Avian sarcoma virus UR2 encodes a transforming protein which is associated with a unique protein kinase activity

Comprehensive Review of ROS1 Tyrosine Kinase Inhibitors-Classified by Structural Designs and Mutation Spectrum (Solvent Front Mutation [G2032R] and Central β-Sheet 6 [Cβ6] Mutation [L2086F])

Despite ROS1 fusion-positive NSCLC accounting for approximately 1% to 2% of NSCLC, there is a long list of ROS1 tyrosine kinase inhibitors (TKIs) being developed in addition to three approved ROS1 TKIs, crizotinib, entrectinib and repotrectinib. Here, we categorized ROS1 TKIs by their structures (cyclic versus noncyclic) and inhibitory abilities (active against solvent front mutation G2032R or central β-sheet #6 [Cβ6] mutation L2086F) and summarized their reported clinical activity in order to provide a dashboard on how to use these ROS1 TKIs in various clinical situations. In addition, the less known Cβ6 mutation ROS1 L2086F confer resistances to next-generation ROS1 TKIs (repotrectinib, taletrectinib, and potentially NVL-520) that can be overcome by cabozantinib as documented in published patient reports and potentially by certain L-shaped type I ROS1 TKIs including ceritinib and gilteritinib, which is approved as a FLT3 inhibitor for relapsed refractory FLT3+ acute myeloid leukemia but have published preclinical activites against ROS1 (and ALK). Future clinical trials should investigate cabozantinib and gilteritinib to repurpose them as ROS1 TKIs that can target ROS1 L2086F Cβ6 mutation.

ROS1-positive non-small cell lung cancer (NSCLC): Biology, Diagnostics, Therapeutics and Resistance

ROS1 is a proto-oncogene encoding a receptor tyrosine protein kinase (RTK), homologous to the v - Ros sequence of University of Manchester tumours virus 2(UR2) sarcoma virus, whose ligands are still being investigated. ROS1 fusion genes have been identified in various types of tumours. As an oncoprotein, it promotes cell proliferation, activation and cell cycle progression by activating downstream signalling pathways, accelerating the development and progression of non-small cell lung cancer (NSCLC). Studies have demonstrated that ROS1 inhibitors are effective in patients with ROS1-positive NSCLC and are used for first-line clinical treatment. These small molecule inhibitors provide a rational therapeutic option for the treatment of ROS1-positive patients. Inevitably, ROS1 inhibitor resistance mutations occur, leading to tumours recurrence or progression. Here, we comprehensively review the identified biological properties and Differential subcellular localization of ROS1 fusion oncoprotein promotes tumours progression. We summarize recently completed and ongoing clinical trials of the classic and new ROS1 inhibitors. More importantly, we classify the complex evolving tumours cell resistance mechanisms. This review contributes to our understanding of the biological properties of ROS1 and current therapeutic advances and resistant tumours cells, and the future directions to develop ROS1 inhibitors with durable effects.

ROS1-dependent cancers - biology, diagnostics and therapeutics

The proto-oncogene ROS1 encodes a receptor tyrosine kinase with an unknown physiological role in humans. Somatic chromosomal fusions involving ROS1 produce chimeric oncoproteins that drive a diverse range of cancers in adult and paediatric patients. ROS1-directed tyrosine kinase inhibitors (TKIs) are therapeutically active against these cancers, although only early-generation multikinase inhibitors have been granted regulatory approval, specifically for the treatment of ROS1 fusion-positive non-small-cell lung cancers; histology-agnostic approvals have yet to be granted. Intrinsic or extrinsic mechanisms of resistance to ROS1 TKIs can emerge in patients. Potential factors that influence resistance acquisition include the subcellular localization of the particular ROS1 oncoprotein and the TKI properties such as the preferential kinase conformation engaged and the spectrum of targets beyond ROS1. Importantly, the polyclonal nature of resistance remains underexplored. Higher-affinity next-generation ROS1 TKIs developed to have improved intracranial activity and to mitigate ROS1-intrinsic resistance mechanisms have demonstrated clinical efficacy in these regards, thus highlighting the utility of sequential ROS1 TKI therapy. Selective ROS1 inhibitors have yet to be developed, and thus the specific adverse effects of ROS1 inhibition cannot be deconvoluted from the toxicity profiles of the available multikinase inhibitors. Herein, we discuss the non-malignant and malignant biology of ROS1, the diagnostic challenges that ROS1 fusions present and the strategies to target ROS1 fusion proteins in both treatment-naive and acquired-resistance settings.

EZH2-mediated upregulation of ROS1 oncogene promotes oral cancer metastasis

Current anti-epidermal growth factor receptor (EGFR) therapy for oral cancer does not provide satisfactory efficacy due to drug resistance or reduced EGFR level. As an alternative candidate target for therapy, here we identified an oncogene, ROS1, as an important driver for oral squamous cell carcinoma (OSCC) metastasis. Among tumors from 188 oral cancer patients, upregulated ROS1 expression strongly correlated with metastasis to lung and lymph nodes. Mechanistic studies uncover that the activated ROS1 results from highly expressed ROS1 gene instead of gene rearrangement, a phenomenon distinct from other cancers. Our data further reveal a novel mechanism that reduced histone methyltransferase EZH2 leads to a lower trimethylation of histone H3 lysine 27 suppressive modification, relaxes chromatin, and promotes the accessibility of the transcription factor STAT1 to the enhancer and the intron regions of ROS1 target genes, CXCL1 and GLI1, for upregulating their expressions. Down-regulation of ROS1 in highly invasive OSCC cells, nevertheless, reduces cell proliferation and inhibits metastasis to lung in the tail-vein injection and the oral cavity xenograft models. Our findings highlight ROS1 as a candidate biomarker and therapeutic target for OSCC. Finally, we demonstrate that co-targeting of ROS1 and EGFR could potentially offer an effective oral cancer therapy.

