Showing papers by "Axel Ullrich published in 1995"

Heregulin-dependent regulation of HER2/neu oncogenic signaling by heterodimerization with HER3.

Abstract: Amplification and/or overexpression of HER2/neu and HER3 genes have been implicated in the development of cancer in humans. The fact that these receptor tyrosine kinases (RTKs) are frequently coexpressed in tumor-derived cell lines and that heterodimers form high affinity binding sites for heregulin (HRG) suggests a novel mechanism for signal definition, diversification or amplification. In cells expressing HER2 and HER3, tyrosine phosphorylation of HER3 is markedly increased upon exposure to recombinant HRG. ATP binding site mutants of HER2 and HER3 demonstrate transphosphorylation of HER3 by HER2, but not vice versa. HRG-induced transphosphorylation of HER3 results in a substrate phosphorylation pattern distinct from HER2 cells and enhances association of the receptor with SHC and phosphoinositol 3-kinase in transfected 293 and mammary carcinoma-derived MCF-7 cells. The physiological relevance of HER2/HER3 heterodimerization is demonstrated by HRG-dependent transformation of NIH 3T3 cells coexpressing the two receptors. These findings demonstrate the acquisition of expanded signaling capacities for HER2 by HRG-induced heterodimerization with HER3 and provide a molecular basis for the involvement of receptor heteroactivation in the development of human malignancies.

Distinct structural characteristics of discoidin I subfamily receptor tyrosine kinases and complementary expression in human cancer.

Abstract: Mammary carcinoma kinase 10 (MCK-10) and colon carcinoma kinase 2 (CCK-2) constitute a subclass of receptor tyrosine kinases characterized by a discoidin I motif in the extracellular domain and a large cytoplasmic juxtamembrane (JM) region. While the ectodomain structure suggests a common role in cell aggregation, the JM domains of MCK-10 and CCK-2 are structurally most divergent and display features that suggest an involvement in signal generation and definition. MCK-10 occurs in at least three isoforms, which contain alternatively spliced consensus sequences for internalization and SH3 domain interaction. The presence of the 37 amino acid insert affects receptor autophosphorylation and changes ectodomain glycosylation. Proteolytic cleavage within the extracellular domain of MCK-10 generates a membrane-anchored kinase domain and releases a soluble ectodomain fragment including the discoidin I homology domain. CCK-2 and MCK-10 expression was found in connective and epithelial tissues, respectively, which in cancers of epithelial origin results in mutually exclusive expression in stroma and tumor cells, indicating a possible involvement of this class of RTKs in tumor invasion.

Expression of a dominant negative mutant of epidermal growth factor receptor in the epidermis of transgenic mice elicits striking alterations in hair follicle development and skin structure.

Abstract: Epidermal growth factor receptor (EGFR) is a key regulator of keratinocyte biology. However, the physiological role of EGFR in vivo has not been well established. To analyze the role of EGFR in skin, we have generated transgenic mice expressing an EGFR dominant negative mutant in the basal layer of epidermis and outer root sheath of hair follicles. Mice expressing the mutant receptor display short and waved pelage hair and curly whiskers during the first weeks of age, but subsequently pelage and vibrissa hairs become progressively sparser and atrophic. Eventually, most mice present severe alopecia. Histological examination of the skin of transgenic mice shows striking alterations in the development of hair follicles, which fail to enter into catagen stage. These alterations eventually lead to necrosis and disappearance of the follicles, accompanied by strong infiltration of the skin with inflammatory elements. The interfollicular epidermis of these mice shows marked hyperplasia, expression of hyperproliferation-associated keratin K6 and increased 5-bromo-2-deoxyuridine incorporation. EGFR function was inhibited in transgenic skin keratinocytes, since in vivo and in vitro autophosphorylation of EGFR was almost completely abolished on EGF stimulation. These results implicate EGFR in the control of hair cycle progression, and provide new information about its role in epidermal growth and differentiation.

Colon carcinoma kinase-4 defines a new subclass of the receptor tyrosine kinase family.

