Septins comprise a conserved family of GTPases important in cytokinesis. These 
proteins polymerize into filaments from rod-shaped heteromeric septin complexes. 
Septins interact with one another at two interfaces (NC and G) that alternate 
within the complex. Here, we show that small mutations at the N terminus greatly 
enhance the formation of SEPT2 homopolymers. Taking advantage of this mutation 
to examine polymer formation using SEPT2 alone, we show that both NC and G 
interfaces are required for filament formation. However, co-expression of wild 
type SEPT2 with SEPT2 containing mutations at either NC or G interfaces revealed 
that only the NC mutant suppressed filament formation. NC mutants are able to 
interact with one another at putative G interfaces, whereas G mutants fail to 
interact at NC interfaces. In addition, all promiscuous septin pairwise 
interactions occur at the G interface. These findings suggest that G interface 
interactions must occur before NC interactions during polymer formation.