Assembly of Mammalian Septins

Septins are a conserved family of polymerizing guanine nucleotide binding proteins associated with diverse processes in dividing and non-dividing cells. In humans, 12 septin genes generate dozens of polypeptides, many of which comprise heterooligomeric complexes. Native and recombinant mammalian septin complexes are purified as approximately 8-nm-thick filaments of variable length. Ultrastructurally, a mammalian septin filament appears an irregular array of structural segments, whose polarity is obscure. The filaments have a potential to self-assemble into higher-order structures by lateral stacking and tandem annealing, eventually forming uniformly curved bundles, i.e., rings and coils. The septin filaments also undergo templated assembly along existing actin bundles containing an adapter protein, anillin. The resultant higher-order assembly of septin filaments may provide scaffolds to recruit other molecules and/or help organize the actin-based structures. The in vitro self-assembly is an irreversible process, which is not coupled with robust nucleotide exchange or hydrolysis. In contrast, septin-based structures rearrange and disassemble in cells, which might be controlled by diverse factors (e.g., the Cdc42-borg system, anillin, syntaxin, phospholipids) and covalent modifications (e.g., phosphorylation, ubiquitination, sumoylation). An immediate goal of septin biochemistry is to define the mechanisms of assembly and disassembly of this elusive cytoskeleton.