Biology subjects

Bracher, A.

Publications and source records attributed to Bracher, A..

6 recordsLinked to original sources

bioRxiv2024

Hydrophobic tails enable diverse functions of the extracellular chaperone clusterin

Clusterin, a conserved secretory glycoprotein abundant in blood plasma and cerebrospinal fluid, functions as a molecular chaperone and apolipoprotein (Wyatt et al. 2013, Raulin et al. 2022). Dysregulation of clusterin is linked to late-onset Alzheimers disease, cardiovascular pathology and cancer (Rohne et al. 2016, Satapathy and Wilson 2021, Wilson et al. 2023). Despite its prominent role in extracellular proteostasis, the chaperone mechanism of clusterin has remained unclear. Here we present crystal structures of human clusterin, revealing a discontinuous three-domain architecture. Structure-based mutational analysis demonstrated that two intrinsically disordered, hydrophobic peptide tails enable diverse clusterin activities. Resembling the N-terminal substrate binding regions of so-called small heat shock proteins, these sequences mediate clusterins chaperone function in suppressing amyloid-{beta}, tau and -synuclein aggregation. In conjunction with highly conserved surface areas, the tail segments also participate in clusterin binding to very low density lipoprotein receptor (VLDLR) and cellular uptake. Moreover, the disordered tails cooperate with an adjacent amphipathic helix in lipoprotein formation, but remain accessible for chaperone function in the lipoprotein complex. The remarkable versatility of these sequences allows clusterin to function alone or bound to lipid in maintaining solubility of aberrant extracellular proteins and facilitating their clearance by endocytosis and lysosomal degradation.

molecular biology↗

bioRxiv2022

Subcellular proteome of radial glia reveals non-muscle myosins control basal endfeet to mediate interneuron organization

Radial glial cells (RGCs) are essential for the generation and organization of neurons in the cerebral cortex. RGCs have an elongated bipolar morphology with basal and apical endfeet which reside in distinct niches. Yet, how this subcellular compartmentalization of RGCs controls cortical development is largely unknown. Here, we employ in vivo proximity labeling using unfused BirA to generate the first subcellular proteome of RGCs and uncover new principles governing local control of cortical development. We discover a cohort of proteins that are significantly enriched in RGC basal endfeet, with MYH9 and MYH10 among the most abundant. Myh9 and Myh10 transcripts also localize to endfeet with distinct temporal dynamics. Although they each encode isoforms of non-muscle myosin II heavy chain, Myh9 and Myh10 have drastically different requirements for RGC integrity. Myh9 loss from RGCs decreases branching complexity and causes endfoot protrusion through the basement membrane. In contrast, Myh10 controls endfoot adhesion, as mutants have unattached apical and basal endfeet. Finally, we show that Myh9- and Myh10-mediated regulation of RGC complexity and endfoot position non-cell autonomously controls interneuron number and organization in the marginal zone. Our study demonstrates the utility of in vivo proximity labeling for dissecting local control of complex systems, and reveals new mechanisms for dictating RGC integrity and cortical architecture.

neuroscience↗

bioRxiv2021

The extracellular chaperone Clusterin enhances Tau aggregate seeding in a cellular model

Spreading of aggregate pathology across brain regions acts as a driver of disease progression in Tau-related neurodegeneration, including Alzheimers disease (AD) and frontotemporal dementia. Aggregate seeds released from affected cells are internalized by naive cells and induce the prion-like templating of soluble Tau into neurotoxic aggregates. Here we show in a cellular model system and in neurons that Clusterin, an abundant extracellular chaperone, strongly enhances Tau aggregate seeding. Upon interaction with Tau aggregates, Clusterin stabilizes highly potent, soluble seed species. Tau/Clusterin complexes enter recipient cells via endocytosis and compromise the endolysosomal compartment, allowing transfer to the cytosol where they propagate aggregation of endogenous Tau. Thus, upregulation of Clusterin, as observed in AD patients, may enhance Tau seeding and possibly accelerate the spreading of Tau pathology.

cell biology↗

bioRxiv2020

The Hsc70 Disaggregation Machinery RemovesMonomer Units Directly from α-Synuclein FibrilEnds

