Reconstitution of high affinity leukaemia inhibitory factor (LIF) receptors in haemopoietic cells transfected with the cloned human LIF receptor

cDNA clones encoding the human leukaemia inhibitory factor (hLIF) receptor were isolated by screening a placental cDNA expression library in COS-7 cells with 125I-hLIF. The cloned LIF receptor is a member of the haemopoietin receptor family and comprises a signal sequence (44 amino acids), an extracellular region of two haemopoietin receptor domains and three fibronectin type III domains (789 amino acids), a transmembrane domain (26 amino acids) and a cytoplasmic domain (238 amino acids). The LIF receptor is expressed in COS-7 cells as a 190 kDa glycoprotein that specifically binds human LIF with low affinity, but does not bind mouse LIF. Clones encoding a soluble form of the homologous mouse LIF receptor have been isolated, suggesting complex interactions between the various forms of LIF ligand and receptor in vivo. The LIF receptor is most related to the gp130 signal-transducing component of the IL-6 receptor, a feature that may provide a molecular basis for the intertwined biologies of LIF and IL-6 in the absence of obvious structural similarly between the ligands. Mouse B9 plasmacytoma cells transfected with the human LIF receptor display novel high affinity LIF receptors that are presumed to consist of transfected receptors in association with endogenous mouse high affinity-converting subunits. Unlike the low affinity human LIF receptor, the mixed species high affinity receptor is capable of binding mouse LIF.

A large-scale in situ screen provides molecular evidence for the induction of eye anterior segment structures by the developing lens

The anterior segment of the vertebrate eye includes the cornea, iris, ciliary body, trabecular meshwork, and lens. Although malformations of these structures have been implicated in many human eye diseases, little is known about the molecular mechanisms that control their development. To identify genes involved in anterior segment formation, we developed a large-scale in situ hybridization screen and examined the spatial and temporal expression of over 1000 genes during eye development. This screen identified 62 genes with distinct expression patterns in specific eye structures, including several expressed in novel patterns in the anterior segment. Using these genes as developmental markers, we tested for the presence of inductive signals that control the differentiation of anterior segment tissues. Organ culture recombination experiments showed that a chick lens is capable of inducing the expression of markers of the presumptive iris and ciliary body in the developing mouse neural retina. The inducing activity from the lens acts only over short ranges and is present at multiple stages of eye development. These studies provide molecular evidence that an evolutionarily conserved signal from the lens controls tissue specification in the developing optic cup.

Repression of p53-mediated transcription by MDM2: a dual mechanism

The oncoprotein MDM2 binds to the activation domain of the tumor suppressor p53 and inhibits its ability to stimulate transcription. This same region of p53 is able to bind several basal transcription factors that appear to be important for the transactivation function of p53. It has therefore been suggested that MDM2 acts to inhibit p53 by concealing its activation domain from the basal machinery. Here we present data suggesting that MDM2 possesses an additional inhibitory function. Our experiments reveal that in addition to a p53-binding domain, MDM2 also contains an inhibitory domain that can directly repress basal transcription in the absence of p53. By fusing portions of MDM2 to a heterologous DNA-binding domain to allow p53-independent promoter recruitment, we have localized this inhibitory domain to a region encompassing amino acids 50-222 of MDM2. Furthermore, the function of this inhibitory domain does not require the presence of either TFIIA or the TAFs. Of the remaining basal factors, both the small subunit of TFIIE and monomeric TBP are bound by the MDM2 inhibitory domain. It is possible that MDM2 inhibits the ability of the preinitiation complex to synthesize RNA through one of these interactions. Our results are consistent with a model in which MDM2 represses p53-dependent transcription by a dual mechanism: a masking of the activation domain of p53 through a protein-protein interaction that additionally serves to recruit MDM2 to the promoter where it directly interferes with the basal transcription machinery.

p53 transcriptional activation mediated by coactivators TAFII40 and TAFII60

The tumor suppressor protein p53 is a transcriptional regulator that enhances the expression of proteins that control cellular proliferation. The multisubunit transcription factor IID (TFIID) is thought to be a primary target for site-specific activators of transcription. Here, a direct interaction between the activation domain of p53 and two subunits of the TFIID complex, TAFII40 and TAFII60, is reported. A double point mutation in the activation domain of p53 impaired the ability of this domain to activate transcription and, simultaneously, its ability to interact with both TAFII40 and TAFII60. Furthermore, a partial TFIID complex containing Drosophila TATA binding protein (dTBP), human TAFII250, dTAFII60, and dTAFII40 supported activation by a Gal4-p53 fusion protein in vitro, whereas TBP or a subcomplex lacking TAFII40 and TAFII60 did not. Together, these results suggest that TAFII40 and TAFII60 are important targets for transmitting activation signals between p53 and the initiation complex.

