Blair, L. J. | bioRxiv Science

bioRxiv2024

Probing tau citrullination in Alzheimer's disease brains and mouse models of tauopathy

Tauopathies, which include Alzheimers disease (AD) share a common defining factor, namely misfolded tau protein. However, the "upstream" etiology and downstream clinical manifestations of tauopathies are quite diverse. Tau deposition elicits different pathological phenotypes and outcomes depending on the tau strain and regional susceptibility. Posttranslational modifications (PTM) can alter tau structure, function, networks, and its pathological sequalae. We uncovered a novel PTM of tau, named citrullination, caused by peptidyl arginine deiminase (PAD) enzymes. PAD induced citrullination irreversibly converts arginine residues to citrulline, producing net loss of positive charge, elimination of pi-pi interactions, and increased hydrophobicity. We observed increased PAD2 and PAD4 in Alzheimers disease (AD) brain and that they both can citrullinate tau. Tau can become citrullinated by PADs at all 14 arginine residues throughout the N-terminal domain (N-term), proline-rich domain (PR), microtubule binding repeat domain (MBR), and C-terminal domain (C-term) on full length tau (2N4R). Citrullination of tau impacts fibrillization and oligomerization rates in aggregation assays. Utilizing a panel of novel citrullinated tau (citR tau) antibodies, we identified citrullination of tau in vitro, several animal models of tauopathies, and Alzheimers disease (AD). CitR tau increased with Braak stage and was enriched in AD brains with higher phospho-tau burden. This work provides a new area of tau biology that signifies further consideration in the emerging spectrum of tauopathies and its clinical understanding.

neuroscience↗

bioRxiv2019

Dynamic Aha1 Co-Chaperone Binding to Human Hsp90

Hsp90 is an essential chaperone that requires large allosteric changes to determine its ATPase activity and client binding. Because of the inherent low ATPase activity of human Hsp90, the co-chaperone Aha1, which is the only known ATPase stimulator in eukaryotes, is important for regulation of Hsp90s allosteric timing. Little is known, however, about the structure of the Hsp90/Aha1 full-length complex. Here, we characterize the solution structure of unmodified human Hsp90 in complex with Aha1 using NMR spectroscopy. We show that the 214 kDa complex adopts multiple conformations in the absence of nucleotide. Interaction with Aha1 induces structural changes near the nucleotide-binding site in Hsp90s N-terminal domain, providing a basis for its ATPase-enhancing activity. Moreover, the E67K mutation in Aha1 strongly diminishes the interaction, supporting a two-step binding mechanism. Our data reveal important aspects of this pivotal chaperone/co-chaperone interaction and emphasize the relevance of characterizing dynamic chaperone structures in solution.

biophysics↗