Molecular chaperones contribute to the maintenance of cellular protein homeostasis through a wide range of mechanisms, including the assistance of de novo protein folding, the rescue of misfolded proteins, and the prevention of amyloid formation. Chaperones of the Hsp70 family have a striking capability of disaggregating otherwise irreversible aggregate structures such as amyloid fibrils that accumulate during the development of neurodegenerative diseases. However, the mechanisms of this key emerging functionality remain largely unknown. Here, we bring together microfluidic measurements with kinetic analysis and show that that the Hsp70 protein heat chock complement Hsc70 together with its two co-chaperones DnaJB1 and the nucleotide exchange factor Apg2 is able to completely reverse the aggregation process of alpha-synuclein, associated with Parkinsons disease, back to its soluble monomeric state. Moreover, we show that this reaction proceeds with first order kinetics in a process where monomer units are taken off directly from the fibril ends. Our results demonstrate that all components of the chaperone triad are essential for fibril disaggregation. Lastly, we quantify the interactions between the three chaperones as well as between the chaperones and the fibrils in solution, yielding both binding stoichiometries and dissociation constants. Crucially, we find that the stoichiometry of Hsc70 binding to fibrils suggests Hsc70 clustering at the fibril ends. Taken together, our results show that the mechanism of action of the Hsc70-DnaJB1-Apg2 chaperone system in disaggregating -synuclein fibrils involves the removal of monomer units without any intermediate fragmentation steps. These findings are fundamental to our understanding of the suppression of amyloid proliferation early in life and the natural clearance mechanisms of fibrillar deposits in Parkinsons disease, and inform on the possibilities and limitations of this strategy in the development of therapeutics against synucleinopathies and related neurodegenerative diseases.

biophysics↗

bioRxiv2020

Dual Role of a Rubisco Activase in Metabolic Repair and Carboxysome Organization

Rubisco, the key enzyme of CO2 fixation in photosynthesis, is prone to inactivation by inhibitory sugar phosphates. Inhibited Rubisco undergoes conformational repair by the hexameric AAA+ chaperone Rubisco activase (Rca) in a process that is not well understood. Here we performed a structural and mechanistic analysis of cyanobacterial Rca, a close homolog of plant Rca. In the Rca:Rubisco complex, Rca is positioned over the Rubisco catalytic site under repair and pulls the N-terminal tail of the large Rubisco subunit (RbcL) into the hexamer pore. Simultaneous displacement of the C-terminus of the adjacent RbcL opens the catalytic site for inhibitor release. An alternative interaction of Rca with Rubisco is mediated by C-terminal domains that resemble the small Rubisco subunit. These domains, together with the N-terminal AAA+ hexamer, ensure that Rca is packaged with Rubisco into carboxysomes. Cyanobacterial Rca is a dual-purpose protein with functions in Rubisco repair and carboxysome organization.

biochemistry↗

bioRxiv2020

Structure and conformational cycle of a bacteriophage-encoded chaperonin

Chaperonins are ubiquitous molecular chaperones found in all domains of life. They form ring-shaped complexes that assist in the folding of substrate proteins in an ATP-dependent reaction cycle. Key to the folding cycle is the transient encapsulation of substrate proteins by the chaperonin. Here we present a structural and functional characterization of the chaperonin gp146 ({phi}EL) from the phage EL of Pseudomonas aeruginosa. {phi}EL, an evolutionary distant homolog of bacterial GroEL, is active in ATP hydrolysis and prevents the aggregation of denatured protein in a nucleotide-dependent manner. However, {phi}EL failed to refold the encapsulation-dependent model substrate rhodanese and did not interact with E. coli GroES, the lid-shaped co-chaperone of GroEL. {phi}EL forms tetradecameric double-ring complexes, which dissociate into single rings in the presence of ATP. Crystal structures of {phi}EL (at 3.54 and 4.03 [A]) in presence of ATP*BeFx revealed two distinct single-ring conformational states, both with open access to the ring cavity. One state showed uniform ATP-bound subunit conformations (symmetric state), whereas the second combined distinct ATP- and ADP-bound subunit conformations (asymmetric state). Cryo-electron microscopy of apo-{phi}EL revealed a double-ring structure composed of rings in the asymmetric state (3.45 [A] resolution). We propose that the phage chaperonin undergoes nucleotide-dependent conformational switching between double- and single rings and functions in aggregation prevention without substrate protein encapsulation. Thus, {phi}EL may represent an evolutionary more ancient chaperonin prior to acquisition of the encapsulation mechanism.

biochemistry↗