CoActivators for PCatenin

P-Catenin exerts its activation potential through communication with components of the basal transcription machinery and recruitment of co-activators. On the other hand, the activity of P-catenin can be counteracted by its antagonists. Interaction domains of P-catenin for those binding partners are illustrated in Fig. 2.

The first P-catenin transcriptional co-factor discovered was the TATA-binding protein (TBP) (Hecht et al. 1999). TBP has been shown to bind three different regions of P-catenin overlapping with the NTAD and CTAD. Although TBP binding to these regions correlates with their activation ability, it remains to be determined whether these interactions are functionally significant in an in vivo setting.

The closely related p300 and CREB-binding protein (CBP) are known to function as transcriptional co-activators by linking a variety of transcription factors to the basal transcription machinery or by altering local chromatin structure through their intrinsic or associated histone acetyltransferase activities (Goodman and Smolik 2000). p300 and CBP have been shown to interact with multiple regions of P-catenin, including the CTAD to enhance transcriptional activity (Hecht et al. 2000; Miyagishi et al. 2000; Sun et al. 2000; Takemaru and Moon 2000). In marked contrast, Drosophila CBP (dCBP) negatively regulates the Wingless (Wg; Drosophila Wnt) signaling pathway (Waltzer and Bienz 1998). dCBP binds to dTcf and acetylates a conserved lysine in the N-terminal Armadillo-binding domain, which weakens the interaction between dTcf and Armadillo. At present, it is not clear if

CBP/p300 could, in some context, similarly elicit a repressive effect on transcription of P-catenin target genes in vertebrates.

Brahma-related gene 1 (Brgl) is a component of mammalian SWI/SNF and Rsc chromatin-remodeling complexes, which utilizes the energy of ATP hydrolysis to mobilize nucleosomes and remodel chromatin to facilitate transcription (Roberts and Orkin 2004). Brgl binds P-catenin through Arm repeats 7-12 (R7-12) (Barker et al. 2001). It synergizes with P-catenin to stimulate gene expression in mammalian cultured cells and also genetically interacts with Armadillo in Drosophila.

Two closely related proteins, Pontin/Tip49 and Reptin/Tip48, are members of a highly conserved family with ATPase and helicase activities (Kanemaki et al. 1997, 1999). Pontin and Reptin can form homo- and hetero-dimers. They both bind TBP and are found in multimeric complexes believed to be important for chromatin remodeling and transcription. They have been shown to interact with P-catenin R2-5 and play opposing roles in P-catenin-mediated transcriptional activation (Bauer et al. 1998, 2000). Pontin potentiates activation by P-catenin, whereas Reptin represses it (Bauer et al. 1998; Rottbauer et al. 2002). By modulating Wnt/P-catenin signaling, these two proteins control wing development in Drosophila (Bauer et al. 1998) and heart growth in zebrafish (Rottbauer et al. 2002). The activity of Pontin appears to be required for P-catenin-mediated neoplastic transformation in vitro (Feng et al. 2003).

Nuclear P-catenin activity is positively regulated by Legless (Lgs) and Pygopus (Pygo). Lgs and Pygo were identified in genetic screens for modifiers for the Wg pathway in fly (Belenkaya et al. 2002; Kramps et al. 2002; Parker et al. 2002; Thompson et al. 2002). Drosophila mutants for these genes display phenotypes highly similar to that of Wg loss-of-function, and ectopic expression of Pygo and/or Lgs stimulates Tcf/P-catenin activity in mammalian cultured cells. Lgs directly binds to P-catenin R1-4 and serves as an adapter molecule to recruit Pygo to target gene promoters (Hoffmans and Basler 2004; Kramps et al. 2002; Sampietro et al. 2006). Pygo harbors a PHD finger that is often found in chromatin remodeling factors. Pygo is exclusively localized in the nucleus and has been proposed to function in the nuclear import/retention of P-catenin (Townsley et al. 2004). Additionally, Pygo and Lgs directly contribute to the ability of P-catenin to enhance gene transcription (Hoffmans et al. 2005). The human ortholog of Lgs, Bcl9, was originally identified as an oncoprotein associated with precursor B-cell acute lymphoblastic leukemia (ALL), which exhibits the chromosomal translocation t(1;14)(q21;q32) (Willis et al. 1998). Although genetic analyses in fly clearly demonstrate that Lgs and Pygo are segment polarity gene products required for Wg signaling events, the biological functions of their vertebrate orthologs remain enigmatic.

More recently, a genetic screen in Drosophila identified Parafibromin/Hyrax as a novel P-catenin binding partner (Mosimann et al. 2006). The polymerase-associ-ated factor 1 (Paf1) complex, which contains parafibromin, interacts with the C-terminal domain of the large subunit of RNA polymerase II (Pol II) and plays a key role in transcription elongation and RNA processing (Rozenblatt-Rosen et al. 2005; Yart et al. 2005). Parafibromin forms a stable complex with P-catenin through R12-C and is essential for activation of Wnt/Wg signaling (Mosimann et al. 2006).

The transactivation potential by parafibromin depends on the recruitment of Pygo to P-catenin, and in agreement with this, Bcl9 and Pygo are present in the parafibromin/ P-catenin complex. Parafibromin is the protein product of the tumor suppressor gene HRPT2 mutated in hyperparathyroism-jaw tumor (HPT-JT) syndrome (Carpten et al. 2002). HPT-JT syndrome is a rare autosomal dominant multi-tumor syndrome characterized by hyperparathyroidism due to parathyroid tumors (Wang et al. 2005). However, it remains unknown if alterations in the Wnt/P-catenin pathway directly contribute to the pathogenesis of this syndrome.

In vitro studies, using a reconstituted transcription system, have shown that P-catenin potently enhances binding and transactivation by Tcf/Lef proteins on chromatin templates (Tutter et al. 2001). In this system, the C-terminal region of P-catenin (R11-C) behaves as a dominant-negative inhibitor that specifically represses transcription by the Tcf/P-catenin complex. This is consistent with the previous notion that the C-terminal region of P-catenin acts as a transactivation domain via recruitment of co-activators. More recent biochemical studies have identified additional factors that associate with the CTAD to stimulate P-catenin-dependent transcription, including the coiled-coil co-activator CocoA (R10-C) (Yang et al. 2006), the mediator subunit Med12 (R12-C) (Kim et al. 2006) and subunits of multiple chromatin remodeling complexes, such as the transformation/ transcription-domain-associated protein (TRAP), the imitation switch nucleosome-remodeling ATPase (ISWI) and the trithorax-family mixed-lineage-leukemia (MLL1/MLL2) SET-type proteins (R11-C) (Sierra et al. 2006). Further studies will be required to validate the physiological relevance of these interactions.

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