Restriction of the in vitro and in vivo tyrosine protein kinase activities of pp60c-src relative to pp60v-src

Unity and diversity among viral kinases

Viral kinases are known to undergo autophosphorylation and also phosphorylate viral and host substrates. Viral kinases have been implicated in various diseases and are also known to acquire host kinases for mimicking cellular functions and exhibit virulence. Although substantial analyses have been reported in the literature on diversity of viral kinases, there is a gap in the understanding of sequence and structural similarity among kinases from different classes of viruses. In this study, we performed a comprehensive analysis of protein kinases encoded in viral genomes. Homology search methods have been used to identify kinases from 104,282 viral genomic datasets. Serine/threonine and tyrosine kinases are identified only in 390 viral genomes. Out of seven viral classes that are based on nature of genetic material, only viruses having double-stranded DNA and single-stranded RNA retroviruses are found to encode kinases. The 716 identified protein kinases are classified into 63 subfamilies based on their sequence similarity within each cluster, and sequence signatures have been identified for each subfamily. 11 clusters are well represented with at least 10 members in each of these clusters. Kinases from dsDNA viruses, Phycodnaviridae which infect green algae and Herpesvirales that infect vertebrates including human, form a major group. From our analysis, it has been observed that the protein kinases in viruses belonging to same taxonomic lineages form discrete clusters and the kinases encoded in alphaherpesvirus form host-specific clusters. A comprehensive sequence and structure-based analysis enabled us to identify the conserved residues or motifs in kinase catalytic domain regions across all viral kinases. Conserved sequence regions that are specific to a particular viral kinase cluster and the kinases that show close similarity to eukaryotic kinases were identified by using sequence and three-dimensional structural regions of eukaryotic kinases as reference. The regions specific to each viral kinase cluster can be used as signatures in the future in classifying uncharacterized viral kinases. We note that kinases from giant viruses Marseilleviridae have close similarity to viral oncogenes in the functional regions and in putative substrate binding regions indicating their possible role in cancer.

A switch in nucleotide affinity governs activation of the Src and Tec family kinases

The Tec kinases, closely related to Src family kinases, are essential for lymphocyte function in the adaptive immune system. Whilst the Src and Abl kinases are regulated by tail phosphorylation and N-terminal myristoylation respectively, the Tec kinases are notable for the absence of either regulatory element. We have found that the inactive conformations of the Tec kinase Itk and Src preferentially bind ADP over ATP, stabilising both proteins. We demonstrate that Itk adopts the same conformation as Src and that the autoinhibited conformation of Src is independent of its C-terminal tail. Allosteric activation of both Itk and Src depends critically on the disruption of a conserved hydrophobic stack that accompanies regulatory domain displacement. We show that a conformational switch permits the exchange of ADP for ATP, leading to efficient autophosphorylation and full activation. In summary, we propose a universal mechanism for the activation and autoinhibition of the Src and Tec kinases.

c-Src phosphorylation and activation of hexokinase promotes tumorigenesis and metastasis

It is well known that c-Src has important roles in tumorigenesis. However, it remains unclear whether c-Src contributes to metabolic reprogramming. Here we find that c-Src can interact with and phosphorylate hexokinases HK1 and HK2, the rate-limiting enzymes in glycolysis. Tyrosine phosphorylation dramatically increases their catalytic activity and thus enhances glycolysis. Mechanistically, c-Src phosphorylation of HK1 at Tyr732 robustly decreases its K_m and increases its V_max by disrupting its dimer formation. Mutation in c-Src phosphorylation site of either HK1 or HK2 remarkably abrogates the stimulating effects of c-Src on glycolysis, cell proliferation, migration, invasion, tumorigenesis and metastasis. Due to its lower K_m for glucose, HK1 rather than HK2 is required for tumour cell survival when glucose is scarce. Importantly, HK1-Y732 phosphorylation level remarkably correlates with the incidence and metastasis of various clinical cancers and may serve as a marker to predict metastasis risk of primary cancers.

The protein tyrosine kinase c-Src is a renowned proto-oncogene with pleiotropic effects. Here, the authors show that c-Src induces the metabolic reprogramming of cancer cells by phosphorylating hexokinases HK1 and HK2, which in turns lead to increased HK catalytic activity and consequent enhancement of glycolysis.

D-Glyceraldehyde-3-Phosphate Dehydrogenase Structure and Function

Aside from its well-established role in glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been shown to possess many key functions in cells. These functions are regulated by protein oligomerization , biomolecular interactions, post-translational modifications , and variations in subcellular localization . Several GAPDH functions and regulatory mechanisms overlap with one another and converge around its role in intermediary metabolism. Several structural determinants of the protein dictate its function and regulation. GAPDH is ubiquitously expressed and is found in all domains of life. GAPDH has been implicated in many diseases, including those of pathogenic, cardiovascular, degenerative, diabetic, and tumorigenic origins. Understanding the mechanisms by which GAPDH can switch between its functions and how these functions are regulated can provide insights into ways the protein can be modulated for therapeutic outcomes.

