Produced in E. coli.

Ubiquitinylation of proteins constitutes an important cellular mechanism for targeting short-lived proteins for degradation by the 26S proteasome. Three classes of enzymes are involved in the conjugation of ubiquitin to proteins. E1, the ubiquitin activating enzyme, activates ubiquitin through the ATP-dependent formation of a high-energy thiol ester bond between the carboxyl terminus of ubiquitin and the active-site cysteine within E1. This E1-activated ubiquitin is transferred to a cysteine residue of an E2, or ubiquitin-conjugating enzyme (UbC). E2 enzymes, either by themselves or in conjunction with E3 enzymes (ubiquitin ligases), then transfer ubiquitin to target proteins forming stable isopeptide bonds resulting in multi-ubiquitin chain formation. It is the diverse combinations of E2-E3 complexes which are thought to define substrate specificity. UbcH3 is a class II enzyme (molecular weight 32kDa) involved in the control of cell cycle and DNA replication being homologous to Cdc34 from Saccharomyces cerevisiae. The human CDC34 gene is expressed in multiple cell lines as a unique species and is highly conserved in higher eukaryotes. The gene has been localised to chromosome 19p13.3. Functional and physical characterisation has been undertaken. It has been demonstrated that the protein itself is a substrate for both ubiquitinylation and phosphorylation. Few in vivo targets of UbcH3 are currently known although investigation continues.