Characterization of DAP1/YPL170W: the Saccharomyces cerevisiae Membrane Associated Progesterone Receptor (MAPR) Homologue Christopher D. Banna 135 pages Directed by Dr. Jung H. Choi MAPRs (Membrane Associated Progesterone Receptors) from several sources have been isolated, studied and minimally characterized in mammalian systems, yet the specific role of this protein family has not been fully determined. Early worked characterized this protein family as a type of steroid binding protein, unrelated to the classical nuclear receptors, and linked this family to non-genomic cellular responses. The MAPR homologues as a group have been suggested to play widely varying roles from axon guidance and neuronal formation, to steroid hydroxylation, to influencing reproductive behavior. Their specific role has not yet been clearly demonstrated in any organism. There is some debate as to whether MAPRs do indeed bind steroid compounds, but there is clear evidence this family of proteins is involved in steroid perception. Recent work has begun to link a specific member of the MAPR family, IZAg from rat, to steroid metabolism/production, specifically, in the hydroxylation step of glucocorticoid production from progesterone. In the yeast Saccharomyces cerevisiae, the MAPR homologue is DAP1. Preliminary work on haploid strains demonstrated several phenotypes associated with the DAP1 deletion mutant, most notably an altered sterol profile. Previous characterization of diploid homozygous mutant strain has shown a differential sensitivity to alcohol, an altered sterol profile, and a strong yeast two-hybrid interaction with Ypr118wp; methylthioribose-1-phosphate isomerase. Work in this study link the localization of Dap1p to lipid particles and on the ER, both sites of sterol synthesis. The sterol profiles of the control strain and the dap1Ġdeletion mutant strain were examined in detail. The most notable difference was the presence of an additional sterol compound associated with the deletion mutant strain. The structure of this compound does not correspond to normal sterols in the ergosterol biosynthetic pathway, but does correspond to structure of sterols in so-called alternate aberrant sterol pathways. The data presented in this study demonstrates that Dap1p was involved in sterol processing, although its specific role is unknown. Two possible scenarios are proposed; one where Dap1p is involved in regulating the flux of sterols from one internal membrane to another, and another where Dap1p is involved in aberrant sterol pathways.