Septins are cytoskeletal elements originally identified and characterized in yeast by the pioneering studies of John Pringle. They are essential proteins involved in cell polarity establishment and bud formation. The septins have been conserved throughout metazoan evolution. They bind GTP, and may function in cytokinesis. They have also been implicated in exocytosis. The MSF septin gene is deleted in breast and ovarian cancers and has been proposed to be an anti-oncogene. The CDCrel-1 septin gene is deleted in some velo-facio DiGeorge syndrome patients. Products of both septin genes are also found fused in a C-terminal position to the MLL protein in acute leukaemia. These observations all point to important physiological functions for septins in metazoa, but do not provide clues to their roles at a molecular level. Progress in understanding septin function in mammalian cells has been hampered by an inability to express functional recombinant proteins, and by a lack of known regulatory factors. We have now overcome these problems. We have devised methods to express septin filaments in bacteria, and identified the Borg proteins as regulators of septin organization. Borgs are also Cdc42 GTPases effector proteins, and hence link two types of GTP-binding protein. We can also express functional septin-GFP fusions and track septin dynamics in vivo. Finally, we can knockdown endogenous septin expression in mammalian cells using ANA interference. These advances will be exploited in the following specific aims:1. We will determine how septin filaments are assembled in vitro, by dynamic light scattering, electron microscopy and biochemical assays. 2. The hypothesis will be tested that Borgs, and other factors, regulate guanine nucleotide binding and filament assembly. Other binding partners of the septins will be purified and identified, using affinity chromatography and yeast two-hybrid screens. 3. It will be determined whether septin filaments are dynamic or stable in intact cells, and how they assemble and disassemble, using septin-GFP fusions, time-lapse fluorescence microscopy and photobleaching.