Tagging C. elegans septins disrupts cytoskeletal scaffolding but not post-embryonic roles

Septins are conserved polymer-forming proteins that scaffold the actomyosin cytoskeleton, its regulators, and other factors to cellular membranes. Septins also sense micron-scale curvature, bind microtubules, and establish membrane diffusion barriers. C. elegans is a powerful animal model to study septins roles because there are only two septin genes: unc-59 and unc-61. In many fungal and animal cell types, septins are required for proper cytokinesis. In the C. elegans zygote, septins scaffolding roles in cytokinesis manifest during the chiral rotation of the cell cortex and the asymmetry of cytokinetic ring closure. Originally named for the uncoordinated movement exhibited by hypomorphic alleles, UNC-59 and UNC-61 are also required for normal postembryonic development, germline development, and fertility. To study C. elegans septins in these various contexts, we sought a fluorescent-protein tagging strategy that minimally perturbed septin function. We examined strains in which GFP, mKate2 or wrmScarlet had been inserted at the unc-59 locus, or coupled to unc-61b/c at an exogenous locus, to encode fluorescently tagged fusion proteins. We compared these tagged septins to classical hypomorphic alleles, and to new null alleles. Null alleles phenocopied hypomorphic alleles in all our assays. Strains bearing fluorescently tagged septins exhibited defects in zygote cytokinesis, qualitatively phenocopying both hypomorphic and null alleles. These findings agreed with recent work with fission yeast, demonstrating the sensitivity of septin function to tagging. Interestingly, tagging septins did not perturb postembryonic development including animal mobility. This suggests that septins play distinct functions in the zygote versus later in development.