SUMMARY The structural basis by which Hsp104 dissolves disordered aggregates and prions is unknown. A single subunit within the Hsp104 hexamer can solubilize disordered aggregates, whereas prion dissolution requires collaboration by multiple Hsp104 subunits. Here, we establish that the poorly understood Hsp104 N-terminal domain (NTD) enables this operational plasticity. Hsp104 lacking the NTD (Hsp104ΔN) dissolves disordered aggregates but cannot dissolve prions or be potentiated by activating mutations. We define how Hsp104ΔN invariably stimulates Sup35 prionogenesis by fragmenting prions without solubilizing Sup35, whereas Hsp104 couples Sup35 prion fragmentation and dissolution. Volumetric reconstruction of Hsp104 hexamers in ATPγS, ADP-AlFx (hydrolysis transition state mimic), and ADP via small-angle X-ray scattering revealed a peristaltic pumping motion upon ATP hydrolysis, which drives directional substrate translocation through the central Hsp104 channel and is profoundly altered in Hsp104ΔN. We establish that the Hsp104 NTD enables cooperative substrate translocation, which is critical for prion dissolution and potentiated disaggregase activity.

Liquid-ordered (Lo) and liquid-disordered (Ld) phase coexistence has been suggested to partition the plasma membrane of biological cells into lateral compartments, allowing for enrichment or depletion of functionally relevant molecules. This dynamic partitioning might be involved in fine-tuning cellular signaling fidelity through coupling to the plasma membrane protein and lipid composition. In earlier work, giant plasma membrane vesicles, obtained by chemically induced blebbing from cultured cells, were observed to reversibly phase segregate at temperatures significantly below 37 °C. In this contribution, we compare the temperature dependence of fluid phase segregation in HeLa and Rat basophilic leukemia (RBL) cells. We find an essentially monotonic temperature dependence of the number of phase separated vesicles in both cell types. We also observe a strikingly broad distribution of phase transition temperatures in both cell types. The binding of peripheral proteins, such as cholera toxin subunit B (CTB), as well as annexin V, is observed to modulate phase transition temperatures, indicating that peripheral protein binding may be a regulator for lateral heterogeneity in vivo. The partitioning of numerous signal protein anchors and full length proteins is investigated. We find Lo phase partitioning for several proteins assumed in the literature to be membrane raft associated, but observe deviations from this expectation for other proteins, including caveolin-1.

Background: How Hsp104 and ClpB coordinate polypeptide handover with Hsp70 to dissolve disordered protein aggregates is unknown.Results: Conserved distal loop residues in the Hsp104 and ClpB middle domain contact NBD2 and enable Hsp70-dependent protein disaggregation.Conclusion: Distal loop does not project out into solution and Hsp104 and ClpB are tuned differently for Hsp70 collaboration.Significance: Understanding how protein disaggregases operate may empower strategies to counter protein-misfolding disorders.

