Phosphorylation of the high-mobility-group-like protein P1 by casein kinase-2

The nuclear protein P1 (molecular mass 53 kDa), found in all mammalian cell types and tissues so far tested, is an excellent substrate for casein kinase-2. The number of phosphate groups on P1 is 20-30/molecule; the phosphorylation sites are distributed throughout the molecule. The phosphate is present as serine phosphate and possibly threonine phosphate. Proteolytic digestion with Staphylococcus aureus V8 protease of 32P-labelled P1 both in vivo and in vitro revealed that casein kinase-2 may be one of the kinases responsible for the phosphorylation in vivo.