UC San Diego

Many of the proteins found in the pathological protein fibrils and aggregates that are associated with neurodegenerative disease also exhibit tendencies for liquid-liquid phase separation (LLPS) both in vitro and in cells. The transition between the LLPS state and the aggregate state can be modulated by protein chaperones that can block the liquid-to-solid transition and help maintain the LLPS state. In this dissertation, we characterize structural features exhibited by FUS LC, a protein that has been demonstrated to facilitate LLPS and form fibrils, as it undergoes a liquid-to-solid transition from the LLPS state to the fibril state. We subsequently examine the α-crystallin chaperone HSPB1 as it interacts with FUS LC in the LLPS state, and compare the structural features of HSPB1 without a client protein to its structural features in the presence of a phase-separated and an aggregated client protein. The heterogeneous nature of the chaperone and client system and the wide range of dynamics adopted by the component proteins present unique challenges for structural interrogation. We therefore integrate solid-state magic angle spinning NMR spectroscopy, coarse-grained molecular dynamics simulations, and cryo-EM to build a model for the structures and dynamics adopted by FUS LC during the liquid-to-solid transition and HSPB1 as it interacts with client proteins, with a specific focus on the role of the N-terminal domain of HSPB1 in client recognition and chaperone function.