[Advances on driver oncogenes of non-small cell lung cancer]

Adenocarcinoma and squamous cell carcinoma are the most common histological types of non-small cell lung cancer (NSCLC). Several molecular alterations have been defined as "driver oncogenes" responsible for both the initiation and maintenance of the malignancy. With next-generation sequencing (NGS) which having multiple and high-throughput genotyping is wildly used and promoted, make the detection of patients gene composition from a tiny tumor biopsy specimens become possible, initiate the clinical studies based on the genetic characteristics, and promote the progress of molecular typing in NSCLC. So far, about 60% of lung adenocarcinoma has been found harbouring driver oncogenes, the rate of lung squamous cell carcinoma driven genes detection has gradually improved, in which epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), fibroblast growth factor receptor 1 (FGFR1), phosphatidylinositol 3-kinase catalytic subunit alpha (PIK3CA) and so on plays important roles. The currently effective targeted therapies is mainly used against EGFR, ALK, etc. In this review, we will report the mainly advances on some latest driver mutations of NSCLC.

Activated RET and ROS: two new driver mutations in lung adenocarcinoma

Rearrangements of ROS1 and RET have been recently described as new driver mutations in lung adenocarcinoma with a frequency of about 1% each. RET and ROS1 rearrangements both represent unique molecular subsets of lung adenocarcinoma with virtually no overlap with other known driver mutations described so far in lung adenocarcinoma. Specific clinicopathologic characteristics have been described and several multitargeted receptor kinase inhibitors have shown in vitro activity against NSCLC cells harbouring these genetic alterations. In addition, the MET/ALK/ROS inhibitor crizotinib has already shown impressive clinical activity in patients with advanced ROS1-positive lung cancer. Currently, several early proof of concept clinical trials are testing various kinase inhibitors in both molecular subsets of lung adenocarcinoma patients. Most probably, personalized treatment of these genetically defined new subsets of lung adenocarcinoma will be implemented in routine clinical care of lung cancer patients in the near future.

[Clinical significance of ROS1 rearrangements in non-small cell lung cancer]

Chromosomal rearrangements involving the ROS1 receptor tyrosine kinase gene have recently been described in multiple malignancies, including non-small cell lung cancer (NSCLC). ROS1 rearrangement defines a new molecular subset of NSCLC with the prevalence of ROS1 rearrangements around 1%-2%. ROS1-positive NSCLCs arise in young never-smokers with adenocarcinoma that are similar to those observed in patients with ALK-rearranged NSCLC. Crizotinib demonstrates in vitro activity and early clinical trial shows marked antitumor activity in ROS1-rearranged patients. The overall response rate is around 56% and the disease control rate at 8 weeks is about 76%. Further understanding the ROS1 fusions in the pathogenesis of NSCLC, methods to detect ROS1 rearrangements, and targeting ROS1-rearranged NSCLC patients with specific kinase inhibitors would lead to an era of personalized medicine.

ROS1 as a 'druggable' receptor tyrosine kinase: lessons learned from inhibiting the ALK pathway

ROS1 is one of 58 receptor tyrosine kinases, and one of two orphan receptor tyrosine kinases where its ligand is unknown. ROS1 is evolutionarily related to ALK. ROS1 rearrangement was discovered in glioblastoma in 1987, in non-small-cell lung cancer (NSCLC) in 2007, and in cholangiocarcinoma in 2011. While the clinicopathologic characteristics of ROS1-rearranged glioblastoma and cholangiocarcinoma patients remain to be defined, the clinicopathologic characteristics of ROS1-rearranged NSCLC patients have recently been described. Although ROS1 shares only 49% amino acid sequence homology with ALK in the kinase domains, several ALK inhibitors have demonstrated in vitro inhibitory activity against ROS1. With the recent US approval of crizotinib, a multi-targeted ALK/MET kinase inhibitor, for the treatment of ALK-rearranged NSCLC, attention has turned to ROS1-rearranged tumors, especially NSCLC. The next few years should witness a rapid pace of clinical research in ROS1-rearranged tumors utilizing available ALK inhibitors.

ROS receptor tyrosine kinase: a new potential target for anticancer drugs

ROS kinase is one of the last two remaining orphan receptor tyrosine kinases with an as yet unidentified ligand. The normal functions of human ROS kinase in different body tissues have not been fully identified so far. However, the ectopic expression, as well as the production of variable mutant forms of ROS kinase has been reported in a number of cancers, such as glioblastoma multiforme, and non-small cell lung cancer, suggesting a role for ROS kinase in deriving such tumors. It is thought also that c-ROS gene may have a role in some cardiovascular diseases, and the fact that homozygous male mice targeted against c-ROS gene are healthy but infertile, has inspired researchers to think about ROS inhibition as a method for development of new male contraceptives. The recent discovery of new selective and potent inhibitors for ROS kinase, along with the development of new specific diagnostic methods for the detection of ROS fusion proteins, raises the importance of using these selective inhibitors for targeting ROS mutations as a new method for treatment of cancers harboring such genes. This review focuses on the ectopic expression of ROS and its fusion proteins in different cancer types and highlights the importance of targeting these proteins for treatment of substantial cancers. It describes also the recent advances in the field of ROS kinase inhibition, and the potential clinical applications of ROS kinase inhibitors.

