Fig. 1.

Septins and their filaments. a A schematic representation of a mammalian septin showing the three domains (N, G and C). The polybasic region (P) is found at the interface between the N- and G-domains while the nucleotide-binding sequences (G1, G3 and G4) are all located within the G-domain. The final 50 residues of the G-domain corresponds to the septin unique element (SUE) which distinguishes septins from other small GTPases. Little is known about the structure of the N- and C-domains although the latter normally includes heptad pseudo-repeats characteristic of coiled-coil structures. b The structure of a septin monomer (center) is shown within the context of its two neighbors along the filament. This gives rise to the NC- and G-interfaces. The nucleotide is bound at the latter, and its full binding site includes residues coming from both of the adjacent subunits. The SUE (purple) can be seen to contribute to both interfaces but most notably to the NC-interface which is unique to septins. It is this interface which allows septins to polymerize. The G1, G3 and G4 motifs are shown according to the same color scheme as used in a. They are essential for the selective recognition of guanosine triphosphate (GTP)