Abstract: Complementary DNA (cDNA) encoding a novel member of the receptor tyrosine kinase (RTK) family has been isolated from colon carcinoma tissue. Colon carcinoma kinase 4 (CCK-4) mRNA is highly expressed in human lung tissue and at lower levels in the thyroid gland and ovary. While no mRNA was found in human adult colon tissues, expression varied remarkably in colon carcinoma-derived cell lines. CCK-4 cDNA encodes a chicken KLG-related, 1071 amino acid-long transmembrane glycoprotein containing several genetic alterations within the RTK consensus sequences. These define CCK-4 as a catalytically inactive member of the RTK family of proteins and, in analogy to HER3, suggest a potentially tumor-characteristic role as a signal amplifier or modulator for an as yet unidentified kinase-competent partner.

Identification of alternatively spliced mRNAs encoding variants of MDK1, a novel receptor tyrosine kinase expressed in the murine nervous system.

Abstract: A novel member of the eck/eph family of receptor tyrosine kinases (RTKs), termed mouse developmental kinase 1 (MDK1), was identified and shown to be closely related to the Eek, Ehk1/Cek7, Ehk2, Cek4/Mek4/hek and Sek/Cek8 subfamily. Northern blot analysis revealed MDK1 mRNA transcripts of 6.8, 5.7, 4.0, 3.2 and 2.6 kb that encode apparent splice variants. Sequence analyses of MDK1 cDNA clones from adult mouse brain predict the existence of at least five isoforms, including two truncated receptor variants lacking the kinase domain. Northern blot and in situ hybridization analysis indicate that in the adult mouse MDK1 RNA expression is restricted to brain, testes and spleen. The distinct patterns of MDK1 gene expression during mouse development suggest an important role in the formation of neuronal structures and possibly other morphogenic processes.

Cloning, characterization, and differential expression of mdk2 and mdk5, two novel receptor tyrosine kinases of the eck/eph family

Abstract: Using a polymerase chain reaction-based strategy for the cloning of developmentally regulated receptor tyrosine kinases, we identified two novel members of the eck/eph-related subfamily which, in analogy with the recently identified mouse developmental kinase 1 (MDK1), were designated MDK2 and MDK5. MDK2 is highly homologous to the mouse kinase Myk-1 and the human kinase Htk, whereas MDK5 represents the mouse homologue of human Hek2. Northern blot analyses of adult mouse tissues revealed a 4.7 kb transcript of MDK2 and a 4.8 kb transcript of MDK5 in various organ systems, including lung, liver, kidney, intestine, muscle, heart, and, in the case of MDK5, also the brain. In addition to the full-length transcripts, smaller fragments were identified that probably represent truncated receptors. Northern blot analysis and in situ hybridization of mouse embryos indicated abundant expression during embryonic development, with preferential involvement of tissues of epithelial and endothelial origin for both kinases and of the spinal cord gray matter for MDK5. Unlike most other members of the eck/eph-related subfamily, the expression of MDK2 and MDK5 is not primarily restricted to neuronal structures, and their abundant presence in various organ systems during embryonic development suggests an important role in gestational growth and differentiation.

Suppression of Tumor Growth in Vivo by Local and Systemic 90K Level Increase

Abstract: Expression levels of the immunostimulatory 90K antigen in mammary carcinoma, glioblastoma, and other tumor-derived cell lines inversely correlate with their tumorigenicity in athymic mice. Engineered enhancement of 90K expression results in significant (>80%) tumor growth inhibition, not by direct action on the tumor cell, but by stimulation of the residual cell-mediated immune defense of the nude mouse. Enhanced 90K level effects are both localized and systemic and involve induction of ICAM-1 and VCAM-1 in the tumor endothelium. The findings presented suggest a role for 90K as a molecular alarm signal for the body's cellular defense against pathogens, which in a subset of tumors is suppressed to allow cancer progression.

Phosphotyrosine Residues in the Nerve‐Growth‐Factor Receptor (Trk‐A)

