the ap-2 family of transcription factors ( ensembl family ensf00000001105 ) consists in humans and mice of five members , ap-2 , ap-2 , ap-2 , ap-2 and ap-2 ; frogs and fish have some of these proteins , and homologs are also known in invertebrates . the chromosomal locations and accession numbers of the family all mammalian ap-2 proteins except ap-2 are encoded by seven exons and share a characteristic domain structure ( reviewed in ; for ap-2 see   and for ap-2 see ) . orthologs show a similarity between 60 and 99% at the amino - acid level , whereas paralogs show a similarity between 56 and 78% . analysis of the phylogenetic tree ( figure 1 ) reveals that the vertebrate ap-2 proteins are grouped together and are divided into five groups . the single xenopus ap-2 is most closely related to mammalian ap-2 proteins . as the genes ap-2 and ap-2 are found on the same chromosome in chickens , rodents and humans ( table 1 ) , it is likely that they are the result of an internal duplication . according to the phylogenetic tree , ap-2 genes appear to have separated from the rest of the family early in the vertebrate clade and to have evolved separately ( figure 1 ) . a blast search of the puffer fish fugu rubripes fourth genome assembly database   suggests that there are orthologs of ap-2 , ap-2 , ap-2 and ap-2 but not ap-2 genes in bony fish , although only orthologs of ap-2 and ap-2 have been found in zebrafish . in the genome of the protochordate ciona intestinalis a single ap-2 gene has been predicted ; the phylogenetic tree shows that the protein evolved before the split of the ap-2 , ap-2 , ap-2 and ap-2 proteins , with the highest sequence similarity with the ap-2 group , suggesting that ap-2 might be most similar to the ancestor of ap-2 proteins . this hypothesis is further supported by the conserved epithelial expression patterns of murine ap-2 , xenopus ap-2   and the amphioxus and lamprey ap-2 genes . as expected , the two caenorhabditis elegans and the single drosophila melanogaster ap-2 proteins show the weakest phylogenetic relationship with vertebrate and protochordate ap-2 transcription factors ; they form an outgroup to the other ap-2 family members ( figure 1 ) . given that no ap-2 gene has been identified in yeast , the family probably originated late in evolution and expanded considerably in the vertebrates . all ap-2 proteins share a highly conserved helix - span - helix dimerization motif at the carboxyl terminus , followed by a central basic region and a less conserved domain rich in proline and glutamine at the amino terminus ( figure 2 ) . the proteins are able to form hetero- as well as homodimers . the helix - span - helix motif together with the basic region mediates dna binding , and the proline- and glutamine - rich region is responsible for transactivation . ap-2 has been shown to bind to the palindromic consensus sequence 5'-gccn3ggc-3 ' , found in various cellular and viral enhancers ( reviewed in ) ; a binding - site selection assay in vitro also revealed the additional binding motifs 5'-gccn3ggc-3 ' , 5'-gccn4ggc-3 ' and 5'-gccn3/4ggg-3 ' . other binding sites differing from these sequence motifs , for example , the sv40 enhancer element 5'-ccccaggc-3 ' , indicate that ap-2 proteins may bind to a range of g / c - rich elements with variable affinities . target genes with ap-2-binding sites in their promoter sequences are involved in biological processes such as cell growth and differentiation and include , for example , those encoding insulin - like growth factor binding protein 5 ( igf - bp5 ) with the binding site 5'-gccaggggc-3 ' , prothymosin- ( 5'-gccggtgggc-3 ' )   and the estrogen receptor ( 5'-gcctgcgggg-3 ' ) . most ap-2 proteins have a py motif ( xppxy ) and other highly conserved critical residues in the transactivation domain ; by contrast , the py motif is missing in ap-2 but the amino- and carboxy - terminal ends of the core sequence of the transactivation domain are still conserved . in addition , the binding affinity of ap-2 to conserved ap-2-binding sites is much lower than that of other ap-2 proteins . this suggests that ap-2 might transactivate genes in vivo by a different mechanism from that used by other ap-2 proteins , probably through interactions with a novel group of coactivators and through a different affinity for ap-2-binding sites . alternatively , ap-2 might act as a negative regulator , inhibiting or modulating the transactivation capability or dna - binding affinity of the other ap-2 family members . ap-2 transcription factors are