Component of the AP-1 transcription factor that represses transactivation mediated by the Jun family of proteins. Involved in a variety of transcriptional responses associated with AP-1, such as UV-induced apoptosis, cell differentiation, tumorigenesis and antitumogeneris. Can also function as a repressor by recruiting histone deacetylase 3/HDAC3 to the promoter region of JUN. May control transcription via direct regulation of the modification of histones and the assembly of chromatin (By similarity). Interacts with IRF2BP1 (By similarity). Forms homodimer or heterodimer with JUN, JUNB, JUND, CEBPG and ATF2 thereby inhibiting transactivation by JUN, ATF2 and CEBPG (By similarity). Binds multiple DNA elements such as cAMP-response element (CRE) and TPA response element (TRE) either as homodimer or heterodimer. Phosphorylation of Thr-148 by MAPK8 in response to different stress conditions such as, UV irradiation, oxidatives stress and anisomycin treatments. Polyubiquitinated; probably by IRF2BP1. Belongs to the bZIP family. ATF subfamily.