Identification of a glycoprotein from rat liver mitochondrial inner membrane and demonstration of its origin in the endoplasmic reticulum

ER-misfolded proteins become sequestered with mitochondria and impair mitochondrial function

Proteostasis is a challenge for cellular organisms, as all known protein synthesis machineries are error-prone. Here we show by cell fractionation and microscopy studies that misfolded proteins formed in the endoplasmic reticulum can become associated with and partly transported into mitochondria, resulting in impaired mitochondrial function. Blocking the endoplasmic reticulum-mitochondria encounter structure (ERMES), but not the mitochondrial sorting and assembly machinery (SAM) or the mitochondrial surveillance pathway components Msp1 and Vms1, abrogated mitochondrial sequestration of ER-misfolded proteins. We term this mitochondria-associated proteostatic mechanism for ER-misfolded proteins ERAMS (ER-associated mitochondrial sequestration). We testify to the relevance of this pathway by using mutant α-1-antitrypsin as an example of a human disease-related misfolded ER protein, and we hypothesize that ERAMS plays a role in pathological features such as mitochondrial dysfunction.

Characterization of the hypersensitive response-like cell death phenomenon induced by targeting antiviral lectin griffithsin to the secretory pathway

Griffithsin (GRFT) is an antiviral lectin, originally derived from a red alga, which is currently being investigated as a topical microbicide to prevent transmission of human immunodeficiency virus (HIV). Targeting GRFT to the apoplast for production in Nicotiana benthamiana resulted in necrotic symptoms associated with a hypersensitive response (HR)-like cell death, accompanied by H2 O2 generation and increased PR1 expression. Mannose-binding lectins surfactant protein D (SP-D), cyanovirin-N (CV-N) and human mannose-binding lectin (hMBL) also induce salicylic acid (SA)-dependent HR-like cell death in N. benthamiana, and this effect is mediated by the lectin's glycan binding activity. We found that secreted GRFT interacts with an endogenous glycoprotein, α-xylosidase (XYL1), which is involved in cell wall organization. The necrotic effect could be mitigated by overexpression of Arabidopsis XYL1, and by co-expression of SA-degrading enzyme NahG, providing strategies for enhancing expression of oligomannose-binding lectins in plants.

Interlink between cholesterol & cell cycle in prostate carcinoma

Background & objectives:

Earlier reports have shown hypocholesterolaemia in cancer patients and high number of lipid rafts in cancer cells. The primary objective of this study was to compare the intracellular cholesterol turnover in non-cancerous (benign) prostatic hyperplasia (BPH) and carcinoma prostate (CAP) with normal prostate cells obtained from patients undergoing radical cystectomy for carcinoma bladder (sham control).

Methods:

ELISA-based estimation of prostate-specific antigen (PSA), evaluation of expression of low-density lipoprotein receptor (LDLR), peripheral-type benzodiazepine receptor (PBR) and cyclin E, immunohistochemistry and confocal microscopy, measurement of integrated optical density of the diaminobenzidine (DAB)-stained immunohistograms, isolation of nucleus and cell cytoplasm from prostate tissue by ultracentrifugation followed by estimation of cholesterol spectrophotometrically in isolated nuclear and cytoplasmic fractions were performed.

Results:

Seventy five individuals, 25 for each group (BPH n=25; CAP n=25 and sham control n=25), were included in the study. Cholesterol was increased in the cytoplasm and nucleus of the prostate cancer cells along with elevated expression of LDLR. Increased cholesterol concentration in the cell nucleus was found comparable with the increased expression of cholesterol transporter viz. PBR in the prostate tumour tissues as compared to its expression in normal prostate cells obtained from individuals undergoing radical cystectomy for carcinoma bladder. Cell cycle protein cyclin E was also highly expressed in cancer tissues.

Interpretation & conclusions:

The present findings along with increased expression of cell cycle protein cyclin E in the cell nucleus of the tumour tissue suggested the possibility of an intriguing role of cholesterol in the mechanism of cell cycle process of prostate cell proliferation.

Linking Glycation and Glycosylation With Inflammation and Mitochondrial Dysfunction in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, affecting about 6.3 million people worldwide. PD is characterized by the progressive degeneration of dopaminergic neurons in the Substantia nigra pars compacta, resulting into severe motor symptoms. The cellular mechanisms underlying dopaminergic cell death in PD are still not fully understood, but mitochondrial dysfunction, oxidative stress and inflammation are strongly implicated in the pathogenesis of both familial and sporadic PD cases. Aberrant post-translational modifications, namely glycation and glycosylation, together with age-dependent insufficient endogenous scavengers and quality control systems, lead to cellular overload of dysfunctional proteins. Such injuries accumulate with time and may lead to mitochondrial dysfunction and exacerbated inflammatory responses, culminating in neuronal cell death. Here, we will discuss how PD-linked protein mutations, aging, impaired quality control mechanisms and sugar metabolism lead to up-regulated abnormal post-translational modifications in proteins. Abnormal glycation and glycosylation seem to be more common than previously thought in PD and may underlie mitochondria-induced oxidative stress and inflammation in a feed-forward mechanism. Moreover, the stress-induced post-translational modifications that directly affect parkin and/or its substrates, deeply impairing its ability to regulate mitochondrial dynamics or to suppress inflammation will also be discussed. Together, these represent still unexplored deleterious mechanisms implicated in neurodegeneration in PD, which may be used for a more in-depth knowledge of the pathogenic mechanisms, or as biomarkers of the disease.

The endoplasmic reticulum is a hub to sort proteins toward unconventional traffic pathways and endosymbiotic organelles

The discovery that much of the extracellular proteome in eukaryotic cells consists of proteins lacking a signal peptide, which cannot therefore enter the secretory pathway, has led to the identification of alternative protein secretion routes bypassing the Golgi apparatus. However, proteins harboring a signal peptide for translocation into the endoplasmic reticulum can also be transported along these alternative routes, which are still far from being well elucidated in terms of the molecular machineries and subcellular/intermediate compartments involved. In this review, we first try to provide a definition of all the unconventional protein secretion pathways in eukaryotic cells, as those pathways followed by proteins directed to an 'external space' bypassing the Golgi, where 'external space' refers to the extracellular space plus the lumen of the secretory route compartments and the inner space of mitochondria and plastids. Then, we discuss the role of the endoplasmic reticulum in sorting proteins toward unconventional traffic pathways in plants. In this regard, various unconventional pathways exporting proteins from the endoplasmic reticulum to the vacuole, plasma membrane, apoplast, mitochondria, and plastids are described, including the short routes followed by the proteins resident in the endoplasmic reticulum.

Metabolic Labeling of Protein Antigens with [35S]Methionine

Metabolic labeling of antigens can be achieved by supplementation of the cell culture medium with radioactive amino acid precursors such as [³⁵S]methionine during the incubation period of target cells. In this protocol, intracellular unlabeled methionine levels are reduced by starvation of cells for 0.5-1 h before the addition of labeled [³⁵S]methionine and incubation for 0.5-4 h. Upon completion of the metabolic labeling process, cells can be prepared for immunoprecipitation by lysis or alternatively pelleted and frozen for cell lysate preparations at a later time.

