Complementation of Homologous Targets

1. S. cerevisiae-Based Screens for Immunosuppressants

Immunosuppressants, such as cyclosporin and FK506, inhibit T cell activation and have made tissue and organ transplantation a reality. These drugs were first identified because of their antifungal activity, and their mechanism of immuno-suppression long remained a mystery. The mechanism of action of cyclosporin and FK506 was determined using S. cerevisiae [20]. Cyclosporin and FK506 bind proteins with peptidyl prolyl isomerase activity (called FK506 binding proteins and cyclosporin binding proteins) and forms a complex that inhibits the calcium-calmodulin phosphatase, calcineurin. The similarity between the activity of the immunosuppressants in T cells and yeast is shown in Fig. 4. In the presence of these immunosuppressants, yeast does not grow and divide after exposure to the yeast pheromone, a-factor. The mechanism of action of Rapamycin, another immunosuppressant, has been elucidated by studies in yeast [21]. The target of rapamycin is known to be TOR, which is involved in phosphorylating protein phosphatase 2A. These phenomena can be used as the basis of screens for new immunosuppressants.

2. Expression of Seven Transmembrane, G-Protein Coupled Receptors in Microbial Systems

Many currently used drug targets are seven transmembrane, G-protein coupled receptors (GPCRs), such as the serotonin receptors, dopamine receptors, and adrenergic receptors. Antagonists have been identified by ligand-displacement assays using mammalian cells or their membrane preparations. Agonists have generally been identified by functional assays. Microbial systems have been adapted to identify agonists and antagonists of GPCRs. The mating factor receptor in S. cerevisiae is called Ste2 and is similar in structure to mammalian GPCRs. Mammalian GPCR can be used to replace Ste2, so that the GPCR can signal

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