The activation function-1 of hepatocyte nuclear factor-4 is an acidic activator that mediates interactions through bulky hydrophobic residues

The hepatocyte nuclear factor-4 (HNF-4) contains two transcription activati on domains. One domain, activation function-1 (AF-1), consists of the extre me N-terminal 24 amino acids and functions as a constitutive autonomous act ivator of transcription. This short transactivator belongs to the class of acidic activators, and it is predicted to adopt an amphipathic alpha -helic al structure. Transcriptional analysis of sequential point mutations of the negatively charged residues (Asp and Glu) revealed a stepwise decrease in activity, while mutation of all acidic residues resulted in complete loss o f transcriptional activity. Mutations of aromatic and hydrophobic amino aci ds surrounding the negatively charged residues had a much more profound eff ect than mutations of acidic amino acids, since even a single mutation of t hese residues resulted in a dramatic decrease in transactivation, thus demo nstrating the importance of hydrophobic residues in AF-1 activity. Like oth er acidic activators, the AF-1 of HNF-4 binds the transcription factor IIB and the TATA-binding protein directly in vitro. In addition, the cAMP-respo nse-element-binding-protein, a transcriptional adapter involved in the tran sactivation of a plethora of transcription factors, interacts with the AF-1 of HNF-4 and co-operates in the process of transactivation by HNF-4. The d ifferent protein targets of AF-1 suggest that the AF-1 of HNF-4 may be invo lved in recruiting both general transcription factors and chromatin remodel ling proteins during activation of gene expression.