Potentiated variants of Hsp104, a protein disaggregase from yeast, can dissolve protein aggregates connected to neurodegenerative diseases such as Parkinson disease and amyotrophic lateral sclerosis. However, the mechanisms underlying Hsp104 potentiation remain incompletely defined. Here, we establish that 2-3 subunits of the Hsp104 hexamer must bear an A503V potentiating mutation to elicit enhanced disaggregase activity in the absence of Hsp70. We also define the ATPase and substratebinding modalities needed for potentiated Hsp104 A503V activity in vitro and in vivo. Hsp104 A503V disaggregase activity is strongly inhibited by the Y257A mutation that disrupts substrate binding to the nucleotide-binding domain 1 (NBD1) pore loop and is abolished by the Y662A mutation that disrupts substrate binding to the NBD2 pore loop. A503V displays a more robust activity that is unperturbed by sensor-1 mutations that greatly reduce Hsp104 activity in vivo. Indeed, ATPase activity at NBD1 or NBD2 is sufficient for Hsp104 potentiation. Our findings will empower design of ameliorated therapeutic disaggregases for various neurodegenerative diseases.Protein misfolding and aggregation are associated with a wide variety of diseases, ranging from type II diabetes (1, 2) to neurodegenerative diseases, such as fatal familial insomnia (3, 4), Parkinson disease, and amyotrophic lateral sclerosis (ALS) (5-7). In Parkinson disease patients, ␣-synuclein (␣-syn) 6 forms toxic soluble oligomers as well as amyloid structures that accumulate in Lewy bodies and contribute to the death of dopaminergic neurons (8 -12). Similarly, toxic soluble oligomers and cytoplasmic inclusions of TDP-43 or FUS are associated with ALS and frontotemporal dementia (13-20). These misfolded protein conformers are recalcitrant and represent a colossal roadblock in the treatment of these diseases.Hsp104 is a 102-kDa AAAϩ ATPase (21) from Saccharomyces cerevisiae capable of dissolving disordered protein aggregates as well as dismantling amyloid fibrils and toxic soluble oligomers (22-33). It assembles into a homohexameric barrel structure with a central channel (34 -39). Hsp104 processes protein aggregates by directly translocating substrates either partially or completely through this channel (35, 36, 40 -46). Hsp104 encompasses an N-terminal domain, two nucleotidebinding domains (NBD1 and NBD2), a coiled-coil middle domain (MD), and a C-terminal domain important for oligomerization (Fig. 1A) (47). Both NBDs contain Walker A and Walker B motifs that are critical for nucleotide binding and hydrolysis, respectively (48). ATP hydrolysis takes place primarily at NBD1, whereas NBD2 has a nucleotide-dependent oligomerization function (29, 34, 49 -52).Remarkably, Hsp104 can remodel amyloid substrates alone, without the aid of any other chaperones (22,24,26,31,33,45,(53)(54)(55)(56). However, to disaggregate amorphous protein aggregates, Hsp104 usually needs to collaborate with the Hsp110, Hsp70, and Hsp40 chaperone system (23,26,30,32,57). Moreover, small heat shock proteins...

Microglia-mediated clearance of amyloid beta-protein (Aβ) via Toll-like receptor 4 (TLR4) signaling may play an important role in the pathogenesis of Alzheimer’s disease (AD). However, as the disease progresses, activated microglia appear to become incapable of clearing Aβ deposits. Because repeated exposure to a TLR4 ligand leads to a diminished response of monocytes/macrophages to lipopolysaccharide (LPS) and because aggregated Aβ is a TLR4 ligand, we hypothesize that chronic exposure of microglia to Aβ deposits may induce a state of Toll-like receptor (TLR) signaling dysfunction, leading to decreased Aβ clearance and accelerated disease progression. LPS or phosphate-buffered saline (PBS) was injected into the hippocampus of AD-model (TgAPP/PS1) and wild-type (non-Tg) mice before and after the onset of Aβ deposition, at age 2 and 12 months, respectively. Brain specimens were collected 7 days post-injection and analyzed for microglial activation and Aβ load. While LPS-injected 2-month-old non-Tg mice showed 48-fold and 11-fold greater Iba1 immunoreactivity in the neocortex and hippocampus, respectively, compared with PBS-injected mice, LPS-injected 2-month-old TgAPP/PS1 mice had 61-fold and 13-fold increases in the neocortex and hippocampus, respectively. LPS injection activated microglia more strongly in TgAPP/PS1 mice than in non-Tg mice at 2 months of age. In contrast, at 12 months of age, Iba1 immunoreactivity of microglia was increased 541-fold and 38-fold in the neocortex and hippocampus, respectively, in LPS-injected non-Tg mice and 2.7-fold and 3.3-fold in the neocortex and hippocampus, respectively, in LPS-injected TgAPP/PS1 mice. Surprisingly, LPS injection decreased CD45 immunoreactivity in TgAPP/PS1 mice but increased it in non-Tg mice at 12 months. Although microglia in 12-month-old non-Tg mice showed stronger response to LPS than 2-month-old non-Tg mice, microglia in TgAPP/PS1 mice exhibited diminished immune response to LPS during aging. Our data indicate that microglial TLR4 signaling is altered in an AD mouse model and suggest that altered TLR4 signaling may contribute to Aβ accumulation in the brain.