Activation of Munc13-1 by Diacylglycerol (DAG)-Lactones.

Munc13-1 is a key protein necessary for vesicle fusion and neurotransmitter release in the brain. Diacylglycerol (DAG)/phorbol ester binds to its C1 domain in the plasma membrane and activates it. The C1 domain of Munc13-1 and protein kinase C (PKC) are homologous in terms of sequence and structure. In order to identify small-molecule modulators of Munc13-1 targeting the C1 domain, we studied the effect of three DAG-lactones, (R,Z)-(2-(hydroxymethyl)-4-(3-isobutyl-5-methylhexylidene)-5-oxotetrahydrofuran-2-yl)methyl pivalate (JH-131e-153), (E)-(2-(hydroxymethyl)-4-(3-isobutyl-5-methylhexylidene)-5-oxotetrahydrofuran-2-yl)methyl pivalate (AJH-836), and (E)-(2-(hydroxymethyl)-4-(4-nitrobenzylidene)-5-oxotetrahydrofuran-2-yl)methyl 4-(dimethylamino)benzoate (130C037), on Munc13-1 activation using the ligand-induced membrane translocation assay. JH-131e-153 showed higher activation than AJH-836, and 130C037 was not able to activate Munc13-1. To understand the role of the ligand-binding site residues in the activation process, three alanine mutants were generated. For AJH-836, the order of activation was wild-type (WT) Munc13-1 > R592A > W588A > I590A. For JH-131e-153, the order of activation was WT > I590 ≈ R592A ≈ W588A. Overall, the Z isomer of DAG-lactones showed higher potency than the E isomer and Trp-588, Ile-590, and Arg-592 were important for its binding. When comparing the activation of Munc13-1 and PKC, the order of activation for JH-131e-153 was PKCα > Munc13-1 > PKCε and for AJH-836, the order of activation was PKCε > PKCα > Munc13-1. Molecular docking supported higher binding of JH-131e-153 than AJH-836 with the Munc13-1 C1 domain. Our results suggest that DAG-lactones have the potential to modulate neuronal processes via Munc13-1 and can be further developed for therapeutic intervention for neurodegenerative diseases.

Analysis of a macrophage carbamylated proteome reveals a function in post-translational modification crosstalk.

BACKGROUND: Lysine carbamylation is a biomarker of rheumatoid arthritis and kidney diseases. However, its cellular function is understudied due to the lack of tools for systematic analysis of this post-translational modification (PTM). METHODS: We adapted a method to analyze carbamylated peptides by co-affinity purification with acetylated peptides based on the cross-reactivity of anti-acetyllysine antibodies. We also performed immobilized-metal affinity chromatography to enrich for phosphopeptides, which allowed us to obtain multi-PTM information from the same samples. RESULTS: By testing the pipeline with RAW 264.7 macrophages treated with bacterial lipopolysaccharide, 7,299, 8,923 and 47,637 acetylated, carbamylated, and phosphorylated peptides were identified, respectively. Our analysis showed that carbamylation occurs on proteins from a variety of functions on sites with similar as well as distinct motifs compared to acetylation. To investigate possible PTM crosstalk, we integrated the carbamylation data with acetylation and phosphorylation data, leading to the identification 1,183 proteins that were modified by all 3 PTMs. Among these proteins, 54 had all 3 PTMs regulated by lipopolysaccharide and were enriched in immune signaling pathways, and in particular, the ubiquitin-proteasome pathway. We found that carbamylation of linear diubiquitin blocks the activity of the anti-inflammatory deubiquitinase OTULIN. CONCLUSIONS: Overall, our data show that anti-acetyllysine antibodies can be used for effective enrichment of carbamylated peptides. Moreover, carbamylation may play a role in PTM crosstalk with acetylation and phosphorylation, and that it is involved in regulating ubiquitination in vitro. Video Abstract.

Probing the Diacylglycerol Binding Site of Presynaptic Munc13-1.