The N2-Src neuronal splice variant of C-Src has altered SH3 domain ligand specificity and a higher constitutive activity than N1-Src

N2-Src is a poorly understood neuronal splice variant of the ubiquitous C-Src tyrosine kinase, containing a 17 amino acid insert in its Src homology 3 (SH3) domain. To characterise the properties of N2-Src we directly compared its SH3 domain specificity and kinase activity with C- and N1-Src in vitro. N2- and N1-Src had a similar low affinity for the phosphorylation of substrates containing canonical C-Src SH3 ligands and synaptophysin, an established neuronal substrate for C-Src. N2-Src also had a higher basal kinase activity than N1- and C-Src in vitro and in cells, which could be explained by weakened intramolecular interactions. Therefore, N2-Src is a highly active kinase that is likely to phosphorylate alternative substrates to C-Src in the brain.

A GAPDH mutant defective in Src-dependent tyrosine phosphorylation impedes Rab2-mediated events

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has multiple intracellular activities in addition to its role in gluconeogenesis. Indeed, we have reported that GAPDH is required for Rab2-mediated retrograde transport from vesicular tubular clusters (VTCs). These diverse GAPDH activities are the result of posttranslational modifications that confer a new function to the enzyme. In that regard, GAPDH is tyrosine phosphorylated by Src. To establish the functional significance of this modification for GAPDH activity in Rab2-dependent events, an amino acid substitution was made at tyrosine 41 (GAPDH Y41F). The inability of Src to phosphorylate purified recombinant GAPDH Y41F was confirmed in an in vitro kinase assay. The mutant was then employed in a quantitative membrane-binding assay that measures Rab2 recruitment of soluble components to VTCs. As we observed with GAPDH wild type, Rab2 promoted GAPDH Y41F binding to membranes in a dose-dependent manner, indicating that GAPDH tyrosine phosphorylation is not required for VTC association. However, GAPDH was tyrosine phosphorylated on VTCs. Importantly, GAPDH Y41F blocked vesicular stomatitis virus-G transport in an assay that reconstitutes endoplasmic reticulum to Golgi trafficking, indicating that phosphorylation of tyrosine 41 is essential for GAPDH activity in the early secretory pathway. The block in transport is because of the decreased binding of atypical protein kinase C iota/lambda to GAPDH Y41F, which reduces beta-coat protein association with the VTC and subsequent formation of Rab2-mediated retrograde vesicles. Our results suggest that Src plays a pivotal role in regulating the interaction of Rab2 effectors on the VTC.

Selected glimpses into the activation and function of Src kinase

Since the discovery of the v-src and c-src genes and their products, much progress has been made in the elucidation of the structure, regulation, localization, and function of the Src protein. Src is a non-receptor protein tyrosine kinase that transduces signals that are involved in the control of a variety of cellular processes such as proliferation, differentiation, motility, and adhesion. Src is normally maintained in an inactive state, but can be activated transiently during cellular events such as mitosis, or constitutively by abnormal events such as mutation (i.e. v-Src and some human cancers). Activation of Src occurs as a result of disruption of the negative regulatory processes that normally suppress Src activity, and understanding the various mechanisms behind Src activation has been a target of intense study. Src associates with cellular membranes, in particular the plasma membrane, and endosomal membranes. Studies indicate that the different subcellular localizations of Src could be important for the regulation of specific cellular processes such as mitogenesis, cytoskeletal organization, and/or membrane trafficking. This review will discuss the history behind the discovery and initial characterization of Src and the regulatory mechanisms of Src activation, in particular, regulation by modification of the carboxy-terminal regulatory tyrosine by phosphatases and kinases. Its focus will then turn to the different subcellular localizations of Src and the possible roles of nuclear and perinuclear targets of Src. Finally, a brief section will review some of our present knowledge regarding Src involvement in human cancers.

Wild-type and mutant forms of v-src differentially alter neuronal migration and differentiation in vivo

The effects of three different forms of v-src on brain cell development were determined in vivo. Recombinant retroviral vectors encoding the marker lacZ (control) and either wild-type v-src or SH2 or SH3 domain-deleted forms of v-src (deltaSH2 or deltaSH3, respectively) were used to infect neuronal progenitor cells in the embryonic chicken midbrain (optic tectum; OT). Embryos were injected in the OT with retroviral concentrates on embryonic day (E) 3 and sacrificed at E6, E9, and later in development. Patterns of cell proliferation, migration, and differentiation of lacZ-marked clonal cell progeny were then analyzed. Relative to lacZ-only controls, cell clone size at E6 was significantly increased for v-src-, unchanged for deltaSH2-, and smaller for deltaSH3-injected embryos. At E9, deltaSH2 cell clones were significantly larger than controls, suggesting increased survival from normal programmed cell death. Radial neuronal migration was impaired for v-src and deltaSH3 clones, whereas tangential neuronal migration was enhanced along fiber tracts in v-src and deltaSH2 clones. Moreover, radial glial cell development and differentiation was hindered in v-src and deltaSH3 clones. These experiments demonstrate that ectopic v-src signaling alters proliferation, migration, survival, and differentiation of developing brain cells and suggest that src signaling pathways are involved in these developmental processes. Furthermore, certain effects of v-src on brain cells require specific src homology domains.

