A distinguishing feature of eukaryotic cells is their internal membrane organization. Internal membrane stuctures are highly dynamic and their integrity and maintenance are dependent upon continual membrane trafficking and protein transport. The small GTPase ARF (ADP-Ribosylation Factor) plays an essential role in multiple steps of protein transport. The GDP/GTP cycle of ARF results in changes in both lipid and protein composition of membranes, thus modulating membrane structure. The best-studied role of ARF is recruitment of cytosolic coat complexes onto membranes, which can deform membranes to produce transport intermediates and concentrate cargo proteins into these transport carriers. A newly-discovered class of ARF effectors is lipid modifying enzymes such as phosphatidylinositol-4-phosphate 5-kinase. ARF relies on a guanine nucleotide exchange factor (GEF) to become activated. Dr. Catherine L. Jackson?s laboratory was the first to identify an ARF GEF, named Gea1p (for Guanine-nucleotide Exchange on ARF) in S. cerevisiae. There are now a dozen ARF GEF proteins characterized to date. They all share a region of approximately 200 amino acids (the """"""""Sec7 domain"""""""") that is homologous to a domain of the Sec7p protein of S. cerevisiae, involved in secretion. The Sec7 domain alone is necessary and sufficient to catalyze GDP/GTP exchange on ARF in vitro. We are investigating the in vivo and in vitro functions of the Sec7 domains of three ARF GEFs in S.cerevisiae, Gea1p, Gea2p, and Sec7p, and two Sec7 orthologues in mammalian cells, BIG1 and BIG2. A number of interacting partners of the Sec7 domains of Gea1/2p, Sec7p an BIG2 have been identified. We are investigating both the physical properties of these interactions and their physiological roles. A second major project is the identification of membrane-binding determinants in the Sec7 domain proteins. All of these proteins are peripherally associated with membranes, and it is not known whether membrane interaction is mediated by direct association with lipids or through protein-protein interaction. Interestingly, one direct binding partner of Gea2p is a transmembrane-domain protein, so this interaction is being explored for a potential membrane-targeting role.
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