NAB1, A COREPRESSOR OF NCFI-A (EGR-1), CONTAINS AN ACTIVE TRANSCRIPTIONAL REPRESSION DOMAIN

Nab proteins constitute an evolutionarily conserved family of corepres sors that specifically interact with and repress transcription mediate d by three members of the NGFI-A (Egr-1, Krox24, zif/268) family of im mediate-early gene transcription factors, which includes NGFI-C, Krox2 0, and Egr3, We explored the mechanism of Nab1 repression and identifi ed structural domains required for Nab1 function. Nab1 does not act by blocking DNA binding or nuclear localization of NGFI-A. In fact, Nab1 repression is not unique to NGFI-A because multiple types of non-NGFI -A activation domains were repressed, as was a heterologous transcript ion factor carrying the NGFI-A RI domain, which is required for Nab1 i nteraction. Additionally, Nab1 tethered directly to DNA repressed cons titutively active promoters. Tethered repression was not dependent on the identity of the basal promoter elements, the presence of a distal enhancer, or the distance separating the binding sites from the promot er. These results suggest that Nab1 repression is not specific to part icular activators and that Nab1 is an active repressor that works by a direct mechanism. We identified a bipartite-like nuclear localization sequence and localized the repression function to the Nab conserved d omain 2 (NCD2), a region found in the carboxy-terminal half of all Nab proteins, Three small regions of homology between Nab1 and previously characterized corepressors, Drl and E1b 55-kDa protein, were identifi ed within NCD2, Replacement mutagenesis of residues conserved between these proteins interfered with Nab1 repression, although Nab1 does not function by the same mechanism as Dr1. The human NAB1 genomic locus w as mapped to chromosome 2q32.3-33.