During erythroid differentiation, β-globin gene expression is regulated by the locus control region (LCR). The transcription factor NF-E2p18/MafK binds within this region and is essential for β-globin expression in murine erythroleukemia (MEL) cells. Here we use the isotope-coded affinity tag (ICAT) technique of quantitative mass spectrometry to compare proteins interacting with NF-E2p18/MafK during differentiation. Our results define MafK as a 'dual-function' molecule that shifts from a repressive to an activating mode during erythroid differentiation. The exchange of MafK dimerization partner from Bach1 to NF-E2p45 is a key step in the switch from the repressed to the active state. This shift is associated with changes in the interaction of MafK with corepressors and co-activators. Thus, our results suggest that in addition to its role as a cis-acting activator of β-globin gene expression in differentiated erythroid cells, the LCR also promotes an active repression of β-globin transcription in committed cells before terminal differentiation.