Bni1p Regulates Microtubule-Dependent Nuclear Migration through the Actin Cytoskeleton in Saccharomyces cerevisiae

TheRHO1 gene encodes a yeast homolog of the mammalian RhoA protein. Rho1p is localized to the growth sites and is required for bud formation. We have recently shown that Bni1p is one of the potential downstream target molecules of Rho1p. TheBNI1 gene is implicated in cytokinesis and the establishment of cell polarity inSaccharomyces cerevisiae but is not essential for cell viability. In this study, we screened for mutations that were synthetically lethal in combination with abni1 mutation and isolated two genes. They were the previously identifiedPAC1 andNIP100 genes, both of which are implicated in nuclear migration inS. cerevisiae . Pac1p is a homolog of human LIS1, which is required for brain development, whereas Nip100p is a homolog of rat p150Glued , a component of the dynein-activated dynactin complex. Disruption ofBNI1 in either thepac1 ornip100 mutant resulted in an enhanced defect in nuclear migration, leading to the formation of binucleate mother cells. Thearp1 bni1 mutant showed a synthetic lethal phenotype while thecin8 bni1 mutant did not, suggesting that Bni1p functions in a kinesin pathway but not in the dynein pathway. Cells of thepac1 bni1 andnip100 bni1 mutants exhibited a random distribution of cortical actin patches. Cells of thepac1 act1-4 mutant showed temperature-sensitive growth and a nuclear migration defect. These results indicate that Bni1p regulates microtubule-dependent nuclear migration through the actin cytoskeleton. Bni1p lacking the Rho-binding region did not suppress thepac1 bni1 growth defect, suggesting a requirement for the Rho1p-Bni1p interaction in microtubule function.