Munc13-1 is a presynaptic active zone protein that acts as a master regulator of synaptic vesicle priming and neurotransmitter release in the brain. It has been implicated in the pathophysiology of several neurodegenerative diseases. Diacylglycerol and phorbol ester activate Munc13-1 by binding to its C1 domain. The objective of this study is to identify the structural determinants of ligand binding activity of the Munc13-1 C1 domain. Molecular docking suggested that residues Trp-588, Ile-590, and Arg-592 of Munc13-1 are involved in ligand interactions. To elucidate the role of these three residues in ligand binding, we generated W588A, I590A, and R592A mutants in full-length Munc13-1, expressed them as GFP-tagged proteins in HT22 cells, and measured their ligand-induced membrane translocation by confocal microscopy and immunoblotting. The extent of 1,2-dioctanoyl-sn-glycerol (DOG)- and phorbol ester-induced membrane translocation decreased in the following order: wild type > I590A > W588A > R592A and wild type > W588A > I590A > R592A, respectively. To understand the effect of the mutations on ligand binding, we also measured the DOG binding affinity of the isolated wild-type C1 domain and its mutants in membrane-mimicking micelles using nuclear magnetic resonance methods. The DOG binding affinity decreased in the following order: wild type > I590A > R592A. No binding was detected for W588A with DOG in micelles. This study shows that Trp-588, Ile-590, and Arg-592 are essential determinants for the activity of Munc13-1 and the effects of the three residues on the activity are ligand-dependent. This study bears significance for the development of selective modulators of Munc13-1.

Effect of ethanol on Munc13-1 C1 in Membrane: A Molecular Dynamics Simulation Study.

BACKGROUND: EtOH has a significant effect on synaptic plasticity. Munc13-1 is an essential presynaptic active zone protein involved in priming the synaptic vesicle and releasing neurotransmitter in the brain. It is a peripheral membrane protein and binds to the activator, diacylglycerol (DAG)/phorbol ester at its membrane-targeting C1 domain. Our previous studies identified Glu-582 of C1 domain as the alcohol-binding residue (Das, J. et al, J. Neurochem., 126, 715-726, 2013). METHODS: Here, we describe a 250 ns molecular dynamics (MD) simulation study on the interaction of EtOH and the activator-bound Munc13-1 C1 in the presence of varying concentrations of phosphatidylserine (PS). RESULTS: In this study, Munc13-1 C1 shows higher conformational stability in EtOH than in water. It forms fewer hydrogen bonds with phorbol 13-acetate in the presence of EtOH than in water. EtOH also affected the interaction between the protein and the membrane and between the activator and the membrane. Similar studies in a E582A mutant suggest that these effects of EtOH are mostly mediated through Glu-582. CONCLUSIONS: EtOH forms hydrogen bonds with Glu-582. While occupancy of the EtOH molecules at the vicinity (4Å) of Glu-582 is 34.4%, the occupancy in the E582A mutant is 26.5% of the simulation time. In addition, the amount of PS in the membrane influences the conformational stability of the C1 domain and interactions in the ternary complex. This study is important in providing the structural basis of EtOH's effects on synaptic plasticity.

Munc13 Is a Molecular Target of Bryostatin 1.

Bryostatin 1 is a natural macrolide shown to improve neuronal connections and enhance memory in mice. Its mechanism of action is largely attributed to the modulation of novel and conventional protein kinase Cs (PKCs) by binding to their regulatory C1 domains. Munc13-1 is a C1 domain-containing protein that shares common endogenous and exogenous activators with novel and conventional PKC subtypes. Given the essential role of Munc13-1 in the priming of synaptic vesicles and neuronal transmission overall, we explored the potential interaction between bryostatin 1 and Munc13-1. Our results indicate that in vitro bryostatin 1 binds to both the isolated C1 domain of Munc13-1 ( Ki = 8.07 ± 0.90 nM) and the full-length Munc13-1 protein ( Ki = 0.45 ± 0.04 nM). Furthermore, confocal microscopy and immunoblot analysis demonstrated that in intact HT22 cells bryostatin 1 mimics the actions of phorbol esters, a previously established class of Munc13-1 activators, and induces plasma membrane translocation of Munc13-1, a hallmark of its activation. Consistently, bryostatin 1 had no effect on the Munc13-1H567K construct that is insensitive to phorbol esters. Effects of bryostatin 1 on the other Munc13 family members, ubMunc13-2 and bMunc13-2, resembled those of Munc13-1 for translocation. Lastly, we observed an increased level of expression of Munc13-1 following a 24 h incubation with bryostatin 1 in both HT22 and primary mouse hippocampal cells. This study characterizes Munc13-1 as a molecular target of bryostatin 1. Considering the crucial role of Munc13-1 in neuronal function, these findings provide strong support for the potential role of Munc13s in the actions of bryostatin 1.

