Evidence that the 16 kDa proteolipid (subunit c) of the vacuolar H(+)-ATPase and ductin from gap junctions are the same polypeptide in Drosophila and Manduca: molecular cloning of the Vha16k gene from Drosophila

Impairment of the Glial Phagolysosomal System Drives Prion-Like Propagation in a Drosophila Model of Huntington's Disease

Protein misfolding, aggregation, and spread through the brain are primary drivers of neurodegenerative disease pathogenesis. Phagocytic glia are responsible for regulating the load of pathological proteins in the brain, but emerging evidence suggests that glia may also act as vectors for aggregate spread. Accumulation of protein aggregates could compromise the ability of glia to eliminate toxic materials from the brain by disrupting efficient degradation in the phagolysosomal system. A better understanding of phagocytic glial cell deficiencies in the disease state could help to identify novel therapeutic targets for multiple neurological disorders. Here, we report that mutant huntingtin (mHTT) aggregates impair glial responsiveness to injury and capacity to degrade neuronal debris in male and female adult Drosophila expressing the gene that causes Huntington's disease (HD). mHTT aggregate formation in neurons impairs engulfment and clearance of injured axons and causes accumulation of phagolysosomes in glia. Neuronal mHTT expression induces upregulation of key innate immunity and phagocytic genes, some of which were found to regulate mHTT aggregate burden in the brain. A forward genetic screen revealed Rab10 as a novel component of Draper-dependent phagocytosis that regulates mHTT aggregate transmission from neurons to glia. These data suggest that glial phagocytic defects enable engulfed mHTT aggregates to evade lysosomal degradation and acquire prion-like characteristics. Together, our findings uncover new mechanisms that enhance our understanding of the beneficial and harmful effects of phagocytic glia in HD and other neurodegenerative diseases.

Impairment of the glial phagolysosomal system drives prion-like propagation in a Drosophila model of Huntington's disease

Protein misfolding, aggregation, and spread through the brain are primary drivers of neurodegenerative diseases pathogenesis. Phagocytic glia are responsible for regulating the load of pathogenic protein aggregates in the brain, but emerging evidence suggests that glia may also act as vectors for aggregate spread. Accumulation of protein aggregates could compromise the ability of glia to eliminate toxic materials from the brain by disrupting efficient degradation in the phagolysosomal system. A better understanding of phagocytic glial cell deficiencies in the disease state could help to identify novel therapeutic targets for multiple neurological disorders. Here, we report that mutant huntingtin (mHTT) aggregates impair glial responsiveness to injury and capacity to degrade neuronal debris in male and female adult Drosophila expressing the gene that causes Huntington's disease (HD). mHTT aggregate formation in neurons impairs engulfment and clearance of injured axons and causes accumulation of phagolysosomes in glia. Neuronal mHTT expression induces upregulation of key innate immunity and phagocytic genes, some of which were found to regulate mHTT aggregate burden in the brain. Finally, a forward genetic screen revealed Rab10 as a novel component of Draper-dependent phagocytosis that regulates mHTT aggregate transmission from neurons to glia. These data suggest that glial phagocytic defects enable engulfed mHTT aggregates to evade lysosomal degradation and acquire prion-like characteristics. Together, our findings reveal new mechanisms that enhance our understanding of the beneficial and potentially harmful effects of phagocytic glia in HD and potentially other neurodegenerative diseases.

Reversible binding of divalent cations to Ductin protein assemblies-A putative new regulatory mechanism of membrane traffic processes

Ductins are a family of homologous and structurally similar membrane proteins with 2 or 4 trans-membrane alpha-helices. The active forms of the Ductins are membranous ring- or star-shaped oligomeric assemblies and they provide various pore, channel, gap-junction functions, assist in membrane fusion processes and also serve as the rotor c-ring domain of V-and F-ATPases. All functions of the Ductins have been reported to be sensitive to the presence of certain divalent metal cations (Me2+), most frequently Cu2+ or Ca2+ ions, for most of the better known members of the family, and the mechanism of this effect is not yet known. Given that we have earlier found a prominent Me2+ binding site in a well-characterised Ductin protein, we hypothesise that certain divalent cations can structurally modulate the various functions of Ductin assemblies via affecting their stability by reversible non-covalent binding to them. A fine control of the stability of the assembly ranging from separated monomers through a loosely/weakly to tightly/strongly assembled ring might render precise regulation of Ductin functions possible. The putative role of direct binding of Me2+ to the c-ring subunit of active ATP hydrolase in autophagy and the mechanism of Ca2+-dependent formation of the mitochondrial permeability transition pore are also discussed.

