Antipeptide antiserum identifies a widely distributed cellular tyrosine kinase related to but distinct from the c-fps/fes-encoded protein

Phosphoproteomics-mediated identification of Fer kinase as a target of mutant Shp2 in Noonan and LEOPARD syndrome

Noonan syndrome (NS) and LEOPARD syndrome (LS) cause congenital afflictions such as short stature, hypertelorism and heart defects. More than 50% of NS and almost all of LS cases are caused by activating and inactivating mutations of the phosphatase Shp2, respectively. How these biochemically opposing mutations lead to similar clinical outcomes is not clear. Using zebrafish models of NS and LS and mass spectrometry-based phosphotyrosine proteomics, we identified a down-regulated peptide of Fer kinase in both NS and LS. Further investigation showed a role for Fer during development, where morpholino-based knockdown caused craniofacial defects, heart edema and short stature. During gastrulation, loss of Fer caused convergence and extension defects without affecting cell fate. Moreover, Fer knockdown cooperated with NS and LS, but not wild type Shp2 to induce developmental defects, suggesting a role for Fer in the pathogenesis of both NS and LS.

Fer tyrosine kinase is required for germinal vesicle breakdown and meiosis-I in mouse oocytes

The control of microtubule and actin-mediated events that direct the physical arrangement and separation of chromosomes during meiosis is critical since failure to maintain chromosome organization can lead to germ cell aneuploidy. Our previous studies demonstrated a role for FYN tyrosine kinase in chromosome and spindle organization and in cortical polarity of the mature mammalian oocyte. In addition to Fyn, mammalian oocytes express the protein tyrosine kinase Fer at high levels relative to other tissues. The objective of the present study was to determine the function of this kinase in the oocyte. Feline encephalitis virus (FES)-related kinase (FER) protein was uniformly distributed in the ooplasm of small oocytes, but became concentrated in the germinal vesicle (GV) during oocyte growth. After germinal vesicle breakdown (GVBD), FER associated with the metaphase-I (MI) and metaphase-II (MII) spindles. Suppression of Fer expression by siRNA knockdown in GV stage oocytes did not prevent activation of cyclin dependent kinase 1 activity or chromosome condensation during in vitro maturation, but did arrest oocytes prior to GVBD or during MI. The resultant phenotype displayed condensed chromosomes trapped in the GV, or condensed chromosomes poorly arranged in a metaphase plate but with an underdeveloped spindle microtubule structure or chromosomes compacted into a tight sphere. The results demonstrate that FER kinase plays a critical role in oocyte meiotic spindle microtubule dynamics and may have an additional function in GVBD.

Essential kinase-independent role of a Fer-like non-receptor tyrosine kinase inCaenorhabditis elegansmorphogenesis

Morphogenesis requires coordination of cell surface activity and cytoskeletal architecture. During the initial stage of morphogenesis in Caenorhabditis elegans, the concerted movement of surface epithelial cells results in enclosure of the embryo by the epidermis. We report that Fer-related kinase-1 (FRK-1), an ortholog of the mammalian non-receptor tyrosine kinase Fer, is necessary for embryonic enclosure and morphogenesis in C. elegans. Expression of FRK-1 in epidermal cells is sufficient to rescue a chromosomal deficiency that removes the frk-1locus, demonstrating its autonomous requirement in the epidermis. The essential function of FRK-1 is independent of its kinase domain, suggesting a non-enzymatic role in morphogenesis. Localization of FRK-1 to the plasma membrane requires β-catenin, but not cadherin or α-catenin, and muscle-expressed β-integrin is non-autonomously required for this localization; in the absence of these components FRK-1 becomes nuclear. Mouse FerT rescues the morphogenetic defects of frk-1 mutants and expression of FRK-1 in mammalian cells results in loss of adhesion, implying a conserved function for FRK-1/FerT in cell adhesion and morphogenesis. Thus,FRK-1 performs a kinase-independent function in differentiation and morphogenesis of the C. elegans epidermis during embryogenesis.