Critical Role of Trp-588 of Presynaptic Munc13-1 for Ligand Binding and Membrane Translocation.

Munc13-1 is a presynaptic active-zone protein essential for neurotransmitter release and presynaptic plasticity in the brain. This multidomain scaffold protein contains a C1 domain that binds to the activator diacylglycerol/phorbol ester. Although the C1 domain bears close structural homology with the C1 domains of protein kinase C (PKC), the tryptophan residue at position 22 (588 in the full-length Munc13-1) occludes the activator binding pocket, which is not the case for PKC. To elucidate the role of this tryptophan, we generated W22A, W22K, W22D, W22Y, and W22F substitutions in the full-length Munc13-1, expressed the GFP-tagged constructs in Neuro-2a cells, and measured their membrane translocation in response to phorbol ester treatment by imaging of the live cells using confocal microscopy. The extent of membrane translocation followed the order, wild-type > W22K > W22F > W22Y > W22A > W22D. The phorbol ester binding affinity of the wild-type Munc13-1C1 domain and its mutants was phosphatidylserine (PS)-dependent following the order, wild-type > W22K > W22A ≫ W22D in both 20% and 100% PS. Phorbol ester affinity was higher for Munc13-1 than the C1 domain. While Munc13-1 translocated to the plasma membrane, the C1 domain translocated to internal membranes in response to phorbol ester. Molecular dynamics (80 ns) studies reveal that Trp-22 is relatively less flexible than the homologous Trp-22 of PKCδ and PKCθ. Results are discussed in terms of the overall negative charge state of the Munc13-1C1 domain and its possible interaction with the PS-rich plasma membrane. This study shows that Trp-588 is an important structural element for ligand binding and membrane translocation in Munc13-1.

Structural determinants of phorbol ester binding activity of the C1a and C1b domains of protein kinase C theta.

The PKC isozymes represent the most prominent family of signaling proteins mediating response to the ubiquitous second messenger diacylglycerol. Among them, PKCθ is critically involved in T-cell activation. Whereas all the other conventional and novel PKC isoforms have twin C1 domains with potent binding activity for phorbol esters, in PKCθ only the C1b domain possesses potent binding activity, with little or no activity reported for the C1a domain. In order to better understand the structural basis accounting for the very weak ligand binding of the PKCθ C1a domain, we assessed the effect on ligand binding of twelve amino acid residues which differed between the C1a and C1b domains of PKCθ. Mutation of Pro9 of the C1a domain of PKCθ to the corresponding Lys9 found in C1b restored in vitro binding activity for [3H]phorbol 12,13-dibutyrate to 3.6 nM, whereas none of the other residues had substantial effect. Interestingly, the converse mutation in the C1b domain of Lys9 to Pro9 only diminished binding affinity to 11.7 nM, compared to 254 nM in the unmutated C1a. In confocal experiments, deletion of the C1b domain from full length PKCθ diminished, whereas deletion of the C1a domain enhanced 5-fold (at 100 nM PMA) the translocation to the plasma membrane. We conclude that the Pro168 residue in the C1a domain of full length PKCθ plays a critical role in the ligand and membrane binding, while exchanging the residue (Lys240) at the same position in C1b domain of full length PKCθ only modestly reduced the membrane interaction.

