Elevating the level of Cdc34/Ubc3 ubiquitin-conjugating enzyme in mitosis inhibits association of CENP-E with kinetochores and blocks the metaphase alignment of chromosomes

Cdc34/Ubc3 is a ubiquitin-conjugating enzyme that functions in targeting pr oteins for proteasome-mediated degradation at the G1 to S cell cycle transi tion. Elevation of Cdc34 protein levels by microinjection of bacterially ex pressed Cdc34 into mammalian cells at prophase inhibited chromosome congres sion to the metaphase plate with many chromosomes remaining near the spindl e poles. Chromosome condensation and nuclear envelope breakdown occurred no rmally, and chromosomes showed oscillatory movements along mitotic spindle microtubules. Most injected cells arrested in a prometaphase-like state. Ki netochores, even those of chromosomes that failed to congress, possessed th e normal trilaminar plate ultrastructure. The elevation of Cdc34 protein le vels in early mitosis selectively blocked centromere protein E (CENP-E), a mitotic kinesin, from associating with kinetochores. Other proteins, includ ing two CENP-E-associated proteins, BubR1 and phospho-p42/p44 mitogen-activ ated protein kinase, and mitotic centromere-associated kinesin, cytoplasmic dynein, Cdc20, and Mad2, all exhibited normal localization to kinetochores . Proteasome inhibitors did not affect the prometaphase arrest induced by C dc34 injection. These studies suggest that CENP-E targeting to kinetochores is regulated by ubiquitylation not involving proteasome-mediated degradati on.