Ubiquitination of full‐length cyclin

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Abstract
Mitotic cyclins are key cell‐cycle regulators that are relatively stable through most of the cell‐cycle then rapidly degraded at mitosis. We have detected ubiquitin conjugates of full‐length Xenopus cyclin B2 strongly suggesting that ubiquitination rather than a proteolytic cleavage 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 system 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 cyclin under these conditions is nuclear.