Abstract: PC12 cells, which lack platelet derived-growth-factor (PDGF) receptors, have been stably transfected with a chimaera consisting of the extracellular domain of the beta-PDGF receptor and the intracellular and transmembrane domains of the nerve-growth-factor receptor Trk-A (termed PT-R). Mutation of the Trk-A residue Tyr490 to phenylalanine prevents the association with Shc, while similar mutations at Tyr751 or Tyr785 are reported to prevent interaction of Trk-A with the p85 subunit of inositol phospholipid 3-kinase and phospholipase C-gamma 1, respectively. The strong and sustained activation of p42 and p44 mitogen-activated-protein kinases induced by PDGF-B/B in PC12/PT-R cells was unaffected by mutation of Tyr785 or Tyr751 to phenylalanine, but was smaller and transient after mutation of Tyr490, and almost abolished by the double mutation of Tyr490 and Tyr785. Mutation of Tyr490 reduced by 70% the PDGF-induced increase in inositol phospholipid 3-kinase activity immunoprecipitated from cell extracts with antiphosphotyrosine monoclonal antibodies and greatly suppressed the PDGF-induced increase in the intracellular products of inositol phospholipid 3-kinase, while mutation of Tyr751 or Tyr785 had no effect. Mutation of Tyr785 (but not mutation of Tyr490 or Tyr751) abolished PDGF-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate. Mutation of Tyr490, alone or in combination with mutation of Tyr751 and Tyr785, had no effect on the PDGF-induced activation of p70 S6 kinase (p70S6K). However, the activation of p70S6K by PDGF (or nerve growth factor), but not the activation of mitogen-activated-protein kinase, was prevented by two structurally unrelated inhibitors of inositol phospholipid 3-kinase, wortmannin or LY294002. Our results demonstrate the following: (1) the phosphorylation of Tyr490 plays a major role in the activation of inositol phospholipid 3-kinase and formation of 3-phosphorylated inositol lipids and confirm that the phosphorylation of Tyr 785 triggers the activation of phospholipase C-gamma 1 in vivo. (2) Tyr490 phosphorylation (but not inositol phospholipid 3-kinase activation) is also required for strong and sustained activation of mitogen-activated-protein kinase and neuronal differentiation, while the smaller and more transient activation of mitogen-activated-protein kinase, produced by the activation of phospholipase C-gamma 1 is insufficient to trigger the neuronal differentiation of PT-R cells. (3) Inositol phospholipid 3-kinase is required for the activation of p70S6K, but only a small increase in inositol phospholipid 3-kinase activity and the level of 3-phosphorylated inositol lipids is required for maximal p70S6K activation.

A tumor-derived protein which provides T-cell costimulation through accessory cell activation

Abstract: A recently described tumor-derived glycoprotein, designated 90K, has been shown to have positive effects on the generation of cytotoxic effector cells (NK/LAK) from human PBMC. To determine the mechanism of these effects, we have examined the effects of 90K on cytokine production by human PBMC. A culture of normal PBMC with 90K alone did not result in IL-2 secretion; however, in coculture with suboptimal doses of ConA, 90K increased IL-2 secretion by PBMC. Coculture of PBMC with 90K and ConA also resulted in increased production of the cytokines IL-1, IL-6, GM-CSF, and TNF alpha. T cells depleted of accessory cells failed to respond to ConA alone, 90K alone, or the combination of ConA and 90K, suggesting that this protein does not have a direct effect on T cells. However, 90K alone was sufficient to induce cytokine production by unfractionated PBMC (IL-1, IL-6, GM-CSF, and TNF alpha) or by CD14-enriched PBMC (IL-1 and IL-6). In addition, expression of ICAM-1 was increased on a human monocytic cell line cultured with purified 90K in the absence of any other stimulus. This 90K-induced upregulation of ICAM-1 expression was accompanied by an increased accessory function of the monocytes, demonstrated by their ability to support ConA-induced activation of peripheral blood T cells. Based on the current data, we propose a model in which 90K activates accessory cells, resulting in the secretion of cytokines and the expression of adhesion molecules, which in turn act as costimulatory signals for T-cell activation. Activated T cells then produce cytokines such as IL-2, which lead to a more vigorous cell-mediated immune response to tumor cells and virus-infected cells. Thus, 90K shows promise as an immunotherapeutic reagent for diseases such as cancer and viral infection.

Protein tyrosine phosphatase

Abstract: A novel protein tyrosine phosphatase designated PTP-S31 and its subfamily are identified, as are nucleic acid molecule coding therefor. Included in this family are PTP-S31 proteins or glycoproteins having one, two, or three identified amino acid changes in previously defined consensus sequences in the catalytic phosphatase domains of known protein tyrosine phosphatases. The PTP-S31 proteins or glycoproteins may be produced by recombinant means. Antibodies to PTP-S31 proteins or glycoproteins and nucleic acid constructs coding therefor, and methods for screening molecules which can bind to PTP-S31 proteins or glycoproteins and inhibit or stimulate their enzymatic activity, are provided.

Oncogenic activation of v-kit involves deletion of a putative tyrosine-substrate interaction site.

