UBIQUITINATION OF FULL-LENGTH CYCLIN

Mitotic cyclins are key cell-cycle regulators that are relatively stab le through most of the cell-cycle then rapidly degraded at mitosis. We have detected ubiquitin conjugates of full-length Xenopus cyclin B2 s trongly suggesting that ubiquitination rather than a proteolytic cleav age is the initiating event in cyclin destruction. The highest levels of ubiquitin conjugates correlate with the phase of rapid proteolysis. This result supports previous findings that implicate the ubiquitin s ystem in cyclin proteolysis. However, we also observe cyclin-ubiquitin conjugates in both cytostatic factor arrested and interphase extracts where cyclin is more stable. The physiologic role of ubiquitinated cy clin under these conditions is unclear.