Cellular prion protein is present in mitochondria of healthy mice

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycophosphatidylinositol (GPI) anchor. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. The precise function of PrP^C remains elusive but may depend upon its cellular localization. Here we show that PrP^C is present in brain mitochondria from 6–12 week old wild-type and transgenic mice in the absence of disease. Mitochondrial PrP^C was fully processed with mature N-linked glycans and did not require the GPI anchor for localization. Protease treatment of purified mitochondria suggested that mitochondrial PrP^C exists as a transmembrane isoform with the C-terminus facing the mitochondrial matrix and the N-terminus facing the intermembrane space. Taken together, our data suggest that PrP^C can be found in mitochondria in the absence of disease, old age, mutation, or overexpression and that PrP^C may affect mitochondrial function.

Comparative Proteomics Reveals Dysregulated Mitochondrial O-GlcNAcylation in Diabetic Hearts

O-linked β-N-acetylglucosamine (O-GlcNAc), a post-translational modification on serine and threonine residues of many proteins, plays crucial regulatory roles in diverse biological events. As a nutrient sensor, O-GlcNAc modification (O-GlcNAcylation) on nuclear and cytoplasmic proteins underlies the pathology of diabetic complications including cardiomyopathy. However, mitochondrial O-GlcNAcylation, especially in response to chronic hyperglycemia in diabetes, has been poorly explored. We performed a comparative O-GlcNAc profiling of mitochondria from control and streptozotocin (STZ)-induced diabetic rat hearts by using an improved β-elimination/Michael addition with isotopic DTT reagents (BEMAD) followed by tandem mass spectrometric analysis. In total, 86 mitochondrial proteins, involved in diverse pathways, were O-GlcNAcylated. Among them, many proteins have site-specific alterations in O-GlcNAcylation in response to diabetes, which suggests that protein O-GlcNAcylation is a novel layer of regulation mediating adaptive changes in mitochondrial metabolism during the progression of diabetic cardiomyopathy.

The Metacaspase (Mca1p) Restricts O-glycosylation During Farnesol-induced Apoptosis in Candida albicans

Protein glycolysation is an essential posttranslational modification in eukaryotic cells. In pathogenic yeasts, it is involved in a large number of biological processes, such as protein folding quality control, cell viability and host/pathogen relationships. A link between protein glycosylation and apoptosis was established by the analysis of the phenotypes of oligosaccharyltransferase mutants in budding yeast. However, little is known about the contribution of glycosylation modifications to the adaptive response to apoptosis inducers. The cysteine protease metacaspase Mca1p plays a key role in the apoptotic response in Candida albicans triggered by the quorum sensing molecule farnesol. We subjected wild-type and mca1-deletion strains to farnesol stress and then studied the early phase of apoptosis release in quantitative glycoproteomics and glycomics experiments on cell-free extracts essentially devoid of cell walls. We identified and characterized 62 new glycosylated peptides with their glycan composition: 17 N-glycosylated, 45 O-glycosylated, and 81 additional sites of N-glycosylation. They were found to be involved in the control of protein folding, cell wall integrity and cell cycle regulation. We showed a general increase in the O-glycosylation of proteins in the mca1 deletion strain after farnesol challenge. We identified 44 new putative protein substrates of the metacaspase in the glycoprotein fraction enriched on concanavalin A. Most of these substrates are involved in protein folding or protein resolubilization and in mitochondrial functions. We show here that key Mca1p substrates, such as Cdc48p or Ssb1p, involved in degrading misfolded glycoproteins and in the protein quality control system, are themselves differentially glycosylated. We found putative substrates, such as Bgl2p (validated by immunoblot), Srb1p or Ugp1p, that are involved in the biogenesis of glycans. Our findings highlight a new role of the metacaspase in amplifying cell death processes by affecting several critical protein quality control systems through the alteration of the protein glycosylation machinery.Data are available via ProteomeXchange with identifier PXD003677.

Superresolution imaging of viral protein trafficking

The endoplasmic reticulum (ER) membrane is closely apposed to the outer mitochondrial membrane (OMM), which facilitates communication between these organelles. These contacts, known as mitochondria-associated membranes (MAM), facilitate calcium signaling, lipid transfer, as well as antiviral and stress responses. How cellular proteins traffic to the MAM, are distributed therein, and interact with ER and mitochondrial proteins are subject of great interest. The human cytomegalovirus UL37 exon 1 protein or viral mitochondria-localized inhibitor of apoptosis (vMIA) is crucial for viral growth. Upon synthesis at the ER, vMIA traffics to the MAM and OMM, where it reprograms the organization and function of these compartments. vMIA significantly changes the abundance of cellular proteins at the MAM and OMM, including proteins that regulate calcium homeostasis and cell death. Through the use of superresolution imaging, we have shown that vMIA is distributed at the OMM in nanometer scale clusters. This is similar to the clusters reported for the mitochondrial calcium channel, VDAC, as well as electron transport chain, translocase of the OMM complex, and mitochondrial inner membrane organizing system components. Thus, aside from addressing how vMIA targets the MAM and regulates survival of infected cells, biochemical studies and superresolution imaging of vMIA offer insights into the formation, organization, and functioning of MAM. Here, we discuss these insights into trafficking, function, and organization of vMIA at the MAM and OMM and discuss how the use of superresolution imaging is contributing to the study of the formation and trafficking of viruses.

Proteomic mapping of the human mitochondrial intermembrane space in live cells via ratiometric APEX tagging

Obtaining complete protein inventories for subcellular regions is a challenge that often limits our understanding of cellular function, especially for regions that are impossible to purify and are therefore inaccessible to traditional proteomic analysis. We recently developed a method to map proteomes in living cells with an engineered peroxidase (APEX) that bypasses the need for organellar purification when applied to membrane-bound compartments; however, it was insufficiently specific when applied to unbounded regions that allow APEX-generated radicals to escape. Here, we combine APEX technology with a SILAC-based ratiometric tagging strategy to substantially reduce unwanted background and achieve nanometer spatial resolution. This is applied to map the proteome of the mitochondrial intermembrane space (IMS), which can freely exchange small molecules with the cytosol. Our IMS proteome of 127 proteins has >94% specificity and includes nine newly discovered mitochondrial proteins. This approach will enable scientists to map proteomes of cellular regions that were previously inaccessible.

The oligomycin-sensitivity conferring protein of mitochondrial ATP synthase: emerging new roles in mitochondrial pathophysiology

The oligomycin-sensitivity conferring protein (OSCP) of the mitochondrial F(O)F1 ATP synthase has long been recognized to be essential for the coupling of proton transport to ATP synthesis. Located on top of the catalytic F1 sector, it makes stable contacts with both F1 and the peripheral stalk, ensuring the structural and functional coupling between F(O) and F1, which is disrupted by the antibiotic, oligomycin. Recent data have established that OSCP is the binding target of cyclophilin (CyP) D, a well-characterized inducer of the mitochondrial permeability transition pore (PTP), whose opening can precipitate cell death. CyPD binding affects ATP synthase activity, and most importantly, it decreases the threshold matrix Ca²⁺ required for PTP opening, in striking analogy with benzodiazepine 423, an apoptosis-inducing agent that also binds OSCP. These findings are consistent with the demonstration that dimers of ATP synthase generate Ca²⁺-dependent currents with features indistinguishable from those of the PTP and suggest that ATP synthase is directly involved in PTP formation, although the underlying mechanism remains to be established. In this scenario, OSCP appears to play a fundamental role, sensing the signal(s) that switches the enzyme of life in a channel able to precipitate cell death.

