Candida maltosa NADPH-cytochrome P450 reductase: cloning of a full-length cDNA, heterologous expression in Saccharomyces cerevisiae and function of the N-terminal region for membrane anchoring and proliferation of the endoplasmic reticulum

HIV-1 Gag release from yeast reveals ESCRT interaction with the Gag N-terminal protein region

The HIV-1 protein Gag assembles at the plasma membrane and drives virion budding, assisted by the cellular endosomal complex required for transport (ESCRT) proteins. Two ESCRT proteins, TSG101 and ALIX, bind to the Gag C-terminal p6 peptide. TSG101 binding is important for efficient HIV-1 release, but how ESCRTs contribute to the budding process and how their activity is coordinated with Gag assembly is poorly understood. Yeast, allowing genetic manipulation that is not easily available in human cells, has been used to characterize the cellular ESCRT function. Previous work reported Gag budding from yeast spheroplasts, but Gag release was ESCRT-independent. We developed a yeast model for ESCRT-dependent Gag release. We combined yeast genetics and Gag mutational analysis with Gag-ESCRT binding studies and the characterization of Gag-plasma membrane binding and Gag release. With our system, we identified a previously unknown interaction between ESCRT proteins and the Gag N-terminal protein region. Mutations in the Gag-plasma membrane-binding matrix domain that reduced Gag-ESCRT binding increased Gag-plasma membrane binding and Gag release. ESCRT knockout mutants showed that the release enhancement was an ESCRT-dependent effect. Similarly, matrix mutation enhanced Gag release from human HEK293 cells. Release enhancement partly depended on ALIX binding to p6, although binding site mutation did not impair WT Gag release. Accordingly, the relative affinity for matrix compared with p6 in GST-pulldown experiments was higher for ALIX than for TSG101. We suggest that a transient matrix-ESCRT interaction is replaced when Gag binds to the plasma membrane. This step may activate ESCRT proteins and thereby coordinate ESCRT function with virion assembly.

Inducible intracellular membranes: molecular aspects and emerging applications

Membrane remodeling and phospholipid biosynthesis are normally tightly regulated to maintain the shape and function of cells. Indeed, different physiological mechanisms ensure a precise coordination between de novo phospholipid biosynthesis and modulation of membrane morphology. Interestingly, the overproduction of certain membrane proteins hijack these regulation networks, leading to the formation of impressive intracellular membrane structures in both prokaryotic and eukaryotic cells. The proteins triggering an abnormal accumulation of membrane structures inside the cells (or membrane proliferation) share two major common features: (1) they promote the formation of highly curved membrane domains and (2) they lead to an enrichment in anionic, cone-shaped phospholipids (cardiolipin or phosphatidic acid) in the newly formed membranes. Taking into account the available examples of membrane proliferation upon protein overproduction, together with the latest biochemical, biophysical and structural data, we explore the relationship between protein synthesis and membrane biogenesis. We propose a mechanism for the formation of these non-physiological intracellular membranes that shares similarities with natural inner membrane structures found in α-proteobacteria, mitochondria and some viruses-infected cells, pointing towards a conserved feature through evolution. We hope that the information discussed in this review will give a better grasp of the biophysical mechanisms behind physiological and induced intracellular membrane proliferation, and inspire new applications, either for academia (high-yield membrane protein production and nanovesicle production) or industry (biofuel production and vaccine preparation).

Endoplasmic reticulum anchored heme-oxygenase 1 faces the cytosol

Heme-oxygenase 1 is an endoplasmic reticulum-anchored enzyme that breaks down heme into iron, carbon monoxide and biliverdin. Heme is a hydrophobic co-factor in many proteins, including hemoglobin. Free heme is highly cytotoxic and, therefore, both heme synthesis and breakdown are tightly regulated. During turnover of heme proteins, heme is released in the phago-lysosomal compartment or the cytosol. The subcellular location of the heme-oxygenase 1 active site has not been clarified. Using constructs of heme-oxygenase 1 with fluorescent proteins, and the endogenous heme-oxygenase 1 in two variations of protease protection assays, we determined that heme-oxygenase 1 is membrane-bound and faces the cytosol in non-activated macrophages in vivo. These findings imply that in quiescent macrophages, heme breakdown products are generated in the cytosol. This facilitates iron recycling to ferroportin for iron export and to ferritin for iron storage.

