ReviewEts ternary complex transcription factors
Introduction
The ets genes are fundamentally important. They have roles in differentiation, development, transformation and cellular proliferation, and their structure has been conserved during evolution [reviewed in Sharrocks, 2001, Oikawa and Yamada, 2003]. The prototype of the family is c-ets-1, the cellular progenitor of oncogenic v-ets. v-ets was first discovered in 1983 as a fusion protein expressed by the avian retrovirus E26, which causes mixed erythroid and myeloid leukemias Leprince et al., 1983, Klempnauer and Bishop, 1984. The viral genome contains truncated gag and env genes, and has two oncogenes v-myb and v-ets (for viral-E-twenty-six). The cloning of the cellular homologue, and of the related gene c-ets-2 in chicken, led to the idea that there was a family of ets genes (Boulukos et al., 1988). The ternary complex factors (TCFs) are probably the most studied Ets factors. Historically, the first TCF was described as a fraction in HeLa cell nuclear extracts that forms a ternary complex with the serum response factor (SRF) on the c-fos serum response element (SRE). This novel component was called p62 due to its 62 000 Da molecular weight (Shaw et al., 1989). The ternary complex factor p62 was then shown to be homologous to Elk-1 (Ets-like transcription factor) that had been cloned previously Rao et al., 1989, Hipskind et al., 1991. Two other related cDNAs were also identified, Sap-1 (SRF accessory protein 1)/Elk-4 (Dalton and Treisman, 1992) and Net/Erp/Sap-2/Elk-3 Giovane et al., 1994, Lopez et al., 1994, Nozaki et al., 1996. The three TCFs, Net, Elk-1 and Sap-1 were shown to be distinct gene products with different chromosomal localizations in both the mouse and human genomes (Giovane et al., 1995). A second human elk-1 chromosomal location was first described as the elk-2 locus (Rao et al., 1989), but has finally been shown to correspond to two elk-1-related processed pseudogenes embedded in the IgH locus (Harindranath et al., 1998). TCF like factors have been identified in Xenopus, (Nentwich et al., 2001), Goldfish (Goldman et al., 1998), Zebrafish (Brown et al., 1999) and Caenorhabditis elegans (Beitel et al., 1995).
Section snippets
Domain structure of the TCFs
The TCFs share four similar regions, A–D (Fig. 1), which were identified by sequence comparison between Elk-1 and Sap-1 (Dalton and Treisman, 1992). The N-terminal A domain corresponds to the Ets DNA binding domain. It has also been demonstrated to act as a transcriptional inhibitor in Elk-1 by recruiting corepressors (Yang et al., 2001) and DNA binding inhibitors (Yates et al., 1999), and to contain a nuclear export signal in Net (Ducret et al., 1999). The B domain interacts with the MADS box
TCF/SRE interaction
Ets proteins share a highly conserved DNA binding domain and as such are used as a paradigm for studying DNA binding specificity. The Ets domain is located at the N-terminus of the TCFs (Fig. 1). Ets domains are about 85 amino acids long (Fig. 2). They bind specifically to Ets binding sites (EBSs) composed of a purine-rich central core sequence 5′-GGA(A/T)-3′ and surrounding more variable sequences that contribute to the binding specificity of individual proteins. The TCFs form complexes with
Transcriptional repression
In the absence of activation of MAP kinase signalling, Net, and to a lesser extent Elk-1, have been shown to be repressors of transcription Giovane et al., 1994, Yang et al., 2001. In NIH3T3 cells, Net is a powerful repressor, Elk-1 activates transcription, whereas Sap-1 is inactive Giovane et al., 1994, Maira et al., 1996. Activation of ERK signalling by Ras expression converts Net to an activator of transcription. Repression is thought to maintain promoters in an inactive state in the absence
Other TCF/protein interactions
In addition to SRF, HLH proteins, corepressors and coactivators complexes, many other proteins have been described to interact with the TCFs. These proteins provide complexity and specificity to the TCFs.
TCF target genes
TCFs can bind autonomously to Ets motifs, suggesting they may regulate genes lacking SREs. Elk-1 and Net interact with Pax5 on a composite element of the mb-1 promoter (Fitzsimmons et al., 1996). Elk-1 binds to an Ets motif site in the TNFα promoter (Tsai et al., 2000). Recently, a cis-acting element in the Cctq gene promoter (θ subunit of chaperonin containing t-complex polypeptide 1) has been shown to be specifically recognized by the three TCFs in the absence of SRF (Yamazaki et al., 2003).
The biological roles of the TCFs
Despite the large number of studies of the TCFs, their physiological roles are still poorly understood. Physiological studies require the generation and analysis of animals, particularly mice, in which the TCFs have been mutated. These studies are in progress and some data is beginning to emerge. Sap-1 mutant animals display immunological defects, including impairment in the generation of single-positive T cells, and a phenotype similar to Castleman's disease (abnormal proliferation of lymphoid
Conclusion/perspectives
The TCFs play a central role in the cellular responses to major conserved signalling pathways, and their study has provided important insights into the control of gene expression. There are many important directions for further investigation. At the cellular level, more studies are required of the signalling pathways that are physiologically relevant for the activation as well as the subsequent inactivation of the TCFs. The spatial and temporal components of this regulation need to be resolved,
Acknowledgements
We would like to thank Dr. I. Davidson for critical reading of the review, and Drs. A. Nordheim and R. Treisman for their permission to cite unpublished data. We also thank members of the Wasylyk laboratory for stimulating discussions, and in particular Hong Zheng, Koji Nakade and Paola Criqi-Filipe for unpublished data cited in this review. GB and CG are recipients of post-graduate fellowships from the Ministère de la Recherche et Technologie. Our laboratory is supported by BioAvenir (Aventis,
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