Biology subjects

Curtis, B. N.

Publications and source records attributed to Curtis, B. N..

3 recordsLinked to original sources

bioRxiv2025

Lipid packing and local geometry influence septin curvature sensing

Septins assemble into scaffolds that direct cell growth and morphology that are often localized to the plasma membrane. While septins preferentially bind convex membranes via amphipathic helices, their assembly on varied geometries in cells suggests additional localization cues. We tested the hypothesis that lipid composition directs septin assembly through lipid packing properties. Lipid mixtures varying in lipid packing were designed by molecular dynamics simulations and incorporated onto supported lipid bilayers to measure septin adsorption in vitro. Septins strongly favor loosely-packed, disordered lipid bilayers but additional geometry cues act in conjunction with this membrane property. Introducing tighter lipid packing in cells disrupted septin structures in a curvature dependent manner, specifically limiting septin assembly and retention along flat regions of the plasma membrane. This work demonstrates that packing defects and geometry jointly regulate septin localization and highlights how multiple membrane properties are integrated to control organization of the septin cytoskeleton. SummaryLocalization of the septin cytoskeleton is controlled by regulatory factors, membrane curvature, and charge. In this study, changes to lipid composition that modulate lipid packing defects are found to impact septin assemblies in vitro and in cells.

biochemistry↗

bioRxiv2024

Charge distribution and helical content tune the binding of septins amphipathic helix domain to lipid membranes

Septins are a class of cytoskeletal proteins that preferentially bind to domains of micron-scale curvature on the cell membrane. Studies have shown that amphipathic helix (AH) domains in septin oligomers are essential for septin curvature sensing. Yet, the underlying mechanochemical interactions that modulate this curvature sensing remain ambiguous. Here we use all-atom molecular dynamics alongside a metadynamics enhanced sampling approach to bridge the gap between time and length scales required to optimize and validate experimental design of amphipathic helices. Simulations revealed that the local charge on the termini of an 18-amino-acid AH peptide impacts its helical content and positioning within lipid membranes. These computational observations are confirmed with experiments measuring the binding of synthetic AH constructs with variable helical content and charged termini to lipid vesicles. Taken together, these results identify the helical content of amphipathic helices as a regulator of septin binding affinity to lipid membranes. Additionally, we examined an extended AH sequence including 8 amino acids upstream and downstream of the minimal 18-amino-acid-long AH domain to more closely mimic the native protein in simulations and experiments. Simulations and experiments show that the extended peptide sequence adopts a strong alpha-helical conformation when free in solution, giving rise to a higher affinity to lipid membranes than that of the shorter AH sequence. Together, these results provide insight into how the native septin proteins interact with membranes, and establish general design principles that can guide the interaction of future synthetic materials with lipid membranes in a programmable manner. STATEMENT OF SIGNIFICANCEUnderstanding how cells sense and react to their shape is necessary for numerous biological processes. Here we explore the interactions between amphipathic helices, a curvature sensing protein motif, and lipid membranes. Using molecular dynamics simulations, enhanced simulation sampling techniques, and experiments, we find that increasing the helical content of the amphipathic helix or adding charged capping sequences yields higher membrane binding affinity. Understanding these parameters for membrane-binding could enable us to interface and regulate native protein functions, as well as guide the design of synthetic curvature-sensing materials that can interact with and deform lipid membranes.

biophysics↗

bioRxiv2022

A kinetic basis for curvature sensing by septins

The ability of cells to sense and communicate their shape is central to many of their functions. Much is known about how cells generate complex shapes, yet how they sense and respond to geometric cues remains poorly understood. Septins are GTP-binding proteins that localize to sites of micron-scale membrane curvature. Assembly of septins is a multi-step and multi-scale process but it is unknown how these discrete steps lead to curvature sensing. Here we experimentally examine the time-dependent binding of septins at different curvatures and septin bulk concentrations. These experiments unexpectedly indicated that the curvature preference of septins is not absolute but rather is sensitive to the combinations of membrane curvatures present in a reaction, suggesting there is competition between different curvatures for septin binding. To understand the basis of this result, we developed a kinetic model that connects septins self-assembly and curvature sensing properties. Our experimental and modeling results are consistent with curvature-sensitive assembly being driven by cooperative associations of septin oligomers in solution with the bound septins. When combined, the work indicates septin curvature sensing is kinetically determined, sensitive to bulk concentration, and the available membrane curvatures. While much geometry-sensitive assembly in biology is thought to be guided by intrinsic material properties of molecules, this is an important example of how kinetics can drive mesoscale curvature-sensitive assembly of polymers. Significance StatementCells use their membrane curvature to coordinate the activation and spatiotemporal compartmentalization of molecules during key cellular processes. Recent works have identified different proteins that can sense or induce membrane curvature from nano- to micron-scale. Septins are nanoscopic cytoskeletal proteins that preferentially bind to membranes with a narrow range of micron-scale curvatures. Yet the sensing mechanism remains ambiguous. Using a combination of microscopy and kinetic modeling, we show that, unlike most proteins that sense curvature in a single protein scale, curvature sensing in septins is determined kinetically through their multi-step hierarchical assembly on the membrane. This introduces a novel kinetic basis of fidelity, where the same protein can be deployed for differential binding sensitivities in different cellular contexts.

biophysics↗