TmpL, a transmembrane protein required for intracellular redox homeostasis and virulence in a plant and an animal fungal pathogen

The regulation of intracellular levels of reactive oxygen species (ROS) is critical for developmental differentiation and virulence of many pathogenic fungi. In this report we demonstrate that a novel transmembrane protein, TmpL, is necessary for regulation of intracellular ROS levels and tolerance to external ROS, and is required for infection of plants by the necrotroph Alternaria brassicicola and for infection of mammals by the human pathogen Aspergillus fumigatus. In both fungi, tmpL encodes a predicted hybrid membrane protein containing an AMP-binding domain, six putative transmembrane domains, and an experimentally-validated FAD/NAD(P)-binding domain. Localization and gene expression analyses in A. brassicicola indicated that TmpL is associated with the Woronin body, a specialized peroxisome, and strongly expressed during conidiation and initial invasive growth in planta. A. brassicicola and A. fumigatus ΔtmpL strains exhibited abnormal conidiogenesis, accelerated aging, enhanced oxidative burst during conidiation, and hypersensitivity to oxidative stress when compared to wild-type or reconstituted strains. Moreover, A. brassicicola ΔtmpL strains, although capable of initial penetration, exhibited dramatically reduced invasive growth on Brassicas and Arabidopsis. Similarly, an A. fumigatus ΔtmpL mutant was dramatically less virulent than the wild-type and reconstituted strains in a murine model of invasive aspergillosis. Constitutive expression of the A. brassicicola yap1 ortholog in an A. brassicicola ΔtmpL strain resulted in high expression levels of genes associated with oxidative stress tolerance. Overexpression of yap1 in the ΔtmpL background complemented the majority of observed developmental phenotypic changes and partially restored virulence on plants. Yap1-GFP fusion strains utilizing the native yap1 promoter exhibited constitutive nuclear localization in the A. brassicicola ΔtmpL background. Collectively, we have discovered a novel protein involved in the virulence of both plant and animal fungal pathogens. Our results strongly suggest that dysregulation of oxidative stress homeostasis in the absence of TmpL is the underpinning cause of the developmental and virulence defects observed in these studies.

The critical roles of reactive oxygen species (ROS) in fungal development and virulence have been well established over the past half a century since the first experimental detection of hydrogen peroxide in fungal cells by Bach (1950). In the cell, ROS act as signaling molecules regulating physiological responses and developmental processes and are also involved in sophisticated virulence processes for many pathogenic fungi. Therefore, uncovering the biological roles of cellular ROS appears to be very important in understanding fungal development and virulence. Currently we have limited knowledge of how intracellular ROS are generated by fungal cells and which cellular ROS regulatory mechanisms are involved in establishing homeostasis. In this study we describe a novel protein, TmpL, involved in development and virulence in both plant and animal pathogenic fungi. In the absence of TmpL, dysregulation of oxidative stress homeostasis in both fungi caused developmental and virulence defects. Therefore, elucidating the role of TmpL presents an opportunity to uncover a common pathogenicity mechanism employed by both plant and animal pathogens and to develop efficient and novel therapeutics for both plant and animal fungal disease. Our findings provide new insights into mechanisms underlying the complex web of interactions between ROS and cell differentiation and the involvement of ROS for both plant and animal fungal pathogenesis.

Dynamic subcompartmentalization of the mitochondrial inner membrane

The inner membrane of mitochondria is organized in two morphologically distinct domains, the inner boundary membrane (IBM) and the cristae membrane (CM), which are connected by narrow, tubular cristae junctions. The protein composition of these domains, their dynamics, and their biogenesis and maintenance are poorly understood at the molecular level. We have used quantitative immunoelectron microscopy to determine the distribution of a collection of representative proteins in yeast mitochondria belonging to seven major processes: oxidative phosphorylation, protein translocation, metabolite exchange, mitochondrial morphology, protein translation, iron-sulfur biogenesis, and protein degradation. We show that proteins are distributed in an uneven, yet not exclusive, manner between IBM and CM. The individual distributions reflect the physiological functions of proteins. Moreover, proteins can redistribute between the domains upon changes of the physiological state of the cell. Impairing assembly of complex III affects the distribution of partially assembled subunits. We propose a model for the generation of this dynamic subcompartmentalization of the mitochondrial inner membrane.

Reevaluation of the 22-1-1 antibody and its putative antigen, EBAG9/RCAS1, as a tumor marker

Background

Tumor-associated antigens are appreciated as diagnostic markers, but they have also prompted tremendous efforts to develop tumor-specific immunotherapy. A previously cloned tumor-associated antigen, EBAG9, was initially defined by reactivity with the monoclonal antibody 22-1-1. Functionally, the EBAG9-encoded gene-product was believed to induce apoptosis in activated immune cells. However, using a cell-biological approach we identified EBAG9 as a Golgi-resident modulator of O-linked glycan expression, the latter product was then recognized by the 22-1-1 antibody. Secondly, EBAG9 expression was found physiologically in all murine tissues examined. This raised the question if EBAG9 is tumor-specific and mediates apoptosis itself or through O-linked glycans generated, among them the cognate 22-1-1 antigen Tn.

