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BMB has established a partnership with the Washington University Center for Cellular Imaging (WUCCI) through the joint acquisition of a new Glacios 200 kV cryo-EM microscope financed by a strategic investment of the Doisy Fund in Biochemistry. The partnership gives all SLU researchers access to the WUCCI with the same priority, costs, and technical assistance available to WU investigators.
The new Glacios complements the WUCCI’s pre-existing Titan Krios 300 kV cryo-EM and has significantly expanded the workflow of many labs at both WU and SLU, thereby benefiting the entire scientific community of the greater St. Louis region.
Neuronal KCNQ2/3 channels are recruited to lipid raft microdomains by palmitoylation of BACE1
β-Secretase 1 (β-site amyloid precursor protein [APP]-cleaving enzyme 1, BACE1) plays a crucial role in the amyloidogenesis of Alzheimer's disease (AD). BACE1 was also discovered to act like an auxiliary subunit to modulate neuronal KCNQ2/3 channels independently of its proteolytic function. BACE1 is palmitoylated at its carboxyl-terminal region, which brings BACE1 to ordered, cholesterol-rich membrane microdomains (lipid rafts). However, the physiological consequences of this specific localization of BACE1 remain elusive. Using spectral Förster resonance energy transfer (FRET), BACE1 and KCNQ2/3 channels were confirmed to form a signaling complex, a phenomenon that was relatively independent of the palmitoylation of BACE1. Nevertheless, palmitoylation of BACE1 was required for recruitment of KCNQ2/3 channels to lipid-raft domains. Two fluorescent probes, designated L10 and S15, were used to label lipid-raft and non-raft domains of the plasma membrane, respectively. Coexpressing BACE1 substantially elevated FRET between L10 and KCNQ2/3, whereas the BACE1-4C/A quadruple mutation failed to produce this effect. In contrast, BACE1 had no significant effect on FRET between S15 probes and KCNQ2/3 channels. A reduction of BACE1-dependent FRET between raft-targeting L10 probes and KCNQ2/3 channels by applying the cholesterol-extracting reagent methyl-β-cyclodextrin (MβCD), raft-disrupting general anesthetics, or pharmacological inhibitors of palmitoylation, all supported the hypothesis of the palmitoylation-dependent and raft-specific localization of KCNQ2/3 channels. Furthermore, mutating the four carboxyl-terminal cysteines (4C/A) of BACE1 abolished the BACE1-dependent increase of FRET between KCNQ2/3 and the lipid raft-specific protein caveolin 1. Taking these data collectively, we propose that the AD-related protein BACE1 underlies the localization of a neuronal potassium channel.
Hepatic stellate cells in physiology and pathology
Kamm DR and McCommis KS
Hepatic stellate cells (HSCs) comprise a minor cell population in the liver but serve numerous critical functions in the normal liver and in response to injury. HSCs are primarily known for their activation upon liver injury and for producing the collagen-rich extracellular matrix in liver fibrosis. In the absence of liver injury, HSCs reside in a quiescent state, in which their main function appears to be the storage of retinoids or vitamin A-containing metabolites. Less appreciated functions of HSCs include amplifying the hepatic inflammatory response and expressing growth factors that are critical for liver development and both the initiation and termination of liver regeneration. Recent single-cell RNA sequencing studies have corroborated earlier studies indictaing that HSC activation involves a diverse array of phenotypic alterations and identified unique HSC populations. This review serves to highlight these many functions of HSCs, and to briefly describe the recent genetic tools that will help to thoroughly investigate the role of HSCs in hepatic physiology and pathology.
Human HELB is a processive motor protein that catalyzes RPA clearance from single-stranded DNA
Hormeno S, Wilkinson OJ, Aicart-Ramos C, Kuppa S, Antony E, Dillingham MS and Moreno-Herrero F
SignificanceSingle-stranded DNA (ssDNA) is a key intermediate in many cellular DNA transactions, including DNA replication, repair, and recombination. Nascent ssDNA is rapidly bound by the Replication Protein A (RPA) complex, forming a nucleoprotein filament that both stabilizes ssDNA and mediates downstream processing events. Paradoxically, however, the very high affinity of RPA for ssDNA may block the recruitment of further factors. In this work, we show that RPA-ssDNA nucleoprotein filaments are specifically targeted by the human HELB helicase. Recruitment of HELB by RPA-ssDNA activates HELB translocation activity, leading to processive removal of upstream RPA complexes. This RPA clearance activity may underpin the diverse roles of HELB in replication and recombination.
