Subnuclear partitioning and functional regulation of the Pit-1 transcription factor

Subnuclear compartmentation is postulated to play an important role in many aspects of nuclear metabolism. To directly test an application of this mod el to transcription factor function, we examined the subnuclear partitionin g behavior of Pit-1, a tissue-specific, POU-class transactivator. Biochemic al and in situ assays indicate the nuclear pool of Pit-1 is normally divide d between two compartments: the majority being differentially soluble in de tergent, and a significant insoluble fraction (similar to 20%) bound to the nuclear matrix. Examination of Pit-1 deletion mutants and chimeric fusions reveal the highly conserved 66 amino acid POU-specific domain contains a n ecessary and sufficient nuclear matrix targeting signal. The nuclear partit ioning behavior of several natural or engineered point mutations of Pit-1 w as also examined. Surprisingly, the inactive point mutants were completely matrix-bound, irrespective of their ability to bind Pit-1 specific DNA. The se results suggest that dynamic partitioning of Pit-1 is a component of its normal transactivator function that takes place upon the insoluble nuclear substructure where transcription occurs. (C) 1999 Wiley-Liss, Inc.