Abstract: The transforming gene of the Hardy-Zuckerman-4 strain of feline sarcoma virus, v-kit, arose by transduction of the cellular c-kit gene, which encodes the receptor tyrosine kinase (RTK) p145c-kit. To gain insight into the molecular basis of the v-kit transforming potential, we characterized the feline c-kit by cDNA cloning. Comparison of the feline v-kit and c-kit sequences revealed, in addition to deletions of the extracellular and transmembrane domains, three additional mutations in the v-kit oncogene product: deletion of tyrosine-569 and valine-570, the exchange of aspartate at position 761 to glycine, and replacement of the C-terminal 50 amino acids by five unrelated residues. Examinations of individual v-kit mutations in the context of chimeric receptors yielded inhibitory effects for some mutants on both autophosphorylation and substrate phosphorylation functions. In contrast, deletion of tyrosine-569 and valine-570 significantly enhanced transforming and mitogenic activities of p145c-kit, while the other mutations had no significant effects. Conservation in subclass III RTKs and the identification of the corresponding residue in beta PDGF-R, Y579, as a binding site for src family tyrosine kinases suggests an important role for Y568 in kit signal regulation and the definition of its oncogenic potential. Repositioning of Y571 by an inframe two codon deletion may be the crucial alteration resulting in enhancement of v-kit oncogenic activity.

Formation of Signal Transfer Complexes between Stem Cell and Platelet-Derived Growth Factor Receptors and SH2 Domain Proteins in Vitro

Abstract: Cellular growth and differentiation signals are generated and defined by the interaction of specific phosphotyrosine residues of activated receptor tyrosine kinases (RTKs) and src homology-2 (SH2) domain-containing intracellular signal transducers. This appears to involve for both the p145c-kit and beta platelet-derived growth factor receptor (PDGF-R) cytoplasmic domains the formation of multiprotein signal transfer complexes, which include combinations of noncatalytic and enzymatically active subunits of phosphatidylinositol 3'-kinase (PI3'-K), phospholipase C-gamma (PLC gamma), and guanosine trisphosphatase activating protein (GAP). In vitro association experiments indicate that PLC gamma and PI3'-K bind the beta PDGF-R simultaneously, while these two SH2 proteins compete for association to p145c-kit binding sites, with p85/PI3'-K exhibiting higher affinity. Interestingly, GAP and p85/PI3'-K binding to distinct p145c-kit phosphotyrosines is cooperative, enhancing formation of a heterotetrameric signaling complex, which may include different combinations of p85 alpha and p85 beta with p110, p112, and p116 by interaction with the same tyrosine 721 docking site. The diversity of molecular interactions observed for PDGF-R and p145c-kit suggests a new mode of signal definition and modulation.

The 90K tumor-associated antigen and clinical progression in human immunodeficiency virus infection.

Abstract: We investigated the possibility that a secreted glycoprotein of approximately 90,000 daltons, termed 90K and identified as a member of the protein superfamily characterized by the scavenger receptor cysteine-rich (SRCR) domain, might have value as a predictor of progression to acquired immunodeficiency syndrome (AIDS) in subjects infected with the human immunodeficiency virus (HIV). Among 488 HIV-seropositive intravenous drug users with a median follow-up of 32.5 months, high levels of serum 90K at baseline proved to be a significant predictor of faster progression to AIDS, either as a continuous variable (log 90K; p or = 500 x 10(6)/L CD4 cells, the proportion in whom AIDS developed was 10.5% for those with 90K levels < or = 30 U/ml as compared with 20% for those with 90K above the cutoff point (p = 0.006). Serum 90K is an independent predictor of the risk for progression to AIDS in HIV-infected subjects, including those whose CD4 counts have not fallen.

Analysis of the protein kinase activity of moss phytochrome expressed in fibroblast cell culture