Relating proton pumps with gap junctions: colocalization of ductin, the channel-forming subunit c of V-ATPase, with subunit a and with innexins 2 and 3 during Drosophila oogenesis

BACKGROUND

Ion-transport mechanisms and gap junctions are known to cooperate in creating bioelectric phenomena, like pH gradients, voltage gradients and ion fluxes within single cells, tissues, organs, and whole organisms. Such phenomena have been shown to play regulatory roles in a variety of developmental and regenerative processes. Using Drosophila oogenesis as a model system, we aim at characterizing in detail the mechanisms underlying bioelectric phenomena in order to reveal their regulatory functions. We, therefore, investigated the stage-specific distribution patterns of V-ATPase components in relation to gap-junction proteins.

RESULTS

We analysed the localization of the V-ATPase components ductin (subunit c) and subunit a, and the gap-junction components innexins 2 and 3, especially in polar cells, border cells, stalk cells and centripetally migrating cells. These types of follicle cells had previously been shown to exhibit characteristic patterns of membrane channels as well as membrane potential and intracellular pH. Stage-specifically, ductin and subunit a were found either colocalized or separately enriched in different regions of soma and germ-line cells. While ductin was often more prominent in plasma membranes, subunit a was more prominent in cytoplasmic and nuclear vesicles. Particularly, ductin was enriched in polar cells, stalk cells, and nurse-cell membranes, whereas subunit a was enriched in the cytoplasm of border cells, columnar follicle cells and germ-line cells. Comparably, ductin and both innexins 2 and 3 were either colocalized or separately enriched in different cellular regions. While ductin often showed a continuous membrane distribution, the distribution of both innexins was mostly punctate. Particularly, ductin was enriched in polar cells and stalk cells, whereas innexin 2 was enriched in the oolemma, and innexin 3 in centripetally migrating follicle cells. In lateral follicle-cell membranes, the three proteins were found colocalized as well as separately concentrated in presumed gap-junction plaques.

CONCLUSIONS

Our results support the notion of a large variety of gap junctions existing in the Drosophila ovary. Moreover, since ductin is the channel-forming part of a proton pump and, like the innexins, is able to form junctional as well as non-junctional membrane channels, a plethora of cellular functions could be realized by using these proteins. The distribution and activity patterns of such membrane channels are expected to contribute to developmentally important bioelectric signals.

Control of lysosomal biogenesis and Notch-dependent tissue patterning by components of the TFEB-V-ATPase axis in Drosophila melanogaster

In vertebrates, TFEB (transcription factor EB) and MITF (microphthalmia-associated transcription factor) family of basic Helix-Loop-Helix (bHLH) transcription factors regulates both lysosomal function and organ development. However, it is not clear whether these 2 processes are interconnected. Here, we show that Mitf, the single TFEB and MITF ortholog in Drosophila, controls expression of vacuolar-type H(+)-ATPase pump (V-ATPase) subunits. Remarkably, we also find that expression of Vha16-1 and Vha13, encoding 2 key components of V-ATPase, is patterned in the wing imaginal disc. In particular, Vha16-1 expression follows differentiation of proneural regions of the disc. These regions, which will form sensory organs in the adult, appear to possess a distinctive endolysosomal compartment and Notch (N) localization. Modulation of Mitf activity in the disc in vivo alters endolysosomal function and disrupts proneural patterning. Similar to our findings in Drosophila, in human breast epithelial cells we observe that impairment of the Vha16-1 human ortholog ATP6V0C changes the size and function of the endolysosomal compartment and that depletion of TFEB reduces ligand-independent N signaling activity. Our data suggest that lysosomal-associated functions regulated by the TFEB-V-ATPase axis might play a conserved role in shaping cell fate.

