Huckebein repressor activity in Drosophila terminal patterning is mediated by Groucho.
The Groucho corepressor mediates negative transcriptional regulation in association with various DNA-binding proteins in diverse developmental contexts. We have previously implicated Groucho in Drosophila embryonic terminal patterning, showing that it is required to confine tailless and huckebein terminal gap gene expression to the pole regions of the embryo. Here we reveal an additional requirement for Groucho in this developmental process by establishing that Groucho mediates repressor activity of the Huckebein protein. Putative Huckebein target genes are derepressed in embryos lacking maternal groucho activity and biochemical experiments demonstrate that Huckebein physically interacts with Groucho. Using an in vivo repression assay, we identify a functional repressor domain in Huckebein that contains an FRPW tetrapeptide, similar to the WRPW Groucho-recruitment domain found in Hairy-related repressor proteins. Mutations in Huckebein's FRPW motif abolish Groucho binding and in vivo repression activity, indicating that binding of Groucho through the FRPW motif is required for the repressor function of Huckebein. Taken together with our earlier results, these findings show that Groucho-repression regulates sequential aspects of terminal patterning in Drosophila. (+info)
Temperature-sensitive RB mutations linked to incomplete penetrance of familial retinoblastoma in 12 families.
The tumor-suppressor activity of the retinoblastoma protein (RB) is encoded within a protein-binding ("pocket") domain that is targeted for mutations in all cases of familial retinoblastoma and in many common adult cancers. Although familial retinoblastoma is a paradigm for a highly penetrant, recessive model of tumorigenesis, the molecular basis for the phenotype of incomplete penetrance of familial retinoblastoma is undefined. We studied the RB pocket-binding properties of three independent, mutant RB alleles that are present in the germline of 12 kindreds with the phenotype of incomplete penetrance of familial retinoblastoma. Each arises from alterations of single codons within the RB pocket domain (designated "delta 480," "661W," or "712R"). Under the same conditions, we studied the properties of wild-type (WT) RB, an RB point mutant isolated from a lung carcinoma sample (706F) and an adjacent, in vitro-generated point mutant (707W). The delta 480, 661W, and 712R mutants lack pocket protein-binding activity in vitro but retain the WT ability to undergo cyclin-mediated phosphorylation in vivo. Each of the low-penetrant RB mutants exhibits marked enhancement of pocket protein binding when the cells are grown at reduced temperature. In contrast, in this temperature range, no change in binding activity is seen with WT RB, the 706F mutant, or the 707W mutant. We have demonstrated that many families with incomplete penetrance of familial retinoblastoma carry unstable, mutant RB alleles with temperature-sensitive pocket protein-binding activity. The variable frequency for tumor development in these families may result from reversible fluctuations in a threshold level of RB pocket-binding activity. (+info)
MEF-2 function is modified by a novel co-repressor, MITR.
The MEF-2 proteins are a family of transcriptional activators that have been detected in a wide variety of cell types. In skeletal muscle cells, MEF-2 proteins interact with members of the MyoD family of transcriptional activators to synergistically activate gene expression. Similar interactions with tissue or lineage-specific cofactors may also underlie MEF-2 function in other cell types. In order to screen for such cofactors, we have used a transcriptionally inactive mutant of Xenopus MEF2D in a yeast two-hybrid screen. This approach has identified a novel protein expressed in the early embryo that binds to XMEF2D and XMEF2A. The MEF-2 interacting transcription repressor (MITR) protein binds to the N-terminal MADS/MEF-2 region of the MEF-2 proteins but does not bind to the related Xenopus MADS protein serum response factor. In the early embryo, MITR expression commences at the neurula stage within the mature somites and is subsequently restricted to the myotomal muscle. In functional assays, MITR negatively regulates MEF-2-dependent transcription and we show that this repression is mediated by direct binding of MITR to the histone deacetylase HDAC1. Thus, we propose that MITR acts as a co-repressor, recruiting a specific deacetylase to downregulate MEF-2 activity. (+info)
MAF1, a novel plant protein interacting with matrix attachment region binding protein MFP1, is located at the nuclear envelope.
The interaction of chromatin with the nuclear matrix via matrix attachment region (MAR) DNA is considered to be of fundamental importance for chromatin organization in all eukaryotic cells. MAR binding filament-like protein 1 (MFP1) from tomato is a novel plant protein that specifically binds to MAR DNA. Its filament protein-like structure makes it a likely candidate for a structural component of the nuclear matrix. MFP1 is located at nuclear matrix-associated, specklelike structures at the nuclear envelope. Here, we report the identification of a novel protein that specifically interacts with MFP1 in yeast two-hybrid and in vitro binding assays. MFP1 associated factor 1 (MAF1) is a small, soluble, serine/threonine-rich protein that is ubiquitously expressed and has no similarity to known proteins. MAF1, like MFP1, is located at the nuclear periphery and is a component of the nuclear matrix. These data suggest that MFP1 and MAF1 are in vivo interaction partners and that both proteins are components of a nuclear substructure, previously undescribed in plants, that connects the nuclear envelope and the internal nuclear matrix. (+info)
Rpn9 is required for efficient assembly of the yeast 26S proteasome.
