NAB proteins as bidirectional coregulators of immediate-early transcription

Early growth response (Egr) genes are induced in mammalian cells following a wide range of stimuli. As zinc finger transcriptional activators that function as immediate-early genes, they are well positioned to influence the phenotypic outcomes of extracellular signals. Previous investigations into the function of Egr1/NGFI-A led to the discovery of the NAB (NGFI- A Binding) family of transcriptional coregulators. NAB proteins are highly conserved throughout evolution and were shown previously to repress Egr-dependent transcriptional activation through an interaction with a domain present in three of the four known Egr proteins.; The work presented here constitutes an investigation both of the interaction between Egr and NAB proteins and of the effects of this interaction upon transcription of Egr target genes. Expression patterns of Egr and NAB proteins are shown to overlap in mammalian tissues and cell lines, with NAB proteins being induced by some of the same stimuli that upregulate Egr protein expression. Moreover, a specific protein-protein interaction between Egr and NAB proteins is demonstrated using a coimmunoprecipitation assay, The possible effect of phosphorylation upon this interaction is explored, and a site within the NAB-binding domain of EgrI is demonstrated to be a target of MAP kinases in vitro, but not in vivo. Although NAB proteins were previously characterized as corepressors, investigations into the effect of NAB proteins upon Egr-dependent transcription reveal an unexpected role for NAB proteins as coactivators. For example, NAB proteins potently coactivate Egr-mediated transcription of the luteinizing hormone beta subunit gene, which had previously been established as a physiological target gene for Egr1. The effect of NAB proteins upon Egr-mediated transcription is demonstrated to depend upon both the number and strength of Egr consensus sites within the gene's promoter. These results suggest that NAB proteins are bidirectional coregulators that can differentially effect transcription of Egr target genes in response to an extracellular stimulus.