Analysis of Septin Structure and Function

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Project Summary/Abstract: Septins from all organisms including yeast and humans form rod-shaped heterooligomeric complexes that are assembled into linear filaments and other higher-order structures such as rings and hourglasses. These structures act as a cellular scaffold and/or diffusion barrier to impact diverse cellular functions including cytokinesis, cell migration, ciliogenesis, dendritic spine morphogenesis, spermiogenesis, and bacterial infection. Mutations in septin genes cause hereditary neuropathy and infertility in humans. Septins are also implicated in tumorigenesis and neurodegenerative diseases such as Alzheimer's and Parkinson's. Thus, understanding septin structure and function is critically important not only for basic science but also for public health. However, it remains largely unknown how septins are assembled and dynamically remodeled into various cellular architectures to perform distinct functions in any system. Since the initial discovery of septins in the budding yeast Saccharomyces cerevisiae, this organism has served as a highly effective model for uncovering the general principles of septin assembly and function. By combining the power of yeast genetics and cell synchronization with cutting-edge imaging technologies including platinum-replica electron microscopy and super-resolution light microscopy, we have determined the architectures of the native septin structures in budding yeast. Septins form an “early hourglass” at the division site that consists of paired septin filaments arranged in parallel to the mother-daughter axis. This structure matures into a “zonal transitional hourglass” in anaphase, with a septin gauze at the outer zones and myosin-II filaments in the mid-zone. The transitional hourglass is then remodeled into a “septin double ring” that consists of circumferential paired and single filaments. The double ring now sandwiches a constricting actomyosin ring. Both structures act together to restrict diffusible factors at the division site during cytokinesis. Recent evidence suggests that septins also undergo architectural remodeling from an hourglass-shaped structure during furrow ingression to a double ring-like structure during abscission in mammalian cells. In this application, we will: (Aim 1) determine how septin high-order assembly and stability at the division site is controlled by a LKB1-like kinase before cytokinesis in yeast; (Aim 2) determine how septin architectural remodeling is controlled by a RhoGEF-anillin module during cytokinesis in yeast; and (Aim 3) determine the septin architectures and their regulation by ArhGEF18 and anillin during furrow ingression and abscission in mammalian cells. The proposed study is expected to significantly advance our mechanistic understanding of septin assembly, remodeling, and function across model systems.