PA1b inhibitor binding to subunits c and e of the vacuolar ATPase reveals its insecticidal mechanism

The vacuolar ATPase (V-ATPase) is a 1MDa transmembrane proton pump that operates via a rotary mechanism fuelled by ATP. Essential for eukaryotic cell homeostasis, it plays central roles in bone remodeling and tumor invasiveness, making it a key therapeutic target. Its importance in arthropod physiology also makes it a promising pesticide target. The major challenge in designing lead compounds against the V-ATPase is its ubiquitous nature, such that any therapeutic must be capable of targeting particular isoforms. Here, we have characterized the binding site on the V-ATPase of pea albumin 1b (PA1b), a small cystine knot protein that shows exquisitely selective inhibition of insect V-ATPases. Electron microscopy shows that PA1b binding occurs across a range of equivalent sites on the c ring of the membrane domain. In the presence of Mg·ATP, PA1b localizes to a single site, distant from subunit a, which is predicted to be the interface for other inhibitors. Photoaffinity labeling studies show radiolabeling of subunits c and e. In addition, weevil resistance to PA1b is correlated with bafilomycin resistance, caused by mutation of subunit c. The data indicate a binding site to which both subunits c and e contribute and inhibition that involves locking the c ring rotor to a static subunit e and not subunit a. This has implications for understanding the V-ATPase mechanism and that of inhibitors with therapeutic or pesticidal potential. It also provides the first evidence for the position of subunit e within the complex.

Estimating the rotation rate in the vacuolar proton-ATPase in native yeast vacuolar membranes

The rate of rotation of the rotor in the yeast vacuolar proton-ATPase (V-ATPase), relative to the stator or steady parts of the enzyme, is estimated in native vacuolar membrane vesicles from Saccharomyces cerevisiae under standardised conditions. Membrane vesicles are formed spontaneously after exposing purified yeast vacuoles to osmotic shock. The fraction of total ATPase activity originating from the V-ATPase is determined by using the potent and specific inhibitor of the enzyme, concanamycin A. Inorganic phosphate liberated from ATP in the vacuolar membrane vesicle system, during ten min of ATPase activity at 20 °C, is assayed spectrophotometrically for different concanamycin A concentrations. A fit of the quadratic binding equation, assuming a single concanamycin A binding site on a monomeric V-ATPase (our data are incompatible with models assuming multiple binding sites), to the inhibitor titration curve determines the concentration of the enzyme. Combining this with the known ATP/rotation stoichiometry of the V-ATPase and the assayed concentration of inorganic phosphate liberated by the V-ATPase, leads to an average rate of ~10 Hz for full 360° rotation (and a range of 6-32 Hz, considering the ± standard deviation of the enzyme concentration), which, from the time-dependence of the activity, extrapolates to ~14 Hz (8-48 Hz) at the beginning of the reaction. These are lower-limit estimates. To our knowledge, this is the first report of the rotation rate in a V-ATPase that is not subjected to genetic or chemical modification and is not fixed to a solid support; instead it is functioning in its native membrane environment.

The binding site of the V-ATPase inhibitor apicularen is in the vicinity of those for bafilomycin and archazolid

The investigation of V-ATPases as potential therapeutic drug targets and hence of their specific inhibitors is a promising approach in osteoporosis and cancer treatment because the occurrence of these diseases is interrelated to the function of the V-ATPase. Apicularen belongs to the novel inhibitor family of the benzolactone enamides, which are highly potent but feature the unique characteristic of not inhibiting V-ATPases from fungal sources. In this study we specify, for the first time, the binding site of apicularen within the membrane spanning V(O) complex. By photoaffinity labeling using derivatives of apicularen and of the plecomacrolides bafilomycin and concanamycin, each coupled to (14)C-labeled 4-(3-trifluoromethyldiazirin-3-yl)benzoic acid, we verified that apicularen binds at the interface of the V(O) subunits a and c. The binding site is in the vicinity to those of the plecomacrolides and of the archazolids, a third family of V-ATPase inhibitors. Expression of subunit c homologues from Homo sapiens and Manduca sexta, both species sensitive to benzolactone enamides, in a Saccharomyces cerevisiae strain lacking the corresponding intrinsic gene did not transfer this sensitivity to yeast. Therefore, the binding site of benzolactone enamides cannot be formed exclusively by subunit c. Apparently, subunit a substantially contributes to the binding of the benzolactone enamides.

