Geometry-dependent assembly of the septin cytoskeleton

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Summary/abstract Cell shape is integral to function and can be described in terms of plasma membrane curvature. Many changes in cell curvature occur on the micrometer scale but proteins are nanometers in size, raising the question as to how cells can perceive, control and use micrometer-scale geometry. The septins are a conserved, filament-forming family of proteins that preferentially assemble at sites of micrometer-scale membrane curvature. Septins assemble on many curved surfaces including at the cytokinetic furrow, dendritic spines, membrane blebs, around intracellular bacteria and bases of cilia and flagella. Given these diverse cell contexts, malfunction of septins is linked to diverse human diseases including many cancers, neuropathies and infertility. At sites of micrometer-scale membrane curvature, septins can influence the diffusion of proteins in the membrane, act as scaffolds to bring together signaling proteins, and impact the rigidity of the cell cortex. How curved septin assemblies form and recruit signaling proteins to the local membrane is critical to understand how septins link cell geometry to responses. Septin filament assembly occurs through annealing of short (~24-32nm) oligomeric rods on lipid bilayers or other cytoskeletal networks. We hypothesize that cells modulate the membrane affinity, length, density, and geometrical arrangement of septins in a curvature-dependent manner. The goal of this proposal is to identify the mechanisms directing assembly of septins on curved surfaces and to measure how curved assemblies regulate signaling networks. We will combine a variety of imaging approaches including high-resolution fluorescence, SEM and high-speed atomic force microscopy (HS-AFM), modeling, proteomics, and molecular genetics. Based on preliminary data, we hypothesize that curvature-dependent septin assembly involves mechanisms at work on several length scales. This work will be directed by three aims: (1) Analyze septin membrane interaction in curvature sensing; (2) Determine the biophysical properties of septin filaments that enable curvature sensing; (3) Identify how curved septin assemblies recruit specific signaling proteins. From the proposed experiments, we will learn how nanometer length scale mechanisms contribute to the emergent mesoscale process of sensing micron- scale curvature. These studies will also reveal how septin scaffolding may change as a function of local curvature. The long-term goal of this proposed study is to identify how septins recognize micrometer-scale curvature and then use shape information to modulate cellular functions.