Tyrosine phosphorylation of connexin 43 by v-Src is mediated by SH2 and SH3 domain interactions

Reduction of gap junctional communication in v-src transformed cells is accompanied by tyrosine phosphorylation of the gap junction protein, connexin 43 (Cx43). Cx43 is phosphorylated on tyrosine by v-Src. The Src homology 3 (SH3) and Src homology 2 (SH2) domains of v-Src mediate interactions with substrate proteins. SH3 domains interact with proline-rich peptide motifs. SH2 domains associate with short amino acid sequences containing phosphotyrosine. We present evidence that the SH3 and SH2 domains of v-Src bind to proline-rich motifs and a phosphorylated tyrosine residue in the C-terminal tail of Cx43. Cx43 bound to the SH3 domain of v-Src, but not c-Src, in vitro. Tyrosine-phosphorylated Cx43 bound to the SH2 domain of v-Src in vitro. v-Src coprecipitated with Cx43 from v-src-transformed Rat-1 fibroblasts. Mutations in the SH3 and SH2 domains of v-Src, and in the proline-rich region or tyrosine 265 of Cx43, reduced interactions between v-Src and Cx43 in vivo. Tyrosine phosphorylation of Cx43 was dependent on the association of v-Src and Cx43. These results provide further evidence for the direct involvement of v-Src in tyrosine phosphorylation of Cx43 and inhibition of gap junctional communication in v-src-transformed cells.

Overexpression of c-src and n-src in the developing Xenopus retina differentially impairs axonogenesis

To compare the roles of the nonreceptor tyrosine kinase c-src and its neuronal splice form n-src in developing neurons, Xenopus retinal precursors were transfected in vivo with c-src, n-src, or constitutively active mutants. Axonogenesis of retinal ganglion cells was markedly impaired by the expression of constitutively active c-src and only mildly affected by the expression of constitutively active n-src. This differential phenotype could not be accounted for by raised levels of intracellular tyrosine phosphorylation alone because the average anti-phosphotyrosine staining intensity of retinal neurons expressing mutant n-src was almost twofold greater than that of neurons expressing mutant c-src. The expression of either constitutively active isoform inhibited photoreceptor differentiation by 72% but did not influence other cell fates. These results suggest that c-src and n-src have both overlapping and distinct activities in differentiating retinal neurons.

Regulation of Btk function by a major autophosphorylation site within the SH3 domain

Bruton's tyrosine kinase (Btk) plays a crucial role in B cell development. Overexpression of Btk with a Src family kinase increases tyrosine phosphorylation and catalytic activity of Btk. This occurs by transphosphorylation at Y551 in the Btk catalytic domain and the enhancement of Btk autophosphorylation at a second site. A gain-of-function mutant called Btk* containing E41 to K change within the pleckstrin homology domain induces fibroblast transformation. Btk* enhances the transphosphorylation of Y551 by endogenous Src family tyrosine kinases and autophosphorylation at the second site. We mapped the major Btk autophosphorylation site to Y223 within the SH3 domain. Mutation of Y223 to F blocks Btk autophosphorylation and dramatically potentiates the transforming activity of Btk* in fibroblasts. The location of Y223 in a potential ligand-binding pocket suggests that autophosphorylation regulates SH3-mediated signaling by Btk.

Degradation of endothelial cell matrix collagen is correlated with induction of stromelysin by an activated ras oncogene

A conditional expression system was established whereby the human K-ras, v-src, and v-mos genes were cloned into a conditional expression vector downstream of the dexamethasone-inducible mouse mammary tumor virus long terminal repeat. Rat-1 fibroblasts were transfected with these constructs and selected in medium containing G418. Cloned transfectants were isolated and characterized for absolute dependence on dexamethasone for expression of oncogene products and anchorage-independent growth in soft agar. Expression of activated p21K-ras(val12) enabled the fibroblasts to degrade extracellular matrix collagen secreted by murine microvessel endothelial cells. Concurrent with p21K-ras(val12) induction a proteinase with the characteristic size and substrate specificity of transin, the murine homologue of the human matrix metalloproteinase stromelysin, was expressed and secreted. Induction of v-mos and v-src oncogenes resulted in little or no detectable transin expression respectively coinciding with a relative or absolute failure to increase degradation of extracellular matrix collagen. This study suggests that in this system the expression of the ras oncogene can contribute to the in vitro invasive behavior of tumor cells by upregulating the production of a metalloproteinase capable of degrading collagen synthesized by vascular endothelial cells.

Disruption of the csk gene, encoding a negative regulator of Src family tyrosine kinases, leads to neural tube defects and embryonic lethality in mice

All Src family non-receptor tyrosine kinases are negatively regulated by phosphorylation at a carboxy-terminal tyrosine. To analyze the significance of this regulation during development, we have generated mice deficient in Csk, a kinase that phosphorylates this tyrosine, by gene targeting in embryonic stem cells. Homozygous mutant embryos exhibit a complex phenotype that includes defects in the neural tube and die between day 9 and day 10 of gestation. Cells derived from these embryos exhibit an order of magnitude increase in activity of Src and the related Fyn kinase. Phosphorylation at the carboxy-terminal tyrosine of Src was reduced but not eliminated and was accompanied by increased phosphorylation at another key tyrosine residue. These results demonstrate that Src family kinase activity is critically dependent on phosphorylation by Csk and suggest that the regulation of kinase activity may be essential during embryogenesis.