Metabolic labeling of Caenorhabditis elegans primary embryonic cells with azido-sugars as a tool for glycoprotein discovery

Glycobiology research with Caenorhabditis elegans (C. elegans) has benefitted from the numerous genetic and cell biology tools available in this system. However, the lack of a cell line and the relative inaccessibility of C. elegans somatic cells in vivo have limited the biochemical approaches available in this model. Here we report that C. elegans primary embryonic cells in culture incorporate azido-sugar analogs of N-acetylgalactosamine (GalNAc) and N-acetylglucosamine (GlcNAc), and that the labeled glycoproteins can be analyzed by mass spectrometry. By using this metabolic labeling approach, we have identified a set of novel C. elegans glycoprotein candidates, which include several mitochondrially-annotated proteins. This observation was unexpected given that mitochondrial glycoproteins have only rarely been reported, and it suggests that glycosylation of mitochondrially-annotated proteins might occur more frequently than previously thought. Using independent experimental strategies, we validated a subset of our glycoprotein candidates. These include a mitochondrial, atypical glycoprotein (ATP synthase α-subunit), a predicted glycoprotein (aspartyl protease, ASP-4), and a protein family with established glycosylation in other species (actin). Additionally, we observed a glycosylated isoform of ATP synthase α-subunit in bovine heart tissue and a primate cell line (COS-7). Overall, our finding that C. elegans primary embryonic cells are amenable to metabolic labeling demonstrates that biochemical studies in C. elegans are feasible, which opens the door to labeling C. elegans cells with other radioactive or azido-substrates and should enable the identification of additional post-translationally modified targets and analysis of the genes required for their modification using C. elegans mutant libraries.

Shifting the paradigm: the putative mitochondrial protein ABCB6 resides in the lysosomes of cells and in the plasma membrane of erythrocytes

ABCB6, a member of the adenosine triphosphate–binding cassette (ABC) transporter family, has been proposed to be responsible for the mitochondrial uptake of porphyrins. Here we show that ABCB6 is a glycoprotein present in the membrane of mature erythrocytes and in exosomes released from reticulocytes during the final steps of erythroid maturation. Consistent with its presence in exosomes, endogenous ABCB6 is localized to the endo/lysosomal compartment, and is absent from the mitochondria of cells. Knock-down studies demonstrate that ABCB6 function is not required for de novo heme biosynthesis in differentiating K562 cells, excluding this ABC transporter as a key regulator of porphyrin synthesis. We confirm the mitochondrial localization of ABCB7, ABCB8 and ABCB10, suggesting that only three ABC transporters should be classified as mitochondrial proteins. Taken together, our results challenge the current paradigm linking the expression and function of ABCB6 to mitochondria.

Multiple proteins with essential mitochondrial functions have glycosylated isoforms

Nucleocytosolic and secreted proteins are commonly glycosylated. However, reports of glycosylated mitochondrial proteins are rare. Using lectin chromatography on bovine heart, we detected low-abundance glycoforms of nuclear-encoded proteins with well-established mitochondrial function: pyruvate dehydrogenase E1α, NADH dehydrogenase [ubiquinone] iron-sulfur protein 3, ADP/ATP translocase, ATP synthase d and oligomycin sensitivity-conferring protein. Notably, the latter two have been previously detected at the plasma membrane. Our findings indicate that glycosylation of classic mitochondrial proteins may be more common than previously appreciated. We discuss the implication that glycosylation could represent an unexplored mechanism for regulating these proteins' functions within mitochondria or at extra-mitochondrial locations.

Precision mapping of an in vivo N-glycoproteome reveals rigid topological and sequence constraints

N-linked glycosylation is a biologically important protein modification, but only a small fraction of modification sites have been mapped. We developed a "filter aided sample preparation" (FASP)-based method in which glycopeptides are enriched by binding to lectins on the top of a filter and mapped 6367 N-glycosylation sites on 2352 proteins in four mouse tissues and blood plasma using high-accuracy mass spectrometry. We found 74% of known mouse N-glycosites and discovered an additional 5753 sites on a diverse range of proteins. Sites almost always have the N-!P-[S|T]-!P (where !P is not proline) and rarely the N-X-C motif or nonconsensus sequences. Combining the FASP approach with analysis of subcellular glycosite localization reveals that the sites always orient toward the extracellular space or toward the lumen of ER, Golgi, lysosome, or peroxisome. The N-glycoproteome contains a plethora of modification sites on factors important in development, organ-specific functions, and disease.

Towards a membrane proteome in Drosophila: a method for the isolation of plasma membrane

Background

The plasma membrane (PM) is a compartment of significant interest because cell surface proteins influence the way in which a cell interacts with its neighbours and its extracellular environment. However, PM is hard to isolate because of its low abundance. Aqueous two-phase affinity purification (2PAP), based on PEG/Dextran two-phase fractionation and lectin affinity for PM-derived microsomes, is an emerging method for the isolation of high purity plasma membranes from several vertebrate sources. In contrast, PM isolation techniques in important invertebrate genetic model systems, such as Drosophila melanogaster, have relied upon enrichment by density gradient centrifugation. To facilitate genetic investigation of activities contributing to the content of the PM sub-proteome, we sought to adapt 2PAP to this invertebrate model to provide a robust PM isolation technique for Drosophila.

Results

We show that 2PAP alone does not completely remove contaminating endoplasmic reticulum and mitochondrial membrane. However, a novel combination of density gradient centrifugation plus 2PAP results in a robust PM preparation. To demonstrate the utility of this technique we isolated PM from fly heads and successfully identified 432 proteins using MudPIT, of which 37% are integral membrane proteins from all compartments. Of the 432 proteins, 22% have been previously assigned to the PM compartment, and a further 34% are currently unassigned to any compartment and represent candidates for assignment to the PM. The remainder have previous assignments to other compartments.

Conclusion

A combination of density gradient centrifugation and 2PAP results in a robust, high purity PM preparation from Drosophila, something neither technique can achieve on its own. This novel preparation should lay the groundwork for the proteomic investigation of the PM in different genetic backgrounds in Drosophila. Our results also identify two key steps in this procedure: The optimization of membrane partitioning in the PEG/Dextran mixture, and careful choice of the correct lectin for the affinity purification step in light of variations in bulk membrane lipid composition and glycosylation patterns respectively. This points the way for further adaptations into other systems.

The Golgi alpha-1,6 mannosyltransferase KlOch1p of Kluyveromyces lactis is required for Ca2+/calmodulin-based signaling and for proper mitochondrial functionality

Background

Protein N-glycosylation is a relevant metabolic pathway in eukaryotes and plays key roles in cell processes. In yeasts, outer chain branching is initiated in the Golgi apparatus by the alpha-1,6-mannosyltransferase Och1p.

