Protein ubiquitination has been suggested as a marking process for ATP- dependent protein degradation and other regulatory roles in cells. The enzymes involved in ubiquitination are: E1, the ubiquitin-activating enzyme; E2 (E2 exists in multiple molecular weight forms), the ubiquitin carrier enzyme which undergoes transthiolation reactions with E1- ubiquitin and transfers the ubiquitin to its target proteins or to the ligase, E3, for ubiquitinating its protein substrates. To investigate the relationship between the phosphorylation cascade and the ubiquitination cascade, we showed previously that E1 and E2 32KDa (an isoform of E2), can be phosphorylated by protein kinase C and by protein tyrosine kinase, respectively. In both cases, phosphorylation leads to activation. Last year, we discovered that E2 20kDa, one of the E2 isozymes, can be extensively phosphorylated by a novel protein kinase from a cytosolic fraction of HeLa cells. Recently, we partially purified this enzyme from HeLa cells. Results of our study showed that this enzyme is not related to protein tyrosine kinase, protein kinase C, or CAMP-dependent protein kinase. This enzyme was not found in brain cells. This suggests that E2 20kDa enzyme may be involved in the metabolism of the nucleus or cell division.
