Identification of novel in vivo phosphorylation sites in high mobility group N1 protein from the MCF-7 human breast cancer cells.

High mobility group N1 (HMGN1) protein is a member of nonhistone chromosomal proteins that binds more strongly with nucleosomes than with DNA. Here we report the identification of the sites of in vivo phosphorylation of HMGN1 isolated from the MCF-7 human breast cancer cells. Our results showed that four serine residues, i.e., Ser6, Ser85, Ser88, and Ser98, can be phosphorylated in this protein. To our knowledge, this is the first demonstration that each of the three serine residues in the acidic C-terminal region of human HMGN1 can be phosphorylated. The additional negative charge resulting from the phosphorylation of the C-terminal serine residues is expected to modulate the interaction between HMGN1 and other proteins, which may enhance transcription and facilitate other cellular functions. In addition, the phosphorylation of HMGN1 at Ser85, which precedes Pro86, might play an important role in cellular signaling.