Phosphorylation of a low Mr high mobility group protein in Chinese hamster ovary cells

Phosphorylation of high mobility group (HMG) chromatin proteins was studied both in intact Chinese hamster ovary cells (strain CHO-P22) and in vitro conditions using isolated HMG proteins from the same cells and purified protein kinases. Prominent phosphorylation of serine in a low Mr HMG protein designated as HMG P was observed in unsynchronized cells. Of the three protein kinases tested, only nuclear type II protein kinase phosphorylated HMG P in vitro. The phosphorylated amino acid was phosphoserine. Cyclic nucleotide dependent protein kinases did not phosphorylate HMG P but phosphorylated HMG 14 with a preference for cGMP-dependent protein kinase. 32P-labeling of HMG 17 was not observed in intact cells or in vitro.