Groucho mediates a Ci-independent mechanism of hedgehog repression in the anterior wing pouch.

Groucho (Gro) is the founding member of a family of transcriptional co-repressors that are recruited by a number of different transcription factors. Drosophila has a single gro gene, whose loss of function affects processes ranging from sex determination to embryonic patterning and neuroblast specification. We have characterized a function of Gro in imaginal development, namely the repression of hedgehog (hh) in anterior wing pouch cells. hh encodes a secreted morphogen with potent patterning activities. In Drosophila thoracic appendages (legs, wings, halteres), hh is expressed in posterior compartments and induces the anteroposterior (AP) pattern organizer in the cells across the AP boundary. hh is repressed in anterior compartments at least partly via Ci[rep], a form of the multifunctional transcription factor Cubitus interruptus (Ci). We show that cells in the wing primordium close to the AP boundary need gro activity to maintain repression of hh transcription, whereas in more anterior cells gro is dispensable. This repressive function of Gro does not appear to be mediated by Ci[rep]. Analysis of mutant gro transgenes has revealed that the Q and WD40 domains are both necessary for hh repression. Yet, deletion of the WD40 repeats does not always abolish Gro activity. Our findings provide new insights both into the mechanisms of AP patterning of the wing and into the function of Gro.

The mammalian basic helix loop helix protein HES-1 binds to and modulates the transactivating function of the runt-related factor Cbfa1.

Drosophila Runt is the founding member of a family of related transcription factors involved in the regulation of a variety of cell-differentiation events in invertebrates and vertebrates. Runt-related proteins act as both transactivators and transcriptional repressors, suggesting that context-dependent mechanisms modulate their transcriptional properties. The aim of this study was to elucidate the molecular mechanisms that contribute to the regulation of the functions of the mammalian Runt-related protein, Cbfa1. Here we provide the first demonstration that Cbfa1 (as well as the related protein, Cbfa2/AML1) physically interacts with the basic helix loop helix transcription factor, HES-1, a mammalian counterpart of the Drosophila Hairy and Enhancer of split proteins. This interaction is mediated by the carboxyl-terminal domains of Cbfa1 and HES-1, but does not require their respective tetrapeptide motifs, WRPY and WRPW. Our studies also show that HES-1 can antagonize the binding of Cbfa1 to mammalian transcriptional corepressors of the Groucho family. Moreover, HES-1 can potentiate Cbfa1-mediated transactivation in transfected cells. Taken together, these findings implicate HES-1 in the transcriptional functions of Cbfa1 and suggest that the concerted activities of Groucho and HES proteins modulate the functions of mammalian Runt-related proteins.

Groucho/transducin-like enhancer of split (TLE) family members interact with the yeast transcriptional co-repressor SSN6 and mammalian SSN6-related proteins: implications for evolutionary conservation of transcription repression mechanisms.

The yeast proteins TUP1 and SSN6 form a transcription repressor complex that is recruited to different promoters via pathway-specific DNA-binding proteins and regulates the expression of a variety of genes. TUP1 is functionally related to invertebrate and vertebrate transcriptional repressors of the Groucho/transducin-like Enhancer of split (TLE) family. The aim was to examine whether similar mechanisms underlie the transcription repression functions of TUP1 and Groucho/TLEs by determining whether TLE family members can interact with yeast SSN6 and mammalian SSN6-like proteins. It is shown in the present work that SSN6 binds to TLE1 and mediates transcriptional repression when expressed in mammalian cells. Moreover, TLE1 and TLE2 interact with two mammalian proteins related to SSN6, designated as the products of the ubiquitously transcribed tetratricopeptide-repeat genes on the Y (or X) chromosomes (UTY/X). These findings suggest that mammalian TLE and UTY/X proteins may mediate repression mechanisms similar to those performed by TUP1-SSN6 in yeast.

Transducin-like Enhancer of split 2, a mammalian homologue of Drosophila Groucho, acts as a transcriptional repressor, interacts with Hairy/Enhancer of split proteins, and is expressed during neuronal development.

Groucho is a Drosophila transcriptional repressor involved in neurogenesis, segmentation, and sex determination together with basic helix-loop-helix proteins of the Hairy/Enhancer of split (HES) family. Several mammalian Groucho homologues, the Transducin-like Enhancer of split (TLE) 1 through 4 proteins, share similar properties with their Drosophila counterpart, suggesting that TLE proteins perform functions analogous to the roles of Groucho in Drosophila. The aim of this study was to examine this possibility by characterizing the properties of TLE2 and extending the analysis of TLE1. It is shown here that TLE2 and TLE1 are transcriptional repressors that contain two separate repression domains, located either within a Gln-rich amino terminal region or within an internal domain characterized by an abundance of Ser, Thr, and Pro residues. In addition, both TLE2 and TLE1 can homo- and heterodimerize through a short region that is part of their amino-terminal transcription repression domains. Finally, TLE2 interacts and is co-expressed with mammalian HES proteins in both neural and non-neural tissues. Taken together, these findings implicate TLE2 in transcriptional repression and define the structural elements that mediate transcriptional and protein-protein interaction functions of Groucho/TLE proteins.

Affinity for the nuclear compartment and expression during cell differentiation implicate phosphorylated Groucho/TLE1 forms of higher molecular mass in nuclear functions.

The Drosophila protein Groucho is involved in embryonic segmentation and neural development, and is implicated in the Notch signal transduction pathway. We are investigating the molecular mechanisms underlying the function of Groucho and of its mammalian homologues, the TLE ('transducin-like Enhancer of split') proteins. We show that Groucho/TLE1 proteins are phosphorylated. We also show that two populations of phosphorylated Groucho proteins can be identified based on their interaction with the nuclear compartment. More slowly migrating proteins with an apparent molecular mass of roughly 110 kDa interact strongly with the nuclei, while faster migrating proteins displaying molecular masses roughly 84-85 kDa show lower affinity for the nuclear compartment. Similarly, TLE1 proteins with an apparent molecular mass of roughly 118 kDa exhibit higher affinity for the nuclear compartment than do faster migrating forms with apparent molecular masses of 90-93 kDa. Moreover, we show that the nuclear, more slowly migrating, TLE1 proteins are induced during neural determination of P19 embryonic carcinoma cells. These results implicate phosphorylation in the activity of Groucho/TLE1 proteins and suggest that phosphorylated forms of higher molecular mass are involved in nuclear functions. Finally, we show that different TLE proteins respond in different ways to the neural commitment of P19 cells, suggesting that individual members of this protein family may have non-redundant functions.

Molecular interaction between TLE1 and the carboxyl-terminal domain of HES-1 containing the WRPW motif.

Groucho is a protein implicated in Notch signaling and involved in segmentation and neural development in Drosophila. Groucho forms transcription complexes with the basic helix-loop-helix proteins encoded by the hairy/Enhancer of split ("hairy-like") gene family. These interactions are mediated by the carboxyl-terminal WRPW motif of Hairy-like proteins. We are interested in determining whether Groucho and its mammalian homologues, the TLE proteins, perform conserved functions. We show that TLE1 interacts with HES-1, a murine homologue of Drosophila Hairy-like proteins, both in the yeast two-hybrid assay and in an interaction assay based on glutathione S-transferase fusion proteins. These results show that Groucho/TLE proteins and Hairy-like/HES proteins are involved in similar interactions in Drosophila and mammals and further suggest that these proteins perform conserved cellular functions.