Resveratrol inhibits phorbol ester-induced membrane translocation of presynaptic Munc13-1.

BACKGROUND: Resveratrol (1) is a naturally occurring polyphenol that has been implicated in neuroprotection. One of resveratrol's several biological targets is Ca2+-sensitive protein kinase C alpha (PKCα). Resveratrol inhibits PKCα by binding to its activator-binding C1 domain. Munc13-1 is a C1 domain-containing Ca2+-sensitive SNARE complex protein essential for vesicle priming and neurotransmitter release. METHODS: To test if resveratrol could also bind and inhibit Munc13-1, we studied the interaction of resveratrol and its derivatives, (E)-1,3-dimethoxy-5-(4-methoxystyryl)benzene, (E)-5,5'-(ethene-1,2-diyl)bis(benzene-1,2,3-triol), (E)-1,2-bis(3,4,5-trimethoxyphenyl)ethane, and (E)-5-(4-(hexadecyloxy)-3,5-dihydroxystyryl)benzene-1,2,3-triol with Munc13-1 by studying its membrane translocation from cytosol to plasma membrane in HT22 cells and primary hippocampal neurons. RESULTS: Resveratrol, but not the derivatives inhibited phorbol ester-induced Munc13-1 translocation from cytosol to membrane in HT22 cells and primary hippocampal neurons, as evidenced by immunoblot analysis and confocal microscopy. Resveratrol did not show any effect on Munc13-1H567K, a mutant which is not sensitive to phorbol ester. Binding studies with Munc13-1 C1 indicated that resveratrol competes with phorbol ester for the binding site. Molecular docking and dynamics studies suggested that hydroxyl groups of resveratrol interact with phorbol-ester binding residues in the binding pocket. CONCLUSIONS AND SIGNIFICANCE: This study characterizes Munc13-1 as a target of resveratrol and highlights the importance of dietary polyphenol in the management of neurodegenerative diseases.

Curcumin Inhibits Protein Kinase Cα Activity by Binding to Its C1 Domain.

Curcumin is a polyphenolic nutraceutical that acts on multiple biological targets, including protein kinase C (PKC). PKC is a family of serine/threonine kinases central to intracellular signal transduction. We have recently shown that curcumin selectively inhibits PKCα, but not PKCε, in CHO-K1 cells [Pany, S. (2016) Biochemistry 55, 2135-2143]. To understand which domain(s) of PKCα is responsible for curcumin binding and inhibitory activity, we made several domain-swapped mutants in which the C1 (combination of C1A and C1B) and C2 domains are swapped between PKCα and PKCε. Phorbol ester-induced membrane translocation studies using confocal microscopy and immunoblotting revealed that curcumin inhibited phorbol ester-induced membrane translocation of PKCε mutants, in which the εC1 domain was replaced with αC1, but not the PKCα mutant in which αC1 was replaced with the εC1 domain, suggesting that αC1 is a determinant for curcumin's inhibitory effect. In addition, curcumin inhibited membrane translocation of PKCε mutants, in which the εC1A and εC1B domains were replaced with the αC1A and αC1B domains, respectively, indicating the role of both αC1A and αC1B domains in curcumin's inhibitory effects. Phorbol 13-acetate inhibited the binding of curcumin to αC1A and αC1B with IC50 values of 6.27 and 4.47 µM, respectively. Molecular docking and molecular dynamics studies also supported the higher affinity of curcumin for αC1B than for αC1A. The C2 domain-swapped mutants were inactive in phorbol ester-induced membrane translocation. These results indicate that curcumin binds to the C1 domain of PKCα and highlight the importance of this domain in achieving PKC isoform selectivity.

Reduction of chemokine CXCL9 expression by omega-3 fatty acids via ADP-ribosylhydrolase ARH3 in MIN6 insulin-producing cells.

