The division cycle of yeast a cells is inhibited by the peptide pheromone, Alpha factor. After prolonged exposure to Alpha-factor, a cells become "desensitized" to the pheromone and resume cell division. The proposed research examines the role of the Alpha-factor receptor in the control of these processes. A genetic approach will be applied to this problem. Mutant a cells that are defective eq. for receptor activity will be isolated and characterized. Two classes of mutants are anticipated--mutants unresponsive to Alpha-factor and mutants super-sensitive to Alpha-factor. Existing methods will be used to examine the number of receptor sites, the ligand affinity, and the thermostability of the receptors in mutant a cells. New methods will be developed to test for cellular internalization of Alpha-factor; electrophoretic techniques will be developed to monitor receptor structure in the mutant and wild-type cells. Alpha-factor-induced alterations of the structure and number of receptors may reflect the intracellular signal that leads to the arrest of cell division; alternatively, these structural changes may represent inactivation of receptors as part of the desensitization process. The phenotype of mutants that block receptor modification should resolve these two possibilities; the signalling model predicts an unresponsive phenotype, whereas the desensitization model predicts a super-sensitive phenotype. The goal of this project is to define the genes which control various aspects of receptor activity (i.e. receptor structural genes, regulators of receptor synthesis, regulators of receptor activity, and the generation of intracellular signals). The long-term objective is to understand in detail the relationship of receptor activity to the control of the cell cycle. Enzymological and cytological characterization of the gene products defined in this present study will facilitate this objective. Detailed understanding of this very basic process in yeast should provide information for a more general understanding of receptor action and cell division control in other organisms as well. The control of cell division by polypeptide hormones is of obvious significance for understanding the biology of cancer cells.