We have isolated the RPN9 gene by two-hybrid screening with, as bait, RPN10 (formerly SUN1), which encodes a multiubiquitin chain receptor residing in the regulatory particle of the 26S proteasome. Rpn9 is a nonessential subunit of the regulatory particle of the 26S proteasome, but the deletion of this gene results in temperature-sensitive growth. At the restrictive temperature, the Deltarpn9 strain accumulated multiubiquitinated proteins, indicating that the RPN9 function is needed for the 26S proteasome activity at a higher temperature. We analyzed the proteasome fractions separated by glycerol density gradient centrifugation by native polyacrylamide gel electrophoresis and found that a smaller amount of the 26S proteasome was produced in the Deltarpn9 cells and that the 26S proteasome was shifted to lighter fractions than expected. The incomplete proteasome complexes were found to accumulate in the Deltarpn9 cells. Furthermore, Rpn10 was not detected in the fractions containing proteasomes of the Deltarpn9 cells. These results indicate that Rpn9 is needed for incorporating Rpn10 into the 26S proteasome and that Rpn9 participates in the assembly and/or stability of the 26S proteasome. (+info)
The Borgs, a new family of Cdc42 and TC10 GTPase-interacting proteins.
The Rho family of GTPases plays key roles in the regulation of cell motility and morphogenesis. They also regulate protein kinase cascades, gene expression, and cell cycle progression. This multiplicity of roles requires that the Rho GTPases interact with a wide variety of downstream effector proteins. An understanding of their functions at a molecular level therefore requires the identification of the entire set of such effectors. Towards this end, we performed a two-hybrid screen using the TC10 GTPase as bait and identified a family of putative effector proteins related to MSE55, a murine stromal and epithelial cell protein of 55 kDa. We have named this family the Borg (binder of Rho GTPases) proteins. Complete open reading frames have been obtained for Borg1 through Borg3. We renamed MSE55 as Borg5. Borg1, Borg2, Borg4, and Borg5 bind both TC10 and Cdc42 in a GTP-dependent manner. Surprisingly, Borg3 bound only to Cdc42. An intact CRIB (Cdc42, Rac interactive binding) domain was required for binding. No interaction of the Borgs with Rac1 or RhoA was detectable. Three-hemagglutinin epitope (HA(3))-tagged Borg3 protein was mostly cytosolic when expressed ectopically in NIH 3T3 cells, with some accumulation in membrane ruffles. The phenotype induced by Borg3 was reminiscent of that caused by an inhibition of Rho function and was reversed by overexpression of Rho. Surprisingly, it was independent of the ability to bind Cdc42. Borg3 also inhibited Jun kinase activity by a mechanism that was independent of Cdc42 binding. HA(3)-Borg3 expression caused substantial delays in the spreading of cells on fibronectin surfaces after replating, and the spread cells lacked stress fibers. We propose that the Borg proteins function as negative regulators of Rho GTPase signaling. (+info)
DOT4 links silencing and cell growth in Saccharomyces cerevisiae.
Transcriptional silencing in Saccharomyces cerevisiae occurs at specific loci and is mediated by a multiprotein complex that includes Rap1p and the Sir proteins. We studied the function of a recently identified gene, DOT4, that disrupts silencing when overexpressed. DOT4 encodes an ubiquitin processing protease (hydrolase) that is primarily located in the nucleus. By two-hybrid analysis, the amino-terminal third of Dot4p interacts with the silencing protein Sir4p. Cells lacking DOT4 exhibited reduced silencing and a corresponding decrease in the level of Sir4p. Together, these findings suggest that Dot4p regulates silencing by acting on Sir4p. In strains with several auxotrophic markers, loss of DOT4 ubiquitin hydrolase activity also results in a slow-growth defect. The defect can be partially suppressed by mutations in a subunit of the 26S proteasome, suggesting that Dot4p has the ability to prevent ubiquitin-mediated degradation. Furthermore, wild-type SIR2, SIR3, and SIR4 are required for full manifestation of the growth defect in a dot4 strain, indicating that the growth defect is caused in part by a silencing-related mechanism. We propose that Dot4p helps to restrict the location of silencing proteins to a limited set of genomic loci. (+info)
Leukemic HRX fusion proteins inhibit GADD34-induced apoptosis and associate with the GADD34 and hSNF5/INI1 proteins.
One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene (also called MLL, ALL-1, or HTRX) at chromosomal locus 11q23, resulting in the formation of HRX fusion proteins. Using the yeast two-hybrid system and human cell culture coimmunoprecipitation experiments, we show here that HRX proteins interact directly with the GADD34 protein. We have found that transfected cells overexpressing GADD34 display a significant increase in apoptosis after treatment with ionizing radiation, indicating that GADD34 expression not only correlates with apoptosis but also can enhance apoptosis. The amino-terminal third of the GADD34 protein was necessary for this observed increase in apoptosis. Furthermore, coexpression of three different HRX fusion proteins (HRX-ENL, HRX-AF9, and HRX-ELL) had an anti-apoptotic effect, abrogating GADD34-induced apoptosis. In contrast, expression of wild-type HRX gave rise to an increase in apoptosis. The difference observed here between wild-type HRX and the leukemic HRX fusion proteins suggests that inhibition of GADD34-mediated apoptosis may be important to leukemogenesis. We also show here that GADD34 binds the human SNF5/INI1 protein, a member of the SNF/SWI complex that can remodel chromatin and activate transcription. These studies demonstrate, for the first time, a gain of function for leukemic HRX fusion proteins compared to wild-type protein. We propose that the role of HRX fusion proteins as negative regulators of post-DNA-damage-induced apoptosis is important to leukemia progression. (+info)