CCAAT/enhancer binding protein alpha assembles essential cooperating factors in common subnuclear domains

The transcription factor CCAAT/enhancer binding protein alpha (C/EBP alpha) is the DNA binding subunit of a multiprotein complex that regulates the pi tuitary-specific GH promoter. C/EBP alpha is absent from the GHFT1-5 pituit ary progenitor cell line in which ectopic C/EBP alpha expression leads to a ctivation of the otherwise dormant GH promoter. Transcriptional regulatory complexes are commonly envisaged as assembling from components that evenly diffuse throughout the nucleoplasm. We show that C/EBP alpha, expressed in GHFT1-5 cells as a fusion with color variants of the green fluorescent prot ein (GFP), concentrated specifically at peri-centromeric chromosomal domain s. Although we found the CREB-binding protein (CBP) to activate C/EBP alpha -dependent transcription, CBP was absent from the pericentromeric chromati n. C/EBP alpha expression was accompanied by the translocation of endogenou s and ectopically expressed CBP to pericentromeric chromatin. The intranucl ear recruitment of CBP required the transcriptional activation domains of C /EBP alpha. C/EBP alpha also caused GFP-tagged TATA binding protein (TBP) t o relocate to the Hoechst-stained domains. The altered intranuclear distrib ution of critical coregulatory factors defines complexes formed upon C/EBP alpha expression. It also identifies an organizational activity, which we l abel "intranuclear marshaling," that may regulate gene expression by determ ining the cooperative and antagonistic interactions available at specific n uclear sites.