Neurotrophin-3 is involved in the formation of apical dendritic bundles in cortical layer 2 of the rat

Apical dendritic bundles from pyramidal neurons are a prominent feature of cortical neuropil but with significant area specializations. Here, we investigate mechanisms of bundle formation, focusing on layer (L) 2 bundles in rat granular retrosplenial cortex (GRS), a limbic area implicated in spatial memory. By using microarrays, we first searched for genes highly and specifically expressed in GRS L2 at postnatal day (P) 3 versus GRS L2 at P12 (respectively, before and after bundle formation), versus GRS L5 (at P3), and versus L2 in barrel field cortex (BF) (at P3). Several genes, including neurotrophin-3 (NT-3), were identified as transiently and specifically expressed in GRS L2. Three of these were cloned and confirmed by in situ hybridization. To test that NT-3-mediated events are causally involved in bundle formation, we used in utero electroporation to overexpress NT-3 in other cortical areas. This produced prominent bundles of dendrites originating from L2 neurons in BF, where L2 bundles are normally absent. Intracellular biocytin fills, after physiological recording in vitro, revealed increased dendritic branching in L1 of BF. The controlled ectopic induction of dendritic bundles identifies a new role for NT-3 and a new in vivo model for investigating dendritic bundles and their formation.

The 16 kDa subunit of vacuolar H+-ATPase is a novel sarcoglycan-interacting protein

The sarcoglycan complex in muscle consists of alpha-, beta-, gamma- and delta-sarcoglycan and is part of the larger dystrophin-glycoprotein complex (DGC), which is essential for maintaining muscle membrane integrity. Mutations in any of the four sarcoglycans cause limb-girdle muscular dystrophies (LGMD). In this report, we have identified a novel interaction between delta-sarcoglycan and the 16 kDa subunit c (16K) of vacuolar H(+)-ATPase. Co-expression studies in heterologous cell system revealed that 16K interacts specifically with delta-sarcoglycan and the highly related gamma-sarcoglycan through the transmembrane domains. In cultured C2C12 myotubes, 16K forms a complex with sarcoglycans at the plasma membrane. Loss of sarcoglycans in the sarcoglycan-deficient BIO14.6 hamster destabilizes the DGC and alters the localization of 16K at the sarcolemma. In addition, the steady state level of beta(1)-integrin is increased. Recent studies have shown that 16K also interacts directly with beta(1)-integrin and our data demonstrated that sarcoglycans, 16K and beta(1)-integrin were immunoprecipitated together in C2C12 myotubes. Since sarcoglycans have been proposed to participate in bi-directional signaling with integrins, our findings suggest that 16K might mediate the communication between sarcoglycans and integrins and play an important role in the pathogenesis of muscular dystrophy.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

ATP6V0C competes with von Hippel-Lindau protein in hypoxia-inducible factor 1alpha (HIF-1alpha) binding and mediates HIF-1alpha expression by bafilomycin A1

