CRP2 (C EBP-BETA) CONTAINS A BIPARTITE REGULATORY DOMAIN THAT CONTROLS TRANSCRIPTIONAL ACTIVATION, DNA-BINDING AND CELL-SPECIFICITY/

Two members of the C/EBP family of basic region-leucine zipper protein s enriched in the liver, C/EBP (C/EBP alpha) and CRP2 (C/EBP beta), we re previously shown to transactivate the albumin promoter in a cell ty pe-dependent manner. These proteins function efficiently in HepG2 hepa toma cells, but inefficiently in HeLa (epithelial) and L (fibroblastic ) cells. Here we have investigated the mechanism for cell-specific con trol of CRP2 activity. We show that CRP2 contains a negative regulator y region composed of two elements, RD1 and RD2. Deletions of RD2 relie ve the inhibition of CRP2 activity in L cells, but do not affect CRP2 function in HepG2 cells. These deletions also increase the DNA binding activity of CRP2 similar to 3-fold, suggesting that RD2-mediated repr ession of DNA binding activity is responsible for CRP2 inhibition in L cells. The adjacent RD1 element functions independently of RD2 and mo dulates the CRP2 activation domain, which we show to be composed of th ree subdomains that are conserved within the C/EBP protein family. RD1 does not affect cell type specificity, but inhibits the transactivati on potential of GAL4-CRP2 hybrid proteins by 50-fold. These findings s uggest that CRP2 assumes a tightly folded conformation in which the DN A binding and activation domains are masked by interactions with the r egulatory domain and that to function efficiently in HepG2 cells the p rotein must undergo an activation step. We propose that relief of inhi bition conferred by the regulatory domains also accounts for CRP2 acti vation in response to extracellular signals.