell cycles of Xenopus laevis oocytes and the mitotic cell cycles of Xenopus eggs. We have identified two proteins that undergo marked changes in tyrosine phosphorylation during these processes: a 42-kDa protein related to mitogen-activated protein kinase or microtubule-associated protein-2 kinase (MAP kinase) and a 34-kDa protein identical or related to p34cdc2. p42 undergoes an abrupt increase in its tyrosine phosphorylation at the onset of meiosis 1 and remains tyrosine phosphorylated until 30 min after fertilization, at which point it is dephosphorylated. p42 also becomes tyrosine phosphorylated after microinjection of oocytes with partially purified M-phase-promoting factor, even in the presence of cycloheximide. These findings suggest that MAP kinase, previously implicated in the early responses of somatic cells to mitogens, is also activated at the onset of meiotic M phase and that MAP kinase can become tyrosine phosphorylated downstream from M-phase-promoting factor activation. We have also found that p34 goes through a cycle of tyrosine phosphorylation and dephosphorylation prior to meiosis 1 and mitosis 1 but is not detectable as a phosphotyrosyl protein during the 2nd through 12th mitotic cell cycles. It may be that the delay between assembly and activation of the cyclin-p34cdc2 complex that p34cdc2 tyrosine phosphorylation provides is not needed in cell cycles that lack G2 phases. Finally, an unidentified protein or group of proteins migrating at 100 to 116 kDa increase in tyrosine phosphorylation throughout maturation, are dephosphorylated or degraded within 10 min of fertilization, and appear to cycle between low-molecular-weight forms and high-molecular-weight forms during early embryogenesis.

Cell cycle tyrosine phosphorylation of p34cdc2 and a microtubule-associated protein kinase homolog in Xenopus oocytes and eggs

We have examined the time course of protein tyrosine phosphorylation in the meiotic cell cycles of Xenopus laevis oocytes and the mitotic cell cycles of Xenopus eggs. We have identified two proteins that undergo marked changes in tyrosine phosphorylation during these processes: a 42-kDa protein related to mitogen-activated protein kinase or microtubule-associated protein-2 kinase (MAP kinase) and a 34-kDa protein identical or related to p34cdc2. p42 undergoes an abrupt increase in its tyrosine phosphorylation at the onset of meiosis 1 and remains tyrosine phosphorylated until 30 min after fertilization, at which point it is dephosphorylated. p42 also becomes tyrosine phosphorylated after microinjection of oocytes with partially purified M-phase-promoting factor, even in the presence of cycloheximide. These findings suggest that MAP kinase, previously implicated in the early responses of somatic cells to mitogens, is also activated at the onset of meiotic M phase and that MAP kinase can become tyrosine phosphorylated downstream from M-phase-promoting factor activation. We have also found that p34 goes through a cycle of tyrosine phosphorylation and dephosphorylation prior to meiosis 1 and mitosis 1 but is not detectable as a phosphotyrosyl protein during the 2nd through 12th mitotic cell cycles. It may be that the delay between assembly and activation of the cyclin-p34cdc2 complex that p34cdc2 tyrosine phosphorylation provides is not needed in cell cycles that lack G2 phases. Finally, an unidentified protein or group of proteins migrating at 100 to 116 kDa increase in tyrosine phosphorylation throughout maturation, are dephosphorylated or degraded within 10 min of fertilization, and appear to cycle between low-molecular-weight forms and high-molecular-weight forms during early embryogenesis.

Identification of a 42-kilodalton phosphotyrosyl protein as a serine(threonine) protein kinase by renaturation

We have surveyed fibroblast lysates for protein kinases that might be involved in mitogenesis. The assay we have used exploits the ability of blotted, sodium dodecyl sulfate-denatured proteins to regain enzymatic activity after guanidine treatment. About 20 electrophoretically distinct protein kinases could be detected by this method in lysates from NIH 3T3 cells. One of the kinases, a 42-kilodalton serine(threonine) kinase (PK42), was found to possess two- to fourfold-higher in vitro activity when isolated from serum-stimulated cells than when isolated from serum-starved cells. This kinase comigrated on sodium dodecyl sulfate-gels with a protein (p42) whose phosphotyrosine content increased in response to serum stimulation. The time courses of p42 tyrosine phosphorylation and PK42 activation were similar, reaching maximal levels within 10 min and returning to basal levels within 5 h. Both p42 tyrosine phosphorylation and PK42 activation were stimulated by low concentrations of phorbol esters, and the responses of p42 and PK42 to TPA were abolished by chronic 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Chronic TPA treatment had less effect on serum-induced p42 tyrosine phosphorylation and PK42 activation. PK42 and p42 bound to DEAE-cellulose, and both eluted at a salt concentration of 250 mM. Thus, PK42 and p42 comigrate and cochromatograph, and the kinase activity of PK42 correlates with the tyrosine phosphorylation of p42. These findings suggest that PK42 and p42 are related or identical, that PK42 is activated by tyrosine phosphorylation, and that this tyrosine phosphorylation can be regulated by protein kinase C.

