Crystal structure of the GDSL family esterase EstL5 in complex with PMSF reveals a branch channel of the active site pocket.

Esterases/lipases from the GDSL family have potential applications in the hydrolysis and synthesis of important esters of pharmaceutical, food, and biotechnical interests. However, the structural and functional understanding of GDSL enzymes is still limited. Here, we report the crystal structure of the GDSL family esterase EstL5 complexed with PMSF at 2.34 Šresolution. Intriguingly, the PMSF binding site is not located at the active site pocket but is situated in a surface cavity. At the active site, we note that there is a trapped crystallization solvent 1,6-hexanediol, which mimics the bound ester chain, allowing for further definition of the active site pocket of EstL5. The most striking structural feature of EstL5 is the presence of a unique channel, which extends approximately 18.9 Å, with a bottleneck radius of 6.8 Å, connecting the active-site pocket and the surface cavity. Replacement of Ser205 with the bulk aromatic residue Trp or Phe could partially block the channel at one end and perturb its access. Reduced enzymatic activity is found in the EstL5 S205W and EstL5 S205F mutants, suggesting the functional relevance of the channel to enzyme catalysis. Our study provides valuable information regarding the properties of the GDSL-family enzymes for designing more efficient and robust biocatalysts.

Ajugol enhances TFEB-mediated lysosome biogenesis and lipophagy to alleviate non-alcoholic fatty liver disease.

Lipophagy is the autophagic degradation of lipid droplets. Dysregulated lipophagy has been implicated in the development of non-alcoholic fatty liver disease (NAFLD). Ajugol is an active alkaloid isolated from the root of Rehmannia glutinosa which is commonly used to treat various inflammatory and metabolic diseases. This study aimed to investigate the effect of ajugol on alleviating hepatic steatosis and sought to determine whether its potential mechanism via the key lysosome-mediated process of lipophagy. Our findings showed that ajugol significantly improved high-fat diet-induced hepatic steatosis in mice and inhibited palmitate-induced lipid accumulation in hepatocytes. Further analysis found that hepatic steatosis promoted the expression of LC3-II, an autophagosome marker, but led to autophagic flux blockade due to a lack of lysosomes. Ajugol also enhanced lysosomal biogenesis and promoted the fusion of autophagosome and lysosome to improve impaired autophagic flux and hepatosteatosis. Mechanistically, ajugol inactivated mammalian target of rapamycin and induced nuclear translocation of the transcription factor EB (TFEB), an essential regulator of lysosomal biogenesis. siRNA-mediated knockdown of TFEB significantly abrogated ajugol-induced lysosomal biogenesis as well as autophagosome-lysosome fusion and lipophagy. We conclude that lysosomal deficit is a critical mediator of hepatic steatosis, and ajugol may alleviate NAFLD via promoting the TFEB-mediated autophagy-lysosomal pathway and lipophagy.

Activated AMPK by metformin protects against fibroblast proliferation during pulmonary fibrosis by suppressing FOXM1.

Pulmonary fibrosis (PF) is a progressive and devastating lung disease of unknown etiology, excessive fibroblast proliferation serves as a key event to promote PF. Transcription factor forkhead box M1 (FOXM1) is not only a well-known proto-oncogene, but also an essential driver of cell proliferation. Recently, 5'-AMP-activated protein kinase (AMPK) is reported to reduce the incidence of PF. However, it remains elusive whether have an underlying relationship between AMPK and FOXM1 in fibroblast proliferation-mediated PF. Here, the progression of lung fibroblast proliferation and the expression levels of AMPK and FOXM1 were observed by intratracheally instilled of bleomycin (BLM) and intraperitoneal injection of metformin in C57BL/6 J mice. Meanwhile, human fetal lung fibroblast1 (HFL1) cells were respectively treated with AMPK activator metformin or AMPK inhibitor Compound C, or FOXM1 depletion by transfected small interfering RNA (siRNA) to unveil roles of AMPK, FOXM1 and the link between them on platelet-derived growth factor (PDGF)-induced fibroblast proliferation. Our results demonstrated that AMPK activated by metformin could down-regulate FOXM1 and alleviate BLM-induced mouse PF model. In vitro, activation of AMPK attenuated PDGF-induced fibroblast proliferation accompanied by the down-regulation of FOXM1. In contrast, inhibition of AMPK enhanced PDGF-induced fibroblast proliferation along with activating FOXM1. These findings suggest that AMPK can ameliorate the progression of fibroblast proliferation during PF via suppressing the expression of FOXM1 and provide new insight into seek PF treatment approaches.

