Actin is the most abundant protein in most eukaryotic cells and is well known for its capability to polymerize into dynamic filaments in the cytoplasm. Actin is now also recognized as a key player in the cell nucleus, where actin monomers function in essential gene-expression processes. Yet, it has been known for decades that actin, together with its regulatory partner cofilin, can also assemble in the nucleus into large-scale, stable ‘rods’, as a consequence of chemical or physical 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. Due to the inherent difficulty in visualizing endogenous actin in live cells, is still not well understood what 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. Here we speculate 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.
The Mahamid team develops and employs advanced cryo-electron microscopy methods for in-cell structural biology, to elucidate the structural principles and cytoplasmic environment driving the dynamic assembly of phase-separated compartments, including ribonucleoprotein stress granules. Our correlative cryo-light and electron microscopy investigations recently lead to the discovery of a structural scaffold at the core of dynamic stress granules in HeLa cells. We combine these studies with a quantitative description of the crowded nature of cytoplasm and of its local variations, to provide a direct readout of the impact of excluded volume on molecular assembly in living cells.
In particular working on the project:
Ievgeniia Zagoriy (Laboratory Officer in Charge) and Mukthi Ammai Sridharan Iyer (PhD Student)
69117 Heidelberg, Germany