SNARE proteins mediate the docking and/or fusion of the vesicle with the plasma membrane. However, it is not clearly understood how this process is regulated. In a search for potential SNARE regulators, we have identified a novel snare interacting protein, the septin CDCrel-1. Septins were first identified as filamentous proteins required for cytokinesis in yeast. However, in mammals little is known about their functions. I show here that cdcrel-1 is predominantly expressed in the brain where it associates with membranes via binding to syntaxin 1A. Wildtype CDCrel-1 transfected into HIT-T15 cells inhibits secretion while mutated forms of CDCrel-1 potentiate secretion, suggesting that cdcrel-1 may be regulating vesicle targeting and/or fusion events. I further map the CDCrel-1 domains important for syntaxin binding and investigate the ability of CDCrel-1 to bind to syntaxin when in various SNARE complexes. CDCrel-1 can bind syntaxin in a SNARE complex, but its binding is occluded by alpha-SNAP. This suggests that CDCrel-1 may act as a novel filamentous element, regulating the delivery and/or fusion of vesicles to the presynaptic membrane through its interaction with syntaxin and the 7S complex. The regulation of filaments may be via post-translational modifications. Indeed we have discovered a novel interaction between SUMO E3 PIAS proteins and CDCrel-1. The conjugation of SUMO to substrates is dependent upon an E1 and E2, whereas specificity is mediated by an E3. Although several SUMO-1 substrates have been characterized, conjugation solely by SUMO-2/3 has not been described. Here I describe the colocalization of CDCrel-1 with SUMO-2 and 3 but not SUMO-1. Transfection of SUMO-2/3 but not SUMO-1 causes a reorganization of CDCrel-1 distribution in CHO cells. Furthermore, CDCrel-1 sequesters the nuclear pool of SUMO-2/3 and of the E2 Ubc9 but not SUMO1 into the cytoplasm. Sumoylation of CDCrel-l is shown in vivo and putative SUMO modification sites on CDCrel-1 are investigated by deletion of lysine residues. These experiments strongly suggest that CDCrel-1 is sumoylated specifically by SUMO-2/3. Sumoylation of CDCrel-1 may therefore play a regulatory role in secretion and septin filament formation. Future work will be aimed at determining the functional significance of SUMO modified CDCrel-1.