MAP30 protein from Momordica charantia is therapeutic and has synergic activity with cisplatin against ovarian cancer in vivo by altering metabolism and inducing ferroptosis.

Increasing evidence shows that Traditional Chinese Medicine (TCM) has an obvious appeal for cancer treatment, but there is still a lack of scientific investigation of its underlying molecular mechanisms. Bitter melon or bitter gourd (Momordica charantia) is an edible fruit that is commonly consumed, and it is used to cure different diseases in various ancient folk medical practices. We report that a bioactive protein, MAP30, isolated from bitter melon seeds exhibited potent anticancer and anti-chemoresistant effects on ovarian cancer cells. Functional studies revealed that MAP30 inhibited cancer cell migration, cell invasion, and cell proliferation in various ovarian cancer cells but not normal immortalized ovarian epithelial cells. When administered with cisplatin, MAP30 produced a synergistic effect on cisplatin-induced cell cytotoxicity in ovarian cancer cells. When low doses of cisplatin and MAP30 were co-injected intraperitoneally, a remarkable reduction of tumor dissemination and tumor growth was observed in an ovarian cancer ascites mouse model. Notably, blood tests confirmed that MAP30 did not cause any adverse effects on liver and kidney functions in the treated mice. MAP30 activated AMP-activated protein kinase (AMPK) signaling via CaMKKß and induced cell cycle arrest in the S-phase. MAP30 modulated cell metabolism of ovarian cancer cells via suppression of GLUT-1/-3-mediated glucose uptake, adipogenesis, and lipid droplet formation in tumor development and progression. MAP30 also induced an increase in intracellular Ca2+ ion concentration, which triggered ROS-mediated cancer cell death via apoptosis and ferroptosis. Collectively, these findings suggest that natural MAP30 is a non-toxic supplement that may enhance chemotherapeutic outcomes and benefit ovarian cancer patients with peritoneal metastases.

Fraxetin inhibits interleukin-1ß-induced apoptosis, inflammation, and matrix degradation in chondrocytes and protects rat cartilage in vivo.

Osteoarthritis (OA) is a disease characterized by degeneration of the joint complex due to cartilage destruction. Fraxetin, a widely used and studied coumarin compound extracted from a traditional Chinese herb (Qin Pi), has shown anti-inflammatory and antioxidant properties, but its effects on OA have not been studied. In the present study, western blotting, immunofluorescence, and terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) were used to evaluate the effects of fraxetin on IL-1ß-induced apoptotic activity, inflammatory responses, and catabolism in rat chondrocytes. The results showed that fraxetin prevented IL-1ß-induced apoptosis of chondrocytes and inhibited inflammatory mediator release by regulating the Toll-like receptor 4 (TLR4)/myeloid differentiation primary response 88 (MyD88)/nuclear factor (NF)-κB pathway in chondrocytes. Additionally, fraxetin suppressed the upregulation of matrix metalloproteinase 13 (MMP13) and degradation of collagen II in the extracellular matrix (ECM). Moreover, the effects of fraxetin in vivo were assessed in a monosodium iodoacetate (MIA)-induced rat model of OA using hematoxylin and eosin (H&E) and Safranin O-fast green staining and magnetic resonance imaging (MRI). The results showed that fraxetin protected the cartilage against destruction. In conclusion, fraxetin could be a potential therapeutic for OA.

Lipid metabolism alterations in the neuronal response to A53T α-synuclein and Fe-induced injury.

