Co-deletion of the MSB3 and MSB4 coding regions affects bipolar budding and perturbs the organization of the actin cytoskeleton.

The proteins Msb3p (Ynl293p) and Msb4p (Yol112p) of the yeast Saccharomyces cerevisiae are very similar in sequence and share a highly conserved domain called TBC. To characterize the cellular functions of these proteins, we constructed single and double yeast mutants by disrupting the MSB3 gene, the MSB4 gene, or both. Co-deletion of the MSB3 and MSB4 coding regions caused growth inhibition in the presence of 10 mM caffeine and 4% dimethyl sulphoxide (DMSO), increased the sensitivity of the yeast strain to latrunculin-A, produced a random budding pattern in diploid cells, and affected the organization of the actin cytoskeleton. Caffeine sensitivity is often associated with defects in mitogen-activated protein (MAP) kinase pathways, highly conserved mechanisms mediating transduction of external signals. The biological effect of DMSO in S. cerevisiae is unclear. The msb3 msb4 mutant's increased sensitivity to latrunculin-A suggests that the double mutation causes destabilization of the actin cytoskeleton. Microscopic observations confirmed this: in haploid and diploid msb3 msb4 mutant cells, polymerized actin was delocalized from the budding sites. Complementation studies suggested that MSB3 and MSB4 encode overlapping activities in the yeast cells. We thus propose that both Msb3p and Msb4p act in budding site selection, probably via their involvement in the organization of the actin cytoskeleton. Copyright 2000 John Wiley & Sons, Ltd.