Abstract: In the moss Ceratodon purpureus a phytochrome gene encodes a phytochrome type (PhyCer) which has a C-terminal domain homologous to the catalytic domain of eukaryotic protein kinases (PKs). PhyCer exhibits sequence conservation to serine/ threonine as well to tyrosine kinases. Since PhyCer is expressed very weakly in moss cells, to investigate the proposed PK activity of PhyCer, we overexpressed PhyCer transiently in fibroblast cells. For this purpose we made a chimeric receptor, EC-R, which consists of the extracellular, the membrane-spanning and the juxtamembrane domains of the human epidermal growth-factor receptor (EGF-R) linked to the PK catalytic domain of PhyCer (CerKin). The expression of EC-R in transiently transfected cells was confirmed with antibodies directed against the extracellular domain of EGF-R or against CerKin. Both EGF-R and EC-R were immunoprecipitated from lysates of overexpressing cells with antibodies against the extracellular domain of EGF-R. Phosphorylation experiments were performed with the immunoprecipitates and the phosphorylation products were subjected to phosphoamino acid analysis. Phosphorylation products specifically obtained with EC-R-transfected cells exhibit phosphorylation on serine and threonine residues. In EC-R transfected cells the endogenous EGF-R showed enhanced phosphorylation of serine and threonine residues compared to EGF-R immuno-precipitated from control cells. Although CerKin is closest to the catalytic domain of a protein tyrosine kinase from Dictyostelium discoideum, EC-R does not appear to phosphorylate tyrosine residues in vitro. From our data we conclude that PhyCer carries an active PK domain capable of phosphorylating serine and threonine residues.

C-Terminus or Juxtamembrane Deletions in the Insulin Receptor do not Affect the Glucose-Dependent Inhibition of the Tyrosine Kinase Activity

Abstract: We have previously shown, in rat-1 fibroblasts which stably overexpress high levels of human insulin receptor (HIR), that high glucose levels induce an inhibition of insulin receptor tyrosine kinase (IRK) activity [Berti, L., Mosthaf, L., Kellerer, M., Tippmer, S., Mushack, J., Seffer, E., Seedorf, K., Haring, H. (1994) J. Biol. Chem. 269, 3381-3386]. This effect appears to be mediated through activation of protein kinase C and phosphorylation of the receptor beta-subunit on threonine or serine residues. The aim of the present study was to determine whether the juxtamembrane region or the C-terminus tail of the receptor are involved in the IRK modulation by glucose. In these domains increased serine and threonine phosphorylation was observed after phorbol ester or insulin stimulation of cells, and a regulatory function for IRK activity seems conceivable. We used an antibody directed against one potential regulatory site in the C-terminus tail, i.e. PSer1315, to study the effect of glucose. An increased signal was detected in HIR from rat-1 fibroblasts treated with phorbol 12-myristate 13-acetate or glucose (25 mM). To investigate whether this site in the C-terminus is essential for glucose-dependent IRK inhibition, rat-1 fibroblasts stably overexpressing a C-terminus-truncated human insulin receptor lacking 43 amino acids (HIR delta CT) were studied in parallel with cells expressing the wild-type receptor. As described earlier, HIR delta CT has lost the ability to stimulate glucose uptake. Glucose (25 mM) inhibited the insulin effect on the autophosphorylation of both receptors to a similar extent. Thus, glucose (25 mM) stimulates phosphorylation of Ser1315, however, this appears not to mediate the inhibitory effect on IRK. To test whether serine residues 955/956 and 962/964 in the juxtamembrane region of the insulin receptor are involved in the inhibitory effect of glucose, 293 cells transiently transfected either with wild-type HIR or HIR with a juxtamembrane deletion spanning amino acids 954-965 [des-(954-965)-HIR] were studied in parallel. As described earlier, the des-(954-965)-HIR has lost the ability to stimulate PI-3 kinase. However, 25 mM glucose equally inhibited the insulin effect on tyrosine phosphorylation of the receptor. Together, the data suggest that the regulatory serine or threonine phosphorylation site(s) involved in the inhibitory effect of hyperglycemia are neither located in the C-terminus nor in the juxtamembrane region of the insulin receptor beta subunit.

Use of Large Combinatorial Chemical Libraries for Anticancer Drug Discovery

Abstract: A new technology for drug discovery (Selectide technology) has been developed which greatly extends the capability for rapid molecular and cellular screening to large libraries containing millions of chemically synthesized compounds. Using a split synthesis approach to condensation chemistry, we recognized that each individual compound of potential interest is located on a different solid phase resin bead. Screening for binding activity on the bead surface: In order to screen for biological activity, we used several cleavable linkers which could release a portion of the compound from each bead into the solution phase in each of two sequential steps. For libraries comprised of peptides, there is sufficient residual peptide on each bead to permit sequence determination via Edman degradation. For non-peptide libraries, a system of peptide encoding is used on each bead so that the structure of the non-peptide compound can be determined by decoding the information conveyed by the peptide (which can be ...