Cryo-EM structure of the prothrombin-prothrombinase complex
Ruben EA, Summers B, Rau MJ, Fitzpatrick J and Di Cera E
The intrinsic and extrinsic pathways of the coagulation cascade converge to a common step where the prothrombinase complex, comprising the enzyme factor Xa (fXa), the cofactor fVa, Ca2+ and phospholipids, activates the zymogen prothrombin to the protease thrombin. The reaction entails cleavage at two sites, R271 and R320, generating the intermediates prethrombin-2 and meizothrombin, respectively. The molecular basis of these interactions that are central to hemostasis remains elusive. We solved two cryo-EM structures of the fVa-fXa complex, one free on nanodiscs at 5.3 Å resolution and the other bound to prothrombin at near atomic 4.1 Å resolution. In the prothrombin-fVa-fXa complex, the Gla domains of fXa and prothrombin align on a plane with the C1 and C2 domains of fVa for interaction with membranes. Prothrombin and fXa emerge from this plane in curved conformations that bring their protease domains in contact with each other against the A2 domain of fVa. The 672ESTVMATRKMHDRLEPEDEE691 segment of the A2 domain closes on the protease domain of fXa like a lid to fix orientation of the active site. The 696YDYQNRL702 segment binds to prothrombin and establishes the pathway of activation by sequestering R271 against D697 and directing R320 toward the active site of fXa. The structure provides a molecular view of prothrombin activation along the meizothrombin pathway and suggests a mechanism for cleavage at the alternative R271 site. The findings advance our basic knowledge of a key step of the coagulation response and bear broad relevance to other macromolecular interactions in the blood.
Alpha-hydroxytropolones are noncompetitive inhibitors of human RNase H1 that bind to the active site and modulate substrate binding
Ponzar NL, Tajwar R, Pozzi N and Tavis JE
The ribonucleases H (RNases H) of HIV and hepatitis B virus are type 1 RNases H that are promising drug targets because inhibiting their activity blocks viral replication. Eukaryotic ribonuclease H1 (RNase H1) is an essential protein and a probable off-target enzyme for viral RNase H inhibitors. α-hydroxytropolones (αHTs) are a class of anti-RNase H inhibitors that can inhibit the HIV, hepatitis B virus, and human RNases H1; however, it is unclear how these inhibitors could be developed to distinguish between these enzymes. To accelerate the development of selective RNase H inhibitors, we performed biochemical and kinetic studies on the human enzyme, which was recombinantly expressed in Escherichia coli. Size-exclusion chromatography showed that free RNase H1 is monomeric and forms a 2:1 complex with a substrate of 12 bp. FRET heteroduplex cleavage assays were used to test inhibition of RNase H1 in steady-state kinetics by two structurally diverse αHTs, 110 and 404. We determined that turnover rate was reduced, but inhibition was not competitive with substrate, despite inhibitor binding to the active site. Given the compounds' reversible binding to the active site, we concluded that traditional noncompetitive and mixed inhibition mechanisms are unlikely. Instead, we propose a model in which, by binding to the active site, αHTs stabilize an inactive enzyme-substrate-inhibitor complex. This new model clarifies the mechanism of action of αHTs against RNase H1 and will aid the development of RNase H inhibitors selective for the viral enzymes.