Results

Here we report that, in Kluyveromyces lactis, this glycosyltransferase is also required to maintain functional mitochondria and calcium homeostasis. Cells carrying a mutation in KlOCH1 gene showed altered mitochondrial morphology, increased accumulation of ROS and reduced expression of calcium signalling genes such as calmodulin and calcineurin. Intracellular calcium concentration was also reduced in the mutant cells with respect to the wild type counterparts.

Phenotypes that occur in cells lacking the alpha-1,6-mannosyltransferase, including oxidative stress and impaired mitochondria functionality, were suppressed by increased dosage of KlCmd1p. This, in turn, acts through the action of calcineurin.

Conclusions

Proper functioning of the alpha-1,6-mannosyltransferase in the N-glycosylation pathway of K. lactis is required for maintaining normal calcium homeostasis; this is necessary for physiological mitochondria dynamics and functionality.

Alternative oxidase: what information can protein sequence comparisons give us?

The finding that alternative oxidase (AOX) is present in most kingdoms of life has resulted in a large number of AOX sequences that are available for analyses. Multiple sequence alignments of AOX proteins from evolutionarily divergent organisms represent a valuable tool and can be used to identify amino acids and domains that may play a role in catalysis, membrane association and post-translational regulation, especially when these data are coupled with the structural model for the enzyme. I validate the use of this approach by demonstrating that it detects the conserved glutamate and histidine residues in AOX that initially led to its identification as a di-iron carboxylate protein and the generation of a structural model for the protein. A comparative analysis using a larger dataset identified 35 additional amino acids that are conserved in all AOXs examined, 30 of which have not been investigated to date. I hypothesize that these residues will be involved in the quinol terminal oxidase activity or membrane association of AOX. Major differences in AOX protein sequences between kingdoms are revealed, and it is hypothesized that two angiosperm-specific domains may be responsible for the non-covalent dimerization of AOX, whereas two indels in the aplastidic AOXs may play a role in their post-translational regulation. A scheme for predicting whether a particular AOX protein will be recognized by the alternative oxidase monoclonal antibody generated against the AOX of Sauromatum guttatum (Voodoo lily) is presented. The number of functional sites in AOX is greater than expected, and determining the structure of AOX will prove extremely valuable to future research.

Campylobacter jejuni PglH is a single active site processive polymerase that utilizes product inhibition to limit sequential glycosyl transfer reactions

Asparagine-linked protein glycosylation is essential for the virulence of the human gut mucosal pathogen Campylobacter jejuni . The heptasaccharide that is transferred to proteins is biosynthesized via the glycosyltransferase-catalyzed addition of sugar units to an undecaprenyl diphosphate-linked carrier. Genetic studies on the heptasaccharide assembly enzymes have shown that PglH, which transfers three terminal N-acetyl-galactosamine (GalNAc) residues to the carrier polyisoprene, is essential for chick colonization by C. jejuni . While it is now clear that PglH catalyzes multiple transfer reactions, the mechanism whereby the reactions cease after the addition of just three GalNAc residues has yet to be understood. To address this issue, a series of mechanistic biochemical studies was conducted with purified native PglH. This enzyme was found to follow a processive mechanism under initial rate conditions; however, product inhibition and product accumulation led to PglH release of intermediate products prior to complete conversion to the native ultimate product. Point mutations of an essential EX(7)E sequence motif were used to demonstrate that a single active site was responsible for all three transferase reactions, and a homology model with the mannosyltransferase PimA, from Mycobacteria smegmatis , establishes the requirement of the EX(7)E motif in catalysis. Finally, increased binding affinity with increasing glycan size is proposed to provide PglH with a counting mechanism that does not allow the transfer of more than three GalNAc residues. These results provide important mechanistic insights into the function of the glycosyl transfer polymerase that is related to the virulence of C. jejuni .

Global analysis of the glycoproteome in Saccharomyces cerevisiae reveals new roles for protein glycosylation in eukaryotes

To further understand the roles of protein glycosylation in eukaryotes, we globally identified glycan-containing proteins in yeast. A fluorescent lectin binding assay was developed and used to screen protein microarrays containing over 5000 proteins purified from yeast. A total of 534 yeast proteins were identified that bound either Concanavalin A (ConA) or Wheat-Germ Agglutinin (WGA); 406 of them were novel. Among the novel glycoproteins, 45 were validated by mobility shift upon treatment with EndoH and PNGase F, thereby extending the number of validated yeast glycoproteins to 350. In addition to many components of the secretory pathway, we identified other types of proteins, such as transcription factors and mitochondrial proteins. To further explore the role of glycosylation in mitochondrial function, the localization of four mitochondrial proteins was examined in the presence and absence of tunicamycin, an inhibitor of N-linked protein glycosylation. For two proteins, localization to the mitochondria is diminished upon tunicamycin treatment, indicating that protein glycosylation is important for protein function. Overall, our studies greatly extend our understanding of protein glycosylation in eukaryotes through the cataloguing of glycoproteins, and describe a novel role for protein glycosylation in mitochondrial protein function and localization.

Endoplasmic reticulum and mitochondria interplay mediates apoptotic cell death: relevance to Parkinson's disease

Sporadic Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra pars compacta. Many cellular mechanisms are thought to be involved in the death of these specific neurons in PD, including oxidative stress, changes of intracellular calcium homeostasis, and mitochondrial dysfunction. Since recent studies have revealed that also endoplasmic reticulum (ER) stress in conjunction with abnormal protein degradation can contribute to the PD pathophysiology, we investigated here the molecular mechanisms underlying the interplay between ER and mitochondria and its relevance in the control of neuronal cell death in PD. We observed that MPP+ induced changes in the mitochondrial function, affecting mitochondrial membrane potential and electron transport chain function. Likewise, it was also evident the unfolded protein response activation by an overexpression of GRP78 protein. Moreover, stress stimuli caused the release of Ca2+ from the ER that consistently induced mitochondrial Ca2+ uptake, with a rise of mitochondrial matrix free Ca2+. Besides, Ca2+ release inhibition prevented MPP+ mediated mitochondria-dependent caspases activation. Our findings show that ER and mitochondria are in a close communication, establishing a dynamic ER-Ca2+-mitochondria interconnection that can play a prominent role in the neuronal cell death induction under particular stressful circumstances of PD pathology.

Organelle dynamics and dysfunction: A closer link between peroxisomes and mitochondria

Mitochondria and peroxisomes are ubiquitous subcellular organelles, which fulfil an indispensable role in the cellular metabolism of higher eukaryotes. Moreover, they are highly dynamic and display large plasticity. There is growing evidence now that both organelles exhibit a closer interrelationship than previously appreciated. This connection includes metabolic cooperations and cross-talk, a novel putative mitochondria-to-peroxisome vesicular trafficking pathway, as well as an overlap in key components of their fission machinery. Thus, peroxisomal alterations in metabolism, biogenesis, dynamics and proliferation can potentially influence mitochondrial functions, and vice versa. In this review, we present the latest progress in the emerging field of peroxisomal and mitochondrial interrelationship with a particular emphasis on organelle dynamics and its implication in diseases.

