26.18: Septins
Septins are protein filaments forming the cytoskeleton along with the microtubules, microfilaments, intermediate filaments, and other accessory proteins. In 1971 while studying the cell division cycle in mutant Saccharomyces cerevisiae Harwell et al. first identified the septin-related genes playing a crucial role in yeast cytokinesis. Fluorescence microscopy revealed that these proteins localize at the budding neck as rings. These ring-like proteins were then named Septins by John Pringle, and the characterization of these highly conserved cytoskeletal proteins began in the 1980s.
Structure and formation of Septins
Septin monomers can be broadly divided into a variable N-terminal domain, a GTP hydrolyzing domain, and a coiled-coil C-terminal domain. The GTP-binding domain consists of three GTP binding motifs, first G1 or P-loop or Walker A box; second the G3 Switch II and the third G4 for selective binding of GTP. The GTP binding domain ends in a septin unique region (SUR), which comprises 50 amino acid residues and is responsible for helping in septin filament formation.
Classification of Septins
The type and number of septins vary among different organisms. For example, S. cerevisiae has seven, while humans have thirteen. Based on protein sequence similarity, septins are classified into four groups: SEPT2, SEPT3, SEPT6, and SEPT7. In humans, group SEPT2 comprises SEPT1, SEPT2, SEPT4, and SEPT5; group SEPT3 has SEPT3, SEPT9, and SEPT 12; group SEPT6 has SEPT6, SEPT8, SEPT10, and SEPT11; and lastly, group SEPT7 only has SEPT7.
