Function and Role of Synaptotagmin Proteins in Yeast

In the genome of the budding yeast, Saccharomyces cerevisiae, a family of proteins is encoded that resembles the Ca2+ sensing synaptotagmins present in neuronal cells of higher eukaryotes. The synaptotagmins couple a rise in cytosolic Ca2+ with synaptic vesicle formation and release during the process of vesicular trafficking and exocytosis. This process is important in higher organisms for neurotransmitter release and signal transmission. At least a dozen synaptotagmin isoforms exist, some that are expressed outside the nervous system, suggesting that the synaptotagmin proteins might regulate membrane trafficking in a variety of different cell types. The synaptotagmin proteins in higher eukaryotes contain two Ca2+ binding domains. However, in yeast there are three Ca2+ binding domains present in each of the three synaptotagmin homologs and the function and role of these proteins and domains is yet to be fully understood. We have studied the three yeast synaptotagmin homologues by examining mutants of them; double and triple mutants of these homologues have been constructed through a homologous recombination method and their growth properties compared to wild type strains under normal and high Ca2+ conditions. By microscopy, we have observed if these mutants have cell-cell fusion defects in the mating process as this step of the yeast life cycle appears to be highly dependent on vesicle trafficking, perhaps in a Ca2+ dependent manner. The responses of wild type and mutant strains to alpha-factor pheromone has also been observed as this step is involves cell surface receptor dependent cell-cell signaling