| The secretory pathway in eukaryotic cells is responsible for the exocytic and endocytic pathways, as well as for the delivery of proteins and lipids from their sites of synthesis to the cell surface and the different compartments of the endomembrane system. Transport between these compartments is largely mediated by vesicles and tubules that bud from a donor membrane and selectively target to and fuse with an acceptor membrane. Structural, genetic, biochemical and morphological studies in yeast and mammalian cells have provided a wealth of information about the protein machinery involved in vesicular transport between the ER and Golgi compartments. However, little is known about the function of lipids in these events. This dissertation investigates the modulation of membrane properties on budding and fusion of COPII vesicles.;Got1p was identified as a multicopy suppressor of genes required for COPII vesicle budding. Further characterization revealed that Got1p is efficiently packaged into COPII vesicles and cycles between the ER and Golgi compartments. Bioinformatics sequence-profiling tools indicate that Got1p contains a lipase-related domain, and preliminary analysis of whole cell phospholipid profiles indicates alterations in acyl chain properties in got1Delta cells compared to wild-type. This suggests that Got1p influences ER membrane composition to facilitate COPII vesicle budding.;A role for PI(4)P in COPII vesicle fusion was determined in a biochemical screen to identify specific lipid requirements for ER to Golgi transport. PI(4)P is the most abundant cellular phosphoinositide and it is most prevalent at the Golgi apparatus. Consistently, PI(4)P function was required at the Golgi apparatus for COPII vesicle fusion. Further characterization revealed that PI(4)P function could be placed downstream of vesicle tethering and most likely in concert with SNARE-mediated membrane fusion. Together, these findings provide evidence for a novel role for PI(4)P in regulation of COPII vesicle fusion with the Golgi apparatus. Models are proposed to explain the role of PI(4)P in this process. |