Proteomics of mitochondrial inner and outer membranes

For the proteomic study of mitochondrial membranes, documented high quality mitochondrial preparations are a necessity to ensure proper localization. Despite the state-of-the-art technologies currently in use, there is no single technique that can be used for all studies of mitochondrial membrane proteins. Herein, we use examples to highlight solubilization techniques, different chromatographic methods, and developments in gel electrophoresis for proteomic analysis of mitochondrial membrane proteins. Blue-native gel electrophoresis has been successful not only for dissection of the inner membrane oxidative phosphorylation system, but also for the components of the outer membrane such as those involved in protein import. Identification of PTMs such as phosphorylation, acetylation, and nitration of mitochondrial membrane proteins has been greatly improved by the use of affinity techniques. However, understanding of the biological effect of these modifications is an area for further exploration. The rapid development of proteomic methods for both identification and quantitation, especially for modifications, will greatly impact the understanding of the mitochondrial membrane proteome.

Differential distribution of Ca2+-activated potassium channel beta4 subunit in rat brain: immunolocalization in neuronal mitochondria

Large conductance Ca(2+)-activated potassium channels (BK(Ca) channels) are expressed in the plasma membrane of various cell types. Interestingly, recent studies provided evidence for the existence of BK(Ca) channels also in mitochondria. However, the molecular composition of these channels as well as their cellular and tissue distribution is still unknown. The goal of the present study was to find a candidate for the regulatory component of the mitochondrial large conductance calcium activated potassium (mitoBK(Ca)) channel in neurons. A combined approach of Western blot analysis, high-resolution immunofluorescence and immunoelectron microscopy with the use of antibodies directed against four distinct beta subunits demonstrated the presence of the BK(Ca) channel beta4 subunit (KCNMB4) in the inner membrane of neuronal mitochondria in the rat brain and cultured neurons. Within the cell, the expression of beta4 subunit was restricted to a subpopulation of mitochondria. The analysis of beta4 subunit distribution throughout the brain revealed that the highest expression levels occur in the thalamus and the brainstem. Our results suggest that beta4 subunit is a regulatory component of mitochondrial BK(Ca) channels in neurons. These findings may support the perspectives for the neuroprotective role of mitochondrial BK(Ca) channel in specific brain structures.

Mitochondrial and secretory human cytomegalovirus UL37 proteins traffic into mitochondrion-associated membranes of human cells

The human cytomegalovirus (HCMV) UL37 exon 1 protein (pUL37x1), also known as vMIA, is the predominant UL37 isoform during permissive infection. pUL37x1 is a potent antiapoptotic protein, which prevents cytochrome c release from mitochondria. The UL37x1 NH(2)-terminal bipartite localization signal, which remains uncleaved, targets UL37 proteins to the endoplasmic reticulum (ER) and then to mitochondria. Based upon our findings, we hypothesized that pUL37x1 traffics from the ER to mitochondria through direct contacts between the two organelles, provided by mitochondrion-associated membranes (MAMs). To facilitate its identification, we cloned and tagged the human phosphatidylserine synthase 1 (huPSS-1) cDNA, whose mouse homologue localizes almost exclusively in the MAM. Using subcellular fractionation of stable HeLa cell transfectants expressing mEGFP-huPSS-1, we found that HCMV pUL37x1 is present in purified microsomes, mitochondria, and MAM fractions. We further examined the trafficking of the full-length UL37 glycoprotein cleavage products, which divergently traffic either through the secretory apparatus or into mitochondria. Surprisingly, pUL37(NH2) and gpUL37(COOH) were both detected in the ER and MAM fraction, even though only pUL37(NH2) is preferentially imported into mitochondria but gpUL37(COOH) is not. To determine the sequences required for MAM importation, we examined pUL37x1 mutants that were partially defective for mitochondrial importation. Deletion mutants of the NH(2)-terminal UL37x1 mitochondrial localization signal were reduced in trafficking into the MAM, indicating partial overlap of MAM and mitochondrial targeting signals. Taken together, these results suggest that HCMV UL37 proteins traffic from the ER into the MAM, where they are sorted into either the secretory pathway or to mitochondrial importation.

Processing of human cytomegalovirus UL37 mutant glycoproteins in the endoplasmic reticulum lumen prior to mitochondrial importation

The human cytomegalovirus (HCMV) UL37 glycoprotein (gpUL37) is internally cleaved and its products divergently traffic to mitochondria or are retained in the secretory pathway. To define the requirements for gpUL37 cleavage, residues -1 and -3 of the consensus endoplasmic reticulum (ER) signal peptidase I site within exon 3 (UL37x3) were replaced by bulky tyrosines (gpUL37 cleavage site mutant I). Internal cleavage of this UL37x3 mutant was inhibited, verifying usage of the consensus site at amino acids (aa) 193/194. The full-length mitochondrial species of gpUL37 cleavage site mutant I was N glycosylated and endoglycosidase H sensitive, indicating that ER translocation and processing took place prior to its mitochondrial importation. Moreover, these results suggest that internal cleavage of gpUL37 is not necessary for its N glycosylation. Partial deletion or disruption of the UL37 hydrophobic core immediately upstream of the cleavage site resulted in decreased protein abundance, suggesting that the UL37x3 hydrophobic alpha-helix contributes to either correct folding or stability of gpUL37. Insertion of the UL37x3 hydrophobic core and cleavage site into pUL37(M), a splice variant of gpUL37 which lacks these sequences and is neither proteolytically cleaved nor N glycosylated, resulted in its internal cleavage and N glycosylation. Its NH(2)-terminal fragment, pUL37(M-NH2), was detected more abundantly in mitochondria, while its N-glycosylated C-terminal fragment, gpUL37(M-COOH), was detected predominantly in the ER in a manner analogous to that of gpUL37 cleavage products. These results indicate that UL37x3 aa 178 to 205 are prerequisite for gpUL37 internal cleavage and alter UL37 protein topology allowing N glycosylation of its C-terminal sequences. In contrast, the NH(2)-terminal UL37x1 hydrophobic leader, present in pUL37x1, pUL37(M), and gpUL37, is not cleaved from mature UL37 protein, retaining a membrane anchor for UL37 isoforms during trafficking. Taken together, these results suggest that HCMV gpUL37 undergoes sequential trafficking, during which it is ER translocated, processed, and then mitochondrially imported.

Mitochondrial glycosidic residues contribute to the interaction between ruthenium amine complexes and the calcium uniporter

The role of glycosidic residues in the inhibitory properties of ruthenium complexes on mitochondrial calcium uptake was determined in mitoplasts. Our results showed that the binding and inhibitory properties of ruthenium amine complexes were modified when mitoplasts were exposed to N-glycosidase F action, but calcium uptake was not altered. N-linked proteins of the mitochondrial inner membrane were identified. We detected an 18-kDa protein that binds labeled Ru360 under control conditions, but failed to bind the inhibitor after deglycosilation. A relationship between this protein and the action of ruthenium amine inhibitors of the mitochondrial uniporter is proposed.

