Project Summary The goal of this proposal is to study the structure and dynamics of the interaction of the AAA+ ATPase, valosin-containing protein (VCP) with disease-relevant tau fibrils to determine VCP-mediated disease progression and therapeutic potential. Alzheimer's Disease (AD) and Corticobasal degeneration (CBD) are 2 of more than 20 “tauopathies” characterized by different forms of fibrillar tau aggregates. The degree of tau pathology on autopsy correlates with cognitive decline, yet the mechanism of neurotoxicity, disease development, and disease progression in tauopathies remains to be elucidated. Diseases of aggregation indicate a dysfunction in proteostasis – the regulation and balance of the proteome. VCP plays an essential role in proteostasis by removing proteins from complexes, membranes, and aggregates such as those formed by fibrillar tau, thus pointing to VCP as an important player in proteostatic dysfunction and as a potential drug target for tauopathies. VCP has a variety of cofactors that enable it to perform an array of vital cellular. Ufd1 and Npl4 cofactors (UN) together specifically enable VCP to recognize poly-ubiquitinated substrates. AD tau fibrils have high rates of ubiquitination, likely because cells have marked these fibrils for removal by proteins such as VCP. Additionally, mutations within VCP are linked to neurodegenerative disease. A hypoactive VCP mutation first described by the Edward Lee lab clinically presents as dementia and has been shown to promote tau fibril pathology. The hypoactive VCP mutation is thought to reduce VCP function causing the accumulation of tau fibrils, leading to neurodegenerative disease. However, a similar mechanism of VCP inadequately clearing tau fibrils may be present in cases of sporadic tauopathies. VCP/UN, mutant or wild-type, may also get stuck on tau fibrils (possibly due to fibrils' high stability) and be unable to break down the fibril or be released, reducing effective VCP function by trapping it on the fibril. This has been seen with a similar AAA+ ATPase in the 26S proteasome and neurodegenerative aggregates. I propose that for productive disaggregase activity VCP/UN binds unstructured ubiquitin at either end of tau fibrils then pulls off one tau monomer at a time. I also propose the mechanism of action will not change but the stoichiometry and rate of reaction will change between tau fibrils due to differences in fibril structure and ubiquitination. To test this model, in Aim 1 I will structurally characterize how VCP/UN interacts with brain- derived AD and CBD tau fibrils. Structurally understanding how VCP/UN interacts with tau fibrils will help elucidate the protein regions that are crucial to VCP/UN function, and knowing how this interaction changes with fibril type will help understand the generalizability of the VCP/UN fibril interaction. In Aim 2, I will characterize the dynamics of the VCP/UN-tau interaction. Understanding the dynamics of VCP/UN interaction ...