Maf family proteins are so named because of their structural similarity to the founding member, the oncoprotein v-Maf. The small Maf proteins (MafF, MafG and MafK), as do all family members, include a characteristic basic region linked to a leucine zipper (b-Zip) domain which mediate DNA binding and subunit dimerization respectively. The small Maf proteins form homodimers or heterodimers with other b-Zip proteins present in the cell and bind to Maf recognition elements (MARE) in DNA. Since they lack known transcriptional activation domains, the small Maf proteins function either as obligatory heterodimeric partner molecules with numerous large subunits, discussed below, or alternatively as homo- or heterodimeric transcriptional repressors. The three small Maf proteins are expressed in a number of overlapping tissues, but their expression profiles nonetheless appear to be under meticulous tissue- and developmental stage-specific control. The MARE bears a striking resemblance to the NF-E2 binding sequence, NF-E2 binding sites in the human β-globin locus control region have been directly implicated as integral components in the circuitry required for eliciting changes in chromatin structure that precede globin gene activation. While the NF-E2 DNA sequence has been shown to be important for erythroid-specific gene regulation, a growing list of other genes may also be regulated through the same, or very similar, cis elements in non-erythroid cells. Taken together, these observations argue that comprehensive analysis of the activities of the small Maf proteins may provide a unique perspective for expanding our understanding of transcriptional regulation that can be elicited through interacting transcription factor networks.
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