Septins are a cytosolic GTP-binding protein family first characterized in yeast, 
but gaining increasing recognition as critical protagonists in higher eukaryotic 
cellular events. Mammalian septins have been associated with cytokinesis and 
exocytosis, along with contributing to the development of neurological 
disorders. Ten different septins, divided into four groups, have been identified 
in mammals, and individual septins are capable of interacting with each other to 
form macromolecular complexes. The present study characterizes the structural 
requirements for human septin-septin interactions using a yeast two-hybrid 
system. We focus on three septins that are highly expressed in platelets and 
neurons, SEPT4 [previously designated H5, CDCrel-2 
(cell-division-control-related-2), PNUTL2], SEPT5 (CDCrel-1, PNUTL1) and SEPT8 
(KIAA0202). Each of these three septins contains a characteristic domain 
structure consisting of unique N- and C-termini, and a central core domain 
conserved among the family of proteins. The yeast two-hybrid system yielded data 
consistent with a model where each of the three septins can interact with itself 
(homotypic assembly) or with one of the other septins (heterotypic assembly). 
For SEPT5 and SEPT8, the results illustrate a model whereby heterotypic septin 
assembly is dependent on the conserved central core domain and homotypic 
interactions require the N- and C-termini of each protein. We also characterized 
a model in which the proper cellular localization of SEPT5 and SEPT8 requires 
concomitant expression of both proteins. Co-transfection of SEPT5 and SEPT8 
results in both proteins targeted to a vesicular-like location. Therefore the 
cellular repertoire of human septins has an impact on function by targeting 
septin macromolecular complexes to specific cellular locations.