Functional domains in the Deformed protein

Molecular characterization of cytochrome P450 CYP6B47 cDNAs and 5'-flanking sequence from Spodoptera litura (Lepidoptera: Noctuidae): its response to lead stress

In insects, P450s are responsible for the oxidative metabolism of structurally diverse endogenous and exogenous compounds including plant allelochemicals and insecticides. A novel full-length P450 cDNA, CYP6B47, was cloned from Spodoptera litura (Lepidoptera: Noctuidae). The sequence is 1718 bp in length with an ORF of 1509 bp encoding 503 amino acid residues. The phylogenetic analysis indicated that CYP6B47 belongs to CYP3 clan and second clade of CYP6Bs which contain 11 P450s from Noctuidae. Quantitative real-time PCR showed that CYP6B47 was expressed only in larvae stages and had a high level of transcription in the midgut and fat body. In addition, we cloned a 2141-bp 5'-flanking regions and presented the basal luciferase activities of promoter. We also predicted multiple putative elements for transcription factors binding in the 5'-flanking region. Interestingly, the expression of CYP6B47 significantly increased in the midgut and fat body after lead (Pb) exposure for 5 generations. Larvae tolerance to the alpha-cypermethrin (35% increased in LC(50)) and fenvalerate (52% increased in LC(50)) were improved after pre-exposure to 50 mg/kg Pb. These dates suggested that lead increased tolerance of larvae to insecticides mainly through transcriptional induction of detoxification genes including CYP6B47.

Dissecting the functional specificities of two Hox proteins

Hox proteins frequently select and regulate their specific target genes with the help of cofactors like Extradenticle (Exd) and Homothorax (Hth). For the Drosophila Hox protein Sex combs reduced (Scr), Exd has been shown to position a normally unstructured portion of Scr so that two basic amino acid side chains can insert into the minor groove of an Scr-specific DNA-binding site. Here we provide evidence that another Drosophila Hox protein, Deformed (Dfd), uses a very similar mechanism to achieve specificity in vivo, thus generalizing this mechanism. Furthermore, we show that subtle differences in the way Dfd and Scr recognize their specific binding sites, in conjunction with non-DNA-binding domains, influence whether the target gene is transcriptionally activated or repressed. These results suggest that the interaction between these DNA-binding proteins and the DNA-binding site determines the architecture of the Hox-cofactor-DNA ternary complex, which in turn determines whether the complex recruits coactivators or corepressors.

Regulation of Hox activity: insights from protein motifs

Deciphering the molecular bases of animal body plan construction is a central question in developmental and evolutionary biology. Genome analyses of a number of metazoans indicate that widely conserved regulatory molecules underlie the amazing diversity of animal body plans, suggesting that these molecules are reiteratively used for multiple purposes. Hox proteins constitute a good example of such molecules and provide the framework to address the mechanisms underlying transcriptional specificity and diversity in development and evolution. Here we examine the current knowledge of the molecular bases of Hox-mediated transcriptional control, focusing on how this control is encoded within protein sequences and structures. The survey suggests that the homeodomain is part of an extended multifunctional unit coordinating DNA binding and activity regulation and highlights the need for further advances in our understanding of Hox protein activity.

Cap ‘n’ collar B cooperates with a small Maf subunit to specify pharyngeal development and suppress deformed homeotic function in the Drosophila head

