A role for Saccharomyces cerevisiae Cul8 ubiquitin ligase in proper anaphase progression

We have undertaken a study of the yeast cullin family members Cul3 and Cul8, as little is known about their biochemical and physiological functions. We demonstrate that these cullins are associated in vivo with ubiquitin ligase activity. We show that Cul3 and Cul8 are functionally distinct from Cdc53 and do not interact with ySkp1, suggesting that they target substrates by Skp1- and possibly F- box protein- independent mechanisms. Whereas null mutants of CUL3 appear normal, yeast cells lacking CUL8 have a slower growth rate and are delayed in their progress through anaphase. The anaphase delay phenotype can be complemented by ectopic expression of Cul8 but not by any other yeast or human cullins, nor by a cul8 mutant deficient in binding to RING finger protein Roc1. Deletion of the RAD9 gene suppressed the anaphase delay phenotype of cul8 Delta, suggesting that loss of Cul8 function may compromise genomic integrity. These results indicate that in addition to the anaphase promoting complex, mitotic progression may involve another E3 ubiquitin ligase mediated by Cul8 protein.