Pathological α-synuclein (α-syn) overexpression and iron (Fe)-induced oxidative stress (OS) are involved in the death of dopaminergic neurons in Parkinson's disease (PD). We have previously characterized the role of triacylglycerol (TAG) formation in the neuronal response to Fe-induced OS. In this work we characterize the role of the α-syn variant A53T during Fe-induced injury and investigate whether lipid metabolism has implications for neuronal fate. To this end, we used the N27 dopaminergic neuronal cell line either untransfected (UT) or stably transfected with pcDNA3 vector (as a transfection control) or pcDNA-A53T-α-syn (A53T α-syn). The overexpression of A53T α-syn triggered an increase in TAG content mainly due to the activation of Acyl-CoA synthetase. Since fatty acid (FA) ß-oxidation and phospholipid content did not change in A53T α-syn cells, the unique consequence of the increase in FA-CoA derivatives was their acylation in TAG moieties. Control cells exposed to Fe-induced injury displayed increased OS markers and TAG content. Intriguingly, Fe exposure in A53T α-syn cells promoted a decrease in OS markers accompanied by α-syn aggregation and elevated TAG content. We report here new evidence of a differential role played by A53T α-syn in neuronal lipid metabolism as related to the neuronal response to OS.

Extraction, purification, and biochemical characterization of serine protease from leaves of Abrus precatorius.

A protease from fresh leaves of Abrus precatorius was purified using two classical chromatography techniques: ion-exchange (DEAE-Sepharose) and Gel filtration (Sephadex G-75). The purified protease showed a molecular weight of ∼ 28 kDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The optimum pH and temperature for the purified protease was 8 and 40°C, respectively. The purified protease was stable throughout a wide temperature range from 10 to 80°C and pH from 2 to 12. Protease activity was inhibited in the presence of Co2+, Ni2+, Hg2+, and Zn2+ while its activity has increased in the presence of Ca2+ and Mg2+. The protease was highly specific to casein when compared to its specificity for gelatin, bovine serum albumin, hemoglobin, and defatted flour of Ricinodendron heudelotii. Its Vmax and Km determined using casein as a substrate were 94.34 U/mL and 349.07 µg/mL respectively. Inhibition studies showed that this purified protease was inhibited by both phenylmethane sulfonyl fluoride and aprotinin which are recognized as competitive inhibitors of serine proteases.

Purification and characterization of a novel trypsin-like protease from green-seeded chickpea (Cicer arientum).

The present study describes the purification and physicochemical and biochemical characterization of trypsin-like protease from green-seeded chickpea (Cicer arientum). The crude extract of chickpea trypsin (CpT) was obtained by homogenization followed by differential ammonium sulfate precipitation. The CpT was purified by ion-exchange chromatography on diethylaminoethyl (DEAE) column, pre-equilibrated with 20 mM tris-CaCl2 buffer (pH 8.2) with a flow rate of 0.5 mL min-1. The molecular weight and purity of ∼23 kDa of CpT were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Activity of protease was determined using Nα-benzoyl-DL-arginine-p-nitroanilide as chromogenic substrate and CpT purified showed a specific inhibitor activity of 26978.7697 U mg-1, fold purity of 9.8, and the yield of 70.2%. The characterization was performed for thermal stability, pH profile, and effect of various inhibitors on enzymatic activity. The protein isolated showed stability in the neutral to mild alkaline pH range and thermostability up to 50°C. CpT confirmed its serine nature as it was appreciably inhibited by serine protease inhibitors (maximum 6%), whereas metalloprotease inhibitors barely affected the activity of the enzyme (85%). To the best of our knowledge, it is first reported on purification of protease with trypsin-like properties, from this source.

Conformational changes of the bacterial type I ATP-binding cassette importer HisQMP2 at distinct steps of the catalytic cycle.

