hMAF, a small human transcription factor that heterodimerizes specifically with Nrf1 and Nrf2

A 1.6-kilobase pair full-length cDNA encoding a transcription factor homologous to the Maf family of proteins was isolated by screening a K562 cDNA library with the NFE2 tandem repeat probe derived from the globin locus control region. The protein, which was designated hMAF, contains a basic DNA binding domain and an extended leucine zipper but lacks any recognizable activation domain. Expressed in vitro, the hMAF protein is able to homodimerize in solution and bandshift the NFE2 tandem repeat probe. In addition to homodimers, hMAF can also form high affinity heterodimers with two members of the NFE2/CNC-bZip family (Nrf1 and Nrf2) but not with a third family member, p45-NFE2. Although hMAF/hMAF homodimers and hMAF/Nrf1 and hMAF/Nrf2 heterodimers bind to the same NFE2 site, they exert functionally opposite effects on the activity of a linked gamma-globin gene. In fact, whereas all hMAF/CNC-bZip heterodimers stimulate the activity of a gamma-promoter reporter construct in K562 cells, the association into homodimers that is induced by overexpressing hMAF inhibits the activity of the same construct. Thus variations in the expression of hMAF may account for the modulation in the activity of the genes that bear NFE2 recognition sites.