O-linked N-acetylglucosamine modification on CCAAT enhancer-binding protein beta: role during adipocyte differentiation.

CCAAT enhancer-binding protein (C/EBP)beta is a basic leucine zipper transcription factor family member, and can be phosphorylated, acetylated, and sumoylated. C/EBPbeta undergoes sequential phosphorylation during 3T3-L1 adipocyte differentiation. Phosphorylation on Thr(188) by MAPK or cyclin A/cdk2 primes the phosphorylations on Ser(184)/Thr(179) by GSK3beta, and these phosphorylations are required for the acquisition of DNA binding activity of C/EBPbeta. Here we show that C/EBPbeta is modified by O-GlcNAc, a dynamic single sugar modification found on nucleocytoplasmic proteins. The GlcNAcylation sites are Ser(180) and Ser(181), which are in the regulation domain and are very close to the phosphorylation sites (Thr(188), Ser(184), and Thr(179)) required for the gain of DNA binding activity. Both in vitro and ex vivo experiments demonstrate that GlcNAcylation on Ser(180) and Ser(181) prevents phosphorylation on Thr(188), Ser(184), and Thr(179), as indicated by the decreased relative phosphorylation and DNA binding activity of C/EBPbeta delayed the adipocyte differentiation program. Mutation of both Ser(180) and Ser(181) to Ala significantly increase the transcriptional activity of C/EBPbeta. These data suggest that GlcNAcylation regulates both the phosphorylation and DNA binding activity of C/EBPbeta.