The multifaceted roles of the receptor tyrosine kinase ROS in development and cancer

The proto-oncogene receptor tyrosine kinase ROS was originally discovered through the identification of oncogenic variants isolated from tumors. These discoveries spearheaded a body of work aimed at elucidating the function of this evolutionarily conserved receptor in development and cancer. Through genetic and biochemical approaches, progress in the characterization of ROS points to distinctive roles in the program of epithelial cell differentiation during the development of a variety of organs. Although substantial, these advances remain hampered by the absence of an identified ligand, making ROS one of the last two remaining orphan receptor tyrosine kinases. Recent studies on the oncogenic activation of ROS as a result of different chromosomal rearrangements found in brain and lung cancers have shed light on the molecular mechanisms underlying ROS transforming activities. ROS and its oncogenic variants therefore constitute clinically relevant targets for cancer therapeutic intervention. This review highlights the various roles that this receptor plays in multiple system networks in normalcy and disease and points to future directions towards the elucidation of ROS function in the context of ligand identification, signaling pathways and clinical applications.

Molecular and biochemical bases for activation of the transforming potential of the proto-oncogene c-ros

The transforming gene of avian sarcoma virus UR2, v-ros, encodes a receptor-like protein tyrosine kinase and differs from its proto-oncogene, c-ros, in its 5' truncation and fusion to viral gag, a three-amino-acid (aa) insertion in the transmembrane (TM) domain, and changes in the carboxyl region. To explore the basis for activation of the c-ros transforming potential, various c-ros retroviral vectors containing those changes were constructed and studied for their biological and biochemical properties. Ufcros codes for the full-length c-ros protein of 2,311 aa, Uppcros has 1,661-aa internal deletion in the extracellular domain, CCros contains the 3' c-ros cDNA fused 150 aa upstream of the TM domain to the UR2 gag, CVros is the same as CCros except that the 3' region is replaced by that of v-ros, and VCros is the same as CCros except that the 5' region is replaced by that of v-ros. The Ufcros, Uppcros, CCros, and CVros are inactive in transforming chicken embryo fibroblasts, whereas VCros is as potent as UR2 in cell-transforming and tumorigenic activities. Upon passages of CCros and CVros viruses, the additional extracellular sequence in comparison with that of v-ros was delected; concurrently, both viruses (named CC5d and CV5d, respectively) attained moderate transforming activity, albeit significantly lower than that of UR2 or VCros. The native c-ros protein has a very low protein tyrosine kinase activity, whereas the ppcros protein is constitutively activated in kinase activity. The inability of CCros and CVros to transform chicken embryo fibroblasts is consistent with the inefficient membrane association, instability, and low kinase activity of their encoded proteins. The CC5d and CV5d proteins are indistinguishable in kinase activity, membrane association, and stability from the v-ros protein. The reduced transforming potency of CC5d and CV5d proteins can be attributed only to their differential substrate interaction, notably the failure to phosphorylate a 88-kDa protein. We conclude that the 5' rather than the 3' modification of c-ros is essential for its oncogenic activation; the sequence upstream of the TM domain has a negative effect on the transforming activity of CCros and CVros and needs to be deleted to activate their biological activity.

Differential modulation of plasminogen activator gene expression by oncogene-encoded protein tyrosine kinases

Urokinase-type plasminogen activator (uPA) gene transcription is increased > or = 50-fold in chicken embryo fibroblasts (CEF) following transformation by the protein tyrosine kinase pp60v-src. Protein phosphorylation appears to play a critical role in uPA gene expression in these cells; protein kinase C-activating phorbol esters cooperate with pp60v-src to synergistically increase uPA mRNA, whereas cyclic AMP (cAMP)-dependent protein kinase-activating agents (e.g., 8-bromo cAMP) repress uPA mRNA levels. To explore the relationship between transforming oncogenes and uPA gene expression, uPA mRNA levels were measured in CEF infected with selected avian retroviruses. We report that v-ras and the transforming protein tyrosine kinases v-src, v-yes, and v-ros all increase cellular uPA mRNAs. However, transformation with the protein tyrosine kinase encoded by v-erbB, or the nuclear proteins encoded by v-jun, v-ski, or v-myc, did not increase uPA mRNA detectably. Ras and all of the protein tyrosine kinases analyzed, including the v-erbB product, but none of the nuclear oncoproteins sensitized cells to phorbol ester induction of uPA gene expression. Thus, increased uPA gene expression is not simply a secondary consequence of cell transformation but, rather, is regulated or comodulated by only a subset of oncogene products. Analysis of cells expressing site-directed mutants of pp60v-src showed that the induction of the uPA gene is dependent on protein tyrosine kinase catalytic activity, myristylation, and plasma membrane localization. However, these properties together are not sufficient; an additional feature in the src homology 2 domain is also required. The major sites of serine phosphorylation, serines 12 and 17, and the autophosphorylation site, tyrosine 416, are not essential for uPA gene induction. However, the reduction of uPA mRNA in pp60v-src-transformed cells by 8-bromo cAMP is dependent on tyrosine 416.

