A novel, highly phosphorylated protein, of the high-mobility group type, present in a variety of proliferating and non-proliferating mammalian cells.

The present work describes a perchloric-acid-soluble high-mobility-group (HMG)-like protein present in HeLa and Ehrlich ascites cells, rat and calf liver. The protein is designated P1 and has, depending on the source, a molecular mass 48-53 kDa and an amino acid composition which, like the HMG proteins, is characterized by a high content of acidic and basic residues and of proline. The protein contains about 10 mol serine/100 mol amino acid residues, is highly phosphorylated and has, in contrast to the known HMG proteins, an acidic isoelectric point of 5.0. An estimate suggests that protein P1 in HeLa interphase cells contains 25-30 residues of phosphate. Like HMG 1 and 2 it is distributed between the nucleus and the cytoplasm. In HeLa metaphase cells P1 is further modified, resulting in an increase in apparent molecular mass from 53 kDa to 56 kDa.