A strong protein-tyrosine kinase activity is associated with a baculovirus-expressed chicken tkl gene

We have previously described a gene named tkl (tyrosine kinase related to lck). It belongs to the src family of protein-tyrosine kinases and among these it has significant homology to the lck gene (lymphoide cell kinase). The tkl gene product may represent the avian homolog of Lck, which is believed to participate in a lymphocyte-specific signal transduction pathway by association with a membrane receptor. To study the biochemical properties of the protein, a nearly complete tkl gene (isolated from a cDNA library from chicken spleen cells) was expressed in a baculovirus system. Approximately 10% of the extracted protein consisted of the soluble 51-kDa Tkl protein (p51tkl) at 40 h post-infection. This protein was found to be phosphorylated on tyrosine and serine residues at a ratio of 5:1. As expected, glycosylation or myristoylation could not be detected. Immunocomplex kinase assays indicated strong autophosphorylation of p51tkl at tyrosine residues and phosphorylation of exogenous substrates such as D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), histones H2b and H4, and casein. This protein-tyrosine kinase activity also exhibited a marked preference for Mn2+ compared to Mg2+. The high level expression of enzymatically active Tkl should provide an excellent tool to further study the biological functions of this class of enzymes.

Zinc-induced tyrosine phosphorylation of hippocampal p60c-src is catalyzed by another protein tyrosine kinase

Tyrosine phosphorylation of p60c-src induced by Zn2+ in rat hippocampal membranes is shown to inhibit Src tyrosine kinase activity. Zn2+ catalyzes the phosphorylation of p60c-src in the membranes but does not activate autophosphorylation of p60c-src immunoprecipitated with anti-Src monoclonal antibody. Moreover, the immunoprecipitated Src kinase has no Zn(2+)-induced activity in phosphorylation of exogenous substrate, enolase. Cyanogen bromide cleavage of p60c-src phosphorylated in the presence of Zn2+ yields a 4-kDa phosphopeptide corresponding to phosphorylation of a carboxy-terminal tyrosine residue of Src kinase. In conclusion, hippocampal membranes contain a Zn(2+)-stimulated protein tyrosine kinase capable of regulating the p60c-src activity.

Kinase activity and genetic characterization of a growth related antigen of Drosophila

The Drosophila developmental antigen recognized by the monoclonal antibody F7D6 is expressed in dividing embryonic and imaginal cells but is lost from all differentiating tissues except electrogenic cells of the nervous system and spontaneously contracting muscles. The 63 kDa antigen is associated with the inner surface of plasma membranes and is expressed in several classes of tumorous mutants of Drosophila. The monoclonal antibody was used for immunoprecipitating the antigen for biochemical characterization and for screening expression vector cDNA libraries. Here we report that this oncodevelopmental antigen is a phosphoprotein and a serine-threonine specific protein kinase. A 1.6 kb cDNA isolated by immunological screening of an ovarian library hybridized to a single band on polytene chromosomes, localizing the gene to 72F on the left arm of the third chromosome. Immunofluorescence assays of deficiency stocks in the region confirmed the location of the gene and identity of the cDNA clone, and mapped the gene between the left breakpoints of Df(3L) st100.62 and Df(3L) stj7, i.e., between 72F3-7 and 73A1-2. The biochemical and genetic properties indicate that this is a novel growth-related kinase of Drosophila.

Factors contributing to the restricted DNA replicating activity of JC virus

The basis for the restricted host range behavior of JC virus (JCV) in vitro was investigated by focusing on its DNA replicating activity and comparing it to that of simian virus 40 (SV40). Prototype, mutant, and hybrid JCV and SV40 DNAs were tested for their replicating activity in cells permissive for one or both of the viruses. Results from these experiments indicated that, relative to its SV40 counterpart, the JCV T antigen functioned less efficiently and was more specific in its interactions with polyomavirus DNA replication origins. The JCV T antigen exhibited a lower specific DNA binding activity than did the SV40 T antigen, which might contribute to this virus' reduced DNA replicating activity. However, the JCV protein did bind to both the JCV and SV40 replication origins with similar efficiency, indicating that the ability of the JCV T antigen to discriminate between the JCV and SV40 origins involved a step subsequent to specific DNA binding. The results also suggested that the failure of JCV to replicate to detectable levels in monkey kidney cells was due to the inefficient interactions of its T protein with the viral origin and the host replication machinery. The inability of the JCV T antigen to carry out one or more of these DNA replication functions efficiently contributes to the restricted lytic behavior of this virus.

Serine/threonine-specific protein kinase activity associated with viral pp60src protein

Three different types of experiments are presented in this paper, the results of which converge to indicate that the viral src protein associates with and modulates the activity and/or the specificity of a serine/threonine protein kinase. Firstly, a 60-kDa protein from extracts of FR3T3 rat fibroblasts transformed by wild-type Rous sarcoma virus (SRD-FR3T3) is shown to be immunoprecipitated with a monoclonal antibody (mAb) raised against bacterially produced pp60v-src, the mAb327 [Lipsich, L. A., Lewis, A. J. & Brugge, J. S. (1983) J. Virol. 48, 352-360] and to be phosphorylated in vitro at serine/threonine/tyrosine residues, in the ratio 25:53:22. Under the same experimental conditions, the pp60c-src protein immunoprecipitated with mAb327 from extracts of NIH c-src overexpresser cells is phosphorylated exclusively on tyrosine residues. Secondly, the results of immunoprecipitation experiments using a tumor-bearing rabbit (TBR) serum and reported in an earlier work [David-Pfeuty, T. & Hovanessian, A. (1984) Eur. J. Biochem. 140, 325-342], together with those reported here, suggest that the TBR-immunoprecipitated pp60v-src coprecipitates with a cellular protein related to the 60-kDa subunit of the Ca2+/calmodulin protein kinase II from brain. Finally, partially purified preparations of pp60v-src, but not of pp60c-src, are shown to contain a Ca2+/calmodulin-dependent protein kinase activity that phosphorylates a 52-kDa protein substrate.

