FUNCTIONAL DOMAINS OF THE NUCLEAR RECEPTOR HEPATOCYTE NUCLEAR FACTOR-4

The hepatocyte nuclear factor 4 (HNF-4) is a member of the nuclear rec eptor superfamily and participates in the regulation of several genes involved in diverse metabolic pathways and developmental processes. To date, the functional domains of this nuclear receptor have not been i dentified, and it is not known whether its transcriptional activity is regulated by a ligand or other signals. In this report, we show that HNF-4 contains two transactivation domains, designated AF-1 and AF-2, which activate transcription in a cell type-independent manner. AF-1 c onsists of the extreme N-terminal 24 amino acids and functions as a co nstitutive autonomous activator of transcription. This short transacti vator belongs to the class of acidic activators, and it is predicted t o adopt an amphipathic alpha-helical structure. In contrast, the AF-2 transactivator is complex, spanning the 128-366 region of HNF-4, and i t cannot be further dissected without impairing activity. The 360-366 region of HNF-4 contains a motif that is highly conserved among transc riptionally active nuclear receptors, and it is essential for AF-2 act ivity, but it is not necessary for dimerization and DNA binding of HNF -4. Thus, HNF-4 deletion mutants lacking the 361-465 region bind effic iently to DNA as homo- and heterodimers and behave as dominant negativ e mutants. Remarkably, the full transactivation potential of AF-2 is i nhibited by the region spanning residues 371-465 (region F). The inhib itory effect of region F on the HNF-4 AF-2 activity is a unique featur e among members of the nuclear receptor superfamily, and we propose th at it defines a distinct regulatory mechanism of transcriptional activ ation by HNF-4.