The ternary complex factor (TCF) subfamily of ETS-domain transcription factors bind with serum response factor (SRF) to the serum response element (SRE) and mediate increased gene expression. The TCF protein Elk-1 is phosphorylated by the JNK and ERK groups of mitogen-activated protein (MAP) kinases causing increased DNA binding, ternary complex formation, and transcriptional activation. Activated SRE-dependent gene expression is induced by JNK in cells treated with interleukin-1 and by ERK after treatment with phorbol ester. The Elk-1 transcription factor therefore integrates MAP kinase signaling pathways in vivo to coordinate biological responses to different extracellular stimuli.

The MADS-box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS-box is a conserved motif found within the DNA-binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum-response factor (SRF) is involved in co-ordinating transcription of the protooncogene c-fos, whilst MCM1 is central to the transcriptional control of cell-type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a sub-family of this class of transcription factors which are key components in muscle-specific gene regulation. Moreover, in plants, MADS-box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS-box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS-box have been defined. The N-terminal half is the major determinant of DNA-binding specificity whilst the C-terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA-binding specificities, from a limited number of genes by heterodimerisation between different MADS-box proteins. The majority of MADS-box proteins bind similar sites based on the consensus sequence CC(A/T)6GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS-box proteins specifically recruit other transcription factors into multi-component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.

The epidermis is a highly organized structure, the integrity of which is central to the protection of an organism. Development and subsequent maintenance of this tissue depends critically on the intricate balance between proliferation and differentiation of a resident stem cell population; however, the signals controlling the proliferation-differentiation switch in vivo remain elusive. Here, we show that mice carrying a homozygous missense mutation in interferon regulatory factor 6 (Irf6), the homolog of the gene mutated in the human congenital disorders Van der Woude syndrome and popliteal pterygium syndrome, have a hyperproliferative epidermis that fails to undergo terminal differentiation, resulting in soft tissue fusions. We further demonstrate that mice that are compound heterozygotes for mutations in Irf6 and the gene encoding the cell cycle regulator protein stratifin (Sfn; also known as 14-3-3sigma) show similar defects of keratinizing epithelia. Our results indicate that Irf6 is a key determinant of the keratinocyte proliferation-differentiation switch and that Irf6 and Sfn interact genetically in this process.

The MADS‐box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS‐box is a conserved motif found within the DNA‐binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum‐response factor (SRF) is involved in co‐ordinating transcription of the proto‐oncogene c‐fos, whilst MCM1 is central to the transcriptional control of cell‐type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a subfamily of this class of transcription factors which are key components in muscle‐specific gene regulation. Moreover, in plants, MADS‐box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS‐box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS‐box have been defined. The N‐terminal half is the major determinant of DNA‐binding specificity whilst the C‐terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA‐binding specificities, from a limited number of genes by heterodimerisation between different MADS‐box proteins. The majority of MADS‐box proteins bind similar sites based on the consensus sequence CC(A/T)6 GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS‐box proteins specifically recruit other transcription factors into multi‐component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.

Transcriptional induction of the c-fos gene in response to epidermal growth factor stimulation is mediated in part by a ternary nucleoprotein complex within the promoter consisting of serum response factor (SRF), p62TCF/Elk-i and the serum response element (SRE). Both A combination of DNase I footprinting, methylation interference, and methylation protection analyses has confirmed the utilization of the SRE both in vitro (27, 36) and in vivo (11). Indeed, genomic footprints indicate that the factors occupying the SRE do not change after c-fos induction (11), suggesting that modification of a preexisting complex occurs. Two nuclear phosphoproteins, the dimeric serum response factor (SRF) and monomeric ternary complex factor (p62TCF), bind cooperatively to the SRE to form a ternary complex (30) and have been shown to be essential for full SRE function (7,18,20,36 (12).The primary amino acid sequences of Elk-1 and SAP-1 are significantly homologous in three regions (Fig. IA) (4). The N-terminal 93 amino acids comprise the A-box which shares homology with the ETS DNA-binding domain (reviewed in references 16 and 44) and is sufficient for Elk-1 to bind autonomously to high-affinity sites (15, 37). The region encompassing a 21-amino-acid segment (B-box) is required for ternary complex formation (15). Studies on SAP-1 indicate that the A-box and B-box play similar roles in this protein (4). The 49-amino-acid C-box, located towards the C terminus of the protein, is the target for phosphorylation by MAP kinase and can act as an autonomous transcriptional activation domain (21).Regions in both SRF and Elk-1 which are essential for the formation of ternary complexes have been identified. The minimal 80-amino-acid MADS box DNA-binding domain of SRF (METcoreSRF) is sufficient for ternary complex formation (33). Extensive mapping indicates that residues in the Cterminal half of this domain are required for ternary complex formation (23,33,34