Identification of a 42-kilodalton phosphotyrosyl protein as a serine(threonine) protein kinase by renaturation

We have surveyed fibroblast lysates for protein kinases that might be involved in mitogenesis. The assay we have used exploits the ability of blotted, sodium dodecyl sulfate-denatured proteins to regain enzymatic activity after guanidine treatment. About 20 electrophoretically distinct protein kinases could be detected by this method in lysates from NIH 3T3 cells. One of the kinases, a 42-kilodalton serine(threonine) kinase (PK42), was found to possess two- to fourfold-higher in vitro activity when isolated from serum-stimulated cells than when isolated from serum-starved cells. This kinase comigrated on sodium dodecyl sulfate-gels with a protein (p42) whose phosphotyrosine content increased in response to serum stimulation. The time courses of p42 tyrosine phosphorylation and PK42 activation were similar, reaching maximal levels within 10 min and returning to basal levels within 5 h. Both p42 tyrosine phosphorylation and PK42 activation were stimulated by low concentrations of phorbol esters, and the responses of p42 and PK42 to TPA were abolished by chronic 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Chronic TPA treatment had less effect on serum-induced p42 tyrosine phosphorylation and PK42 activation. PK42 and p42 bound to DEAE-cellulose, and both eluted at a salt concentration of 250 mM. Thus, PK42 and p42 comigrate and cochromatograph, and the kinase activity of PK42 correlates with the tyrosine phosphorylation of p42. These findings suggest that PK42 and p42 are related or identical, that PK42 is activated by tyrosine phosphorylation, and that this tyrosine phosphorylation can be regulated by protein kinase C.

A 42-kD tyrosine kinase substrate linked to chromaffin cell secretion exhibits an associated MAP kinase activity and is highly related to a 42-kD mitogen-stimulated protein in fibroblasts

The localization of the protein tyrosine kinase pp60c-src to the plasma membrane and to the membrane of secretory vesicles in neurally derived bovine chromaffin cells has suggested that tyrosine phosphorylations may be associated with the process of secretion. In the present study we have identified two cytosolic proteins of approximately 42 and 45 kD that become phosphorylated on tyrosine in response to secretagogue treatment. Phosphorylation of these proteins reached a maximum (3 min after stimulation) before maximum catecholamine release was observed (5-10 min after stimulation). Both secretion and tyrosine phosphorylation of p42 and p45 required extracellular Ca2+. Tyrosine-phosphorylated proteins of similar Mr have previously been identified in 3T3-L1 adipocytes stimulated with insulin (MAP kinase; Ray, L. B., and T. W. Sturgill. 1987. Proc. Natl. Acad. Sci. USA. 84:1502-1506) and in avian and rodent fibroblasts stimulated with a variety of mitogenic agents (Cooper, J. A., D. F. Bowen-Pope, E. Raines, R. Ross, and T. Hunter. 1982. Cell. 31:263-273; Nakamura, K. D., R. Martinez, and M. J. Weber. 1983. Mol. Cell. Biol. 3:380-390). Comparisons of the secretion-associated 42-kD protein of chromaffin cells with the 42-kD protein of Swiss 3T3 fibroblasts and 3T3-L1 adipocytes provide evidence that these three proteins are highly related. This evidence includes comigration during one-dimensional SDS-PAGE, cochromatography using ion exchange and hydrophobic matrices, similar isoelectric points, identical cyanogen-bromide peptide maps, and cochromatography of MAP kinase activity with the tyrosine-phosphorylated form of pp42. This protein(s), which appears to be activated in a variety of cell types, may serve a common function, perhaps in signal transduction involving a cascade of kinases.