A gene duplication of a septin provides a developmentally-regulated filament length control mechanism

Septins are a family of conserved filament-forming proteins that function in a variety of processes including cell cycle progression, cell morphogenesis and autophagy. Despite their conservation from yeast to humans, the number of septin genes within an organism varies and higher eukaryotes express many septin isoforms due to alternative splicing. It is unclear how variability in septin complex composition influences the biophysical properties of septin polymers. Here we report that a complex duplication event within the CDC11 locus in the fungus, Ashbya gossypii, gave rise to two similar, but distinct Cdc11 proteins, Cdc11a and Cdc1b. CDC11b transcription is developmentally regulated producing different ratios of Cdc11a and b complexes during Ashbyas lifecycle. Moreover, deletion of either CDC11a or CDC11b results in distinct cell polarity defects. Remarkably, despite substantial identity in amino acid sequence, Cdc11a and Cdc11b complexes have distinct biophysical properties with clear filament length and membrane-binding ability differences. Thus, septin subunit composition has functional consequences for filament properties and such functional plasticity can be exploited for distinct biophysical properties and cell functions.