Septins are a conserved family of eukaryotic GTP-binding, filament-forming 
proteins. In Saccharomyces cerevisiae, five septins (Cdc3p, Cdc10p, Cdc11p, 
Cdc12p, and Shs1p) form a complex and colocalize to the incipient bud site and 
as a collar of filaments at the neck of budded cells. Septins serve as a 
scaffold to localize septin-associated proteins involved in diverse processes 
and as a barrier to diffusion of membrane-associated proteins. Little is known 
about the role of nucleotide binding in septin function. Here, we show that 
Cdc3p, Cdc10p, Cdc11p, and Cdc12p all bind GTP and that P-loop and G4 motif 
mutations affect nucleotide binding and result in temperature-sensitive defects 
in septin localization and function. Two-hybrid, in vitro, and in vivo analyses 
show that for all four septins nucleotide binding is important in septin-septin 
interactions and complex formation. In the absence of complete complexes, 
septins do not localize to the cortex, suggesting septin localization factors 
interact only with complete complexes. When both complete and partial complexes 
are present, septins localize to the cortex but do not form a collar, perhaps 
because of an inability to form filaments. We find no evidence that nucleotide 
binding is specifically involved in the interaction of septins with 
septin-associated proteins.