Characterization of purified pp60c-src protein tyrosine kinase from human platelets

Intact pp60c-src, the cellular homologue of the transforming protein of Rous sarcoma virus, was purified from human platelets. The purified fractions also contained small amounts of a 54-kDa proteolytic degradation product of pp60c-src. We investigated some of the biochemical and kinetic properties of pp60c-src protein tyrosine kinase. Maximum kinase activity occurred at pH 6.5 and required a mixture of 2 mM Mn2+/Mg2+ as divalent cations. The enzyme most strongly phosphorylated casein, followed by enolase and alcohol dehydrogenase. The Km value for ATP was 4 microM for substrate phosphorylation and for autophosphorylation. Using casein, we determined a Vmax for substrate phosphorylation by pp60c-src in the range of 1.9-3.4 nmol.min-1.mg-1. Since the Vmax value for the purified 54-kDa fragment of pp60c-src was also included in this value, we conclude that proteolytic degradation of a 6-kDa fragment from the N-terminus of pp60c-src did not affect its kinase activity. Tryptic phosphopeptide analysis identified Tyr-416 as the major autophosphorylation site. Preincubation of purified pp60c-src with ATP increased the amount of autophosphorylation accompanied by an increase in Vmax, whereas the Km values were not altered. Our data directly demonstrate that autophosphorylation at Tyr-416 exerts, in contrast to phosphorylation at Tyr-527, a positive regulatory effect on the pp60c-src kinase activity.

Different pattern of expression of cellular oncogenes in human non-small-cell lung cancer cell lines

Altered and deregulated cellular oncogenes were found in many human solid tumors. Except for a few types of tumors that consistently exhibited specific altered proto-oncogenes, the majority of tumors are associated with a number of transcriptionally activated cellular oncogenes. In the heterologous group of non-small-cell lung cancer (NSCLC), nothing about a specific pattern of proto-oncogene expression is known. Therefore, we investigated the expression of a panel of cellular oncogenes in NSCLC cell lines. DNA and RNA from 11 established NSCLC cell lines (4 adenocarcinoma cell lines, 3 squamous cell carcinoma cell lines, 3 large-cell carcinoma cell lines and 1 mesothelioma cell line) were isolated and analysed using the Southern, dot blot and Northern hybridization technique. c-myc RNA expression was found in all NSCLC cell line, L-myc expression only in 1 adenocarcinoma cell line, N-myc and c-myb expression in none of the 11 cell lines examined. No c-myc amplification could be detected in the DNAs. v-sis-related mRNA was observed in 5/11 cell lines without association to a specific NSCLC subtype. v-src-related mRNA, found in all tested cells, exhibited increased levels in 1 adenocarcinoma cell line (A-549) compared to the other cell lines. Binding sites for epidermal growth factor (EGF) had been described previously in NSCL, therefore we found erbB homologue transcripts coding for the EGF receptor in all NSCLC cell lines. Also, c-raf1-, N-ras-, Ki-ras-, and H-ras-related RNA expression was observed in all lines. We conclude that L-myc, N-myc, and c-myb expression does occur less frequently in NSCLC than in SCLC. Also amplification does not appear to be an important mechanism by which the c-myc proto-oncogene is activated in NSCLC. A specific pattern of oncogene expression could not be detected in NSCLC cells; each cell line examined showed its own pattern. However, transcriptional activation of a proto-oncogene like erbB, ras, raf, src, and c-myc, which are all involved in the progression pathway of EGF, may be a common feature of NSCLC.

Regulation of pp60c-src and its interaction with polyomavirus middle T antigen in insect cells

High yields of soluble, biologically active pp60c-src and middle t antigen (MTAg) of polyomavirus were produced in insect cells, using a baculovirus expression system. In mammalian cells, pp60c-src undergoes a regulatory phosphorylation on Tyr-527 in vivo and is autophosphorylated on Tyr-416 in vitro. In insect cells, pp60c-src was phosphorylated primarily on Tyr-416, although Tyr-527 was detectable at a low level. A kinase-negative mutant of pp60c-src was not phosphorylated on either Tyr-527 or Tyr-416 in insect cells and thus is an excellent biochemical reagent to search for the regulatory kinase that usually phosphorylates Tyr-527 in mammalian cells. MTAg synthesized in insect cells was not phosphorylated on tyrosine residues in vivo or in vitro, suggesting that it did not associate with any endogenous tyrosine kinases. However, MTAg isolated from cells coinfected with viruses encoding both MTAg and pp60c-src was phosphorylated on tyrosine residues both in vivo and in vitro.