localized predominantly in the nucleus , where they bind to target sequences and regulate transcription of target genes . ap-2 proteins have also been shown to interfere with other signal transduction pathways ; for example , it has been proposed that they modulate the pathway downstream of the developmental signaling molecule wnt by associating with the adenomatous polyposis coli ( apc ) tumor suppressor protein in the nucleus . the activity of ap-2 proteins can be controlled at multiple levels : their transactivation potential , their dna binding , their subcellular localization [ 17 - 19 ] and their degradation   can all be modified . mechanisms of regulation include post - translational modifications , such as protein kinase a - mediated phosphorylation , sumoylation   and redox regulation , as well as physical interaction with various proteins ( see table 3 for a comprehensive list ) . interacting proteins either modulate the activity of ap-2 proteins or are influenced in their function by binding to ap-2 proteins . the tissue distribution and developmental functions of ap-2 transcription factors have been studied extensively in several species . drosophila ap-2 ( dap-2 ) is expressed in the maxillary segment and neural structures during embryogenesis , and in the central nervous system ( cns ) and the leg , antennal and labial imaginal disks during larval development . mutation of the dap-2 gene leads to defects in proboscis development and leg - joint formation . the multiple overlapping and diverging expression patterns of ap-2 family proteins suggest that , following the expansion of the family during vertebrate evolution , redundant and non - redundant functions of the individual ap-2 family members evolved . although the single ap-2 protein in the cephalochordate amphioxus is expressed mainly in non - neuronal ectoderm , in the lamprey , a primitive vertebrate , ap-2 has co - opted a second expression domain , the neural crest . the single ap-2 homolog described so far in xenopus is expressed in the epidermis and neural crest and has been shown to be critical for the development of these structures [ 7,31 - 33 ] . in zebrafish , the two ap-2 family members , tfap2a and tfap2b , are coexpressed in the neural tube , the ectoderm and the pronephric ducts of the developing kidney , but only tfap2a is expressed in neural crest cells . positional cloning revealed that the zebrafish point mutants named mont blanc   and lockjaw   encode tfap2a ; the mutant animals display impaired development of neural - crest derivatives , such as the facial skeleton , the peripheral nervous system and pigment cells . it is also interesting to note that ap-2 proteins are expressed in the primitive ectoderm of both invertebrates and vertebrates , suggesting an evolutionarily conserved role for the family in the formation of this tissue . in mice , three of the five ap-2 family members ( ap-2 , ap-2 and ap-2 ) are coexpressed in neural - crest cells , the peripheral nervous system , facial and limb mesenchyme , various epithelia of the developing embryo and the extraembryonic trophectoderm [ 2,39 - 41 ] . ap-2 expression is restricted mainly to the developing heart , cns and retina , whereas ap-2 expression is detected in cells of the olfactory bulb . despite the overlapping expression patterns of ap-2 , ap-2 and ap-2 , disruption of these ap-2 genes reveals non - redundant roles during development . mutation of ap-2 predominantly affects the cranial neural crest and the limb mesenchyme , leading to disturbances of facial and limb development in a manner reminiscent of the defects described in dap2 mutant flies . ap-2 and ap-2 , on the other hand , are essential for kidney development   or placentation of the embryo , respectively . in humans , mutations generating a dominant negative allele of ap-2 have been shown to be the cause of char syndrome ( online mendelian inheritance in man ( omim ) i d 169100 ) ; the hallmarks of this syndrome are patent ductus arteriosus ( abnormal persistence of a normal fetal heart structure after birth ) with facial dysmorphism and abnormal fifth digits . comparing all mutant phenotypes , it can be seen that loss of ap-2 transcription factor activity generally impairs proliferation and induces premature differentiation and/or apoptosis in various cell types during development . this conclusion is further substantiated by results from a screen for ap-2 target genes   and supported by gain - of - function studies in xenopus and mice . as uncontrolled proliferation leads to