Purification and characterization of a ubiquitin carrier protein kinase from HeLa cells

Protein ubiquitination plays an important role in ATP-dependent protein turnover, and it also may regulate other cellular events. Covalent attachment of ubiquitin to other proteins is catalyzed by three different enzymes, E1, E2, and E3. We have previously shown that protein ubiquitination can be regulated by phosphorylation. In the present study, we show that 20-kDa E2, an E2 molecular mass isoform, is phosphorylated by a protein kinase from the cytosolic fraction of HeLa cells. This protein kinase was purified by a procedure involving ammonium sulfate precipitation and three column chromatographies (phenyl-Sepharose, Superose gel filtration, and DEAE-Sephacel). Gel-filtration chromatography indicated that the molecular mass of this protein kinase was about 300 kDa. However, SDS/PAGE showed that the purified protein kinase consists of three subunits with molecular masses of 120, 105, and 70 kDa, respectively. The stoichiometry of the phosphorylated 20-kDa E2 isozyme was found to be 0.45 mol of phosphate per mol of protein. The phosphorylation of 20-kDa E2 occurred only at the serine residue. The activity of this protein kinase required the presence of Mg2+; however, the enzyme was inhibited by a high concentration of Mg2+.