Identification of a novel casein kinase activity in HeLa cell nuclei

Three casein kinase activities have been resolved by column chromatography of HeLa cell nuclear extracts. In addition to casein kinases NI and NII, which have been described in other cell types, HeLa nuclei contain a third casein kinase activity which we have named NIII. NIII is a cyclic nucleotide-independent casein kinase which uses either Mg2+ or Mn2+ as a divalent cation, but is inhibited by increasing NaCl concentrations in the presence of Mg2+ and has optimal activity at 50 mM NaCl in the presence of Mn2+. In Mg2+, NIII uses only ATP as a phosphate donor, but in Mn2+ NIII transfers phosphate from either ATP or GTP. NIII phosphorylates the serine and threonine residues of casein, but does not phosphorylate phosvitin or calf thymus histones.

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.

Human cdc2 protein kinase is a major cell-cycle regulated tyrosine kinase substrate

cdc2 is a catalytic subunit of a protein kinase complex, called the M-phase promoting factor, that induces entry into mitosis and is universal among eukaryotes. In HeLa cells, cdc2 is shown to be the most abundant phosphotyrosine-containing protein and its phosphotyrosine content is subject to cell-cycle regulation. One site of cdc2 tyrosine phosphorylation in vivo is selectively phosphorylated by pp60c-src in vitro.

Identification of a novel protein tyrosine kinase that phosphorylates pp60c-src and regulates its activity in neonatal rat brain

A novel protein tyrosine kinase not related to pp60c-src, designated as N-PTK, has recently been found in neonatal rat brain. In the present study, the enzyme was purified further by heparin-Sepharose column chromatography, and identified as a monomer protein with a Mr of 47 K and a pI of 7.0 by two-dimensional gel electrophoresis. The enzyme was found to phosphorylate purified pp60c-src at a tyrosine residue(s). The major phosphorylation site was shown by alpha-chymotryptic peptide mapping to be in the carboxy terminal V8 protease fragment (V2), but to be different from the autophosphorylation site, Tyr-416. The phosphorylation significantly suppressed pp60c-src activity with enolase as a substrate. These findings strongly suggest that N-PTK is a specific kinase that phosphorylates pp60c-src and regulates its function in the cell.

Altered phosphorylation and activation of pp60c-src during fibroblast mitosis

At least half the pp60c-src in NIH 3T3-derived c-src overexpresser cells in modified by novel threonine and, possibly, serine phosphorylation within its amino 16 kd region during mitosis. At the same time, the tryptic phosphopeptide containing Ser 17, the site of cyclic AMP-dependent phosphorylation, is either modified or dephosphorylated. While the amount of pp60c-src is not significantly altered, the in vitro-specific kinase activity of modified pp60c-src is enhanced 4- to 7-fold. Modified pp60c-src has the same tyrosine-containing tryptic phosphopeptides as pp60c-src from unsynchronized cells, indicating that activation is independent of Tyr 416/Tyr 527 phosphorylation. Electrophoretic mobility retardations indicated that endogenous pp60c-src and pp60v-src are similarly modified during mitosis. The modifications and enhanced activity disappear near the time of cell division. These results suggest that pp60c-src is regulated by and, in turn, may regulate mitosis-specific events in fibroblasts.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.

Epidermal growth factor receptor tyrosine kinase phosphorylation of glucose-6-phosphate dehydrogenase in vitro

The specific tyrosine phosphorylation of glucose-6-phosphate dehydrogenase (G6PDH) by the epidermal growth factor (EGF) receptor in vitro is demonstrated. The Km values of the substrate G6PDH and of ATP for the receptor tyrosine kinase were ca. 1 and 10 microM, respectively. The rate of phosphorylation was EGF dependent, with a four-fold increase in Vmax in the presence of EGF. The phosphorylation was stimulated maximally by 0.2 microM or greater EGF, with an ED50 of ca. 20 nM which is consistent with the affinity of the solubilized receptor for EGF. Using conditions of 5 microM G6PDH, 100 microM ATP, 5 mM Mg2+, and 1 mM Mn2+, up to 0.3 mol phosphate was incorporated into 1 mol of the 55-kDa subunit of Baker's yeast G6PDH. Tryptic peptide mapping revealed several unique phosphopeptides for both Baker's yeast and bovine adrenal G6PDH. The patterns of phosphopeptides for a given enzyme were identical for basal and EGF-stimulated phosphorylation.

Analysis of pp60c-src tyrosine kinase activity and phosphotyrosyl phosphatase activity in human colon carcinoma and normal human colon mucosal cells

