Septins: a ring to part mother and daughter

The septins are well conserved GTPases found in animals and fungi. In yeast, they are required for the formation of 10-nm filaments, with which they co-localize at the bud neck. Therefore, septins have been proposed to be components of the neck filaments and to have polymerization properties. In support of this hypothesis, septin complexes purified from yeast and flies form filaments in vitro. However, recent studies have questioned the relevance of septin filament formation for septin function. Particularly, septin polymerization may not be required for their function in cytokinesis. New septin functions have also been recently uncovered: in budding yeast, the septin ring is required for the maintenance of cell polarity. It forms a cortical barrier that prevents lateral diffusion of membrane-associated proteins through the bud neck. Here, we review the most recent functional and biochemical data, to discuss whether there is a link between septin polymerization properties and septin function. reported to associate with focal adhesion complexes in fibroblasts (Kinoshita et al. 1997) and to control the organization of the cell cortex in yeast (Barral et al. 2000; Takizawa et al. 2000). Septins have also been shown to be abundant in neurons (Neufeld and Rubin 1994; Fares et al. 1995; Kinoshita et al. 1997), a cell type that does not divide actively. While we have made progress in the analysis of septin function at the cellular level, the biochemical and structural characterization of these proteins is still in its infancy. This situation, though, is now changing rapidly, as several groups put efforts in biochemical and ultrastructural characterization of various septin molecules and complexes.