Fes tyrosine kinase promotes survival and terminal granulocyte differentiation of factor-dependent myeloid progenitors (32D) and activates lineage-specific transcription factors

The c-fps/fes proto-oncogene encodes a 92-kDa protein-tyrosine kinase that is involved in myeloid cell development and function. We have recently shown that expression of an activated allele of Fes (Fes(act)) in monocyte precursors resulted in their differentiation into functional macrophages through the activation of lineage-specific transcription factors. We now report that this kinase also plays a role in the survival and terminal differentiation of granulocyte progenitors. The expression of Fes(act) in factor-dependent 32D cells prevented their apoptotic death after interleukin-3 removal, but Fes(act)-expressing cells remained factor-dependent for proliferation. Removal of interleukin-3 from the Fes(act)-expressing cells was followed by granulocytic differentiation in the absence of granulocyte colony-stimulating factor within 4-8 days. The differentiated cells had distinctive granulocyte morphology and there was up-regulation of CD11b, Gr-1, and late differentiation markers such as lactoferrin, suggesting that this kinase induced terminal granulocytic differentiation. Concomitantly, Fes(act) down-regulated the macrophage marker F4/80, suggesting that the biological activity of Fes was coordinated in a lineage-specific manner. Further analysis showed that Fes(act) caused activation of CCAAT/enhancer-binding protein-alpha and STAT3, two transcription factors that are involved in granulocyte differentiation. Our results provide evidence that Fes may be a key component of the granulocyte differentiation machinery, and suggest a potential mechanism by which this kinase may regulate granulocyte-specific gene expression.

The protein-tyrosine kinase fer associates with signaling complexes containing insulin receptor substrate-1 and phosphatidylinositol 3-kinase

In a screen for 3T3-F442A adipocyte proteins that bind SH2 domains, we isolated a cDNA encoding Fer, a nonreceptor protein-tyrosine kinase of the Fes/Fps family that contains a functional SH2 domain. A truncated splicing variant, iFer, was also cloned. iFer is devoid of both the tyrosine kinase domain and a functional SH2 domain but displays a unique 42-residue C terminus and retains the ability to form oligomers with Fer. Expression of both Fer and iFer proteins are strikingly increased upon differentiation of 3T3-L1 fibroblasts to adipocytes. Platelet-derived growth factor treatment of the cultured adipocytes caused rapid tyrosine phosphorylation of Fer and its recruitment to complexes containing platelet-derived growth factor receptor and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase. Insulin treatment of 3T3-L1 adipocytes stimulated association of Fer with complexes containing tyrosine phosphorylated IRS-1 and PI 3-kinase but did not stimulate tyrosine phosphorylation of Fer. PI 3-kinase activity in anti-Fer immunoprecipitates was also acutely activated by insulin treatment of cultured adipocytes. These data demonstrate the presence of Fer tyrosine kinase in insulin signaling complexes, suggesting a role of Fer in insulin action.

Role of positive and negative regulation in modulation of the Fer promoter activity

p94(fer) is a cytoplasmic and nuclear tyrosine kinase whose function has been linked to cell growth. p94(fer) accumulates at different levels in various cell types and is not detected in pre-B, pre-T and T-cells (Halachmy, S., Bern, O., Schreiber, L., Carmel, M., Sharabi, Y., Shoham, J., Nir, U., 1997. p94(fer) facilitates cellular recovery of gamma irradiated pre-T cells. Oncogene 14, 2871-2880). The fer RNA, encoding p94fer, is transcribed from the FER locus in human rat and mouse. In the present work, a Fer gene transcription initiation point was determined, and the Fer promoter was cloned. A DNA genomic fragment, extending 3698bp upstream of the fer RNA start site, was isolated, sequenced and functionally characterized. A transient transfection assay, carried out in fibroblastic cell lines, revealed the presence of the Fer promoter within the cloned genomic fragment. The Fer promoter contains neither an obvious 'TATA' element nor a putative initiator sequence, but is composed of positive and negative, cis-acting elements. Negative regulation was found to be the main cause for dysfunctioning of the Fer promoter in a T-cell leukemia cell line (Jurkat). The minimal Fer promoter that is still active in fibroblasts consists of an AP1 binding site located 14bp upstream of the fer transcription initiation point. This minimal promoter was not active in the Jurkat T-cell leukemia cells and did not bind AP1 in these cells. Three additional AP1 sites were identified in functional sequences of the Fer promoter. Thus, the availability of AP1 activity may contribute as well to the modulation of the Fer promoter activity. The presumed regulatory role of AP1 in modulating the Fer promoter activity implies a link between cell growth and the Fer gene expression level. Indeed, exposure of fibroblasts to low serum growth conditions reduced the cellular level of the fer RNA.