Differential modulation of plasminogen activator gene expression by oncogene-encoded protein tyrosine kinases

Urokinase-type plasminogen activator (uPA) gene transcription is increased > or = 50-fold in chicken embryo fibroblasts (CEF) following transformation by the protein tyrosine kinase pp60v-src. Protein phosphorylation appears to play a critical role in uPA gene expression in these cells; protein kinase C-activating phorbol esters cooperate with pp60v-src to synergistically increase uPA mRNA, whereas cyclic AMP (cAMP)-dependent protein kinase-activating agents (e.g., 8-bromo cAMP) repress uPA mRNA levels. To explore the relationship between transforming oncogenes and uPA gene expression, uPA mRNA levels were measured in CEF infected with selected avian retroviruses. We report that v-ras and the transforming protein tyrosine kinases v-src, v-yes, and v-ros all increase cellular uPA mRNAs. However, transformation with the protein tyrosine kinase encoded by v-erbB, or the nuclear proteins encoded by v-jun, v-ski, or v-myc, did not increase uPA mRNA detectably. Ras and all of the protein tyrosine kinases analyzed, including the v-erbB product, but none of the nuclear oncoproteins sensitized cells to phorbol ester induction of uPA gene expression. Thus, increased uPA gene expression is not simply a secondary consequence of cell transformation but, rather, is regulated or comodulated by only a subset of oncogene products. Analysis of cells expressing site-directed mutants of pp60v-src showed that the induction of the uPA gene is dependent on protein tyrosine kinase catalytic activity, myristylation, and plasma membrane localization. However, these properties together are not sufficient; an additional feature in the src homology 2 domain is also required. The major sites of serine phosphorylation, serines 12 and 17, and the autophosphorylation site, tyrosine 416, are not essential for uPA gene induction. However, the reduction of uPA mRNA in pp60v-src-transformed cells by 8-bromo cAMP is dependent on tyrosine 416.

Oncogenes: cause or consequence in the development of glial tumors

Recent developments in the field of oncogenes and growth stimulatory factors have provided limited but essential models in neuro-oncology. The observation in gliomas of platelet growth factor (PDGF)-like immunoreactivity fits with the autocrine secretion model, rising the possibility for the growth factor independence of the cancer cells. The discovery of the tumor suppressor genes, for which loss of function mutations are oncogenic as in the RB gene of the retinoblastoma and p53 gene, has introduced a new concept of oncogenesis which could be useful even in the cure of the neoplasms. Several oncogenes are amplified and/or expressed in brain tumors, some associated with polymorphism leading to abnormal protein products. Therefore, corresponding functions, such as production of deficient epidermal growth factor receptor (EGFR) encoded by erb-B, are impaired. Abnormal chromosomal patterns have been recognized in brain tumors and found mainly in chromosomes 7 and 22 on which oncogenes erb-B and sis are located, respectively. Location of proto-oncogenes, which are normally expressed in the brain, indicate that they share common distribution patterns mainly involving the cerebellum, hippocampus and olfactory bulbs. These proto-oncogenes may be regulated by physiological and pathological events. The concept of oncogene involvement in brain tumors must be extended to include the other factors such as G-proteins, growth factor receptors, membrane-associated and cytoplasmic protein kinases, which are all responsible for the control of the cell growth and their response to external signals including chemotherapeutic drigs.

Transforming properties and substrate specificities of the protein tyrosine kinase oncogenes ros and src and their recombinants

To determine the sequences of the oncogenes src (encoded by Rous sarcoma virus [RSV]) and ros (encoded by UR2) that are responsible for causing different transformation phenotypes and to correlate those sequences with differences in substrate recognition, we constructed recombinants of the two transforming protein tyrosine kinases (PTKs) and studied their biological and biochemical properties. A recombinant with a 5' end from src and a 3' end from ros, called SRC x ROS, transformed chicken embryo fibroblasts (CEF) to a spindle shape morphology, mimicking that of UR2. Neither of the two reverse constructs, ROS x SRC I and ROS x SRC II, could transform CEF. However, a transforming variant of ROS x SRC II appeared during passages of the transfected cells and was called ROS x SRC (R). ROS x SRC (R) contains a 16-amino-acid deletion that includes the 3' half of the transmembrane domain of ros. Unlike RSV, ROS x SRC (R) also transformed CEF to an elongated shape similar to that of UR2. We conclude that distinct phenotypic changes of RSV- and UR2-infected cells do not depend solely on the kinase domains of their oncogenes. We next examined cellular proteins phosphorylated by the tyrosine kinases of UR2, RSV, and their recombinants as well as a number of other avian sarcoma viruses including Fujinami sarcoma virus Y73, and some ros-derived variants. Our results indicate that the UR2-encoded receptorlike PTK P68gag-ros and its derivatives have a very restricted substrate specificity in comparison with the nonreceptor PTKs encoded by the rest of the avian sarcoma viruses. Data from ros and src recombinants indicate that sequences both inside and outside the catalytic domains of ros and src exert a significant effect on the substrate specificity of the two recombinant proteins. Phosphorylation of most of the proteins in the 100- to 200-kDa range correlated with the presence of the 5' src domain, including the SH2 region, but not with the kinase domain in the recombinants. This corroborates the conclusion given above that the kinase domain of src or ros per se is not sufficient to dictate the transforming morphology of these two oncogenes. High-level tyrosyl phosphorylation of most of the prominent substrates of src is not sufficient to cause a round-shape transformation morphology.

