"The septins are a conserved family of GTPases that interact with membranes and the cytoskeleton. Several different septin molecules combine into nonpolar dimeric rods. These rods are thought to assemble end-over-end into higher order structures that orchestrate complex cellular events at the interface of the cytoskeleton and the plasma membrane. In this way, septin complexes organize the cleavage furrow in dividing cells, serve as a corset to maintain cell shape in cell motility, engulf invading bacteria and control the morphological differentiation of nerve cells. The integrity of the complexes is essential for septin function. However, the architecture of septin complexes and the rules guiding their assembly, while seemingly conserved from yeast to mammals, are not clear. The main objective of this study is thus to understand the rules controlling the assembly of septin complexes from subunit rods in yeast and mammals and to find out, whether there are alternative models to end-over-end assembly of septin rods into complexes. Individual septin rods will be localized within complexes with nanometer precision using ultra-resolution fluorescence microscopy methods. The septin complex architecture will then be modeled from the resulting pattern of individual rod positions. As a starting point, the best-understood septin complex, the ring-shaped complex at the mother bud neck in yeast involved in cytokinesis, will be investigated. First conclusions from the analysis of this complex will aid in the investigation of a mammalian septin complex at the neck of dendritic spines in neurons. Neither the precise composition nor the global structure of this septin complex are known yet, but morphological and functional analogies of the spine neck with the yeast bud neck suggest that a similar septin-ring structure may be present. The expected results will clarify the role of the septin complexes in yeast cell division and define the rules for septin complex assembly in neurons."