My research has focused on two major areas. First, I have been studying membrane trafficking in mammalian cells. My interests have focused on identifying molecules that regulate the fusion and fission of endocytic vesicles. To address this question I have developed an in vitro fusion assay and successfully identified several molecules involved in late endocytic membrane trafficking. I have focused my efforts biochemically and genetically dissecting the mechanisms of vesicle fusion with lysosomes, as many human disease results from alterations in the delivery of internalized molecules to lysosomes. In particular I have focused on understanding Chediak-Higashi syndrome, a rare autosomal recessive disorder typified by enlarged granules in all cells. Patients with this disease have hypopigmentation, immunologic defects, and neurologic problems. The absence of the Chediak-Higashi protein results in enlarged lysosomes. We have cloned the mouse orthologue (Beige) of the Chediak-Higashi syndrome gene and are examining the role of this protein in vesicle trafficking. These studies have lead to the identification of proteins required for multivesicular body formation, a step required for the degradation of internalized membrane proteins. In collaborations with the Sundquist laboratory we demonstrated that many of the proteins we identified in MVB formation are also required for the budding of HIV. While my initial studies were on membrane trafficking in mammalian cells, I have further expanded my research by utilizing the budding yeast Saccharomyces cerevisiae to study membrane trafficking. In particular, I have made use of the tractable genetics of this organism to identify proteins required not only for the endocytic process but also for the assembly of the vacuolar H+ATPase. We are continuing to use yeast to identify proteins involved in membrane trafficking as a vehicle to the study of these proteins in mammalian cells.

The second area of research I have focused on is iron metabolism. Iron is an element required by virtually all organisms. The transport of iron into cells and within cells requires appropriate membrane trafficking. Our studies on iron metabolism have focused primarily on the mechanisms that regulate iron import/export in the model organism Saccharomyces cerevisiae with the goal of identifying homologues in higher eukaryotes. Malregulation of iron metabolism in humans leads to anemia when iron uptake into the body is inadequate and iron overload disease when iron transport cannot be regulated. Recent work in the lab resulted in a discovery that has lead to a mechanistic understanding of both iron overload and of the anemia of chronic disease. Mammalian iron metabolism is regulated by a hormone, termed hepcidin, which is secreted by the liver in response to inflammation and iron. We discovered that this hormone binds to the iron exporter Ferroportin and induces the internalization and degradation of Ferroportin resulting in retention of iron in cells. Since that award, we discovered the molecular basis of an iron overload disease that is due to mutations in ferroportin. We showed that there are two classes of mutations and these classes can account for patients phenotypes. Our current studies are addressing the mechanism of hepcidin mediated ferroportin internalization.