Type 1 diabetes (T1D) results from the autoimmune destruction of the insulin producing ß cells of the pancreas. Omega-3 fatty acids protect ß cells and reduce the incident of T1D. However, how omega-3 fatty acids act on ß cells is not well understood. We have shown that omega-3 fatty acids reduce pro-inflammatory cytokine-mediated ß-cell apoptosis by upregulating the expression of the ADP-ribosylhydrolase ARH3. Here, we further investigate the ß-cell protection mechanism by ARH3 by performing siRNA of its gene Adprhl2 in MIN6 insulin-producing cells followed by treatment with a cocktail of the pro-inflammatory cytokines IL-1ß + IFN-γ + TNF-α, and proteomics analysis. ARH3 regulated proteins from several pathways related to the nucleus (splicing, RNA surveillance and nucleocytoplasmic transport), mitochondria (metabolic pathways) and endoplasmic reticulum (protein folding). ARH3 also regulated the levels of cytokine-signaling proteins related to the antigen processing and presentation, and chemokine-signaling pathway. We further studied the role of ARH in regulating the chemokine CXCL9. We confirmed that ARH3 reduces the cytokine-induced expression of CXCL9 by ELISA. We also found that CXCL9 expression is regulated by omega-3 fatty acids. In conclusion, we showed that omega-3 fatty acids regulate CXCL9 expression via ARH3, which might have a role in protecting ß cells from immune attack and preventing T1D development.

Regulation of bacterial stringent response by an evolutionarily conserved ribosomal protein L11 methylation.

Lysine and arginine methylation is an important regulator of enzyme activity and transcription in eukaryotes. However, little is known about this covalent modification in bacteria. In this work, we investigated the role of methylation in bacteria. By reanalyzing a large phyloproteomics data set from 48 bacterial strains representing six phyla, we found that almost a quarter of the bacterial proteome is methylated. Many of these methylated proteins are conserved across diverse bacterial lineages, including those involved in central carbon metabolism and translation. Among the proteins with the most conserved methylation sites is ribosomal protein L11 (bL11). bL11 methylation has been a mystery for five decades, as the deletion of its methyltransferase PrmA causes no cell growth defects. Comparative proteomics analysis combined with inorganic polyphosphate and guanosine tetra/pentaphosphate assays of the ΔprmA mutant in Escherichia coli revealed that bL11 methylation is important for stringent response signaling. In the stationary phase, we found that the ΔprmA mutant has impaired guanosine tetra/pentaphosphate production. This leads to a reduction in inorganic polyphosphate levels, accumulation of RNA and ribosomal proteins, and an abnormal polysome profile. Overall, our investigation demonstrates that the evolutionarily conserved bL11 methylation is important for stringent response signaling and ribosomal activity regulation and turnover. IMPORTANCE: Protein methylation in bacteria was first identified over 60 years ago. Since then, its functional role has been identified for only a few proteins. To better understand the functional role of methylation in bacteria, we analyzed a large phyloproteomics data set encompassing 48 diverse bacteria. Our analysis revealed that ribosomal proteins are often methylated at conserved residues, suggesting that methylation of these sites may have a functional role in translation. Further analysis revealed that methylation of ribosomal protein L11 is important for stringent response signaling and ribosomal homeostasis.

Exploring new frontiers in type 1 diabetes through advanced mass-spectrometry-based molecular measurements.

Type 1 diabetes (T1D) is a devastating autoimmune disease for which advanced mass spectrometry (MS) methods are increasingly used to identify new biomarkers and better understand underlying mechanisms. For example, integration of MS analysis and machine learning has identified multimolecular biomarker panels. In mechanistic studies, MS has contributed to the discovery of neoepitopes, and pathways involved in disease development and identifying therapeutic targets. However, challenges remain in understanding the role of tissue microenvironments, spatial heterogeneity, and environmental factors in disease pathogenesis. Recent advancements in MS, such as ultra-fast ion-mobility separations, and single-cell and spatial omics, can play a central role in addressing these challenges. Here, we review recent advancements in MS-based molecular measurements and their role in understanding T1D.