Prokaryotic solute binding protein-dependent ATP-binding cassette import systems are divided into type I and type II and mechanistic differences in the transport process going along with this classification are under intensive investigation. Little is known about the conformational dynamics during the catalytic cycle especially concerning the transmembrane domains. The type I transporter for positively charged amino acids from Salmonella enterica serovar Typhimurium (LAO-HisQMP2) was studied by limited proteolysis in detergent solution in the absence and presence of co-factors including ATP, ADP, LAO/arginine, and Mg(2+) ions. Stable peptide fragments could be obtained and differentially susceptible cleavage sites were determined by mass spectrometry as Lys-258 in the nucleotide-binding subunit, HisP, and Arg-217/Arg-218 in the transmembrane subunit, HisQ. In contrast, transmembrane subunit HisM was gradually degraded but no stable fragment could be detected. HisP and HisQ were equally resistant under pre- and post-hydrolysis conditions in the presence of arginine-loaded solute-binding protein LAO and ATP/ADP. Some protection was also observed with LAO/arginine alone, thus reflecting binding to the transporter in the apo-state and transmembrane signaling. Comparable digestion patterns were obtained with the transporter reconstituted into proteoliposomes and nanodiscs. Fluorescence lifetime spectroscopy confirmed the change of HisQ(R218) to a more apolar microenvironment upon ATP binding and hydrolysis. Limited proteolysis was subsequently used as a tool to study the consequences of mutations on the transport cycle. Together, our data suggest similar conformational changes during the transport cycle as described for the maltose ABC transporter of Escherichia coli, despite distinct structural differences between both systems.

Biphasic regulation of type II phosphatidylinositol-4 kinase by sphingosine: cross talk between glycero- and sphingolipids in the kidney.

Phosphatidylinositol-4 kinase (PI-4K) is responsible for the generation of phosphatidylinositol-4 phosphate (PtdIns(4)P), a bioactive signaling molecule involved in several biological functions. In this study, we show that sphingosine modulates the activity of the PI-4K isoform associated with the basolateral membranes (BLM) from kidney proximal tubules. Immunoblotting with an anti-α subunit PI-4K polyclonal antibody revealed the presence of two bands of 57 and 62kDa in the BLM. BLM-PI-4K activity retains noteworthy biochemical properties; it is adenosine-sensitive, not altered by wortmanin, and significantly inhibited by Ca(2+) at the µM range. Together, these observations indicate the presence of a type II PI-4K. Endogenous phosphatidylinositol (PI) alone reaches PI-4K half-maximal activity, revealing that even slight modifications in PI levels at the membrane environment promote significant variations in BLM-associated-PI-4K activity. ATP-dependence assays suggested that the Mg.ATP(2-) complex is the true substrate of the enzyme and that free Mg(2+) is an essential cofactor. Another observation indicated that higher concentrations of free ATP are inhibitory. BLM-associated-PI-4K activity was ~3-fold stimulated in the presence of increasing concentration of sphingosine, while in concentrations higher than 0.4mM, in which S1P is pronouncedly formed, there was an inhibitory effect on PtdIns(4)P formation. We propose that a tightly coupled regulatory network involving phosphoinositides and sphingolipids participate in the regulation of key physiological processes in renal BLM carried out by PI-4K.

Identification and active site analysis of the 1-aminocyclopropane-1-carboxylic acid oxidase catalysing the synthesis of ethylene in Agaricus bisporus.

BACKGROUND: 1-Aminocyclopropane-1-carboxylate oxidase (ACO) is a key enzyme that catalyses the final step in the biosynthesis of the plant hormone ethylene. Recently, the first ACO homologue gene was isolated in Agaricus bisporus, whereas information concerning the nature of the ethylene-forming activity of this mushroom ACO is currently lacking. METHODS: Recombinant ACO from A. bisporus (Ab-ACO) was purified and characterised for the first time. Molecular modelling combined with site-directed mutagenesis and kinetic and spectral analysis were used to investigate the property of Ab-ACO. RESULTS: Ab-ACO has eight amino acid residues that are conserved in the Fe (II) ascorbate family of dioxygenases, including four catalytic residues in the active site, but Ab-ACO lacks a key residue, S289. In comparison to plant ACOs, Ab-ACO requires ACC and Fe (II) but does not require ascorbate. In addition, Ab-ACO had relatively low activity and was completely dependent on bicarbonate, which could be ascribed to the replacement of S289 by G289. Moreover, the ferrous ion could induce a change in the tertiary, but not the secondary, structure of Ab-ACO. CONCLUSIONS: These results provide crucial experimental support for the ability of Ab-ACO to catalyse ethylene formation in a similar manner to that of plant ACOs, but there are differences between the biochemical and catalytic characteristics of Ab-ACO and plant ACOs. GENERAL SIGNIFICANCE: This work enhances the understanding of the ethylene biosynthesis pathways in fungi and could promote profound physiological research of the role of ethylene in the regulation of mushroom growth and development.

