Dynamics and Buffering Functions of Nuclear Actin-Cofilin Assemblies in Cellular Stress Response

• Applicant: Julia Mahamid, Ph.D.
• Subject Area: Structural Biology ; Biophysics ; Cell Biology
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 419138452

Actin is the most abundant protein in eukaryotic cells and is well known for its capability to polymerize into dynamic, semi-flexible filaments in the cytoplasm. These filaments are an integral part of the cytoskeleton and are essential in determining cell shape, division and motility. Actin is now also recognized as a key player in the cell nucleus, where the actin monomers function in essential gene-expression processes. Dynamic nuclear actin filaments have also been shown to play a critical role in specific gene regulation and DNA-damage repair mechanisms. Therefore, shuttling of actin in and out of the nucleus, together with its regulatory partner cofilin, and maintaining their steady-state levels are of importance to normal cellular function. It has been known for decades that actin and cofilin can also assemble into large-scale, stable ‘rods’ within the nucleus, which form as a consequence of stress conditions. While the transient formation of rods is thought to be part of a normal cell response and may have protective functions, persistence of rods is implicated in a variety of pathological conditions, such as certain myopathies and some neurological disorders. However, due to the difficulty in visualizing endogenous nuclear actin in live cells, it is still not well understood what exactly triggers the formation of intranuclear rods, what are the underlying mechanisms of rod assembly or disassembly, and what is their function in the cellular stress response. Intriguingly, the emerging concept of liquid-liquid phase separation processes in cell biology is implicated in many cellular stress response mechanisms due to its exquisite sensitivity to physical and chemical intracellular perturbations and capability to rapidly reorganize cellular functions. Here we speculate that the dynamic assembly of nuclear actin-cofilin rods is triggered upon cellular perturbation through phase separation of monomers into a condensed state. The assemblies mature into stable, liquid-crystalline rods that sequester most of the free actin in the nucleus. The stable assembly may serve a buffering function affecting monomeric actin-dependent gene expression processes that are required as part of the stress response. We propose to study the phase separation dynamics of nuclear actin-cofilin assembly and disassembly, elucidate their structural basis in intact cells, and their relation to other phase separated nuclear compartments.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2191: Molecular Mechanisms of Functional Phase Separation