Methods

We have used immunohistochemistry to detect the expression of 22-1-1 and EBAG9 in various tissues. Correlation between expression of both antigens in cell lines was analysed by immunoblot and flow cytometry. Apoptosis was studied by using flow cytometry and Caspase-Glo™ 3/7 assay kit. Cellular distribution of EBAG9 was analysed by electron and confocal microscopy.

Results

Here, we compared expression of the 22-1-1 and EBAG9-defined antigens in normal and neoplastic tissues in situ. In contrast to 22-1-1 staining, EBAG9 is a ubiquitously expressed antigen in all normal and cancerous tissues. Functional studies on the role of 22-1-1 reactive material did not support any evidence for apoptosis induction. Employing electron and confocal microscopy, a refined subcellular localization of EBAG9 at the Golgi was obtained.

Conclusion

We suggest that the estrogen-inducible EBAG9 gene-product and the 22-1-1 defined antigen are structurally and functionally separate antigens.

Mdm31 and Mdm32 are inner membrane proteins required for maintenance of mitochondrial shape and stability of mitochondrial DNA nucleoids in yeast

The MDM31 and MDM32 genes are required for normal distribution and morphology of mitochondria in the yeast Saccharomyces cerevisiae. They encode two related proteins located in distinct protein complexes in the mitochondrial inner membrane. Cells lacking Mdm31 and Mdm32 harbor giant spherical mitochondria with highly aberrant internal structure. Mitochondrial DNA (mtDNA) is instable in the mutants, mtDNA nucleoids are disorganized, and their association with Mmm1-containing complexes in the outer membrane is abolished. Mutant mitochondria are largely immotile, resulting in a mitochondrial inheritance defect. Deletion of either one of the MDM31 and MDM32 genes is synthetically lethal with deletion of either one of the MMM1, MMM2, MDM10, and MDM12 genes, which encode outer membrane proteins involved in mitochondrial morphogenesis and mtDNA inheritance. We propose that Mdm31 and Mdm32 cooperate with Mmm1, Mmm2, Mdm10, and Mdm12 in maintenance of mitochondrial morphology and mtDNA.

Role for lipid signaling and the cell integrity MAP kinase cascade in yeast septum biogenesis

Polarized deposition of chitin at the bud neck is essential for cell separation in yeast. Chitin septum biogenesis is catalyzed by two distinct chitin synthase activities encoded by the CHS2 and CHS3 genes. The phosphoinositide phosphatase Sac1p is required for proper trafficking of the Chs3p chitin synthase. sac1 mutants also display a severe synthetic growth defect, with mutations in the SLT2 gene which encodes a MAP kinase involved in cell integrity. We characterized the defect that underlies this genetic interaction and found that sac1 Delta slt2 Delta cells arrest as large-budded cells because they fail to separate at the end of mitosis. This inability to complete cell division appears to be caused by an increased deposition of chitin at the septum area and correlates with a mislocalized accumulation of the Chs2p chitin synthase at the cell periphery. Our data therefore indicate that Sac1p and Slt2p have synergistic roles in regulating chitin septum biogenesis.

The inner membrane protein Mdm33 controls mitochondrial morphology in yeast

Mitochondrial distribution and morphology depend on MDM33, a Saccharomyces cerevisiae gene encoding a novel protein of the mitochondrial inner membrane. Cells lacking Mdm33 contain ring-shaped, mostly interconnected mitochondria, which are able to form large hollow spheres. On the ultrastructural level, these aberrant organelles display extremely elongated stretches of outer and inner membranes enclosing a very narrow matrix space. Dilated parts of Delta mdm33 mitochondria contain well-developed cristae. Overexpression of Mdm33 leads to growth arrest, aggregation of mitochondria, and generation of aberrant inner membrane structures, including septa, inner membrane fragments, and loss of inner membrane cristae. The MDM33 gene is required for the formation of net-like mitochondria in mutants lacking components of the outer membrane fission machinery, and mitochondrial fusion is required for the formation of extended ring-like mitochondria in cells lacking the MDM33 gene. The Mdm33 protein assembles into an oligomeric complex in the inner membrane where it performs homotypic protein-protein interactions. Our results indicate that Mdm33 plays a distinct role in the mitochondrial inner membrane to control mitochondrial morphology. We propose that Mdm33 is involved in fission of the mitochondrial inner membrane.

