Schematic showing the common structure of septin protein family and the structures of individual septin subgroups. A. The septin protein structure consists of a GTP-binding domain composed of conserved motifs G1 (GxxxxGK[S/T]), G3 (DxxG) and the GTP-specificity motif G4 (xKxD). D, G, K, S, and T represent aspartic acid, glycine, lysine, serine, and threonine residues, respectively, and x indicates any amino acid. The N-terminus consists of a proline-rich domain and a polybasic region. The C-terminus contains a septin unique element (SUE) and a varying α-helical coiled-coil domain. B. Based on sequence homology and domain composition, the 13 septins have been divided in to four subgroups (SEPT2, SEPT3, SEPT6, and SEPT7). Septins of subgroup SEPT6 lack a threonine residue (T78*), which is needed for the hydrolyzation of GTP to GDP. C. The formation of septin filaments mediated by the interaction between GTP-binding domains (G-dimer) and the N- and C-termini containing faces (NC-dimer). Formation of septin filament structures require different conformational changes mediated by the GTP/GDP molecules, allowing the assembly and disassembly of stable septin complexes. These conformational changes are also influencing the N-terminal helix and thus affect the formation of the NC-dimer. Therefore, the lack of a threonine residue (T78*), resulting in the inability of the septin protein to hydrolyze GTP to GDP, enables the formation of e.g. SEPT2-6-7 complex.