THEME AND VARIATIONS OF SALT AND SHAPE: INVESTIGATING INFLUENCES OF SEPTIN ASSEMBLY IN MULTIPLE YEAST SPECIES

Septins are a conserved family of GTP-binding proteins that assemble into filaments and play critical roles in cellular organization and morphogenesis. Understanding their polymerization mechanisms is essential for elucidating how cells spatially and temporally regulate their assembly. In this thesis, we investigated the intrinsic polymerization modes of septins and the influence of electrostatic interactions and membrane geometry on their assembly. Our results reveal that septin polymerization exhibits hallmarks of cooperative assembly, with a strong dependence on salt concentration and membrane binding. We demonstrate that increasing ionic strength modulates the polymerization mode, assembly rate, and critical concentration, underscoring the sensitivity of septin dynamics to electrostatic conditions. Furthermore, we find that septins preferentially bind to curved membranes, which enhances membrane engagement and influences polymerization behavior compared to planar membranes. While cooperative assembly dominates in solution, septin behavior on membranes suggests a transition to more isodesmic-like polymerization, potentially due to altered conformational constraints or competitive binding interactions. These findings support a model in which septin assembly is highly tunable by ionic conditions and membrane characteristics, allowing cells to regulate filament formation through localized changes in salt concentrations and lipid composition. Additionally, we explored septin localization and function in three black yeasts, Aureobasidium pullulans, Hortaea werneckii, and Knufia petricola, expanding our understanding of septin contributions to fungal morphology and polarization. Comparative sequence analyses suggest that black yeast septins possess subtle modifications at key polymerization interfaces, potentially affecting filament stability and membrane association. This study provides a framework for future investigations into septin biophysics and regulation in diverse fungal species, particularly in emerging model systems with newly developed genetic and recombinant tools.