On page 707, Topper et al. report that an excess of Cdc34, a ubiquitin ligase, prevents the association of CENP-E with kinetochores. A reduction in the levels of kinetochore-localized CENP-E occurs normally during the progression from prometaphase to metaphase (Hoffman, D.B., et al. 2001. Mol. Biol. Cell. 12:1995–2009), as kinetochores reduce in size while switching from microtubule-capture to microtubule-maintenance mode. Overexpression of Cdc34 may be accelerating and exaggerating this reduction process to the extent that kinetochores cannot set up proper connections to the spindle. The result is a failure to get beyond prometaphase.
Progression from prometaphase to metaphase is also inhibited by the recently discovered protein Emi1. One simple model—that excess Cdc34 triggers premature destruction of Emi1and thus premature loss of CENP-E—does not appear to be tenable. Although a ubiquitin ligase is exerting the (possibly indirect) effect on CENP-E, Topper et al. show that ubiquitin-mediated proteolysis is not required. In their hands, excess Cdc34 plus a proteasome inhibitor still results in CENP-E loss and prometaphase arrest. This is yet another case where ubiquitin conjugation is acting as a regulator of protein function rather than as a marker for degradation.
Any link from the overexpression result to a physiologically relevant process remains speculative. But with new antibodies to Cdc34, Topper et al. hope to address the true function of Cdc34 in mitosis. ▪