Enhancement of transforming potential of human insulinlike growth factor 1 receptor by N-terminal truncation and fusion to avian sarcoma virus UR2 gag sequence

The human insulinlike growth factor 1 (hIGF-1) receptor (hIGFR) is a transmembrane protein tyrosine kinase (PTK) molecule which shares high sequence homology in the PTK domain with the insulin receptor and, to a lesser degree, the ros transforming protein of avian sarcoma virus UR2. To assess the transforming potential of hIGFR, we introduced the intact and altered hIGFR into chicken embryo fibroblasts (CEF). The full-length hIGFR cDNA (fIGFR) was cloned into a UR2 retroviral vector, replacing the original oncogene v-ros. fIGFR was able to promote the growth of CEF in soft agar and cause morphological alteration in the absence of added hIGF-1 to medium containing 11% calf and 1% chicken serum. The transforming ability of hIGFR was not further increased in the presence of 10 nM exogenous hIGF-1. The 180-kDa protein precursor of hIGFR was synthesized and processed into alpha and beta subunits. The overexpressed hIGFR in CEF bound hIGF-1 with high affinity (Kd = 5.4 x 10(-9) M) and responded to ligand stimulation with increased tyrosine autophosphorylation. The cDNA sequence coding for part of the beta subunit of hIGFR, including 36 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains, was fused to the 5' portion of the gag gene in the UR2 vector to form an avian retrovirus. The resulting virus, named UIGFR, was able to induce morphological transformation and promote colony formation of CEF with a stronger potency than did fIGFR. The UIGFR genome encodes a membrane-associated, glycosylated gag-IGFR fusion protein. The specific tyrosine phosphorylation of the mature form of the fusion protein, P75, is sixfold higher in vitro and threefold higher in vivo than that of the native IGFR beta subunit, P95. In conclusion, overexpression of the native or an altered hIGFR can induce transformation of CEF with the gag-IGFR fusion protein possessing enhanced transforming potential, which is consistent with its increased in vitro and in vivo tyrosine phosphorylation.

Two point mutations in the transmembrane domain of P68gag-ros inactive its transforming activity and cause a delay in membrane association

The transforming protein of the avian sarcoma virus UR2 is a 68-kDa transmembrane tyrosine protein kinase. We examined the relationship between membrane localization and transforming activity of P68 by changing Val-168-Val-169 in its hydrophobic domain into Asp-168-Glu-169. The resulting transmembrane (TM) mutant (P68TM) lost transforming activity toward chicken embryo fibroblasts (CEF). We found that the mutant protein was expressed and rapidly degraded into a smaller form which was still membrane associated and kinase active. The instability of the TM mutant protein is a phenomenon only manifested in CEF, because the same mutant protein was expressed with efficiency and stability similar to those of the wild-type protein in a transient expression system in COS cells. However, there are several differences between the wild-type and the TM mutant proteins in COS cells. The wild-type protein is more heavily phosphorylated and associated with membrane fractions in a cotranslational manner. It is enzymatically active when recovered from membrane fractions. The TM mutant protein is less phosphorylated, more labile toward protease degradation, and delayed in membrane association, with a lag period of 30 min or longer, and has little kinase activity when recovered from membrane fractions. Most of the kinase-active TM mutant protein was found in the cytosol fractions. Despite the delay, most of the TM protein in COS cells was found to be membrane associated, and its orientation on the cell surface was similar to that of the wild-type protein. It is probable that loss of the CEF-transforming activity of the TM mutant protein is due to its susceptibility to protease degradation resulting from improper membrane association of the newly synthesized product. The differences in the kinetics of membrane association and the distribution of kinase activity in COS cells might not be directly applicable in explaining the inability of the TM mutant to transform CEF but are intriguing as regards protein biosynthesis and translocation. The difference between CEF and COS cells implies that different factors or pathways are involved in the biosynthesis and processing of the TM mutant protein in these two cellular environments. Changes of P68TM in the kinetics of membrane association indicate that the transmembrane domain of ros, besides functioning as a membrane anchor, also plays a role in directing initial membrane association.

Role of gag sequence in the biochemical properties and transforming activity of the avian sarcoma virus UR2-encoded gag-ros fusion protein

The transforming protein P68gag-ros of avian sarcoma virus UR2 is a transmembrane tyrosine protein kinase molecule with the gag portion protruding extracellularly. To investigate the role of the gag moiety in the biochemical properties and biological functions of the P68gag-ros fusion protein, retroviruses containing the ros coding sequence of UR2 were constructed and analyzed. The gag-free ros protein was expressed from one of the mutant retroviruses at a level 10 to 50% of that of the wild-type UR2. However, the gag-free ros-containing viruses were not able to either transform chicken embryo fibroblasts or induce tumors in chickens. The specific tyrosine protein kinase activity of gag-free ros protein is about 10- to 20-fold reduced as judged by in vitro autophosphorylation. The gag-free ros protein is still capable of associating with membrane fractions including the plasma membrane, indicating that sequences essential for recognition and binding membranes must be located within ros. Upon passages of the gag-free mutants, transforming and tumorigenic variants occasionally emerged. The variants were found to have regained the gag sequence fused to the 5' end of the ros, apparently via recombination with the helper virus or through intramolecular recombination between ros and upstream gag sequences in the same virus construct. All three variants analyzed code for gag-ros fusion protein larger than 68 kDa. The gag-ros recombination junction of one of the transforming variants was sequenced and found to consist of a p19-p10-p27-ros fusion sequence. We conclude that the gag sequence is essential for the transforming activity of P68gag-ros but is not important for its membrane association.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Tissue-specific expression of rat c-ros-1 gene and partial structural similarity of its predicted products with sev protein of Drosophila melanogaster