ER-golgi traffic is a prerequisite for efficient ER degradation

Protein quality control is an essential function of the endoplasmic reticulum. Misfolded proteins unable to acquire their native conformation are retained in the endoplasmic reticulum, retro-translocated back into the cytosol, and degraded via the ubiquitin-proteasome system. We show that efficient degradation of soluble malfolded proteins in yeast requires a fully competent early secretory pathway. Mutations in proteins essential for ER-Golgi protein traffic severely inhibit ER degradation of the model substrate CPY*. We found ER localization of CPY* in WT cells, but no other specific organelle for ER degradation could be identified by electron microscopy studies. Because CPY* is degraded in COPI coat mutants, only a minor fraction of CPY* or of a proteinaceous factor required for degradation seems to enter the recycling pathway between ER and Golgi. Therefore, we propose that the disorganized structure of the ER and/or the mislocalization of Kar2p, observed in early secretory mutants, is responsible for the reduction in CPY* degradation. Further, we observed that mutations in proteins directly involved in degradation of malfolded proteins (Der1p, Der3/Hrd1p, and Hrd3p) lead to morphological changes of the endoplasmic reticulum and the Golgi, escape of CPY* into the secretory pathway and a slower maturation rate of wild-type CPY.

Nup2p, a yeast nucleoporin, functions in bidirectional transport of importin alpha

Import of proteins containing a classical nuclear localization signal (NLS) into the nucleus is mediated by importin alpha and importin beta. Srp1p, the Saccharomyces cerevisiae homologue of importin alpha, returns from the nucleus in a complex with its export factor Cse1p and with Gsp1p (yeast Ran) in its GTP-bound state. We studied the role of the nucleoporin Nup2p in the transport cycle of Srp1p. Cells lacking NUP2 show a specific defect in both NLS import and Srp1p export, indicating that Nup2p is required for efficient bidirectional transport of Srp1p across the nuclear pore complex (NPC). Nup2p is located at the nuclear side of the central gated channel of the NPC and provides a binding site for Srp1p via its amino-terminal domain. We show that Nup2p effectively releases the NLS protein from importin alpha-importin and beta and strongly binds to the importin heterodimer via Srp1p. Kap95p (importin beta) is released from this complex by a direct interaction with Gsp1p-GTP. These data suggest that besides Gsp1p, which disassembles the NLS-importin alpha-importin beta complex upon binding to Kap95p in the nucleus, Nup2p can also dissociate the import complex by binding to Srp1p. We also show data indicating that Nup1p, a relative of Nup2p, plays a similar role in termination of NLS import. Cse1p and Gsp1p-GTP release Srp1p from Nup2p, which suggests that the Srp1p export complex can be formed directly at the NPC. The changed distribution of Cse1p at the NPC in nup2 mutants also supports a role for Nup2p in Srp1p export from the nucleus.

Golgi structure correlates with transitional endoplasmic reticulum organization in Pichia pastoris and Saccharomyces cerevisiae

Golgi stacks are often located near sites of "transitional ER" (tER), where COPII transport vesicles are produced. This juxtaposition may indicate that Golgi cisternae form at tER sites. To explore this idea, we examined two budding yeasts: Pichia pastoris, which has coherent Golgi stacks, and Saccharomyces cerevisiae, which has a dispersed Golgi. tER structures in the two yeasts were visualized using fusions between green fluorescent protein and COPII coat proteins. We also determined the localization of Sec12p, an ER membrane protein that initiates the COPII vesicle assembly pathway. In P. pastoris, Golgi stacks are adjacent to discrete tER sites that contain COPII coat proteins as well as Sec12p. This arrangement of the tER-Golgi system is independent of microtubules. In S. cerevisiae, COPII vesicles appear to be present throughout the cytoplasm and Sec12p is distributed throughout the ER, indicating that COPII vesicles bud from the entire ER network. We propose that P. pastoris has discrete tER sites and therefore generates coherent Golgi stacks, whereas S. cerevisiae has a delocalized tER and therefore generates a dispersed Golgi. These findings open the way for a molecular genetic analysis of tER sites.

Protein kinase C bound to the Golgi apparatus supports the formation of constitutive transport vesicles

Constitutive secretion of heparan sulphate proteoglycans (HSPGs) was stimulated in human hepatoma HepG2 cells by phorbol 12-myristate 13-acetate (PMA) and inhibited by calphostin C, a specific inhibitor of protein kinase C (PKC). To delineate more closely the site of PKC action, the packaging in vitro of 35SO4-labelled HSPGs into transport vesicles was investigated. Formation of transport vesicles at the trans-Golgi network was stimulated by PMA and inhibited by calphostin C or Ro 31-8220 by using a post-nuclear supernatant. Treatment of either isolated Golgi-enriched membranes or cytosolic proteins with calphostin C provided evidence that membrane-bound PKC forms strongly supported vesicle formation, whereas cytosolic PKC forms showed a marginal effect. The PKC isoforms PKC-alpha and PKC-zeta were attached to highly purified Golgi membranes, as shown by Western blotting. Both isoforms were localized by confocal immunofluorescence microscopy in the Golgi area of HepG2 cells. Immunoelectron microscopy of ultrathin cryosections of HepG2 cells showed that PKC-zeta predominantly attaches to the trans-Golgi region, whereas PKC-alpha binds to the cis- and trans-Golgi area.