Identification of a novel ubiquitin-conjugating enzyme involved in mitotic cyclin degradation

Background: The destruction of cyclin B is required for exit from mitosis, and is mediated by the ubiquitin pathway. Recently, a 20S complex, termed the anaphase-promoting complex (APC) or the cyclosome, has been genetically and biochemically identified as the cyclin-specific ubiquitin ligase (E3). In addition, a ubiquitin-conjugating enzyme (E2), UBC4, was shown to be involved in cyclin ubiquitination in Xenopus egg extracts. Another E2 activity, designated UBCx, can independently support cyclin ubiquitination in Xenopus. A similar activity (E2-C) has also been observed in clams. However, the molecular identity of Xenopus UBCx or clam E2-C has not been established. Results: We have purified and cloned Xenopus UBCx. Sequence comparisons with known E2s reveal that UBCx is a novel ubiquitin-conjugating enzyme. Purified recombinant UBCx is sufficient to complement purified APC and E1 in destruction box-dependent cyclin ubiquitination. UBCx and UBC4 are active in a similar concentration range and with similar kinetics. At saturating enzyme concentrations, UBCx converts twice as much substrate into ubiquitin conjugates, but generates conjugates of lower molecular mass than UBC4. Conclusions: UBCx is a novel ubiquitin-conjugating enzyme involved in cyclin ubiquitination in Xenopus. Like UBC4, ubiquitination catalyzed by UBCx is dependent on both the destruction box and the APC, suggesting that these E2s function through a similar mechanism. However, as the patterns of conjugates generated by these E2s are distinct, these enzymes may play different roles in promoting cyclin proteolysis in mitosis.