Comparison of the ligand binding site of C1 domains: a molecular dynamics simulation study of the C1 domain-phorbol 13-acetate-membrane system.

C1 domains are lipid-binding structural units of about 50 residues. Typical C1 domains associate with the plasma membrane and bind to diacylglycerol/phorbol ester during the activation of the proteins containing these domains. Although the overall structure of the C1 domains are similar, there are differences in their primary sequence and in the orientation of the ligand/lipid binding residues. To gain structural insights into the ligand/lipid binding, we performed molecular docking of phorbol 13-acetate into the C1 domain and 1.0 µs molecular dynamics simulation on the C1 domain-ligand-lipid ternary system for PKCθ C1A, PKCδ C1B, PKCßII C1B, PKCθ C1B, Munc13-1 C1, and ßII-Chimaerin C1. We divided these C1 domains into three types based on the orientations of Gln-27 and Trp/Tyr-22. In type 1, Trp/Tyr-22 is outside and Gln-27 is inside the ligand binding pocket. In type 2, both Trp/Tyr-22 and Gln-27 are outside the ligand binding pocket, and in type 3, Trp/Tyr-22 is inside and Gln-27 is outside the pocket. The type 1 C1 domains showed higher ligand binding and higher membrane binding with a shorter distance between the C1 domain and the membrane than the type 2 and type 3. For ligand binding, Pro-11 plays a major role in the type 1 and 2, and Gly-23 in the type 1 and type 3 C1 domains. This study elucidates the role of Gln-27, Trp-22, Pro-11 and Gly-23 in ligand/lipid binding in typical C1 domains and bears significance in developing selective modulators of C1 domain-containing proteins.Communicated by Ramaswamy H. Sarma.

Analysis of a macrophage carbamylated proteome reveals a function in post-translational modification crosstalk.

Background. Lysine carbamylation is a biomarker of rheumatoid arthritis and kidney diseases. However, its cellular function is understudied due to the lack of tools for systematic analysis of this post-translational modification (PTM). Methods. We adapted a method to analyze carbamylated peptides by co-affinity purification with acetylated peptides based on the cross-reactivity of anti-acetyllysine antibodies. We integrated this method into a mass spectrometry-based multi-PTM pipeline to simultaneously analyze carbamylated and acetylated peptides in addition to phosphopeptides were enriched by sequential immobilized-metal affinity chromatography. Results. By testing the pipeline with RAW 264.7 macrophages treated with bacterial lipopolysaccharide, 7,299, 8,923 and 47,637 acetylated, carbamylated, and phosphorylated peptides were identified, respectively. Our analysis showed that carbamylation occurs on proteins from a variety of functions on sites with similar as well as distinct motifs compared to acetylation. To investigate possible PTM crosstalk, we integrated the carbamylation data with acetylation and phosphorylation data, leading to the identification 1,183 proteins that were modified by all 3 PTMs. Among these proteins, 54 had all 3 PTMs regulated by lipopolysaccharide and were enriched in immune signaling pathways, and in particular, the ubiquitin-proteasome pathway. We found that carbamylation of linear diubiquitin blocks the activity of the anti-inflammatory deubiquitinase OTULIN. Conclusions Overall, our data show that anti-acetyllysine antibodies can be used for effective enrichment of carbamylated peptides. Moreover, carbamylation may play a role in PTM crosstalk with acetylation and phosphorylation, and that it is involved in regulating ubiquitination in vitro .

A proteomic meta-analysis refinement of plasma extracellular vesicles.

