SACCHAROMYCES-CEREVISIAE CDC15 MUTANTS ARRESTED AT A LATE-STAGE IN ANAPHASE ARE RESCUED BY XENOPUS CDNAS ENCODING N-RAS OR A PROTEIN WITH BETA-TRANSDUCIN REPEATS

We have constructed a Xenopus oocyte cDNA library in a Saccharomyces c erevisiae expression vector and used this library to isolate genes tha t can function in yeast cells to suppress the temperature sensitivity defect of the cdc15 mutation. Two maternally expressed Xenopus cDNAs w hich fulfill these conditions have been isolated. One of these clones encodes Xenopus N-ras. In contrast to the yeast RAS genes, Xenopus N-r as rescues the cdc15 mutation. Moreover, overexpression of Xenopus N-r as in S. cerevisiae does not activate the RAS-cyclic AMP (cAMP) pathwa y; rather, it results in decreased levels of intracellular cAMP in bot h mutant cdc15 and wild-type cells. Furthermore, we show that lowering cAMP levels is sufficient to allow cells with a nonfunctional Cdc15 p rotein to complete the mitotic cycle. These results suggest that a key step of the cell cycle is dependent upon a phosphorylation event cata lyzed by cAMP-dependent protein kinase. The second clone, betaTrCP (be ta-transducin repeat-containing protein), encodes a protein of 518 ami no acids that shows significant homology to the beta subunits of G pro teins in its C-terminal half. In this region, betaTrcp is composed of seven beta-transducin repeats. betaTrCP is not a functional homolog of S. cerevisiae CDC20, a cell cycle gene that also contains beta-transd ucin repeats and suppresses the cdc15 mutation.