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, such as NFE2, NFE2L1 and NFE2L2, and recruiting them to specific DNA-binding sites. Small Maf proteins heterodimerize with Fos and may act as competitive repressors of the NFE2L2 transcription factor. Transcription factor, component of erythroid-specific transcription factor NFE2L2. Activates globin gene expression when associated with NFE2L2. May be involved in signal transduction of extracellular H+.