Collaborative Research: Multiscale Mechanics of Septin Assembly and Curvature Sensing

Duke University, Durham NC

Investigators

Abstract

Biological cells need to sense their own shape and size for a variety of their functions, such as deciding when to divide and how to fit in narrow spaces. Studies over the past decade have shown that the filament-forming protein septin can sense the cell membrane’s shape and curvature by preferentially binding and assembling to areas of cell membrane curvature. This is surprising, since septin protein is only several billionths of a meter in length but can sense broad curvature over 100 times that length. This would be akin to using your foot to measure the curvature of a sphere the width of a football field. Recent studies by this team show that septin curvature sensing is determined through septin polymerization and assembly on the membrane, rather than by single molecules of septin. However, little is known about the interplay between the molecular makeup of these septin bundles, filament formation and the whole-cell scale organization of septins on the membrane, and how the assembly process relates to membrane curvature. This project will study this question in three scales of septin assembly, namely the scale of a few septin moelcules bound together, larger septin filaments, and the whole-cell scale. In order to tell the story of this research to the broader public, the team will design and conduct activities around the theme of “Self-Organization in Biology'' for middle school and high school students and their teachers. Lastly, integral to this research plan is the training of multidisciplinary undergraduate and graduate student scientists to work at the interface of physics, mathematics, and biology. Cellular surfaces often adopt shallow micron-scale curvatures, such as in fungal branches and cytokinetic furrows, whereas the proteins that sense these shapes are only several nanometers in size. This project will determine how cells sense geometry on the micron-scale with nano-scale proteins. The researchers have discovered that septin, a highly conserved filament-forming cytoskeletal protein, is localized to areas of micron-scale curvature on the membrane, and that septin curvature sensing is determined through its multistep, multiscale assembly on membranes. Yet, the relationships between septin’s molecular structure, its packing and organization, and curvature sensing ability remain poorly understood. This project will develop a multiscale mechanical model of septin assembly and curvature sensing in three aims, that correspond to three scales of septin assembly. In Aim 1 (molecular scale), the researchers will determine the effect of septin molecular structure on the binding, unbinding, and polymerization rates of a single oligomer. Aim 2 (filament scale) focuses on determining how this molecular-scale information influences filament-scale structure and transport. Aim 3 (system scale) focuses on analyzing the processes that determine the system-scale assembly - the density, packing, and layering of septin filaments. In order to measure and model these couplings, the researchers will combine multiple simulation and experimental tools across scales, including atomistic and coarse-grain particle simulations and kinetic modeling, with experimentally derived parameters from single-molecule imaging, confocal microscopy, and scanning electron microscopy. This project is funded by the Cellular Dynamics and Function Program of the Division of Molecular and Cellular Biology. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.