A TRUNCATED FORM OF THE PHO80 CYCLIN OF SACCHAROMYCES-CEREVISIAE INDUCES EXPRESSION OF A SMALL CYTOSOLIC FACTOR WHICH INHIBITS VACUOLE INHERITANCE

Vacuoles project streams of vesicles and membranous tubules into the y east bud where they fuse, founding the daughter cell organelle, vac5-1 , which encodes a truncated form of the Pho80 cyclin, inhibits normal vacuole inheritance. An in vitro inheritance assay which measures the fusion of vacuoles serves as a model for several steps of this process , We find that cytosol isolated from the vac5-1 mutant is unable to pr omote the fusion of wild-type vacuoles in the in vitro assay, Wild-typ e vacuoles are irreversibly inactivated in a time- and temperature-dep endent manner if preincubated with vac5-1 cytosol and ATP, suggesting the presence of a soluble inhibitory factor. When mixed with wild-type cytosol, vac5-1 cytosol inhibits the activity of wild-type cytosol. v ac5-1 cytosol treated with trypsin or papain is still able to inhibit the activity of wild-type cytosol. Partial fractionation of vac5-1 cyt osol reveals that the protein fraction (G25 void volume) can promote f usion if wild-type small molecules are included in the fusion reaction . In contrast, the vac5-1 small-molecule fraction retains the full abi lity to inhibit fusion. Thus, the vad-l allele of PHO80 induces the sy nthesis of a small molecule that is an inhibitor of vacuole inheritanc e.