Stereospecific requirement of cholesterol in the function of the serotonin1A receptor.

The serotonin1A receptor is an important member of the G protein-coupled receptor (GPCR) family. It is involved in the generation and modulation of a variety of cognitive and behavioral functions and serves as a drug target. Previous work from our laboratory has established the sensitivity of the function of the serotonin1A receptor to membrane cholesterol. Solubilization of the hippocampal serotonin1A receptor utilizing the zwitterionic detergent CHAPS is accompanied by loss of cholesterol and results in reduction in specific ligand binding. Replenishment of cholesterol to solubilized membranes restores specific ligand binding to the receptor. We utilized this strategy of sterol replenishment of solubilized membranes to explore the stereospecific stringency of cholesterol for receptor function. We used two stereoisomers of cholesterol, ent-cholesterol (enantiomer of cholesterol) and epi-cholesterol (a diastereomer of cholesterol), for this purpose. Importantly, we show here that while ent-cholesterol could replace cholesterol in supporting receptor function, epi-cholesterol could not. These results imply that the requirement of membrane cholesterol for the serotonin1A receptor function is diastereospecific, yet not enantiospecific. Our results extend and help define specificity of the interaction of membrane cholesterol with the serotonin1A receptor, and represent the first report utilizing ent-cholesterol to examine stereospecificity of GPCR-cholesterol interaction.

Quercetin modulates OTA-induced oxidative stress and redox signalling in HepG2 cells - up regulation of Nrf2 expression and down regulation of NF-κB and COX-2.

BACKGROUND: Ochratoxin A (OTA), a mycotoxin, causes extensive cell damage, affecting liver and kidney cells. OTA toxicity is fairly well characterized where oxidative stress is believed to play a role, however, the sequence of molecular events after OTA-exposure, have not been characterized in literature. Further, antidotes for alleviating the toxicity are sparsely reported. The aim of this study was to understand the sequence of some molecular mechanisms for OTA-induced toxicity and the cytoprotective effect of quercetin on OTA-induced toxicity. METHODS: Time course studies to evaluate the time of intracellular calcium release and ROS induction were carried out. The time of activation and induction of two key redox- sensitive transcription factors, NF-κB and Nrf-2 were determined by nuclear localization and expression respectively. The time of expression of inflammatory marker COX-2 was determined. Oxidative DNA damage by comet assay and micronucleus formation was studied. The ameliorative effect of quercetin on OTA-induced toxicity was also determined on all the above-mentioned parameters. RESULTS: OTA-induced calcium release, ROS generation and activated NF-κB nuclear translocation and expression. Pre-treatment with quercetin ameliorated ROS and calcium release as well as NF-κB induction and expression. Quercetin induced Nrf-2 nuclear translocation and expression. Quercetin's anti-inflammatory property was exhibited as it down regulated COX-2. Anti-genotoxic effect of quercetin was evident in prevention of DNA damage and micronucleus formation. CONCLUSION: Quercetin modulated OTA-induced oxidative stress and redox-signaling in HepG2 cells. GENERAL SIGNIFICANCE: The results of the study demonstrate for the first time that quercetin prevents OTA-induced toxicity in HepG2 cells.

Identification of post-transcriptional regulatory networks during myeloblast-to-monocyte differentiation transition.

Treatment of leukemia cells with 1,25-dihydroxyvitamin D3 may overcome their differentiation block and lead to the transition from myeloblasts to monocytes. To identify microRNA-mRNA networks relevant for myeloid differentiation, we profiled the expression of mRNAs and microRNAs associated to the low- and high-density ribosomal fractions in leukemic cells and in their differentiated monocytic counterpart. Intersection between mRNAs shifted across the fractions after treatment with putative target genes of modulated microRNAs showed a series of molecular networks relevant for the monocyte cell fate determination, as for example the post-transcriptional regulation of the Polo-like kinase 1 (PLK1) by miR-22-3p and let-7e-5p.

