RECRUITMENT OF HEPATOCYTE NUCLEAR FACTOR 4 INTO SPECIFIC INTRANUCLEARCOMPARTMENTS DEPENDS ON TYROSINE PHOSPHORYLATION THAT AFFECTS ITS DNA-BINDING AND TRANSACTIVATION POTENTIAL

Hepatocyte nuclear factor 4 (HNF-4) is a prominent member of the famil y of liver-enriched transcription factors, playing a role in the expre ssion of a large number of liver-specific genes, We report here that H NF-4 is a phosphoprotein and that phosphorylation at tyrosine residue( s) is important for its DNA-binding activity and, consequently, for it s transactivation potential both in cell-free systems and in cultured cells, Tyrosine phosphorylation did not affect the transport of HNF-4 from the cytoplasm to the nucleus but had a dramatic effect on its sub nuclear localization. HNF-4 was concentrated in distinct nuclear compa rtments, as evidenced by in situ immunofluorescence and electron micro scopy, This compartmentalization disappeared when tyrosine phosphoryla tion was inhibited by genistein, The correlation between the intranucl ear distribution of HNF-4 and its ability to activate endogenous targe t genes demonstrates a phosphorylation signal-dependent pathway in the regulation of transcription factor activity.