The expression and predicted products of rat c-ros-1 gene, the proto-oncogene of v-ros in UR2 sarcoma virus, were characterized. The c-ros-1 gene was found to be expressed in a tissue-specific manner, and the sizes of its transcripts were heterogeneous: 8.2 kilobases (kb) long in lung and kidney tissues, 6.9 kb in heart tissue, and 2.4 kb and 1.9 kb in testis tissue. The c-ros-1 cDNAs were isolated from lung and heart tissues. The predicted product of the c-ros-1 gene in lung tissue was a receptor-type tyrosine kinase 2,317 amino acids long (including a very large extracellular domain of approximately 1,800 amino acids) which showed a partial but significant structural homology with the sev gene product of Drosophila melanogaster. An alternatively sliced lung transcript was found to encode a protein with external and transmembrane domains but not a tyrosine kinase catalytic domain. The predicted product in heart tissue was essentially identical to that in lung tissue except for a shorter amino-terminal region and a 21-amino-acid insertion in the extracellular domain. On the basis of these results, the c-ros-1 gene appears to be active in the lungs and kidneys and probably in the hearts of rats.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Modulation of arachidonic acid metabolism by Rous sarcoma virus

Arachidonic acid (C20:4) metabolites were released constitutively from wild-type Rous sarcoma virus-transformed chicken embryo fibroblasts (CEF). 3H-labeled C20:4 and its metabolites were released from unstimulated and uninfected CEF only in response to stimuli such as serum, phorbol ester, or the calcium ionophore A23187. High-pressure liquid chromatography analysis showed that the radioactivity released from [3H]arachidonate-labeled transformed cells was contained in free arachidonate and in the cyclooxygenase products prostaglandin E2 and prostaglandin F2 alpha; no lipoxygenase products were identified. The release of C20:4 and its metabolites from CEF infected with pp60src deletion mutants was correlated with serum-independent DNA synthesis and with the expression of the mRNA for 9E3, a gene expressed in Rous sarcoma virus-transformed cells which has homology with several mitogenic and inflammatory peptides. 3H-labeled C20:4 release was not correlated with p36 phosphorylation, which argues against a role for this protein as a phospholipase A2 inhibitor. CEF infected with other oncogenic viruses encoding a tyrosine kinase also released C20:4, as did CEF infected with viruses that contained mos and ras; however, infection with a crk-containing virus did not result in stimulation of 3H-labeled C20:4 release, suggesting that utilization of this signaling pathway is specific for particular transformation stimuli.

Amplification of the structurally and functionally altered epidermal growth factor receptor gene (c-erbB) in human brain tumors

By using Southern blot analysis, we found that in two cases of human glioblastoma multiforme, cells carried amplified c-erbB genes which bore short deletion mutations within the ligand-binding domain of the epidermal growth factor (EGF) receptor. The products of these mutated c-erbB genes were about 30 kilodalton (kDa) smaller than the normal 170-kDa EGF receptor, and the tumor cell membrane fractions containing the 140-kDa abnormal EGF receptor showed a significant elevation of tyrosine kinase activity without its ligand. In view of the similarity to the activated viral and cellular erbB genes in the avian system, these mutated and overexpressed EGF receptors might play a role in the onset or development of human glioblastoma cells.

Amplification of the structurally and functionally altered epidermal growth factor receptor gene (c-erbB) in human brain tumors

By using Southern blot analysis, we found that in two cases of human glioblastoma multiforme, cells carried amplified c-erbB genes which bore short deletion mutations within the ligand-binding domain of the epidermal growth factor (EGF) receptor. The products of these mutated c-erbB genes were about 30 kilodalton (kDa) smaller than the normal 170-kDa EGF receptor, and the tumor cell membrane fractions containing the 140-kDa abnormal EGF receptor showed a significant elevation of tyrosine kinase activity without its ligand. In view of the similarity to the activated viral and cellular erbB genes in the avian system, these mutated and overexpressed EGF receptors might play a role in the onset or development of human glioblastoma cells.

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.

Additional member of the protein-tyrosine kinase family: the src- and lck-related protooncogene c-tkl

We report the isolation and nucleotide sequence of a 3.7-kilobase (kb) cDNA clone from chicken spleen corresponding to a previously undescribed member of the src family of protooncogenes. It encodes a protein with a C-terminal domain related to the src family of protein-tyrosine kinases (EC 2.7.1.112) and, among these, has most significant homology to the lck gene isolated from a murine leukemia virus-induced thymoma cell line. The gene is therefore referred to as c-tkl for cellular tyrosine kinase related to lck. Analysis of genomic DNA reveals that c-tkl is a chromosomal locus distinct from c-src and c-lck. Furthermore, the size of c-tkl mRNA as well as its pattern of expression indicates that it is not the chicken homologue of lck but a different gene. A 3.8-kb transcript of the c-tkl gene, identical to the size determined for c-src mRNA, was observed in cultured chicken embryo fibroblasts and in chicken spleen and brain. In contrast, detection of a definite c-src mRNA signal with mRNA from spleen was not possible under the hybridization conditions employed when the 5' end of v-src was used as the probe, and none of the 11 clones obtained from the cDNA library corresponded to a c-src transcript. Thus previous studies of c-src mRNA expression in spleen may have actually detected c-tkl transcripts.