Hodges WT, Jarasvaraparn C, Ferguson D, Griffett K, Gill LE, Chen Y, Ilagan MXG, Hegazy L, Elgendy B, Cho K, Patti GJ, McCommis KS and Finck BN
The mitochondrial pyruvate carrier (MPC) is an inner mitochondrial membrane complex that plays a critical role in intermediary metabolism. Inhibition of the MPC, especially in liver, may have efficacy for treating type 2 diabetes mellitus. Herein, we examined the antidiabetic effects of zaprinast and 7ACC2, small molecules which have been reported to act as MPC inhibitors. Both compounds activated a bioluminescence resonance energy transfer-based MPC reporter assay (reporter sensitive to pyruvate) and potently inhibited pyruvate-mediated respiration in isolated mitochondria. Furthermore, zaprinast and 7ACC2 acutely improved glucose tolerance in diet-induced obese mice in vivo. Although some findings were suggestive of improved insulin sensitivity, hyperinsulinemic-euglycemic clamp studies did not detect enhanced insulin action in response to 7ACC2 treatment. Rather, our data suggest acute glucose-lowering effects of MPC inhibition may be due to suppressed hepatic gluconeogenesis. Finally, we used reporter sensitive to pyruvate to screen a chemical library of drugs and identified 35 potentially novel MPC modulators. Using available evidence, we generated a pharmacophore model to prioritize which hits to pursue. Our analysis revealed carsalam and six quinolone antibiotics, as well as 7ACC1, share a common pharmacophore with 7ACC2. We validated that these compounds are novel inhibitors of the MPC and suppress hepatocyte glucose production and demonstrated that one quinolone (nalidixic acid) improved glucose tolerance in obese mice. In conclusion, these data demonstrate the feasibility of therapeutic targeting of the MPC for treating diabetes and provide scaffolds that can be used to develop potent and novel classes of MPC inhibitors.
Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology
Aspatwar A, Tolvanen MEE, Barker H, Syrjänen L, Valanne S, Purmonen S, Waheed A, Sly WS and Parkkila S
During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.
Identification of Novel Mitochondrial Pyruvate Carrier Inhibitors by Homology Modeling and Pharmacophore-Based Virtual Screening
Hegazy L, Gill LE, Pyles KD, Kaiho C, Kchouk S, Finck BN, McCommis KS and Elgendy B
The mitochondrial pyruvate carrier (MPC) is an inner-mitochondrial membrane protein complex that has emerged as a drug target for treating a variety of human conditions. A heterodimer of two proteins, MPC1 and MPC2, comprises the functional MPC complex in higher organisms; however, the structure of this complex, including the critical residues that mediate binding of pyruvate and inhibitors, remain to be determined. Using homology modeling, we identified a putative substrate-binding cavity in the MPC dimer. Three amino acid residues (Phe66 (MPC1) and Asn100 and Lys49 (MPC2)) were validated by mutagenesis experiments to be important for substrate and inhibitor binding. Using this information, we developed a pharmacophore model and then performed a virtual screen of a chemical library. We identified five new non-indole MPC inhibitors, four with IC values in the nanomolar range that were up to 7-fold more potent than the canonical inhibitor UK-5099. These novel compounds possess drug-like properties and complied with Lipinski's Rule of Five. They are predicted to have good aqueous solubility, oral bioavailability, and metabolic stability. Collectively, these studies provide important information about the structure-function relationships of the MPC complex and for future drug discovery efforts targeting the MPC.
Structural Insight into the Mechanism of PALB2 Interaction with MRG15
Redington J, Deveryshetty J, Kanikkannan L, Miller I and Korolev S
The tumor suppressor protein partner and localizer of BRCA2 (PALB2) orchestrates the interactions between breast cancer susceptibility proteins 1 and 2 (BRCA1, -2) that are critical for genome stability, homologous recombination (HR) and DNA repair. PALB2 mutations predispose patients to a spectrum of cancers, including breast and ovarian cancers. PALB2 localizes HR machinery to chromatin and links it with transcription through multiple DNA and protein interactions. This includes its interaction with MRG15 (Morf-related gene on chromosome 15), which is part of many transcription complexes, including the HAT-associated and the HDAC-associated complexes. This interaction is critical for PALB2 localization in actively transcribed genes, where transcription/replication conflicts lead to frequent replication stress and DNA breaks. We solved the crystal structure of the MRG15 MRG domain bound to the PALB2 peptide and investigated the effect of several PALB2 mutations, including patient-derived variants. PALB2 interacts with an extended surface of the MRG that is known to interact with other proteins. This, together with a nanomolar affinity, suggests that the binding of MRG15 partners, including PALB2, to this region is mutually exclusive. Breast cancer-related mutations of PALB2 cause only minor attenuation of the binding affinity. New data reveal the mechanism of PALB2-MRG15 binding, advancing our understanding of PALB2 function in chromosome maintenance and tumorigenesis.