Since they lack a putative transactivation domain, the small Mafs behave as transcriptional repressors when they dimerize among themselves. However, they seem to serve as transcriptional activators by dimerizing with other (usually larger) basic-zipper proteins and recruiting them to specific DNA-binding sites. Small Maf proteins heterodimerize with Fos and may act as competitive repressors of the NF-E2 transcription factor. Transcription factor, component of erythroid-specific transcription factor NF-E2. May be involved in signal transduction of extracellular H(+) (By similarity). Homodimer or heterodimer. Sumoylation at Lys-14 is required for active transcriptional repression. Belongs to the bZIP family. Maf subfamily.