Role of PGRMC1 in intracellular sterol transport.

Cholesterol is an essential component of mammalian cells. Cellular cholesterol levels are tightly regulated by various, well-characterized, homeostatic mechanisms that reside primarily in the endoplasmic reticulum. As the majority of cellular cholesterol resides in the plasma membrane and membrane derived vesicles, regulation of cholesterol homeostasis depends upon the transport of sterols from the plasma membrane to the endoplasmic reticulum. There are also other specific cellular membranes and structures that must be enriched with cholesterol for proper function. The regulation of sterol distribution between various membranes within the cell, and mechanisms behind specific intracellular sterol transport remain poorly understood. Elucidation of these processes remains key to our understanding of cholesterol metabolism and will be important for diagnosis and treatment of lipid related diseases. In this dissertation I describe my work with a putative progesterone binding protein, PGRMCl, and detail a novel role for this protein in intracellular sterol transport. Additionally, I report a previously uncharacterized protein function associated with PGRMCl, that of catalyzing the incorporation of cholesterol into membranes. In Chapter 1 I review what is currently known about regulation of cholesterol homeostasis and intracellular sterol transport. In Chapter 2 I describe my initial work to characterize PGRMCl, its ligand, and its cellular function, and suggest a possible role in sterol metabolism. In Chapter 3 I describe the development of a large scale, high throughput, Synthetic Genetic Array screen designed to uncover genetic interactions between yeast PGRMCl and other genes, and report the results of that screen which further reveal its biologic function. In Chapter 4 I describe an in vitro assay developed to study PGRMCl ligand binding which provided important insights into PGRMCl's role in intracellular cholesterol transport. In Chapter 5 I describe how my work with PGRMCl helps define the role of an entire family of putative sterol binding proteins, and uncovers a possible drug target for the treatment of diseases of cholesterol metabolism.