Disruption of coiled-coil domains in Fer protein-tyrosine kinase abolishes trimerization but not kinase activation

The protein-tyrosine kinase Fer and the highly homologous proto-oncoprotein Fps/Fes are implicated in signaling from a variety of growth factor and cytokine receptors. Here we examine the molecular basis of Fer kinase activation with an emphasis on the role of oligomerization. We show that Fer forms trimers in vivo and that disruption of either the first or second coiled-coil domain abolishes oligomerization, suggesting a cooperative interaction between these two domains. Although Fps/Fes also forms homotypic oligomers, probably via homologous coiled-coil domains, no heterotypic interactions were observed between Fer and Fps/Fes. Incorporation of catalytically inactive Fer peptides into the oligomeric complex caused only mild reduction of wild type Fer kinase activity, suggesting that kinase-inactive Fer would not behave as a potent dominant negative. Although oligomerization of Fer can potentiate autophosphorylation in trans at three major phosphorylation sites, these residues can likely also be phosphorylated in cis. In contrast, the testis-specific FerT isomer does not oligomerize and is able to autophosphorylate in cis at two of the same three residues autophosphorylated in Fer. These results suggest that although oligomerization potentiates autophosphorylation in trans, this is apparently not necessary for Fer activation.

Tyrosine phosphorylation of the TATA element modulatory factor by the FER nuclear tyrosine kinases

The FER locus in the mouse encodes two tyrosine kinases, p94fer and p51ferT. While p94fer accumulates in the cytoplasm and nucleus of most mammalian cells the expression of p51ferT is restricted to the nucleus of meiotic primary spermatocytes. The cellular function of the FER kinases is not understood, nor has a substrate for these enzymes been characterized. To identify putative substrates of p94fer and p51ferT, the two enzymes were used as 'baits' in the yeast two-hybrid screening system. cDNAs encoding the mouse TATA element modulatory factor (TMF) were repeatedly isolated in this assay. TMF was previously shown to bind the TATA element in RNA polymerase II promoters and impaired their functioning in a cell free transcription system. Both p94fer and p51ferT phosphorylated the TMF protein in in vitro and in vivo kinase assays. Sequential deletions showed that the carboxy-terminal region of TMF was essential for phosphorylation. In situ hybridization analysis revealed the preferential accumulation of TMF transcripts in meiotic spermatogenic and oogenic cells. p94fer and p51ferT may thus modulate the suppressive activity of TMF during cellular growth and in defined differentiation processes.

The bovine papillomavirus E5 transforming protein can stimulate the transforming activity of EGF and CSF-1 receptors

The bovine papillomavirus E5 transforming gene encodes a 44 amino acid protein product that is localized to cytoplasmic membranes, including the plasma membrane. We now report that E5 can cooperate with human EGF receptors and with human CSF-1 receptors to induce cellular transformation of NIH 3T3 cells. Cooperation occurred in the absence of receptor stimulation by ligand, and it was further augmented by treatment with ligand. Cooperation was not seen between E5 and either c-fes or c-src. The cooperation between E5 and high levels of EGF receptors was associated with inhibition of receptor degradation and persistence of activated receptors on the cell surface. We conclude that E5 may enhance the receptor activity via inhibition of receptor down-modulation.

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.