Septins constitute a eukaryotic family of guanine nucleotide-binding proteins, most of which polymerise to form filaments []. Members of the family were first identified by genetic screening for Saccharomyces cerevisiae (Baker's yeast) mutants defective in cytokinesis []. Temperature-sensitive mutations in four genes, CDC3, CDC10, CDC11 and CDC12, were found to cause cell-cycle arrest and defects in bud growth and cytokinesis. The protein products of these genes localise at the division plane between mother and daughter cells, indicating a role in mother-daughter separation during cytokinesis []. Members of the family were therefore termed septins to reflect their role in septation and cell division. The identification of septin homologues in higher eukaryotes, which localise to the cleavage furrow in dividing cells, supports an orthologous function in cytokinesis. Septins have since been identified in most eukaryotes, except plants [].Septins are approximately 40-50kDa in molecular mass, and typically comprise a conserved central core domain (more than 35% sequence identity between mammalian and yeast homologues) flanked by more divergent N- and C-termini. Most septins possess a P-loop motif in their N-terminal domain (which is characteristic of GTP-binding proteins), and a predicted C-terminal coiled-coil domain [].A number of septin interaction partners have been identified in yeast, many of which are components of the budding site selection machinery, kinase cascades or of the ubiquitination pathway. It has been proposed that septins may act as a scaffold that provides an interaction matrix for other proteins [, ]. In mammals, septins have been shown to regulate vesicle dynamics []. Mammalian septins have also been implicated in a variety of other cellular processes, including apoptosis, carcinogenesis and neurodegeneration [].Northern blotting studies indicate that septin 3 expressionis high in the brain but undetectable in other tissues, suggesting that theprotein is primarily neuronal.