Involvement of alpha-amylase I-1 in starch degradation in rice chloroplasts

To determine the role of alpha-amylase isoform I-1 in the degradation of starch in rice leaf chloroplasts, we generated a series of transgenic rice plants with suppressed expression or overexpression of alpha-amylase I-1. In the lines with suppressed expression of alpha-amylase I-1 at both the mRNA and protein levels, seed germination and seedling growth were markedly delayed in comparison with those in the wild-type plants. However, the growth retardation was overcome by supplementation of sugars. Interestingly, a significant increase of starch accumulation in the young leaf tissues was observed under a sugar-supplemented condition. In contrast, the starch content of leaves was reduced in the plants overexpressing alpha-amylase I-1. In immunocytochemical analysis with specific anti-alpha-amylase I-1 antiserum, immuno-gold particles deposited in the chloroplasts and extracellular space in young leaf cells. We further examined the expression and targeting of alpha-amylase I-1 fused with the green fluorescent protein in re-differentiated green cells, and showed that the fluorescence of the expressed fusion protein co-localized with the chlorophyll autofluorescence in the transgenic cells. In addition, mature protein species of alpha-amylase I-1 bearing an oligosaccharide side chain were detected in the isolated chloroplasts. Based on these results, we concluded that alpha-amylase I-1 targets the chloroplasts through the endoplasmic reticulum-Golgi system and plays a significant role in the starch degradation in rice leaves.

A hypomorphic allele of the first N-glycosylation gene, ALG7, causes mitochondrial defects in yeast

The modification of proteins at asparagine residues with oligosaccharides (N-glycans) plays critical roles in diverse cell functions. N-glycans originate from a common lipid-linked oligosaccharide (LLO) precursor whose synthesis is initiated by the Dol-P-dependent GlcNAc-1-P transferase (GPT) encoded by an essential ALG7 gene. To identify cellular processes affected by ALG7 and N-glycosylation, we replaced the genomic copy of ALG7 with its hypomorphic allele in two genetically distinct haploid yeast cells. We show that ALG7 knockdown gave rise to an unexpected phenotype of mitochondrial dysfunction. The alg7 mutants did not grow on glycerol and DNA arrays revealed the absence of mitochondrial genes' expression. Accordingly, the alg7 mutants displayed no detectable mtDNA and respiratory activity. Both mutants exhibited diminished abundance of LLO and under-glycosylation of carboxypeptidase Y (CPY). Moreover, another N-glycosylation mutant with a LLO defect, alg6, was respiratory deficient. Collectively, our studies provide evidence that the dysregulation of N-glycosylation in haploid yeast cells leads to mitochondrial dysfunction.

Membrane lipids and cell death: an overview

In this article we overview major aspects of membrane lipids in the complex area of cell death, comprising apoptosis and various forms of programmed cell death. We have focused here on glycerophospholipids, the major components of cellular membranes. In particular, we present a detailed appraisal of mitochondrial lipids that attract increasing interest in the field of cell death, while the knowledge of their re-modelling and traffic remains limited. It is hoped that this review will stimulate further studies by lipid experts to fully elucidate various aspects of membrane lipid homeostasis that are discussed here. These studies will undoubtedly reveal new and important connections with the established players of cell death and their action in promoting or blocking membrane alteration of mitochondria and other organelles. We conclude that the new dynamic era of cell death research will pave the way for a better understanding of the 'chemistry of apoptosis'.

Role of the Putative Structural Protein Sed1p in Mitochondrial Genome Maintenance

The nuclear gene MIP1 encodes the mitochondrial DNA polymerase responsible for replicating the mitochondrial genome in Saccharomyces cerevisiae. A number of other factors involved in replicating and segregating the mitochondrial genome are yet to be identified. Here, we report that a bacterial two-hybrid screen using the mitochondrial polymerase, Mip1p, as bait identified the yeast protein Sed1p. Sed1p is a cell surface protein highly expressed in the stationary phase. We find that several modified forms of Sed1p are expressed and the largest of these forms interacts with the mitochondrial polymerase in vitro. Deletion of SED1 causes a 3.5-fold increase in the rate of mitochondrial DNA point mutations as well as a 4.3-fold increase in the rate of loss of respiration. In contrast, we see no change in the rate of nuclear point mutations indicating the specific role of Sed1p function in mitochondrial genome stability. Indirect immunofluorescence analysis of Sed1p localization shows that Sed1p is targeted to the mitochondria. Moreover, Sed1p is detected in purified mitochondrial fractions and the localization to the mitochondria of the largest modified form is insensitive to the action of proteinase K. Deletion of the sed1 gene results in a reduction in the quantity of Mip1p and also affects the levels of a mitochondrially-expressed protein, Cox3p. Our results point towards a role for Sed1p in mitochondrial genome maintenance.

Targeting of hepatitis C virus core protein to mitochondria through a novel C-terminal localization motif

The hepatitis C virus (HCV) core protein represents the first 191 amino acids of the viral precursor polyprotein and is cotranslationally inserted into the membrane of the endoplasmic reticulum (ER). Processing at position 179 by a recently identified intramembrane signal peptide peptidase leads to the generation and potential cytosolic release of a 179-amino-acid matured form of the core protein. Using confocal microscopy, we observed that a fraction of the mature core protein colocalized with mitochondrial markers in core-expressing HeLa cells and in Huh-7 cells containing the full-length HCV replicon. Subcellular fractionation confirmed this observation and showed that the core protein associates with purified mitochondrial fractions devoid of ER contaminants. The core protein also fractionated with mitochondrion-associated membranes, a site of physical contact between the ER and mitochondria. Using immunoelectron microscopy and in vitro mitochondrial import assays, we showed that the core protein is located on the mitochondrial outer membrane. A stretch of 10 amino acids within the hydrophobic C terminus of the processed core protein conferred mitochondrial localization when it was fused to green fluorescent protein. The location of the core protein in the outer mitochondrial membrane suggests that it could modulate apoptosis or lipid transfer, both of which are associated with this subcellular compartment, during HCV infection.

The mitochondria-associated endoplasmic-reticulum subcompartment (MAM fraction) of rat liver contains highly active sphingolipid-specific glycosyltransferases

Although most glycosphingolipids (GSLs) are thought to be located in the outer leaflet of the plasma membrane, recent evidence indicates that GSLs and their precursor, ceramide, are also associated with intracellular organelles and, particularly, mitochondria. GSL biosynthesis starts with the formation of ceramide in the endoplasmic reticulum (ER), which is transported by controversial mechanisms to the Golgi apparatus, where stepwise addition of monosaccharides on to ceramides takes place. We now report the presence of GSL-biosynthetic enzymes in a subcompartment of the ER previously characterized and termed 'mitochondria-associated membrane' (MAM). MAM is a membrane bridge between the ER and mitochondria that is involved in the biosynthesis and trafficking of phospholipids between the two organelles. Using exogenous acceptors coated on silica gel, we demonstrate the presence of ceramide glucosyltransferase (Cer-Glc-T), glucosylceramide galactosyltransferase and sialyltransferase (SAT) activities in the MAM. Estimation of the marker-enzyme activities showed that glycosyltransferase activities could not be ascribed to cross-contamination of MAM by Golgi membranes. Cer-Glc-T was found to have a marked preference for ceramide bearing phytosphingosine as sphingoid base. SAT activities in MAM led to the synthesis of G(M3) ganglioside and small amounts of G(D3). G(M1) was also synthesized along with G(M3) upon incubation of the fraction with exogenous unlabelled G(M3), underlying the presence of other sphingolipid-specific glycosyltransferases in MAM. On the basis of our results, we propose MAM as a privileged compartment in providing GSLs for mitochondria.

