Identification of the C/EBPα C-terminal tail residues involved in the protein interaction with GABP and their potency in myeloid differentiation of K562 cells

The CCAAT/enhancer-binding protein α (C/EBPα) is the member of a family of related basic leucine zipper (bZIP) transcription factors and is critical for granulopoiesis. We previously demonstrated that C/EBPα interacts with the ETS domain of widely expressed GABPα, which leads to cooperative transcriptional activation of the myeloid-specific promoter for human FCAR encoding the Fc receptor for IgA (FcαR, CD89) in part by facilitating recruitment of C/EBPα to the promoter. The C/EBPα molecule contains transactivation domains (TADs) at its N-terminus and a DNA-binding and dimerization bZIP structure at its C-terminus. We demonstrate here that GABPα interacts with the last 18 residues of the C/EBPα C-terminus beyond the bZIP DNA-binding and dimerizing region. Deletion of this C-terminus resulted in loss of GABPα interaction but not affecting its DNA binding ability, indicating that it is not required for homodimer formation. Moreover, the C-terminus confers the ability to functionally synergize with GABP on a heterologous TAD when fused to the C-terminus of the VP16 TAD. We identified a three-amino acid stretch (amino acids 341-343) that is important for both functional and protein interactions with GABP. Ectopic expression in K562 cells of C/EBPα mutant incapable of interacting with GABPα does not induce expression of granulocytic differentiation markers including CD15, CD11b, GCSF-R and C/EBPε, and does not inhibit proliferation, whereas wild type does. These results demonstrate the functional importance of the C/EBPα C-terminus beyond the bZIP DNA-binding and dimerization region, which may mediate cooperative activation by C/EBPα and GABP of myeloid-specific genes involved in C/EBPα-dependent granulopoiesis.