The basic-leucine zipper protein Cap ‘n’ collar B (CncB) suppresses the segmental identity function of the Hox gene Deformed (Dfd) in the mandibular segment of Drosophila embryos. CncB is also required for proper development of intercalary, labral and mandibular structures. In this study, we provide evidence that the CncB-mediated suppression of Dfd requires the Drosophila homolog of the mammalian small Maf proteins, Maf-S, and that the suppression occurs even in the presence of high amounts of Dfd protein. Interestingly, the CncB/Maf-S suppressive effect can be partially reversed by overexpression of Homothorax (Hth), suggesting that Hth and Extradenticle proteins antagonize the effects of CncB/Maf-S on Dfd function in the mandibular segment. In embryos, multimers of simple CncB/Maf-S heterodimer sites are transcriptionally activated in response to CncB, and in tissue culture cells the amino-terminal domain of CncB acts as a strong transcriptional activation domain. There are no good matches to CncB/Maf binding consensus sites in the known elements that are activated in response to Dfd and repressed in a CncB-dependent fashion. This suggests that some of the suppressive effect of CncB/Maf-S proteins on Dfd protein function might be exerted indirectly, while some may be exerted by direct binding to as yet uncharacterized Dfd response elements. We also show that ectopic CncB is sufficient to transform ventral epidermis in the trunk into repetitive arrays of ventral pharynx. We compare the functions of CncB to those of its vertebrate and invertebrate homologs, p45 NF-E2, Nrf and Skn-1 proteins, and suggest that the pharynx selector function of CncB is highly conserved on some branches of the evolutionary tree.

Divergent homeobox gene hex regulates promoter of the Na(+)-dependent bile acid cotransporter

The divergent homeobox gene Hex is expressed in both developing and mature liver. A putative Hex binding site was identified in the promoter region of the liver-specific Na(+)-bile acid cotransporter gene (ntcp), and we hypothesized that Hex regulates the ntcp promoter through this site. Successive 5'-deletions of the ntcp promoter in a luciferase reporter construct transfected into Hep G2 cells confirmed a Hex response element (HRE) within the ntcp promoter (nt -733/-714). Moreover, p-CMHex transactivated a heterologous promoter construct containing HRE multimers (p4xHRELUC), whereas a 5-bp mutation of the core HRE eliminated transactivation. A dominant negative form of Hex (p-Hex-DN) suppressed basal luciferase activity of p-4xHRELUC and inhibited activation of this construct by p-CMHex. Interestingly, p-CMHex transactivated the HRE in Hep G2 cells but not in fibroblast-derived COS cells, suggesting the possibility that Hex protein requires an additional liver cell-specific factor(s) for full activity. Electrophoretic mobility shift assays confirmed that liver and Hep G2 cells contain a specific nuclear protein that binds the native HRE. We have demonstrated that the liver-specific ntcp gene promoter is the first known target of Hex and is a useful tool for evaluating function of the Hex protein.

Distinct hox protein sequences determine specificity in different tissues

Hox genes encode evolutionarily conserved transcription factors that control the morphological diversification along the anteroposterior (A/P) body axis. Expressed in precise locations in the ectoderm, mesoderm, and endoderm, Hox proteins have distinct regulatory activities in different tissues. How Hox proteins achieve tissue-specific functions and why cells lying at equivalent A/P positions but in different germ layers have distinctive responses to the same Hox protein remains to be determined. Here, we examine this question by identifying parts of Hox proteins necessary for Hox function in different tissues. Available genetic markers allow the regulatory effects of two Hox proteins, Abdominal-A (AbdA) and Ultrabithorax (Ubx), to be distinguished in the Drosophila embryonic epidermis and visceral mesoderm (VM). Chimeric Ubx/AbdA proteins were tested in both tissues and used to identify protein sequences that endow AbdA with a different target gene specificity from Ubx. We found that distinct protein sequences define AbdA, as opposed to Ubx, function in the epidermis vs. the VM. These sequences lie mostly outside the homeodomain (HD), emphasizing the importance of non-HD residues for specific Hox activities. Hox tissue specificity is therefore achieved by sensing distinct Hox protein structures in different tissues.

