Abstract
The fungal vacuole is an acidic organelle that is involved in a variety of physiological processes, such as protein turnover, ion and pH homeostasis and osmoregulation. The function of the vacuole largely depends on vesicle transport providing the organelle with enzymes and substrates. The process of vesicle transportation has been studied best in Saccharomyces cerevisiae, where several proteins that are crucial for intracellular vesicle sorting have been identified. One such protein is Pep3 (Vps18). In pep3 mutants vacuole function and vacuole morphology are affected. We cloned the gene for a potential homolog of Pep3 from the filamentous fungus Aspergillus nidulans. The gene, digA (for dichotomous growth), was identified in a screen for nuclear migration mutants. A. nidulans digA encodes a protein of 108.3 kDa, which includes a 122-amino acid clathrin repeat motif, two short coiled-coil regions, and a RING finger Zn-binding motif at the C-terminus. All three sequence motifs suggest interaction of DigA with other proteins. DigA is 25% identical to a homolog from Schizosaccharomyces pombe, 23% to a protein from human, 21% to the product of the Drosophila melanogaster gene deep orange (dor) and 18% to S. cerevisiae Pep3 (Vps18). We localized DigA as a HA-epitope-tagged protein in the cytoplasm of A. nidulans vegetative cells, by secondary immunofluorescence. A digA mutant displays a pleiotropic phenotype with clustered mitochondria, clustered nuclei and a defect in polarization of the actin cytoskeleton. These results suggest that vacuolar functions are required for organelle positioning and polarized growth.