Repression and coactivation of CCAAT/enhancer-binding protein ε by sumoylation and protein inhibitor of activated STATx proteins

CCAAT/enhancer-binding protein epsilon (C/EBP epsilon) is a neutrophil-specific transcription factor whose activity is controlled by juxtaposed activating and regulatory domains. We previously determined that the function of the major regulatory domain (RD1) in C/EBP epsilon was dependent on the integrity of a five-amino acid motif that was identical to the recognition site for members of the small ubiquitin-like modifier (SUMO) family of ubiquitin-related proteins. We show here that the SUMO attachment site (the regulatory domain motif) is necessary and sufficient both for the intrinsic inhibitory function of RD1 and for coactivation by PIASx alpha and PIASx beta, two members of the protein inhibitor of activated STAT (PIAS) family of SUMO E3 ligases. PIASx beta was a more potent coactivator than PIASx alpha of both full-length C/EBP epsilon and fusion proteins containing the N-terminal portion of C/EBP epsilon, whereas PIASx alpha was more active on fusion proteins containing a heterologous activation domain. Two modes of coactivation were observed, one that was dependent on the integrity of the RING finger (RF) domain and was shared by both PIASx alpha and PIASx beta and a second mode that was independent of the RF and was only observed with PIASx beta. Sumoylation of C/EBP epsilon was enhanced by coexpression of PIASx alpha, suggesting that this modification is associated with the enhanced activity of the target protein. These results suggest that a complex interplay of accessory factors, including SUMO and PIAS proteins, modulates the activity of C/EBP epsilon.