We have compared the level of phosphotyrosyl phosphatase activity in lysates from normal human colon mucosal cells and human colon carcinoma cells and analyzed the effect of incubating these cells with sodium orthovanadate, an inhibitor of phosphotyrosyl phosphatase activity, on the relative abundance of acid-stable phosphotyrosine and on in vitro protein kinase activity of pp60c-src. Additionally, we compared the effect of lysing these cells in buffer containing only nonionic detergents with RIPA buffer, which contains both sodium dodecyl sulfate and deoxycholate, on the in vitro kinase activity of pp60c-src. Our results show that the level of detectable phosphotyrosyl phosphatase activity in lysates derived from normal colon cells and colon carcinoma cells is very similar. Additionally, the abundance of acid-stable phosphotyrosine in these cells cultured in the absence or presence of vanadate is not significantly different. However, incubation of these cells with vanadate significantly stimulates the activity of pp60c-src derived from the normal colon cells in immune-complex kinase assays, while having no detectable effect on the activity of pp60c-src from the colon tumor cells. The in vitro protein kinase activity of pp60c-src derived from RIPA buffer lysates of colon carcinoma cells was found to be elevated five- to sevenfold when compared with pp60c-src from these same cells lysed in buffer containing only Nonidet-P 40 as a detergent. The type of lysis buffer did not effect the activity of pp60c-src from normal colon mucosal cells. These results provide additional evidence that the activity of pp60c-src may be regulated differently in colon carcinoma and normal colon mucosal cells.

Demonstration that LSTRA cells have an elevated level of proteins phosphorylated on tyrosine residues

The LSTRA cell line has been shown to have an exceptionally high level of a tyrosine protein kinase (pp56lck). We now report that LSTRA cells also have a much higher level of proteins phosphorylated on tyrosine residues in comparison to several other cell lines with normal levels of pp56lck. The level of phosphotyrosine-containing proteins in LSTRA cells was comparable to that seen in K562 cells, a cell line known to have a constitutively active tyrosine protein kinase. These results provide evidence that LSTRA cells have an elevated level of in vivo tyrosine protein kinase activity, probably due to the overexpression and activation of pp56lck.

Enzymatically inactive p60c-src mutant with altered ATP-binding site is fully phosphorylated in its carboxy-terminal regulatory region

Cellular src protein, p60c-src, is phosphorylated on tyrosine 527 in chicken embryo fibroblasts, and this phosphorylation is implicated in suppressing the protein-tyrosine kinase activity and transforming potential of p60c-src. To determine whether tyrosine 527 phosphorylation is dependent on p60c-src kinase activity, the ATP-binding site of chicken p60c-src was destroyed by substitution of lysine 295 with methionine. The resultant protein, p60c-src(M295), expressed either in chicken cells or in yeast, lacked detectable kinase activity. Nevertheless, tyrosine and serine phosphorylation of p60c-src(M295) overproduced in chicken cells were indistinguishable from that of authentic p60c-src. By contrast, p60c-src(M295) was not phosphorylated on tyrosine in yeast. These results suggest that a protein kinase present in chicken cells but not in yeast phosphorylates tyrosine 527 in trans, and are consistent with the possibility that this kinase is distinct from p60c-src.

The mechanism of transduction of proto-oncogene c-src by avian retroviruses

Chicken c-src sequences have been transduced by avian leukosis viruses (ALV) and by partial src-deletion (td) mutants of Rous sarcoma virus in several independent events. Analyses of the recombination junctions in the genomes of src-containing viruses and the c-src DNA have shed light on the mechanism of transduction, which involves at least two steps of recombination. The initial recombination between a viral genome and the 5' region of c-src appears to occur at the DNA level. This step does not require extensive homology and can be mediated by stretches of sequences with only partial homology. The 5' recombination junction can also be formed by splicing between viral and c-src sequences. The second recombination is presumed to occur between the transducing ALV or td viral RNA and the viral-c-src hybrid RNA molecule generated from the initial recombination. This step involving recombination at the 3' ends of those molecules restores the 3' viral sequences essential for replication to the viral-c-src hybrid molecule. High frequency of c-src transduction by partial td mutants suggests that the second recombination is greatly enhanced when there is sequence homology between the transducing virus and the 3' region of c-src. Incorporation of the c-src sequences into an ALV genome results in greatly elevated expression of the gene. However, increased expression of c-src alone is insufficient to activate its transforming potential. Structural changes in c-src are necessary to convert it into a transforming gene. The changes can be as small as single nucleotide changes resulting in single amino aid substitutions at certain positions. Mutations can occur rapidly during viral replication after c-src is incorporated into the viral genome. Therefore, it is most likely that transduction of c-src by ALV is followed by subsequent mutation and selection for the sarcomagenic virus. In the case of transduction by td viruses that retain certain src sequences, joining of these sequences with the transduced c-src apparently is sufficient to activate its transforming potential.

Amiloride inhibits the protein tyrosine kinases associated with the cellular and the transforming src-gene products

Amiloride inhibits a protein tyrosine kinase from rat brain extracts. The kinase activity is characterized by an anti-serum (TBR-serum) which immunoprecipitates pp60c-src, the cellular counterpart of the transforming protein pp60v-src of Rous sarcoma virus. In immunocomplexes, TBR-IgG serves as an artificial but specific phosphate acceptor. The phosphate incorporation into TBR-IgG is a time- and temperature-dependent process. In the presence of amiloride the TBR-IgG phosphorylation is reduced. The drug does not influence the immunocomplexes formed by TBR-IgG and pp60src and no amiloride-activated protein tyrosine phosphatase can be detected in the immunocomplex system. Half-maximal inhibition of the tyrosine kinase occurs at 300 microM amiloride and is competitive with respect to ATP. Viral pp60src kinase of transformed cells is more sensitive to amiloride (IC50: 50-100 microM). Furthermore, normal cellular tyrosine kinases are to a lesser extent inhibited by amiloride as compared to the transforming viral pp60src kinase. These results may indicate different amiloride-sensitive forms of cellular pp60src kinases.