HIF-1alpha not only enables cells to survive under hypoxic conditions but also promotes cell cycle arrest and apoptosis. Therefore, its expression should be controlled at optimal levels in growing tumors. We recently reported that bafilomycin A1 exorbitantly expressed HIF-1alpha and induced the p21(WAF1/Cip1)-mediated growth arrest of tumors (Mol Pharmacol 70:1856-1865, 2006). In the present study, we addressed the mechanism underlying bafilomycin-induced HIF-1alpha expression. Bafilomycin stabilized HIF-1alpha under normoxic conditions without changes in intracellular pH. However, when ATP6V0C, the target protein of bafilomycin, was knocked down, this bafilomycin effect was significantly attenuated. Inversely, ATP6V0C expression increased HIF-1alpha levels in a gene dose-dependent manner. ATP6V0C competed with Von Hippel-Lindau protein in HIF-1alpha binding by directly interacting with HIF-1alpha, which was stimulated by bafilomycin. In confocal images, ATP6V0C was normally present in the cytoplasm but was translocated in company with HIF-1alpha to the nucleus by bafilomycin. The N-terminal end (amino acids 1-16) of HIF-1alpha was identified as the ATP6V0C-interacting motif. These results suggest that ATP6V0C, a novel regulator of HIF-1alpha, mediates HIF-1alpha expression by bafilomycin.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

Suppression of tumor-related glycosylation of cell surface receptors by the 16-kDa membrane subunit of vacuolar H+-ATPase

The glycosylation of integrins and other cell surface receptors is altered in many transformed cells. Notably, an increase in the number of beta1,6-branched N-linked oligosaccharides correlates strongly with invasive growth of cells. An ectopic expression of the Golgi enzyme N-acetylglucosaminyltransferase V (GlcNAc-TV), which forms beta1,6 linkages, promotes metastasis of a number of cell types. It is shown here that the 16-kDa transmembrane subunit (16K) of vacuolar H(+)-ATPase suppresses beta1,6 branching of beta(1) integrin and the epidermal growth factor receptor. Overexpression of 16K inhibits cell adhesion and invasion. 16K contains four hydrophobic membrane-spanning alpha-helices, and its ability to influence glycosylation is localized primarily within the second and fourth membrane-spanning alpha-helices. 16K also interacts directly with the transmembrane domain of beta(1) integrin, but its effects on glycosylation were independent of its binding to beta(1) integrin. These data link cell surface tumor-related glycosylation to a component of the enzyme responsible for acidification of the exocytic pathway.

Requirement of V-ATPase for ovulation and embryogenesis in Caenorhabditis elegans

Immunofluorescence analysis indicated that VHA-11, the C subunit of Caenorhabditis elegans V-ATPase, was localized in dot-like structures around the nuclei of early embryonic cells and was also detected in embryonic intestinal cells after comma stage. Vital staining with acridine orange showed that the intestinal cells had acidic compartments generated by V-ATPase, consistent with the intracellular localization of VHA-11. RNA interference could efficiently silence vha-11 gene expression: introduction of vha-11 double strand RNA led to embryonic lethality. Worms injected with the vha-11 double strand RNA produced embryos that became lethal. The development of embryos was arrested at various stages. However, their numbers gradually decreased, and the worms eventually became sterile due to the failure of ovulation. Similar results were obtained for RNA interference of the V-ATPase proteolipid genes. These results suggest that V-ATPases, and thus inside-acidic organelles, are required for ovulation and embryogenesis.

dtrap-1 encodes a novel member of the heat shock super family of proteins and is expressed in derivatives of all three germ layers during Drosophila embryogenesis

Heat shock proteins (Hsps) comprise a highly conserved superfamily of proteins that are required for stress tolerance in living cells. At physiological conditions, these proteins act as chaperones during protein folding and protein assembly processes (Parsell and Lindquist, 1993. Annu. Rev. Genet. 27, 427-496). Members of the subfamily of Hsp90 proteins were shown to be additionally involved in the presentation and structural modification of components of diverse cellular signal transduction pathways including steroid hormone reception and regulatory kinase activities (Pratt, 1993. J. Biol. Chem. 268, 21455-21458); Cutforth et al., 1994. Cell 77, 1027-1036; van der Straten et al., 1997. EMBO J. 16, 1961-1997; Hunter and Poon, 1997. Trends Cell Biol. 7, 157-161). We have identified a Drosophila gene, called dtrap-1, which encodes a Hsp of a novel subfamily that is related to the Hsp90 family of proteins. During oogenesis dtrap-1 is expressed in nurse cells and its transcripts accumulate in the oocyte. The maternal transcripts remain in the egg but rapidly degrade during early embryogenesis, except in the posterior pole region. Zygotic expression is initiated after the onset of gastrulation showing dynamic patterns of transcripts in the developing mid- and hindgut as well as a subset of mesoderm derivatives.

