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Presentation

This group studies the molecular mechanisms of organelle structure and membrane trafficking in the secretory and endocytic pathways, and the mechanisms by which these pathways are misregulated or subverted in human disease. Eukaryotic cells are characterized by their internal membrane-bound compartments (organelles). Membrane trafficking pathways assure communication between these intracellular organelles and between the cell and its external environment. The Golgi apparatus, the central sorting and processing organelle of the cell, is at the intersection of both secretory and endocytic pathways. Our group studies a family of proteins (called Arf GEFs) whose function is required to maintain Golgi structure and which play crucial roles in numerous trafficking pathways. Arf GEFs activate the small G protein Arf1, which acts as a molecular switch to regulate membrane remodeling. Our group focuses primarily on studies of the large Golgi-localized Arf GEFs, GBF1, BIG1 and BIG2. We have shown that the large multi-domain Arf GEFs have multiple interacting partners, and we are studying the assembly of these large complexes on membranes at sites where Arf is to be activated. We are also studying the role of the Arf system, including Arf effectors, in regulation of Golgi structure and in the mechanisms by which proteins are trafficked through the secretory pathway.

Arf Guanine Nucleotide Exchange Factors (GEFs)

 

Localization of Arf GEFs GBF1 and BIG2 in mammalian cells

Enteroviruses subvert the Arf GEF machinery to create membrane structures essential for viral replication. A second major axis of investigation within our group is to elucidate the mechanisms by which viral proteins recruit Arf GEF complexes to membranes to generate the membrane structures required for viral replication. Both poliovirus and coxsackievirus absolutely require the function of the large Golgi-localized GEFs for their replication, and also for their capacity to block secretion to suppress the immune response of their hosts. The goal of this project is to elucidate the molecular mechanisms by which viral proteins use the Arf GEFs for viral replication.

Lipid droplets (LDs) are organelles that store neutral lipids for use as an energy supply in times of nutrient deprivation and for membrane assembly. Misregulation of LD function leads to many human diseases including lipodystrophy, obesity and neutral lipid storage disorders. Much progress has been made in understanding the molecular mechanisms of lipid storage and mobilization in mammals, but there are still many unanswered questions. It has recently been shown that two lipases, Adipose TriGlyceride Lipase (ATGL) and Hormone-Sensitive Lipase (HSL), are responsible for most of the TG hydrolase activity present in adipose tissue. ATGL plays the major role in controlling TG mobilization, as ATGL knockout mice become obese. Our group recently discovered that the Arf GEF GBF1 localizes to the surface of LDs in mammalian cells, in addition to its well-known localization to the Golgi apparatus. Moreover, we have demonstrated that depletion of GBF1 in HeLa cells by RNA interference leads to redistribution of the TG lipase ATGL into the cytosol, a significant increase in TG levels, and an increase in the size of LDs. The third theme of research in our group is aimed at understanding GBF1 regulation of ATGL recruitment to LDs, and its function in the upstream regulatory cascade that controls ATGL-mediated lipolysis.

Approaches: Mammalian cell culture, confocal microscopy, live cell imaging, co-immunoprecipitation of proteins from cells, purification of recombinant proteins, yeast 2-hybrid and cell biology.