Reversible oxidation of PRL family protein-tyrosine phosphatases.

Recent studies have revealed that reactive oxygen species (ROS) are actively generated in cells and function as second messengers to mediate physiological intracellular signaling. ROS exert their effects on intracellular signaling via ROS effector proteins, which are sensitively and reversibly oxidized by ROS. Among various ROS effector proteins, the protein tyrosine phosphatase (PTP) family is of special interest. In the catalytic pocket, PTP proteins commonly possess a highly reactive cysteine (Cys) residue, which is susceptible to oxidation by ROS. Phosphatase of regenerating liver (PRL) belongs to the PTP family and is oxidized by ROS to form an intramolecular disulfide bond. In general, disulfide bonds in proteins can be reduced in cells with the help of various reducing enzymes, which enables the reversible redox regulation of PRL proteins. In the case of PRL proteins, thioredoxin-related protein 32 specifically catalyzes the reducing reaction, indicating the importance of redox regulation for ROS effector proteins.

Novel off-target effect of tamoxifen--inhibition of acid ceramidase activity in cancer cells.

Acid ceramidase (AC), EC 3.5.1.23, a lysosomal enzyme, catalyzes the hydrolysis of ceramide to constituent sphingoid base, sphingosine, and fatty acid. Because AC regulates the levels of pro-apoptotic ceramide and mitogenic sphingosine-1-phosphate, it is considered an apt target in cancer therapy. The present study reveals, for the first time, that the prominent antiestrogen, tamoxifen, is a pan-effective AC inhibitor in the low, single digit micromolar range, as demonstrated in a wide spectrum of cancer cell types, prostate, pancreatic, colorectal, and breast. Prostate cancer cells were chosen for the detailed investigations. Treatment of intact PC-3 cells with tamoxifen produced time- and dose-dependent inhibition of AC activity. Tamoxifen did not impact cell viability nor did it inhibit AC activity in cell-free assays. In pursuit of mechanism of action, we demonstrate that tamoxifen induced time-, as early as 5min, and dose-dependent, as low as 5µM, increases in lysosomal membrane permeability (LMP), and time- and dose-dependent downregulation of AC protein expression. Assessing various protease inhibitors revealed that a cathepsin B inhibitor blocked tamoxifen-elicited downregulation of AC protein; however, this action failed to restore AC activity unless assayed in a cell-free system at pH4.5. In addition, pretreatment with tamoxifen inhibited PC-3 cell migration. Toremifene, an antiestrogen structurally similar to tamoxifen, was also a potent inhibitor of AC activity. This study reveals a new, off-target action of tamoxifen that may be of benefit to enhance anticancer therapies that either incorporate ceramide or target ceramide metabolism.

The human topoisomerase 1B Arg634Ala mutation results in camptothecin resistance and loss of inter-domain motion correlation.

Human topoisomerase 1B, the unique target of the natural anticancer compound camptothecin, catalyzes the unwinding of supercoiled DNA by introducing transient single strand nicks and providing covalent protein-DNA adducts. The functional properties and the drug reactivity of the single Arg634Ala mutant have been investigated in comparison to the wild type enzyme. The mutant is characterized by an identical relaxation and cleavage rate but it displays resistance to camptothecin as indicated by a viability assay of the yeast cells transformed with the mutated protein. The mutant also displays a very fast religation rate that is only partially reduced by the presence of the drug, suggesting that this is the main reason for its resistance. A comparative analysis of the structural-dynamical properties of the native and mutant proteins by molecular dynamics simulation indicates that mutation of Arg634 brings to a loss of motion correlation between the different domains and in particular between the linker and the C-terminal domain, containing the catalytic tyrosine residue. These results indicate that the loss of motion correlation and the drug resistance are two strongly correlated events.

Residues of a proposed gate region in type I ATP-binding cassette import systems are crucial for function as revealed by mutational analysis.