Extracellular vesicles play major roles in cell-to-cell communication and are excellent biomarker candidates. However, studying plasma extracellular vesicles is challenging due to contaminants. Here, we performed a proteomics meta-analysis of public data to refine the plasma EV composition by separating EV proteins and contaminants into different clusters. We obtained two clusters with a total of 1717 proteins that were depleted of known contaminants and enriched in EV markers with independently validated 71% true-positive. These clusters had 133 clusters of differentiation (CD) antigens and were enriched with proteins from cell-to-cell communication and signaling. We compared our data with the proteins deposited in PeptideAtlas, making our refined EV protein list a resource for mechanistic and biomarker studies. As a use case example for this resource, we validated the type 1 diabetes biomarker proplatelet basic protein in EVs and showed that it regulates apoptosis of ß cells and macrophages, two key players in the disease development. Our approach provides a refinement of the EV composition and a resource for the scientific community.

Molecular dynamics simulation studies on binding of activator and inhibitor to Munc13-1 C1 in the presence of membrane.

Munc13-1 is a presynaptic active zone protein that plays a critical role in priming the synaptic vesicle and releasing neurotransmitters in the brain. Munc13-1 acts as a scaffold and is activated when diacylglycerol (DAG)/phorbol ester binds to its C1 domain in the plasma membrane. Our previous studies showed that bryostatin 1 activated the Munc13-1, but resveratrol inhibited the phorbol ester-induced Munc13-1 activity. To gain structural insights into the binding of the ligand into Munc13-1 C1 in the membrane, we conducted 1.0 µs molecular dynamics (MD) simulation on Munc13-1 C1-ligand-lipid ternary system using phorbol 13-acetate, bryostatin 1 and resveratrol as ligands. Munc13-1 C1 shows higher conformational stability and less mobility along membrane with phorbol 13-acetate and bryostatin 1 than with resveratrol. Bryostatin 1 and phorbol ester remained in the protein active site, but resveratrol moved out of Munc13-1 C1 during the MD simulation. While bryostatin 1-bound Munc13-1 C1 showed two different positioning in the membrane, phorbol 13-acetate and resveratrol-bound Munc13-1 C1 only showed one positioning. Phorbol 13-acetate formed hydrogen bond with Ala-574 and Gly-589. Bryostatin 1 had more hydrogen bonds with Trp-588 and Arg-592 than with other residues. Resveratrol formed hydrogen bond with Ile-590. This study suggests that different ligands control Munc13-1 C1's mobility and positioning in the membrane differently. Ligand also has a critical role in the interaction between Munc13-1 C1 and lipid membrane. Our results provide structural basis of the pharmacological activity of the ligands and highlight the importance of membrane in Munc13-1 activity.Communicated by Ramaswamy H. Sarma.

Modulation of proteasome activity by curcumin and didemethylcurcumin.

Modulation of proteasome function by pharmacological interventions and molecular biology tools is an active area of research in cancer biology and neurodegenerative diseases. Curcumin (diferuloylmethane) is a naturally occurring polyphenol that affects multiple signaling pathways. Curcumin shows anti-inflammatory, antioxidant, anti-angiogenic, or anti-apoptotic properties. Recent research suggests that the therapeutic efficacy of curcumin may be due to its activity as a potent inhibitor of the proteasome. Using in vitro cell culture and molecular docking methods, here we show that both curcumin and its synthetic polyphenolic derivative (didemethylcurcumin, CUIII) modulated proteasome activity in a biphasic manner. Curcumin and CUIII increased proteasome activity at nanomolar concentrations, but inhibited proteasome activity at micromolar concentrations. Curcumin was more effective than CUIII in increasing relative proteasome activity at nanomolar concentrations. Also, curcumin was more effective than CUIII in inhibiting relative proteasome activity at micromolar concentrations. Docking simulations of curcumin and didemethylcurcumin binding to the 20S proteasome catalytic subunit estimated Kd values of 0.0054 µM and 1.3167 µM, respectively. These values correlate well with the results of the effectiveness of modulation by curcumin compared to CUIII. The small size of CUIII allows it to dock to the narrow cavity of the active site, but the binding interaction is not strong compared to curcumin. These results indicate that curcumin and its didemethyl derivative can be used to modulate proteasome activity and suggest that curcumin and its didemethyl derivative may be useful in treating two different disease classes: neurodegeneration and cancer.Communicated by Ramaswamy H. Sarma.