Detection technologies in proteome analysis

Common strategies employed for general protein detection include organic dye, silver stain, radiolabeling, reverse stain, fluorescent stain, chemiluminescent stain and mass spectrometry-based approaches. Fluorescence-based protein detection methods have recently surpassed conventional technologies such as colloidal Coomassie blue and silver staining in terms of quantitative accuracy, detection sensitivity, and compatibility with modern downstream protein identification and characterization procedures, such as mass spectrometry. Additionally, specific detection methods suitable for revealing protein post-translational modifications have been devised over the years. These include methods for the detection of glycoproteins, phosphoproteins, proteolytic modifications, S-nitrosylation, arginine methylation and ADP-ribosylation. Methods for the detection of a range of reporter enzymes and epitope tags are now available as well, including those for visualizing beta-glucuronidase, beta-galactosidase, oligohistidine tags and green fluorescent protein. Fluorescence-based and mass spectrometry-based methodologies are just beginning to offer unparalleled new capabilities in the field of proteomics through the performance of multiplexed quantitative analysis. The primary objective of differential display proteomics is to increase the information content and throughput of proteomics studies through multiplexed analysis. Currently, three principal approaches to differential display proteomics are being actively pursued, difference gel electrophoresis (DIGE), multiplexed proteomics (MP) and isotope-coded affinity tagging (ICAT). New multiplexing capabilities should greatly enhance the applicability of the two-dimensional gel electrophoresis technique with respect to addressing fundamental questions related to proteome-wide changes in protein expression and post-translational modification.

Complex glycosylation of Skp1 in Dictyostelium: implications for the modification of other eukaryotic cytoplasmic and nuclear proteins

Recently, complex O-glycosylation of the cytoplasmic/nuclear protein Skp1 has been characterized in the eukaryotic microorganism Dictyostelium. Skp1's glycosylation is mediated by the sequential action of a prolyl hydroxylase and five conventional sugar nucleotide-dependent glycosyltransferase activities that reside in the cytoplasm rather than the secretory compartment. The Skp1-HyPro GlcNAcTransferase, which adds the first sugar, appears to be related to a lineage of enzymes that originated in the prokaryotic cytoplasm and initiates mucin-type O-linked glycosylation in the lumen of the eukaryotic Golgi apparatus. GlcNAc is extended by a bifunctional glycosyltransferase that mediates the ordered addition of beta1,3-linked Gal and alpha1,2-linked Fuc. The architecture of this enzyme resembles that of certain two-domain prokaryotic glycosyltransferases. The catalytic domains are related to those of a large family of prokaryotic and eukaryotic, cytoplasmic, membrane-bound, inverting glycosyltransferases that modify glycolipids and polysaccharides prior to their translocation across membranes toward the secretory pathway or the cell exterior. The existence of these enzymes in the eukaryotic cytoplasm away from membranes and their ability to modify protein acceptors expose a new set of cytoplasmic and nuclear proteins to potential prolyl hydroxylation and complex O-linked glycosylation.

Myocilin is associated with mitochondria in human trabecular meshwork cells

The trabecular meshwork (TM) is a specialized tissue located at the chamber angle of the eye next to the cornea. This tissue is believed to be responsible for regulation of the aqueous humor outflow and control of the intraocular pressure (IOP). Alterations in functions of the TM may lead to IOP elevation and development of glaucoma, a major cause of blindness. The myocilin gene has recently been directly linked to open-angle glaucomas. The gene product was originally identified as a protein inducible in TM cells by treatment with glucocorticoids such as dexamethasone (DEX) and termed TIGR (TM inducible-glucocorticoid response). The exact nature and function of the myocilin protein so far still remain elusive. In this study, myocilin was localized to the perinuclear region of both DEX-treated and control TM cells. Its distribution overlapped considerably with that of mitochondria. Subcellular fractionation and Western blot analyses suggested a rather extensive association of myocilin with mitochondria. The DEX-treated TM cells were found to undergo apoptosis, when exposed to anti-Fas antibody, to a significantly higher degree than the untreated control cells. It appears that the TM cell integrity remains intact after DEX treatment. However, the induced myocilin or myocilin-mitochondria association seems to render the cells more susceptible to a second stress or challenge. This vulnerability may be the basis that ultimately leads to pathological consequences.

Targeting of a tail-anchored protein to endoplasmic reticulum and mitochondrial outer membrane by independent but competing pathways

Many mitochondrial outer membrane (MOM) proteins have a transmembrane domain near the C terminus and an N-terminal cytosolic moiety. It is not clear how these tail-anchored (TA) proteins posttranslationally select their target, but C-terminal charged residues play an important role. To investigate how discrimination between MOM and endoplasmic reticulum (ER) occurs, we used mammalian cytochrome b(5), a TA protein existing in two, MOM or ER localized, versions. Substitution of the seven C-terminal residues of the ER isoform or of green fluorescent protein reporter constructs with one or two arginines resulted in MOM-targeted proteins, whereas a single C-terminal threonine caused promiscuous localization. To investigate whether targeting to MOM occurs from the cytosol or after transit through the ER, we tagged a MOM-directed construct with a C-terminal N-glycosylation sequence. Although in vitro this construct was efficiently glycosylated by microsomes, the protein expressed in vivo localized almost exclusively to MOM, and was nearly completely unglycosylated. The small fraction of glycosylated protein was in the ER and was not a precursor to the unglycosylated form. Thus, targeting occurs directly from the cytosol. Moreover, ER and MOM compete for the same polypeptide, explaining the dual localization of some TA proteins.

High-throughput profiling of the mitochondrial proteome using affinity fractionation and automation

Recent studies have demonstrated the need for complementing cellular genomic information with specific information on expressed proteins, or proteomics, since the correlation between the two is poor. Typically, proteomic information is gathered by analyzing samples on two-dimensional gels with the subsequent identification of specific proteins of interest by using trypsin digestion and mass spectrometry in a process termed peptide mass fingerprinting. These procedures have, as a rule, been labor-intensive and manual, and therefore of low throughput. The development of automated proteomic technology for processing large numbers of samples simultaneously has made the concept of profiling entire proteomes feasible at last. In this study, we report the initiation of the (eventual) complete profile of the rat mitochondrial proteome by using high-throughput automated equipment in combination with a novel fractionation technique using minispin affinity columns. Using these technologies, approximately one hundred proteins could be identified in several days. In addition, separate profiles of calcium binding proteins, glycoproteins, and hydrophobic or membrane proteins could be generated. Because mitochondrial dysfunction has been implicated in numerous diseases, such as cancer, Alzheimer's disease and diabetes, it is probable that the identification of the majority of mitochondrial proteins will be a beneficial tool for developing drug and diagnostic targets for associated diseases.