Molecular characterization of the yeast vacuolar H+-ATPase proton pore

The Saccharomyces cerevisiae vacuolar ATPase (V-ATPase) is composed of at least 13 polypeptides organized into two distinct domains, V(1) and V(0), that are structurally and mechanistically similar to the F(1)-F(0) domains of the F-type ATP synthases. The peripheral V(1) domain is responsible for ATP hydrolysis and is coupled to the mechanism of proton translocation. The integral V(0) domain is responsible for the translocation of protons across the membrane and is composed of five different polypeptides. Unlike the F(0) domain of the F-type ATP synthase, which contains 12 copies of a single 8-kDa proteolipid, the V-ATPase V(0) domain contains three proteolipid species, Vma3p, Vma11p, and Vma16p, with each proteolipid contributing to the mechanism of proton translocation (Hirata, R., Graham, L. A., Takatsuki, A., Stevens, T. H., and Anraku, Y. (1997) J. Biol. Chem. 272, 4795-4803). Experiments with hemagglutinin- and c-Myc epitope-tagged copies of the proteolipids revealed that each V(0) complex contains all three species of proteolipid with only one copy each of Vma11p and Vma16p but multiple copies of Vma3p. Since the proteolipids of the V(0) complex are predicted to possess four membrane-spanning alpha-helices, twice as many as a single F-ATPase proteolipid subunit, only six V-ATPase proteolipids would be required to form a hexameric ring-like structure similar to the F(0) domain. Therefore, each V(0) complex will likely be composed of four copies of the Vma3p proteolipid in addition to Vma11p and Vma16p. Structural differences within the membrane-spanning domains of both V(0) and F(0) may account for the unique properties of the ATP-hydrolyzing V-ATPase compared with the ATP-generating F-type ATP synthase.

Role of mediatophore in connection with proteins of the active zone in synaptic transmission

Mediatophore is a protein purified from Torpedo electric organ synaptosomes, which translocates acetylcholine (ACh) upon calcium action after reconstitution in artificial membranes. After expression in transfected cells, it endows these cells with a calcium-dependent release mechanism displaying clear quantal properties. The role of mediatophore in synaptic transmission is discussed in relation to the ultrastructural organization of the active zone and the cytosolic high calcium microdomains that transiently appear after presynaptic membrane depolarization.

Stage-dependent redistribution of the V-ATPase during bovine implantation

The 16-kD subunit of the vacuolar H(+)-ATPase (V-ATPase), or ductin, is essential for the activity of this proton pump and has roles in intercellular communication and control of cell growth and differentiation. The V-ATPase is important for acidification-dependent degradation of tissue matrices through which some cell types move, and for pH regulation across some epithelial cell layers. Placentation involves intricate signaling, cell proliferation, and controlled invasion. We examined the distribution of three subunits of the V-ATPase in bovine trophoblast and endometrium at the time of implantation to determine the relationship of ductin expression to that of two other subunits, A (approximately 73 kD) and B (approximately 58 kD). Epithelial expression of all three subunits was observed, and in nonpregnant animals this expression was apical. As pregnancy proceeded, expression of all subunits became pericellular in luminal but not glandular epithelium, suggesting a redistribution of V-ATPase activity. The trophoblast expressed all three subunits during initial contact with the epithelium. In the stroma, ductin expression was reduced after implantation, and we discuss the possibility that ductin plays a role in the shifting communication between stromal and epithelial cells induced by embryo attachment. (J Histochem Cytochem 47:1247-1254, 1999)

Membrane topology of Alzheimer's disease-related presenilin 1. Evidence for the existence of a molecular species with a seven membrane-spanning and one membrane-embedded structure

