A screen for targets of the Xenopus T-box gene Xbra

Identification of functional domains involved in BTG1 cell localization

We have previously shown that BTG1 stimulates myoblast differentiation. In addition, this protein displays a major nuclear localization in confluent myoblasts, decreasing during the early steps of differentiation, and is essentially detected in the cytoplasm of mature myotubes. To identify the domains involved in the cellular trafficking of BTG1, we observed the localization of several BTG1 sequences fused to betaGalactosidase. The highly conserved B box among all members of the BTG family induces a significant nuclear localization of the betaGal moiety, enhanced by presence of the BTG1 carboxy-terminal sequence. In addition, a functional Nuclear Export Signal (NES) overlaps the B box. Moreover, presence of the first 43 NH(2)-terminal amino acids reduced the nuclear localization of each chimeric protein tested. Last, the BTG1 amino-terminal domain bears an LxxLL motif favouring nuclear accumulation, and another region encompassing the A box inhibiting nuclear localization. In contrast to a BTG1 mutant exclusively localized in the cytoplasm, transient expression of a mutant displaying a nuclear localization enhanced myoblasts withdrawal from the cell cycle and terminal differentiation, thus mimicking the myogenic influence of BTG1. In conclusion, several regions of BTG1 are implicated in its cellular localization, and BTG1 myogenic activity is induced at the nuclear level.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.

T-targets: clues to understanding the functions of T-box proteins

Members of the T-box gene family have been identified in both vertebrates and invertebrates, where they play key roles in the regulation of embryonic development, and particularly in morphogenesis and the assignment of cell fate. T-box proteins act as transcription factors which regulate the expression of downstream effector genes. This review focuses on the identification of T-box target genes and the basis of T-box functional specificity.

T-box genes in development: from hydra to humans

The T-box gene family was uncovered less than a decade ago but has been recognized as important in controlling many and varied aspects of development in metazoans from hydra to humans. Extensive screening and database searching has revealed several subfamilies of genes with orthologs in species as diverse as Caenorhabditis elegans and humans. The defining feature of the family is a conserved sequence coding for a DNA-binding motif known as the T-box, named after the first-discovered T-box gene, T or Brachyury. Although several T-box proteins have been shown to function as transcriptional regulators, to date only a handful of downstream target genes have been discovered. Similarly, little is known about regulation of the T-box genes themselves. Although not limited to the embryo, expression of T-box genes is characteristically seen in dynamic and highly specific patterns in many tissues and organs during embryogenesis and organogenesis. The essential role of several T-box genes has been demonstrated by the developmental phenotypes of mutant animals.

Differential DNA binding and transcription modulation by three T-box proteins, T, TBX1 and TBX2

T-box genes encode a family of phylogenetically conserved DNA-binding proteins that regulate gene expression during embryogenesis. While the developmental importance of many T-box genes has been well documented, little is known about how family members differ in their DNA binding properties and ability to modulate transcription. Here we show that although TBX1, TBX2 and the Xenopus T protein (Xbra) share only 50-60% identity within their DNA-binding domains they can bind the same DNA sequence in vitro. However, the proteins differ in three important respects. While TBX1 protein binds a palindromic T oligonucleotide as a dimer, as had been previously reported for Xbra, TBX2 appears to bind the same DNA sequence as a monomer. Also, T protein/DNA complexes are stabilized in vitro by the addition of specific antibodies, whereas TBX2/DNA complexes are not stabilized by antibodies. Most importantly, TBX2 represses while Xbra activates transcription of the same chimeric reporter plasmid. TBX1, although capable of binding to the chimeric promoter, has no effect on transcription. Thus, while the DNA binding domains of T-box proteins share substantial homology, these proteins differ in both their DNA binding and transcriptional modulation properties. These results suggest that the various T-box proteins, while highly conserved, likely use different mechanisms to modulate transcription and may have different targets in vivo.

Xwnt11 and the regulation of gastrulation in Xenopus

The molecular basis of gastrulation is poorly understood. In this paper we address this problem by taking advantage of the observation that the transcription activator Brachyury is essential for gastrulation movements in Xenopus and mouse embryos. We infer from this observation that amongst the target genes of Brachyury are some that are involved in the regulation of gastrulation. In the course of a screen for Brachyury targets we identified Xwnt11. Use of a dominant-negative Xwntll construct confirms that signalling by this class of Wnts is essential for normal gastrulation movements, and further investigation suggests that Xwntll signals not through the canonical Wnt signalling pathway involving GSK-3 and beta-catenin but through another route, which may require small GTPases such as Rho and Rac. Future work will concentrate on elucidating the Xwnt11 signal transduction pathway and on investigating its influence on cell shape and polarity during Xenopus gastrulation.