malignancies , ap-2 transcription factors are not only implicated in normal development , but also seem to be involved in cellular neoplasia , and enhanced ap-2 levels have been reported in various types of cancer [ 19,54 - 60 ] . in a murine breast - cancer model , tumor progression thus , ap-2 proteins can be viewed as gatekeepers controlling the balance between proliferation and differentiation during embryogenesis . the lethal phenotypes of the ap-2 mutants generated so far have precluded an analysis of the roles of ap-2 transcription factors in adult tissues . we and others are currently exploiting the power of conditional mouse mutants to overcome these restrictions [ 61 - 63 ] . such approaches will not only shed light on normal ap-2 functions but will probably also lead to unique insights into human disorders . complementary approaches currently include the identification of ap-2 target genes ; this might give a better understanding of developmental disturbances and pave the way to novel treatment options . at the molecular level , one major challenge will be the identification of specific ap-2 homo- or hetero - dimeric complexes bound to a particular promoter and the identification of the specific properties of each complex with respect to gene regulation . also , the signaling pathways responsible for induction of ap-2 genes are currently under investigation . a cross - species comparison of the various ap-2 promoters may give insights into the evolution of tissue specificity and help to determine important enhancer elements . moreover , given that cpg islands are present in ap-2 promoters , epigenetic regulation such as dna methylation also needs to be considered . ap-2 transcription factors are currently being studied extensively in human cancer , and they may be of diagnostic value , as has been demonstrated for mammary or testicular carcinoma . it is tempting to speculate that ap-2 transcription factors might not only be molecular markers for certain types of cancer , but could also be causally involved in their etiologies and would therefore represent a potential target for therapeutic intervention . we thank roland dosch and michael pankratz for critical reading of the manuscript . this work was supported by funding from the deutsche forschungsgemeinschaft ( # 503/6 and 503/7 ) that was awarded to h.s . amino - acid sequence alignments were performed using clustalw implemented in sequence data explorer of the mega3 software . the phylogenetic tree was created using the neighbor - joining method ( gaps setting : pairwise deletion ; distance method : number of differences ) . numbers at selected nodes indicate the percentage frequencies of branch association on the basis of 1,000 bootstrap repetitions . species : caenorhabditis elegans ( nematode ) ; ciona intestinalis ( sea squirt ) ; drosophila melanogaster ( fruit fly ) ; danio rerio ( zebrafish ) ; gallus gallus ( chicken ) ; homo sapiens ( human ) ; mus musculus ( mouse ) ; pan troglodytes ( chimpanzee ) ; rattus norvegicus ( rat ) ; xenopus laevis and xenopus tropicalis ( frog ) . a schematic representation of the protein structure of an ap-2 dimer , showing the proline- and glutamine ( p / q)-rich transactivation domain ( 89 amino acids , red ) , the py motif within this domain ( 5 amino acids , green ) , the basic domain ( 20 amino acids , yellow ) and the helix - span - helix motif ( 131 amino acids , blue ) . the helix - span - helix motif is responsible for dimerization of the proteins and mediates dna binding together with the basic domain . chromosomal locations of ap-2 genes from selected species * the ap-2 genes of c. elegans and d. melanogaster are not orthologous to any of the five mammalian genes . accession numbers for ap-2 proteins from selected species * the ap-2 genes of c. elegans , d. melanogaster and c. intestinalis are not orthologous to any of the five mammalian genes . proteins that physically interact with ap-2 transcription factors * abbreviations : dbd , dna - binding domain ; dd , dimerization domain ; n.d . , not determined . it is currently not entirely clear whether rb binds ap-2 only via the amino terminus , or whether the dna - binding domain is also necessary .
ap-2 transcription factors are involved in cell - type - specific stimulation of proliferation and the suppression of terminal differentiation during embryonic development . 
 members of the family are found in mammals ( with five different proteins in human and mice ) , frogs and fish , as well as protochordates , insects and nematodes .