A significant member of early-onset familial type of Alzheimer's disease cases has been shown to be caused by dominant mutations in either of the two genes encoding presenilin 1 (PS1) and presenilin 2 (PS2). These two proteins are highly homologous to each other and have been reported to be mainly localized to the membranes of intracellular compartments such as the endoplasmic reticulum. Information about the membrane topological structures of these proteins is indispensable for understanding their physiological and pathological roles. Although several models have been proposed previously, their precise membrane topologies remain unknown. In this study, we examined this issue in detail by expressing a series of C-terminally deleted PS1 mutants fused to the hydrophilic portion of Escherichia coli leader peptidase in vitro using a reticulocyte lysate in the presence of microsomal membranes. Our results predict that PS1 exists mainly in a seven membrane-spanning structure with its C-terminal end exposed to the luminal space. This was also confirmed by expressing these fusion proteins in cultured cells. We further showed that a ninth hydrophobic segment is tightly bound to the membrane without spanning it. Based on the above observations, we propose a novel "seven membrane-spanning and one membrane-embedded" topological model for presenilins.

beta(1) integrin binds the 16-kDa subunit of vacuolar H(+)-ATPase at a site important for human papillomavirus E5 and platelet-derived growth factor signaling

Integrins mediate adhesive interactions between cells and the extracellular matrix, and play a role in cell migration, proliferation, differentiation, cytoskeletal organization, and signal transduction. We have identified an interaction between the beta(1) integrin and the 16-kDa subunit of vacuolar H(+)-ATPase (16K). This interaction was first isolated in a yeast two-hybrid screen and confirmed by coimmunoprecipitation and in in vitro binding assays using bacterially expressed proteins. Immunofluorescent studies performed in L6 myoblasts expressing both native and epitope-tagged 16K demonstrate co-localization with beta(1) integrin in focal adhesions. Deletion of the fourth of four transmembrane helices in 16K results in loss of interaction with beta(1) integrin in vitro and in the two-hybrid system, and less prominent staining in focal adhesions. This helix is also required for ligand-independent activation of platelet-derived growth factor-beta receptor signaling by the human papillomavirus E5 oncoprotein. Overexpression of 16K or expression of 16K lacking this helix alters the morphology of myoblasts and fibroblasts, suggesting that the interaction of 16K with integrins could be important for cell growth control. We also discuss the possible role 16K might play in integrin movement.

Coupled translocation events generate topological heterogeneity at the endoplasmic reticulum membrane

Topogenic determinants that direct protein topology at the endoplasmic reticulum membrane usually function with high fidelity to establish a uniform topological orientation for any given polypeptide. Here we show, however, that through the coupling of sequential translocation events, native topogenic determinants are capable of generating two alternate transmembrane structures at the endoplasmic reticulum membrane. Using defined chimeric and epitope-tagged full-length proteins, we found that topogenic activities of two C-trans (type II) signal anchor sequences, encoded within the seventh and eighth transmembrane (TM) segments of human P-glycoprotein were directly coupled by an inefficient stop transfer (ST) sequence (TM7b) contained within the C-terminus half of TM7. Remarkably, these activities enabled TM7 to achieve both a single- and a double-spanning TM topology with nearly equal efficiency. In addition, ST and C-trans signal anchor activities encoded by TM8 were tightly linked to the weak ST activity, and hence topological fate, of TM7b. This interaction enabled TM8 to span the membrane in either a type I or a type II orientation. Pleiotropic structural features contributing to this unusual topogenic behavior included 1) a short, flexible peptide loop connecting TM7a and TM7b, 2) hydrophobic residues within TM7b, and 3) hydrophilic residues between TM7b and TM8.

Site-selected mutagenesis of the Drosophila second chromosome via plasmid rescue of lethal P-element insertions

This paper describes a fast and efficient approach to correlating cloned genes with mutant phenotypes in Drosophila. We make use of a large collection D. melanogaster lines with recessive lethal insertions of a P[lacW] transposon on their second chromosome. Within this collection there clearly must be many insertions corresponding to Drosophila genes that have been cloned and characterized, e.g., via homology with cloned mammalian genes, but for which mutant phenotypes have yet to be identified. We have made use of the fact that P[lacW] contains a plasmid replicon to establish a collection of rescued plasmids containing genomic DNA flanking the sites of transposon insertion. Plasmids representing a total of 1836 lines were independently rescued and pooled in batches of 10 and 100. Pools of 100 plasmids were screened by hybridization with cDNAs corresponding to cloned second chromosome loci. Hybridizing pools were then narrowed down to single plasmids by a process of subdivision and rehybridization, and corresponding mutant lines were obtained. The success rate was better than one in four. This rate would undoubtedly be improved by the use of genomic DNA probes.

