The cytoskeleton, secreted peptides, and disease models: A multidisciplinary approach to yeast cell biology

Saccharomyces cerevisiae MATalpha cells carrying mutations that disrupt microtubule cytoskeleton function produce reduced alpha-factor halos when plated on lawns of MATalpha cells, indicating reduced production, activity, or persistence of alpha-factor. Consistent with previous results, the secretory pathway of these mutants appears to function normally. Four lines of evidence indicate that this phenomenon is caused by mating type switching in a subpopulation of the colony, an indirect result of the decreased mitotic fidelity of microtubule cytoskeleton mutants.;Calmodulin (Cmd1p) is required in yeast for regulation of the actin (Act1p) cytoskeleton. In order to identify potential calmodulin targets that mediate actin function, we took a three-part genetic approach: comparative phenotypic analysis of cmd1 and act1 mutants, genetic interactions between cmd1 and act1, and genetic interactions between cmd1 and mutations in genes encoding actin-associated proteins. This combined approach confirmed that calmodulin regulates multiple actin-dependent pathways and identified putative Cmd1p targets including multiple myosins and Arp2p.;Mass spectrometry was used to identify a secreted peptide from yeast that is, encoded by the YGP1 gene. Although the strong induction of YGP1 transcription under starvation and stress conditions suggested a role in stress tolerance, strains carrying a deletion allele of this gene exhibited wild-type stress tolerance. By monitoring global gene expression patterns, no evidence was found for a role in intercellular signaling.;Huntington's disease (HD) is caused by expansion of a polyglutamine repeat near the N-terminus of the huntingtin protein. In order to further understand the molecular pathogenesis of huntingtin polyglutamine expansion, we expressed in yeast a disease-length huntingtin fragment. However, unlike in neuronal cells, expression of this protein in yeast did not result in aggregation and was not toxic. Comparison of global transcriptional profiles indicated that huntingtin expression causes differential expression of a small set of genes, implying a subtle effect on yeast physiology and suggesting a potential screen for HD therapeutics.;The combined results of these projects demonstrate the variety of powerful techniques at the disposal of the modern yeast laboratory and illustrate the suitability of S. cerevisiae as an experimental organism for genetics and cell biology, and as a model for human disease.