Phosphorylation of high mobility group proteins 14 and 17 by nuclear protein kinase NII in rat O6 glioma cells

High mobility group (HMG) proteins 14 and 17 of rat C₆ glioma cells are phosphorylated $\text{in}\text{vivo}$ on both serine and threonine. In HMG 14 about 60% of the total ³²P]phosphate was identified as phosphoserine and 40% as phosphothreonine. In HMG 17, there was 88% phosphoserine and 12% phosphothreonine. Glioma cell nuclear protein kinase NII phosphorylates HMG 14 and 17 $\text{in}\text{vitro}$ on serine as well as threonine and the relative percentages of ³²P]phosphoamino acid are similar to those seen $\text{in}\text{vivo}$. Nuclear protein kinase NI and the type I and II cAMP-dependent protein kinases exhibit only minor phosphorylating activity towards HMG 14 and 17. We conclude that nuclear protein kinase NII is responsible for the phosphorylation of HMG 14 and 17 $\text{in}\text{vivo}$.