Activity regulation of Hox proteins, a mechanism for altering functional specificity in development and evolution

The closely related Hox transcription factors Ultrabithorax (Ubx) and Antennapedia (Antp) respectively direct first abdominal (A1) and second thoracic (T2) segment identities in Drosophila. It has been proposed that their functional differences derive from their differential occupancy of DNA target sites. Here we show that a hybrid version of Ubx (Ubx-VP16), which possesses an enhanced transcriptional activation function, no longer directs A1 denticle pattern in embryonic epidermal cells. Instead, it mimics Antp in directing T2 denticle pattern, and it can rescue the cuticular loss-of-function phenotype of Antp mutants. In cells that do not produce denticles, Ubx-VP16 appears to have largely retained its normal repressive regulatory functions. These results suggest that the modulation of Hox activation and repression functions can account for segment-specific morphological differences that are controlled by different members of the Hox family. Our results also are consistent with the idea that activity regulation underlies the phenotypic suppression phenomenon in which a more posterior Hox protein suppresses the function of a more anterior member of the Hox cluster. The acquisition of novel activation and repression potentials in Hox proteins may be an important mechanism underlying the generation of subtle morphological differences during evolution.

Interactions between Wingless and DFz2 during Drosophila wing development

Drosophila Wingless (Wg) is a secreted signaling protein of the Wnt family. Mutations in the wg gene disrupt the patterning of embryonic segments and their adult derivatives. Wg protein has been shown in cell culture to functionally interact with DFz2, a receptor that is structurally related to the tissue polarity protein Frizzled (Fz). However, it has not been determined if DFz2 functions in the Wg signaling pathway during fly development. Here we demonstrate that overexpression of DFz2 increases Wg-dependent signaling to induce ectopic margin bristle formation in developing Drosophila wings. Overexpression of a truncated form of DFz2 acts in a dominant-negative manner to block Wg signaling at the wing margin, and this block is rescued by co-expression of full-length DFz2 but not full-length Fz. Our results suggest that DFz2 and not Fz acts in the Wg signaling pathway for wing margin development. However, a truncated form of Fz also blocks Wg signaling in embryo and wing margin development, and the truncated form of DFz2 affects ommatidial polarity during eye development. These observations suggest that a single dominant-negative form of Fz or DFz2 can block more than one type of Wnt signaling pathway and imply that truncated proteins of the Fz family lose some aspect of signaling specificity.

Photoreceptor cell differentiation requires regulated proteolysis of the transcriptional repressor Tramtrack

The transcription repressor Tramtrack (TTK) is found in cone cells but not photoreceptor cells of the Drosophila eye. We show that down-regulation of TTK expression occurs in photoreceptor cells and is required for their fate determination. Down-regulation requires the presence of Phyllopod (PHYL), which is induced by the RAS pathway, and Seven In Absentia (SINA). Loss of either gene causes accumulation of TTK in photoreceptor cells, and TTK does not accumulate in cone cells if both PHYL and SINA are present. We report that SINA and PHYL promote ubiquitination and rapid degradation of TTK by the proteasome pathway in cell culture, and both SINA and PHYL bind to the N-terminal domain of TTK. These results argue that photoreceptor differentiation is regulated by the RAS pathway through targeted proteolysis of the TTK repressor.

Homeodomain interaction with the beta subunit of the general transcription factor TFIIE

Homeodomain-containing proteins play a crucial role as transcriptional regulators in the process of cell differentiation. The homeodomain performs a dual function in this regard, acting as a DNA binding domain and participating in protein-protein interactions that enhance DNA binding specificity or regulatory activity. Here we describe a homeodomain class-specific interaction with the beta subunit of the general transcription factor TFIIE. We show that the Antennapedia and Abdominal-B homeodomains bind to TFIIEbeta, but the even-skipped homeodomain does not. Using a two-hybrid assay performed in cultured cells, we demonstrate that the homeodomain-TFIIEbeta interaction occurs in vivo. The Abdominal-B homeodomain is shown to activate transcription in vitro, and this activation can be blocked with anti-TFIIEbeta antibody without affecting basal transcription levels. Together with published data demonstrating an interaction between proteins containing even-skipped class homeodomains and the TATA-binding protein (Um, M., Li, C., and Manley, J. L. (1995) Mol. Cell. Biol. 15, 5007-5016; Zhang, H., Catron, K. M., and Abate-Shen, C. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1764-1769), these results suggest various homeodomain containing proteins interact with different general transcription factors, an observation that may have important implications for transcriptional regulation.