Novel Functions of the Flippase Drs2 and the Coat Protein COPI in Protein Trafficking

Abstract

Within a eukaryotic cell is a steady stream of vesicles budding from and fusing to different membranes to generate distinct organelles. The formation and movement of these vesicles is a complicated and interconnected process orchestrated by many proteins. Many components of the trafficking machinery have been identified, but there remains much to be elucidated as new proteins and new functions for known proteins are continuously identified. The goal of my dissertation research has been to identify novel roles of two proteins involved in protein trafficking: the flippase Drs2 and the protein coat COPI. The first protein, Drs2, is known to support a variety of coat-dependent trafficking pathways stemming from the trans-Golgi network (TGN) and early endosomes. I have determined that Drs2 also has a novel role in coat-independent, domain-mediated protein sorting into exocytic vesicles at the TGN. The primary function of Drs2 in this trafficking pathway appears to be to control the activity of the oxysterol binding protein homologue Kes1/Osh4 to regulate ergosterol homeostasis at the TGN. The second protein, COPI, is known to support retrograde trafficking from the Golgi. However, a small subset of COPI has long been known to localize to early endosomes, but no clear function has been attributed to early endosome-associated COPI. Described in this work is a novel function of COPI in ubiquitin-dependent protein trafficking from early endosomes to the TGN.