Regulation of TIGR/MYOC gene expression in human trabecular meshwork cells

Glucocorticoid (GC) treatment of human trabecular meshwork (HTM) cells produces delayed, progressive cellular and extracellular protein/glycoprotein inductions with characteristics matching those for intraocular pressure elevation with corticosteroid eyedrops. The cloning of the Trabecular Meshwork Inducible Glucocorticoid Response (TIGR) gene from this system has suggested possible environmental and genetic influences in relation to glaucoma mechanisms. As reported here, the major GC-induced increase of TIGR expression in HTM cells is reduced approximately 4-fold by basic fibroblast growth factor (bFGF, 100-1000 pM), with a somewhat smaller inhibition noted with the thyroid hormone triiodothyronine (T3, 100 nM). Such endogenous 'protective' factors could help balance stimulatory effects on TIGR gene expression from 'stress' and/or mechanical perturbations in the trabecular meshwork. TIGR coding region mutations affecting the gene's olfactomedin (OLF) homology domain may also perturb biosynthetic pathways and cellular homeostatic functions. Our recent studies have shown the OLF domain corresponds to a major translocational 'pause', an area where critical processes for normal TIGR biogenesis are expected to take place. Observations that Glu323Lys (and other mutations early in the OLF domain) altered the pattern of paused protein intermediates provide possible clues to previously unexplained pathogenetic mechanisms. HTM cell transfection studies using TIGR-green fluorescent protein (GFP) fusions showed increased and altered distribution of the expressed protein with constructs missing the OLF domain, an effect also found with the Pro370 Leu mutation for early-onset glaucoma. The data suggest an activation of stress/apoptotic pathways in HTM cells as a potential mechanism for environmental/genetic interactions in glaucoma pathogenesis.

Temporal and spatial regulation of expression of two galectins during kidney development of the chicken

Organogenesis and the establishment of the mature phenotype require an interplay between diverse recognition systems. Concerning protein-carbohydrate interactions, galectins are known to be involved in several extra- and intracellular functions. Due to the occurrence of two avian galectins in liver (chicken galectin-16 CG-16) and intestine (chicken galectin-14; CG-14) with different developmental regulation. the questions addressed are to what extent and where these galectins are present during chicken kidney development. Using Western blot analysis, the presence of both activities in tissue extracts was ascertained. A solid-phase assay showed peak levels at day 12 followed by a decline. A histochemical analysis was carried out in combination with routine staining. Epithelial cells of the mesonephric proximal tubules were immunoreactive in the cytoplasm for CG-14 from day 5 of incubation onwards. Additionally, epithelial cells of the metanephric collecting ducts were stained. For CG-16 a rather similar pattern of staining was seen, additional positivity in early glomerular podocytes being notable. At the electron microscopical level, a diffuse staining for CG-14 was seen in the cytoplasm, whereas immunoreactivity for CG-16 was observed mainly in mitochondria. These results demonstrate quantitative differences in the developmental regulation of the two avian galectins with obvious similarities in the cell-type pattern but with a disparate intracellular localisation profile.

Apoptotic crosstalk between the endoplasmic reticulum and mitochondria controlled by Bcl-2

Apoptosis involves mitochondrial steps such as the release of the apoptogenic factor cytochrome c which are effectively blocked by Bcl-2. Although Bcl-2 may have a direct action on the mitochondrial membrane, it also resides and functions on the endoplasmic reticulum (ER), and there is increasing evidence for a role of the ER in apoptosis regulation as well. Here we uncover a hitherto unrecognized, apoptotic crosstalk between the ER and mitochondria that is controlled by Bcl-2. After triggering massive ER dilation due to an inhibition of secretion, the drug brefeldin A (BFA) induces the release of cytochrome c from mitochondria in a caspase-8- and Bid-independent manner. This is followed by caspase-3 activation and DNA/nuclear fragmentation. Surprisingly, cytochrome c release by BFA is not only blocked by wild-type Bcl-2 but also by a Bcl-2 variant that is exclusively targeted to the ER (Bcl-2/cb5). Similar findings were obtained with tunicamycin, an agent interfering with N-linked glycosylations in the secretory system. Thus, apoptotic agents perturbing ER functions induce a novel crosstalk between the ER and mitochondria that can be interrupted by ER-based Bcl-2.

Use of recombinant endomannosidase for evaluation of the processing of N-linked oligosaccharides of glycoproteins and their oligosaccharide-lipid precursors

Although glucose residues in a triglucosyl sequence are essential for the N-glycosylation of proteins and in their monoglucosyl form have been implicated in lectin-like interactions with chaperones, their removal is required for the formation of mature carbohydrate units and represents the initial steps in the glycoprotein processing sequence. In order to provide a probe for the glucosylation state of newly synthesized glycoproteins obtained from normal or altered cells, we have evaluated the usefulness of recombinant endo-alpha-mannosidase employing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to monitor the change in molecular mass brought about by the release of glucosylated mannose (Glc(1-3)Man). With this approach the presence of two triglucosylated-N-linked oligosaccharides in vesicular stomatis virus (VSV) G protein formed by castanospermine-treated CHO cells or the glucosidase I deficient Lec23 mutant could be clearly demonstrated and an even more pronounced change in migration was observed upon endomannosidase treatment of their more heavily N-glycosylated lysosomal membrane glycoproteins. Furthermore, the G protein of the temperature sensitive VSV ts045 mutant was found to be sensitive to endomannosidase, resulting in a change in electrophoretic mobility consistent with the presence of mono-glucosylated-N-linked oligosaccharides. The finding that endomannosidase also acts effectively on oligosaccharide lipids, as assessed by SDS-PAGE or thin layer chromatography, indicated that it would be a valuable tool in assessing the glucosylation state of these biosynthetic intermediates in normal cells as well as in mutants or altered metabolic states, even if the polymannose portion is truncated. Endomannosidase can also be used to determine the glucosylation state of the polymannose oligosaccharides released during glycoprotein quality control and when used together with endo-beta-N- acetylglucosaminidase H can distinguish between those terminating in a single N-acetylglucosamine or in a di-N-acetylchitobiose sequence.

The dynamin-like protein DLP1 is essential for normal distribution and morphology of the endoplasmic reticulum and mitochondria in mammalian cells

The dynamin family of large GTPases has been implicated in vesicle formation from both the plasma membrane and various intracellular membrane compartments. The dynamin-like protein DLP1, recently identified in mammalian tissues, has been shown to be more closely related to the yeast dynamin proteins Vps1p and Dnm1p (42%) than to the mammalian dynamins (37%). Furthermore, DLP1 has been shown to associate with punctate vesicles that are in intimate contact with microtubules and the endoplasmic reticulum (ER) in mammalian cells. To define the function of DLP1, we have transiently expressed both wild-type and two mutant DLP1 proteins, tagged with green fluorescent protein, in cultured mammalian cells. Point mutations in the GTP-binding domain of DLP1 (K38A and D231N) dramatically changed its intracellular distribution from punctate vesicular structures to either an aggregated or a diffuse pattern. Strikingly, cells expressing DLP1 mutants or microinjected with DLP1 antibodies showed a marked reduction in ER fluorescence and a significant aggregation and tubulation of mitochondria by immunofluorescence microscopy. Consistent with these observations, electron microscopy of DLP1 mutant cells revealed a striking and quantitative change in the distribution and morphology of mitochondria and the ER. These data support very recent studies by other authors implicating DLP1 in the maintenance of mitochondrial morphology in both yeast and mammalian cells. Furthermore, this study provides the first evidence that a dynamin family member participates in the maintenance and distribution of the ER. How DLP1 might participate in the biogenesis of two presumably distinct organelle systems is discussed.