A simple method for immunoaffinity purification of nondenatured avian sarcoma and leukemia virus gag-containing proteins

We have developed a one-step purification procedure for proteins containing the N-terminal portion of the gag protein of avian sarcoma and leukemia viruses. In this procedure, a resin with a covalently attached monoclonal antibody to the gag protein p19 is used to bind gag-containing proteins from crude extracts. After washing of the resin, the bound proteins are eluted with 2 M MgCl2. For the transforming protein kinase encoded by Fujinami sarcoma virus p130gag-fps, this procedure gave an enrichment of several thousand-fold, a yield of over 10%, a final purity of over 20%, and no significant loss of protein kinase activity. Similar purifications were obtained with three other gag-containing proteins. The immunoaffinity purification described may be of general utility as a first step in purification of the several other avian retroviral transforming proteins that are synthesized from fusions of an oncogene with the viral gag gene.

Infection of neuroretinal cells in vitro by avian sarcoma viruses UR1 and UR2: transformation, cell growth stimulation, and changes in transducin levels

Infection in vitro of differentiating chick embryo neuroretinal cells with avian sarcoma viruses UR1 and UR2 results in mitogenic stimulation and morphologic conversion of both support neuronal cells. This was shown by the continuous propagation of transformed cells for over 4 months and growth of reaggregated colonies in liquid medium as well as in soft agar. Production of the transforming proteins p 150 gag-fps and p68 gag-ros of UR1 and UR2, respectively, was similar to that of transformed chick embryo fibroblasts, as judged from in vitro kinase activity assays. The two protein subunits, T beta and T gamma, but not T alpha of the GTP binding protein transducin, found in the retina of many animal species, were present in control neuroretinal cells. Infection with Rous sarcoma virus or UR2 resulted in an inhibition of T gamma synthesis and enhancement of T beta-like protein production.

Heterologous transmembrane signaling by a human insulin receptor-v-ros hybrid in Chinese hamster ovary cells

A hybrid receptor molecule composed of the extracellular ligand-binding domain of the human insulin receptor and the transmembrane and cytoplasmic (protein-tyrosine kinase) domains of the chicken sarcoma virus UR2 transforming protein p68gag-ros has been constructed and expressed in Chinese hamster ovary (CHO) cells. The hybrid is processed normally into alpha and hybrid beta subunits, is expressed on the cell surface at high levels, and binds insulin with near-wild-type affinity. Furthermore, insulin stimulates the phosphorylation on tyrosine residues of the hybrid beta subunit in vivo and the phosphorylation of an exogenous substrate [poly(Glu,Tyr)] in vitro. Thus the hybrid is capable of heterologous transmembrane signaling. However, the hybrid mediates neither the insulin-activated uptake of 2-deoxyglucose nor the incorporation of [3H]thymidine into DNA, suggesting that the physiological response(s) mediated by ligand-activated protein-tyrosine kinases may utilize distinct intracellular mechanisms for postreceptor signaling.

Association of p60src with Triton X-100-resistant cellular structure correlates with morphological transformation

More than 70% of wild-type Rous sarcoma virus p60v-src was found to be associated with a cellular structure resistant to nonionic detergent extraction that consists primarily of cytoskeletal proteins. On the other hand, nontransforming src proteins, including cellular p60c-src, nonmyristoylated forms, and those inactive in protein kinase, were found in the fraction solubilized by the detergent extraction. p60c-src was detergent-soluble even in transformed cells, suggesting that the association of p60v-src is not a result of cell transformation. Analyses with a variety of Rous sarcoma virus mutants showed a good correlation between the degree of association with the detergent-resistant structure and the extent of cell transformation caused by mutant src proteins, suggesting that this association may be significant for the process of cell transformation by Rous sarcoma virus.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

Human c-ros-1 gene homologous to the v-ros sequence of UR2 sarcoma virus encodes for a transmembrane receptorlike molecule

We isolated a human gene (designated c-ros-1) homologous to the v-ros sequence of UR2 sarcoma virus. Ten exons, 1,414 base pairs spanning 26 kilobases, contained a tyrosine kinase domain, a transmembrane domain, and a part of an extracellular domain carrying an N glycosylation site which was not acquired by UR2 sarcoma virus. The predicted structure of c-ros-1 is unique among the src family and clearly distinct from the human insulin receptor.

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

We raised antibodies directed against a synthetic peptide representing an amino acid sequence of the conserved kinase domain of the transforming protein of Fujinami sarcoma virus (FSV) (P140). The antiserum obtained specifically recognized FSV-P140 and its cellular homolog and in addition, it recognized a new cellular protein of 94,000 daltons (NCP94) in avian and mammalian cells. NCP94 was found to be associated with a cyclic nucleotide-independent protein kinase activity that was specific for tyrosine residues. Although NCP94 and FSV-P140 share antigenic determinants, NCP94 is not a cellular homolog of FSV-P140: NCP94 and the previously identified c-fps/fes product were different in their tryptic fingerprints and in their tissue specificities. Thus, the function of NCP94 in normal cells is probably different than that of the c-fps/fes product. NCP94 was expressed in every tissue and cell line that was examined. In chickens, NCP94 levels were highest during embryonic development and NCP94 expression was high in gizzard, brain, and spleen throughout embryonic and adult life. The universal expression of NCP94 suggests that this protein may be involved in an essential function of normal cells. NCP94 may be a new cellular tyrosine kinase of the src gene family.