IGF1R: IGF1 receptor (vertebrates) (Somatomedin receptor)

Abstract: The IGF1 receptor a subunit binds insulin-like growth factor-1 with high affinity. Binding leads to activation of the tyrosine kinase, autophosphorylation of the receptor itself, and tyrosine phosphorylation of cytoplasmic proteins. In cultured cells, within minutes, membrane transport processes (for example, glucose transport) are stimulated, and within hours physiological activities such as glycogen synthesis and DNA synthesis are stimulated. IGF1R overexpression can lead to cellular transformation. In vivo IGF1R is regulated in its expression with embryonic development or cell differentiation. Its function is presumed to be primarily mitogenic. The protein is synthesized as a single precursor that is dimerized by disulphide bonds and then processed into a heterotetrameric protein with two α and two β subunits. The α subunits are connected to each other and to the β subunits via disulphide bonds and contain the binding site for the ligand, IGF1. The extracellular domain contains 16 potential N-glycosylation sites, and its size varies depending on cell type. An alternative transcript with the substitution of Arg at position 899 for Thr-Gly is expressed ubiquitously. Presence of IGF1R can be assayed on intact cells by binding of [125I]-IGF1. Kinase activity of purified receptor can be monitored by autophosphorylation.

- InsR: Insulin receptor (vertebrates)

Abstract: InsR is the high-affinity receptor for the polypeptide hormone insulin and is a glycosylated cell surface protein with tyrosine kinase activity. It mediates the cellular actions of insulin, including stimulation of membrane transport processes (ions, glucose, and amino acids), stimulation of glycogen synthesis, lipogenesis and protein synthesis, inhibition of lipolysis, proteolysis and glycogenolysis, and growth-promoting effects in certain cell types. InsR functions as a membrane-bound PTK. Binding of insulin to the a subunits stimulates the tyrosine kinase activity of the β subunit, which leads torapid autophosphorylation of the β subunits, and phosphorylation of cytoplasmic polypeptides that are thought to be involved in receptor-specific signal transduction. The various functions of InsR can be assayed using binding of I-labeled insulin to calculate the number of receptors per cell, autophosphorylation to assess kinase activity, or stimulation of deoxyglucose uptake. InsR is expressed at low levels in all mammalian cell types but is expressed at much higher levels in liver, fat, and muscle.

- EGFR: EGF receptor (vertebrates) (c-erbB)

Abstract: The chapter describes the EGFR that is the high-affinity receptor for epidermal growth factor (EGF), transforming growth factor-a (TGFa), and VGF. Activated EGFRs stimulate the proliferation of numerous cell types in vitro and epithelial cells in vivo. Binding of the corresponding ligand induces receptor dimerization and activates the intrinsic protein tyrosine kinase, which results in receptor phosphorylation by an intermolecular mechanism. The tyrosine-phosphorylated residues are crucial for normal receptor signaling and function as high-affinity binding sites for cellular proteins with SH2 domains. In domain structure of protein, the juxtamembrane region contains T654 (phosphorylated by PKC and T669 (phosphorylated by Erkl/2). The C-terminal tail contains all known tyrosine phosphorylation sites and, in addition, two serine residues (S1046/S1047), which are phosphorylated by CaMKII. EGFR has been sequenced from human, chicken, avian erythroblastosis virus (v-erbB), and Drosophila melanogaster. EGFR homologues have been identified in Caenorhabditis elegans (LET-23) and Xiphophorus.

- CSF1R: CSF-1 receptor (vertebrates) (Macrophage-CSF receptor1, c-fms gene product2)

Abstract: CSF1R is a receptor PTK that is required for proliferation and survival of monocytes, macrophages, and committed bone marrow progenitors. Binding of the ligand (colony-stimulating factor-1 [CSF-1] or macrophage—CSF) to the extracellular domain mediates dimerization of receptor monomers, followed by the activation of the intrinsic PTK activity. Transmission of the stimulus from the cell membrane to the nucleus is accomplished by a signaling cascade, triggered by the interaction of the activated kinase with a specific subset of cellular proteins, followed by, in many cases, phosphorylation of the proteins on tyrosine residues. CSF1R is a monomer. It forms noncovalently linked dimers upon ligand binding. Noncovalently linked dimers are subsequently covalently linkedby disulphide bonds (covalently modified forms of the receptor are selectively internalized). The extracellular domain of CSG1R is characterized by five Ig-like domains. The PTK domain is split by a hydrophilic insertion sequence.