Ta. Nicolson et al., A TRUNCATED FORM OF THE PHO80 CYCLIN REDIRECTS THE PHO85 KINASE TO DISRUPT VACUOLE INHERITANCE IN SACCHAROMYCES-CEREVISIAE, The Journal of cell biology, 130(4), 1995, pp. 835-845
Partitioning of the vacuole during cell division in Saccharomyces cere
visiae begins during early S phase and ends in late G2 phase before th
e yeast nucleus migrates into the bud neck. We have isolated and chara
cterized a new mutant, vac5-1, which is defective in vacuole segregati
on. Cells with the vac5-1 mutation can form large buds without vacuole
s. The VAC5 gene was cloned and is identical to PHO80. PHO80 encodes a
cyclin which acts in a complex with a cdc-like kinase, PHO85, as a ne
gative regulator of two transcription factors (PHO2 and PHO4) that gov
ern the expression of metabolic phosphatases. The vacuole inheritance
defect in vac5-1 cells is dependent on the presence of the Pho85 kinas
e and its targets Pho4p and Pho2p. As with other alleles of PHO80, pho
sphatase levels are elevated in vac5-1 mutants. A suppressor, the COOH
-terminal half of the Gal11 transcription factor, rescues the vac5-1 p
henotype of defective vacuole inheritance without altering the vac5-1
phenotype of elevated phosphatase levels. In addition, neither maximal
nor minimal levels of expression of the inducible ''PHO'' system phos
phatases causes a vacuole inheritance defect. Though vac5-1 is recessi
ve, pho80 Delta or pho85 Delta strains do not show a defect in vacuole
inheritance, suggesting that vac5-1 is not a complete loss-of-functio
n allele. Sequence analysis shows that the vac5-1 allele encodes a tru
ncated form of the Pho80 cyclin and overexpression of vac5-1 in pho80
Delta cells causes a vacuole inheritance defect. We conclude that the
vac5-1 allele directs the Pho85 kinase to regulate, via transcription
factors Pho4 and Pho2, genes that affect vacuole inheritance but which
are not known to be under normal PHO pathway control.