Members of the OSBP Homologue (Osh) Protein Family in S. cerevisiae Require Specific Lipids to Regulate Polarized Exocytosis

The Oxysterol Binding Proteins (OSBPs) are an evolutionarily conserved protein family in eukaryotes implicated in many cellular functions. One function, supported by a large body of in vitro data is lipid transfer between membranes. In addition, in vivo changes in lipid distribution among cellular membranes have been shown to be OSBP dependent, further suggesting a role for OSBPs in lipid homeostasis. Despite these observations, data supporting a role for OSBPs as dedicated lipid transfer proteins in vivo is less developed. It is not known whether lipid binding and transfer is the main function of the OSBP family or whether lipid binding and transfer also regulates OSBP activity in other processes. Beyond lipid transfer, in the yeast, S. cerevisiae, OSBPs have been found necessary for support of polarized exocytosis, the exocytosis of cellular materials to the site of polarized growth. Because OSBP activity supports polarized exocytosis, I focused on two questions regarding OSBP function: i) is lipid binding by the yeast OSBP Osh4p required for polarized exocytosis and ii) what processes within polarized exocytosis does Osh4p support. In this study, I establish that lipid binding by Osh4p, is required for polarized exocytosis. I also determined that lipid binding by Osh4p is required for exocytic vesicle docking at the plasma membrane and proposed a two-step model of Osh4p function in vesicle docking. These results establish support of polarized exocytosis as the first essential cellular function that has been shown to require lipid binding by a yeast OSBP in vivo. These findings describe an essential function for lipid binding by yeast OSBPs which, while not excluding a role for yeast OSBPs as dedicated lipid transfer proteins, do highlight that OSBPs support other essential cellular functions in a lipid binding dependent manner.