CK2 phosphorylation of C/EBPδ regulates its transcription factor activity

• The CCAAT/enhancer binding protein C/EBPδ is a new substrate for protein kinase CK2.
• The CK2 phosphorylation site is mapped to serine 57 within the transactivation domain.
• C/EBPδ binds to CK2.
• CK2 phosphorylation of C/EBPδ stimulates transcriptional activity of C/EBPδ.

Protein kinase CK2 plays an essential role in cell viability in lower and higher eukaryotes. As a global regulator it phosphorylates and thereby regulates a broad array of cellular targets including a large number of transcription factors. Here, we have identified the CCAAT/enhancer binding protein δ (C/EBPδ) as a new substrate for CK2. Using point mutants of C/EBPδ the major phosphorylation site for CK2 was mapped to serine 57, which is located within the transactivation domain of C/EBPδ. For proper functioning as a transcription factor C/EBPδ has to be translocated into the nucleus where it forms heterodimers with other members of the C/EBP family of proteins and ATF4. Here, we found that CK2 phosphorylation does neither influence the subcellular localization of C/EBPδ nor its interaction with C/EBPβ, but rather does CK2 phosphorylation modulate the transcriptional activity of C/EBPδ. Moreover, we found that CK2 bound to C/EBPδ, which might help to target CK2 to the transcriptional machinery where it can phosphorylate other transcription factors or co-activators.

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