The central vacuolar system of eucaryotic cells ia an interlocking membrane system composed of distinct organelles (including ER, Golgi apparatus, secretory vesicles, secretory granules, plasma membrane, endosomes and lysosomes) and selective transport pathways. Proper functioning of this system is vital for regulating surface expression and secretion of proteins. Using the fungal metabolite brefeldin A (BFA), which dramatically alters both the distribution and flow of membrane through the central vacuolar system, this group has focused on how organelles maintain their structure and identity, and how membrane trafficking between organelles is regulated. These questions have been approached from three different areas: (1) Understanding the biochemical basis of BFA action. We have found that BFA appears to affect the Golgi apparatus by acting on specific membrane targets that regulate the assembly of cytosolic coat proteins onto the cytoplasmic face of this organelle. Altered interaction of the coats with the organelle leads to organelle disassembly, tubule formation and mixing of organelle components within a defined """"""""homotypic"""""""" membrane system. The biochemistry of two BFA sensitive coat proteins, betaCOP and ARF are under investigation. (2) Identification of BFA's binding site(s). We have carried out studies on the structure and function of BFA using radiolabeled BFA, and through the identification of drugs and BFA analogues that antagonize and/or alter the effects of BFA within the cell. (3) Characterization of the distribution and flow of membrane within the vacuolar system. We have found that BFA causes the various compartments of the vacuolar system to collapse into new steady states which produce isolated but functioning new organelle units. Traffic within but not between these units continues in the presence of BFA. In summary, our studies with BFA provide new insights about the properties of sets of organelle-specific coat proteins and presents a framework for relating the biochemical regulation of membrane transport to the structure and maintenance or organelles. We have continued studies on the mechanisms of retention of proteins within the early organelles of the vacuolar system.
