Transcription factor that activates insulin gene expression (PubMed:15665000). Acts synergistically with NEUROD1/BETA2 and PDX1 (By similarity). Binds the insulin enhancer C1/RIPE3b element (PubMed:15665000). Binds to consensus TRE-type MARE 5'-TGCTGACTCAGCA-3' DNA sequence (By similarity). Forms homodimers. Monomers and dimers are able to bind DNA, but the off-rate is faster for monomers (By similarity). Interacts with NEUROD1 and PDX1 (By similarity). May interact with MAFB, FOS, JUN and PCAF (By similarity). Up-regulated by glucose in Langerhans islets (at protein level). Ubiquitinated, leading to its degradation by the proteasome. Phosphorylated at tyrosines. Belongs to the bZIP family.