Specific inhibition of tyrosine kinase activity by an antibody to the v-ros oncogene product

Antibodies present in two peritoneal exudates of rats bearing abdominal tumors induced by UR2-transformed rat cells were characterized. The ability to immunoprecipitate p68gag-ros and to inhibit the protein and phospholipid kinase activities of this protein was investigated. One of the exudates specifically inhibited tyrosyl phosphorylation by p68gag-ros but not the activity of other known tyrosyl kinases, such as p150gag-fps of UR1 avian sarcoma virus, p60src, and the insulin receptor. It precipitated p68gag-ros but not Pr76 or other gag-related proteins from UR2-infected cells. Phosphorylation of phosphatidylinositol was not affected by this exudate, suggesting that this activity is not intrinsic to p68gag-ros. Another exudate precipitated p68gag-ros but not gag-related proteins from UR2-infected cells or p140gag-fps from Fujinami sarcoma virus-infected cells. These results demonstrated that the antibodies in these exudates recognized epitopes present in the ros portion of the fused protein p68gag-ros, but only one of the two exudates inhibited the intrinsic tyrosyl kinase of p68gag-ros.

Transfection and recombination with molecularly cloned derivatives of avian sarcoma virus UR2

A cloned version of avian sarcoma virus UR2, plasmid pKD6, which includes the full, nonpermuted proviral sequence between two LTR regions, has been prepared. The plasmid is biologically active in transfection experiments, even when intact. Two transformation-defective mutants with nonoverlapping deletions within the transforming gene ros were constructed from pKD6. These mutants recombine to produce transforming virus when mixed DNA from both is used to transfect chick embryo fibroblasts along with helper virus DNA. However, recombination was not readily detected when cells were coinfected with fluids harvested from cultures separately transfected with DNA from each mutant. This, and marker rescue experiments with a temperature-sensitive mutant of UR2 defective in transformation but able to replicate, suggest that deletion mutants of UR2 do not propagate efficiently.

The human c-ros gene (ROS) is located at chromosome region 6q16----6q22

The human homolog, c-ros, of the transforming gene, v-ros, of the avian sarcoma virus, UR2, has been isolated from a human genomic library. A single-copy fragment from the human c-ros genomic clone has been used to map the human c-ros homolog (ROS) to human chromosome region 6q16----6q22 by somatic cell hybrid analysis and chromosomal in situ hybridization. Thus, the c-ros gene joins the c-myb oncogene, which is distal to the c-ros gene on the long arm of human chromosome 6, as a candidate for involvement in chromosome 6q deletions and rearrangements seen in various malignancies.

Human c-ros-1 gene homologous to the v-ros sequence of UR2 sarcoma virus encodes for a transmembrane receptorlike molecule

We isolated a human gene (designated c-ros-1) homologous to the v-ros sequence of UR2 sarcoma virus. Ten exons, 1,414 base pairs spanning 26 kilobases, contained a tyrosine kinase domain, a transmembrane domain, and a part of an extracellular domain carrying an N glycosylation site which was not acquired by UR2 sarcoma virus. The predicted structure of c-ros-1 is unique among the src family and clearly distinct from the human insulin receptor.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

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

We raised antibodies directed against a synthetic peptide representing an amino acid sequence of the conserved kinase domain of the transforming protein of Fujinami sarcoma virus (FSV) (P140). The antiserum obtained specifically recognized FSV-P140 and its cellular homolog and in addition, it recognized a new cellular protein of 94,000 daltons (NCP94) in avian and mammalian cells. NCP94 was found to be associated with a cyclic nucleotide-independent protein kinase activity that was specific for tyrosine residues. Although NCP94 and FSV-P140 share antigenic determinants, NCP94 is not a cellular homolog of FSV-P140: NCP94 and the previously identified c-fps/fes product were different in their tryptic fingerprints and in their tissue specificities. Thus, the function of NCP94 in normal cells is probably different than that of the c-fps/fes product. NCP94 was expressed in every tissue and cell line that was examined. In chickens, NCP94 levels were highest during embryonic development and NCP94 expression was high in gizzard, brain, and spleen throughout embryonic and adult life. The universal expression of NCP94 suggests that this protein may be involved in an essential function of normal cells. NCP94 may be a new cellular tyrosine kinase of the src gene family.

Mapping surface structures of the human insulin receptor with monoclonal antibodies: localization of main immunogenic regions to the receptor kinase domain

A panel of 37 monoclonal antibodies to the human insulin receptor has been used to characterize the receptor's major antigenic regions and their relationship to receptor functions. Three antibodies recognized extracellular surface structures, including the insulin binding site and a region not associated with insulin binding. The remaining 34 monoclonal antibodies were directed against the cytoplasmic domain of the receptor beta subunit. Competitive binding studies demonstrated that four antigenic regions (beta 1, beta 2, beta 3, and beta 4) are found on this domain. Sixteen of the antibodies were found to be directed against beta 1, nine against beta 2, seven against beta 3, and two against beta 4. Antibodies to all four regions inhibited the receptor-associated protein kinase activity to some extent, although antibodies directed against the beta 2 region completely inhibited the kinase activity of the receptor both in the autophosphorylation reaction and in the phosphorylation of an exogenous substrate, histone. Antibodies to the beta 2 region also did not recognize autophosphorylated receptor. In addition, antibodies to this same region recognized the receptor for insulin-like growth factor I (IGF-I) as well as the insulin receptor. In contrast, antibodies to other cytoplasmic regions did not recognize the IGF-I receptor as well as the insulin receptor. These results indicate that the major immunogenic regions of the insulin receptor are located on the cytoplasmic domain of the receptor beta subunit and are associated with the tyrosine-specific kinase activity of the receptor. In addition, these results suggest that a portion of the insulin receptor is highly homologous to that of the IGF-I receptor.