pp60c-src has less affinity for the detergent-insoluble cellular matrix than do pp60v-src and other viral protein-tyrosine kinases

A difference in affinity for a Nonidet P-40-insoluble cellular matrix was observed between the products of the viral and cellular src genes. It has previously been demonstrated that pp60v-src is associated with a detergent-insoluble matrix containing the cellular cytoskeleton (J. G. Burr, G. Dreyfuss, S. Penman, and J. M. Buchanan, Proc. Natl. Acad. Sci. USA 77:3484-3488, 1980). We observed a similar association of the transforming proteins of Fujinami sarcoma virus (P130gag-fps) and Yamaguchi 73 avian sarcoma virus (P90gag-yes), both of which are tyrosine-specific protein kinases. However, we found that the endogenous c-src product, pp60c-src, was not tightly bound to the detergent-insoluble matrix. This does not appear to have been due to differences in the cytoskeleton between transformed and nontransformed cells since pp60c-src was also solubilized by nonionic detergent in cells transformed by Rous sarcoma virus. This difference in the affinities of the v-src and c-src products for cytoskeletal proteins may contribute to the inability of pp60c-src to transform cells.

Cell transformation by pp60c-src mutated in the carboxy-terminal regulatory domain

We introduced two mutations into the carboxy-terminal regulatory region of chicken pp60c-src. One, F527, replaces tyrosine 527 with phenylalanine. The other, Am517, produces a truncated pp60c-src protein lacking the 17 carboxy-terminal amino acids. Both mutant proteins were phosphorylated at tyrosine 416 in vivo. The specific activity of the Am517 mutant protein kinase was similar to that of wild-type pp60c-src whereas that of the F527 mutant was 5- to 10-fold higher. Both mutant c-src genes induced focus formation on NIH 3T3 cells, but the foci appeared at lower frequency, and were smaller than foci induced by polyoma middle tumor antigen (mT). The wild-type or F527 pp60c-src formed a complex with mT, whereas the Am517 pp60c-src did not. The results suggest that one, inability to phosphorylate tyrosine 527 increases pp60c-src protein kinase activity and transforming ability; two, transformation by mT involves other events besides lack of phosphorylation at tyrosine 527 of pp60c-src; three, activation of the pp60c-src protein kinase may not be required for transformation by the Am517 mutant; and four, the carboxyl terminus of pp60c-src appears to be required for association with mT.

Tyrosine phosphorylation regulates the biochemical and biological properties of pp60c-src

To investigate the importance of tyrosine phosphorylation in the regulation of pp60c-src, we have substituted phenylalanine for tyrosine at positions 416, 519, and 527. Cells expressing the 527 or the 519/527 mutant but not the 416 or the 519 mutant were morphologically transformed, grew in soft agar, and formed foci. In addition, the 527 and 519/527 mutants had elevated kinase activities in vitro. Modifying Tyr 416 to phenylalanine in the 527 or the 519/527 mutants only partially inhibited their kinase activities yet abolished their ability to induce focus formation and promote growth in soft agar. These results suggest that two events must occur to activate the full transforming potential of pp60c-src: hypophosphorylation at Tyr 527 and hyperphosphorylation at Tyr 416.

Activation and suppression of pp60c-src transforming ability by mutation of its primary sites of tyrosine phosphorylation

pp60c-src is phosphorylated in vivo at tyrosine 527, a residue not present in pp60v-src (its transforming homolog), and not at tyrosine 416, its site of in vitro autophosphorylation. To test the hypothesis that tyrosine phosphorylation regulates pp60c-src biological activity, we constructed and studied pp60c-src mutants in which Tyr 527 and Tyr 416 were separately or coordinately altered to phenylalanine. Tyr----Phe 527 mutation strongly activated pp60c-src transforming and kinase activities, whereas the additional introduction of a Tyr----Phe 416 mutation suppressed these activities. Tyr----Phe 416 mutation of normal pp60c-src eliminated its partial transforming activity, which suggests that transient or otherwise restricted phosphorylation of Tyr 416 is important for pp60c-src function even though stable phosphorylation is not observed in vivo.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes elevation of the levels of the protein tyrosine kinase pp60c-src

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been found to cause increases in cellular levels of pp60src, a protein tyrosine kinase in hepatocytes from the rat and guinea pig, in the thymus of the mouse in vivo and in NIH-3T3 mouse fibroblast cell lines in vitro. Such cellular changes take place in vivo at early stages of TCDD poisoning (as early as one day after treatment in the case of mouse thymus) and at very low doses (single intraperitoneal injections of 1 microgram/kg for guinea pigs, 25 micrograms/kg for rats, and 30 micrograms/kg for mice). In addition such an effect of TCDD was observed only in a TCDD-responsive mouse strain but not in a nonresponsive strain. This effect of TCDD is a long-lasting one (eg, even 25 days after single dosing, the levels of pp60src in the hepatic membrane remained high). In vitro this effect was observed in a wild-type 3T3 cell line but was more pronounced in one of the transfected lines with a v-src gene, a virus-derived oncogene known to code for pp60src protein.