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 cerevisiae expression vector and used this library to isolate genes that can function in yeast cells to suppress the temperature sensitive [corrected] defect of the cdc15 mutation. Two maternally expressed Xenopus cDNAs which fulfill these conditions have been isolated. One of these clones encodes Xenopus N-ras. In contrast to the yeast RAS genes, Xenopus N-ras rescues the cdc15 mutation. Moreover, overexpression of Xenopus N-ras in S. cerevisiae does not activate the RAS-cyclic AMP (cAMP) pathway; rather, it results in decreased levels of intracellular cAMP in both mutant cdc15 and wild-type cells. Furthermore, we show that lowering cAMP levels is sufficient to allow cells with a nonfunctional Cdc15 protein to complete the mitotic cycle. These results suggest that a key step of the cell cycle is dependent upon a phosphorylation event catalyzed by cAMP-dependent protein kinase. The second clone, beta TrCP (beta-transducin repeat-containing protein), encodes a protein of 518 amino acids that shows significant homology to the beta subunits of G proteins in its C-terminal half. In this region, beta Trcp is composed of seven beta-transducin repeats. beta TrCP is not a functional homolog of S. cerevisiae CDC20, a cell cycle gene that also contains beta-transducin repeats and suppresses the cdc15 mutation.