The Epstein-Barr virus nuclear protein 2 acidic domain can interact with TFIIB, TAF40, and RPA70 but not with TATA-binding protein

The Epstein-Barr virus nuclear antigen 2 (EBNA-2) acidic domain is essential for B-lymphocyte growth transformation and can activate transcription when brought to a promoter by a sequence-specific DNA-binding domain. We now show that the EBNA-2 acidic domain has slightly less activity than the proteotypic acidic transactivator VP16 in depleting nuclear extracts of basal transcription activity. Like VP16, EBNA-2 associates with TFIIB, TAF40, and RPA70. However, EBNA-2 has much less avidity for TATA-binding protein. A Trp-to-Thr mutation within the acidic domain abolishes EBNA-2 transactivating activity and greatly compromises the association with TFIIB, TAF40, and RPA70, establishing a genetic linkage between transactivating activity and these associations.

Drosophila TAFII40 interacts with both a VP16 activation domain and the basal transcription factor TFIIB

Enhancement of RNA polymerase II transcription by the viral transactivator VP16 requires the TFIID complex, which consists of the TATA-binding protein (TBP) and TBP-associated factors (TAFs). Here we report the molecular cloning, expression, and biochemical characterization of Drosophila TAFII40 (dTAFII40), a subunit of TFIID. In vitro protein-protein interaction assays revealed direct binding between dTAFII40 and a 39 amino acid VP16 activation domain. In addition, affinity chromatography indicated a direct binding of the basal factor TFIIB to immobilized dTAFII40. Since VP16 also binds TFIIB, our results suggest a ternary interaction among an activator, a coactivator, and a basal transcription factor. Antibodies directed against dTAFII40 inhibited activation by GAL4-VP16 without affecting basal transcription. These results, taken together with previous studies of Sp1 and dTAFII110, establish that different activators interact with distinct TAFs in the TFIID complex and that TAFs can contact both activators and basal factors.

The IL-6 signal transducer, gp130: an oncostatin M receptor and affinity converter for the LIF receptor

Leukemia inhibitory factor (LIF) and interleukin-6 (IL-6) are multifunctional cytokines with many similar activities. LIF is structurally and functionally related to another cytokine, Oncostatin M (OSM), that binds to the high-affinity LIF receptor but not to the low-affinity LIF receptor. A complementary DNA was isolated that encodes the high-affinity converting subunit of the LIF receptor. The converter conferred high-affinity binding of both LIF and OSM when expressed with the low-affinity LIF receptor and is identical to the signal transducing subunit of the IL-6 receptor, gp130. The gp130 subunit alone confers low-affinity binding of OSM when expressed in COS-7 cells. This receptor system resembles the high-affinity receptors for granulocyte-macrophage colony-stimulating factor, IL-3, and IL-5, which share a common subunit.

Leukemia inhibitory factor receptor is structurally related to the IL-6 signal transducer, gp130

Leukemia inhibitory factor (LIF) is a cytokine with a broad range of activities that in many cases parallel those of interleukin-6 (IL-6) although LIF and IL-6 appear to be structurally unrelated. A cDNA clone encoding the human LIF receptor was isolated by expression screening of a human placental cDNA library. The LIF receptor is related to the gp130 'signal-transducing' component of the IL-6 receptor and to the G-CSF receptor, with the transmembrane and cytoplasmic regions of the LIF receptor and gp130 being most closely related. This relationship suggests a common signal transduction pathway for the two receptors and may help to explain similar biological effects of the two ligands. Murine cDNAs encoding soluble LIF receptors were isolated by cross-hybridization and share 70% amino acid sequence identity to the human sequence.