[3H]Bafilomycin as a probe for the transmembrane proton channel of the osteoclast vacuolar H(+)-ATPase

Bone resorption by the osteoclast is dependent on acidification of the bone surface by a vacuolar type H+-ATPase (V-ATPase) present in the ruffled membrane of the actively resorbing cell. V-ATPases are a highly conserved family of proton pumps consisting of two functional complexes: a cytoplasmic catalytic sector (VC) and a membrane bound proton channel (VB). Bafilomycin A1, a macrolide antibiotic, is a highly potent inhibitor of V-ATPases, and inhibits bone resorption in vitro in isolated rat calvariae. In order to investigate the binding of bafilomycin to the osteoclast V-ATPase, we used a tritiated bafilomycin which had been prepared by acetylating the 21-hydroxyl group of bafilomycin A1. Osteoclast ruffled membrane vesicles were prepared from purified chicken osteoclasts by differential centrifugation and proton transport in these vesicles was shown to be inhibited by [3H]bafilomycin (IC50 approximately 2 nM). Control membrane vesicles or membrane vesicles partially inhibited with [3H]bafilomycin were solubilized and separated by centrifugation on 15-30% glycerol gradients. V-ATPase activity and reconstitutable proton transport activity could be recovered in high density fractions of the gradient. However, the peak of [3H]bafilomycin radioactivity (>70% of total radioactivity in the gradient) was present in a single peak at lower density. Antibodies against subunits of VC (70, 56 and 40 kDa) reacted only in fractions containing the peak V-ATPase activity whereas an antibody to the 39 kDa subunit of VB reacted both with fractions containing the peak V-ATPase activity but also, and more strongly, in fractions containing the peak [3H]bafilomycin. The fractions in the control gradient corresponding to the peak of [3H]bafilomycin were reconstituted into liposomes and shown to mediate passive bafilomycin A1-inhibitable proton conductance. SDS-PAGE followed by autoradiography, indicated that the bafilomycin was not covalently bound to the V-ATPase or the proton channel. Quantification of VB by [3H]bafilomycin binding or by antibody staining suggested an excess of the free proton channel to that of the intact holoenzyme. A corresponding amount of free catalytic sector could not be found in any fraction throughout the isolation procedure of the V-ATPase from the initial homogenate. Thus, in conclusion, bafilomycin inhibits the V-ATPase by binding tightly but non-covalently to the proton channel region of the V-ATPase which appears to be present in excess over the intact holoenzyme in the osteoclast. The possible role of an excess of the proton channel subcomplex in the osteoclast is discussed.

Ductin--a proton pump component, a gap junction channel and a neurotransmitter release channel

Ductin is the highest conserved membrane protein yet found in eukaryotes. It is multifunctional, being the subunit c or proteolipid component of the vacuolar H(+)-ATPase and at the same time the protein component of a form of gap junction in metazoan animals. Analysis of its structure shows it to be a tandem repeat of two 8-kDa domains derived from the subunit c of the F0 proton pore from the F1F0 ATPase. Each domain contains two transmembrane alpha-helices, which together may form a four-helix bundle. In both the V-ATPase and gap junction channel, ductin is probably arranged as a hexamer of subunits forming a central channel of gap junction-like proportions. The two functions appear to be seggregated by ductin having two orientations in the bilayer. Ductin is also the major component of the mediatophore, a protein complex which may aid in the release of neurotransmitters across the pre-synaptic membrane. It is also a target for a class of poorly understood viral polypeptides. These polypeptides are small and highly hydrophobic and some have oncogenic activity. Ductin thus appears to be at the crossroads of a number of biological processes.