359 Lorenza Trabalzini and Saverio Francesco Retta (eds.), Ras Signaling: Methods and Protocols, Methods in Molecular Biology, vol. 1120, DOI 10.1007/978-1-62703-791-4_23, © Springer Science+Business Media, LLC 2014 Chapter 23 Yeast as a Model for Ras Signalling Renata Tisi, Fiorella Belotti, and Enzo Martegani Abstract For centuries yeast species have been popular hosts for classical biotechnology processes, such as baking, brewing, and wine making, and more recently for recombinant proteins production, thanks to the advan- tages of unicellular organisms (i.e., ease of genetic manipulation and rapid growth) together with the abil- ity to perform eukaryotic posttranslational modifications. Moreover, yeast cells have been used for few decades as a tool for identifying the genes and pathways involved in basic cellular processes such as the cell cycle, aging, and stress response. In the budding yeast S. cerevisiae the Ras/cAMP/PKA pathway is directly involved in the regulation of metabolism, cell growth, stress resistance, and proliferation in response to the availability of nutrients and in the adaptation to glucose, controlling cytosolic cAMP levels and consequently the cAMP-dependent protein kinase (PKA) activity. Moreover, Ras signalling has been identified in several pathogenic yeasts as a key controller for viru- lence, due to its involvement in yeast morphogenesis. Nowadays, yeasts are still useful for Ras-like proteins investigation, both as model organisms and as a test tube to study variants of heterologous Ras-like proteins. Key words Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, Cyclic AMP, Protein kinase A, Filamentous growth Abbreviations cAMP Cyclic AMP ER Endoplasmic reticulum GAP GTPase activating protein GDP Guanosine diphosphate GEF Guanine nucleotide exchange factor GPCR G-Protein coupled receptor GTP Guanosine triphosphate PKA Protein kinase A ROS Reactive oxygen species VPS Vacuolar protein-sorting