The type I ATP-binding cassette (ABC) importer for positively charged amino acids of the thermophilic bacterium Geobacillus stearothermophilus consists of the extracellular solute binding protein, ArtJ, and a homodimer each of the transmembrane subunit, ArtM, and the nucleotide-binding and -hydrolyzing subunit, ArtP. We have investigated the functional consequences of mutations affecting conserved residues from two peptide regions in ArtM, recently proposed to form a 'gate' by which access of a substrate to the translocation path is controlled (Hollenstein et al., 2007 [14]). Transporter variants were reconstituted into proteoliposomes and assayed for ArtJ/arginine-stimulated ATPase activity. Replacement of residues from region 1 (Arg-63, Pro-66) caused no or only moderate reduction in ATPase activity. In contrast, mutating residues from gate region 2 (Lys-159, Leu-163) resulted in a substantial increase in ATPase activity which, however, as demonstrated for variants ArtM(K159I) and ArtM(K159E), is not coupled to transport. Replacing homologous residues in the closely related histidine transporter of Salmonella enterica serovar Typhimurium (HisJ-QMP2) caused different phenotypes. Mutation to isoleucine of HisQ(K163) or HisM(H172), both homologous to ArtM(K159), abolished ATPase activity. The mutations most likely caused a structural change as revealed by limited proteolysis. In contrast, substantial, albeit reduced, enzymatic activity was observed with variants of HisQ(L167→G) or HisM(L176→G), both homologous to ArtM(L163). Our study provides the first experimental evidence in favor of a crucial role of residues from the proposed gate region in type I ABC importer function.

Protein aggregation propensity is a crucial determinant of intracellular inclusion formation and quality control degradation.

Protein aggregation is linked to many pathological conditions, including several neurodegenerative diseases. The aggregation propensities of proteins are thought to be controlled to a large extent by the physicochemical properties encoded in the primary sequence. We have previously exploited a set of amyloid ß peptide (Aß42) variants exhibiting a continuous gradient of intrinsic aggregation propensities to demonstrate that this rule applies in vivo in bacteria. In the present work we have characterized the behavior of these Aß42 mutants when expressed in yeast. In contrast to bacteria, the intrinsic aggregation propensity is gated by yeast, in such a way that this property correlates with the formation of intracellular inclusions only above a specific aggregation threshold. Proteins displaying solubility levels above this threshold escape the inclusion formation pathway. In addition, the most aggregation-prone variants are selectively cleared by the yeast quality control degradation machinery. Thus, both inclusion formation and proteolysis target the same aggregation-prone variants and cooperate to minimize the presence of these potentially dangerous species in the cytosol. The demonstration that sorting to these pathways in eukaryotes is strongly influenced by protein primary sequence should facilitate the development of rational approaches to predict and hopefully prevent in vivo protein deposition.

Microtubule-associated protein tau in bovine retinal photoreceptor rod outer segments: comparison with brain tau.

Recent studies have suggested a possible involvement of abnormal tau in some retinal degenerative diseases. The common view in these studies is that these retinal diseases share the mechanism of tau-mediated degenerative diseases in brain and that information about these brain diseases may be directly applied to explain these retinal diseases. Here we collectively examine this view by revealing three basic characteristics of tau in the rod outer segment (ROS) of bovine retinal photoreceptors, i.e., its isoforms, its phosphorylation mode and its interaction with microtubules, and by comparing them with those of brain tau. We find that ROS contains at least four isoforms: three are identical to those in brain and one is unique in ROS. All ROS isoforms, like brain isoforms, are modified with multiple phosphate molecules; however, ROS isoforms show their own specific phosphorylation pattern, and these phosphorylation patterns appear not to be identical to those of brain tau. Interestingly, some ROS isoforms, under the normal conditions, are phosphorylated at the sites identical to those in Alzheimer's patient isoforms. Surprisingly, a large portion of ROS isoforms tightly associates with a membranous component(s) other than microtubules, and this association is independent of their phosphorylation states. These observations strongly suggest that tau plays various roles in ROS and that some of these functions may not be comparable to those of brain tau. We believe that knowledge about tau in the entire retinal network and/or its individual cells are also essential for elucidation of tau-mediated retinal diseases, if any.