The geometry of cells is well described by membrane curvature. Septins are filament forming, GTP-binding proteins that assemble on positive, micrometer-scale curvatures. Here, we examine the molecular basis of curvature sensing by septins. We show that differences in affinity and the number of binding sites drive curvature-specific adsorption of septins. Moreover, we find septin assembly onto curved membranes is cooperative and show that geometry influences higher-order arrangement of septin filaments. Although septins must form polymers to stay associated with membranes, septin filaments do not have to span micrometers in length to sense curvature, as we find that single septin complexes have curvature-dependent association rates.We trace this ability to an amphipathic helix (AH) located on the C-terminus of Cdc12. The AH domain is necessary and sufficient for curvature sensing both in vitro and in vivo. These data show that curvature sensing by septins operates at much smaller length scales than the micrometer curvatures being detected. We thank the Gladfelter lab and Danny Lew for useful discussions, Matthias Garten for ideas in setting up the rod assay, the UNC EM facility (Victoria Madden and Kristen White) for support with SEM and HHMI Faculty Scholars award to ASG.
Author contributionsKSC, BLW conducted experiments, BLW and JMC constructed strains for experiments, BLW and JMC constructed plasmids for experiments, KSC, BLW, and ASG designed experiments.