Molecular structure of the gap junctional channel

The proteins in various gap junctional preparations from rodent liver have been analysed by two-dimensional peptide mapping and immunoblotting. Only the protein of relative molecular mass (Mr) 16,000 (16K) is found in all gap junctional isolates, and it is unrelated to the 27K protein. The absence of the 27K protein and any of its fragments from trypsin-treated preparations suggests that this protein does not directly contribute to gap junctional structure. Peptide mapping and immunoblotting of the 16K proteins isolated from various tissues and species and of the arthropod 18K protein present in gap junctional preparations from Nephrops norvegicus show that these proteins constitute a family of related junctional proteins. A site-specific antiserum raised against the N-terminal octapeptide of the 16K protein from mouse liver cross-reacts with all 16K and 18K forms of the junctional protein so far tested, suggesting that this particular antigenic determinant is highly conserved. Immuno-localization studies show that the N-terminus is most likely located on the cytoplasmic aspect of the junction and is available to Pronase digestion.

Interactions between normal mammary epithelial cells and mammary tumour cells in a model system

Normal mammary epithelial (NME) cells and MCF-7 cells aggregate and grow as spheroids when cultured on extracellular matrix derived from Engelbreth/ Holmes/Swarth (EHS) tumour. NME cells stop dividing and differentiate but MCF-7 cells continue to proliferate, although growth is counterbalanced by cell death. In mixed cultures of NME cells and MCF-7 cells, the two cell types form mixed aggregates but then segregate to form well separated domains, often joined by only a narrow neck of cells. In these mixed cultures the growth of MCF-7 cells is inhibited by a factor secreted by NME cells into the medium.

Polyamines regulate gap junction communication in connexin 43-expressing cells

The control of cell-cell communication through gap junctions is thought to be crucial in normal tissue function and during various stages of tumorigenesis. However, few natural regulators of gap junctions have been found. We show here that increasing the activity of ornithine decarboxylase, or adding polyamines to the outside of cells, increases the level of gap junction communication between various epithelial cells. Conversely, reduction of ornithine decarboxylase activity decreases the level of gap junction communication. This regulation is dependent upon the expression of connexin 43 (Cx43 or Cxalpha1), which is a major connexin expressed in many different cell types, and involves an increase in Cx43 and its cellular re-distribution.

Binding of human papillomavirus 16 E5 to the 16 kDa subunit c (proteolipid) of the vacuolar H+-ATPase can be dissociated from the E5-mediated epidermal growth factor receptor overactivation

Human papillomavirus type 16 E5 protein (HPV16 E5) upregulates ligand-mediated activation of the epidermal growth factor receptor (EGFR) in transfected human keratinocytes. HPV16 E5 binds to the 16 kDa proteolipid (subunit c) of the vacuolar H+-ATPase (16K), responsible for endosomal acidification, and this binding has been suggested to be responsible for increased recycling of the EGFRs. Using mutant deletions we show here that amino acids 54-78, but not 79-83 are necessary for binding to the 16K proteolipid. EGF treatment of cells expressing wild type or mutants of the E5 protein show that deletion of the last carboxy terminal 5 amino acids results in loss of E5-mediated EGFR overactivation. Thus, our results show that the binding capacity of HPV16 E5 to 16K can be dissociated from the effect of the viral protein on EGFR activation.

Identification of lipid-accessible sites on the nephrops 16-kDa proteolipid incorporated into a hybrid vacuolar H(+)-ATPase: site-directed labeling with N-(1-Pyrenyl)cyclohexylcarbodiimide and fluorescence quenching analysis

Proton translocation by the vacuolar H(+)-ATPase is mediated by a multicopy transmembrane protein, the 16-kDa proteolipid. It is proposed to assemble in the membrane as a hexameric complex, with each polypeptide comprising four transmembrane helices. The fourth helix of the proteolipid contains an intramembrane acidic residue (Glu140) which is essential for proton translocation and is reactive toward N,N'-dicyclohexylcarbodiimide (DCCD). Current theoretical models of proton translocation by the vacuolar ATPase require that Glu140 should be protonated and in contact with the membrane lipid. In this study we present direct support for this hypothesis. Modification with the fluorescent DCCD analogue N-(1-pyrenyl)cyclohexylcarbodiimide, coupled to fluorescence quenching studies and bilayer depth measurements using the parallax method, was used to probe the position of Glu140 with respect to the bilayer. Glutamate residues were also introduced mutagenically as targets for the fluorescent probe in order to map additional lipid-accessible sites on the 16-kDa proteolipid. These data are consistent with a structural model of the 16-kDa proteolipid oligomer in which the key functional residue Glu140 and discrete faces of the second and third transmembrane helices of the 16-kDa proteolipid are exposed at the lipid-protein interface.

Binding of bovine papillomavirus type 4 E8 to ductin (16K proteolipid), down-regulation of gap junction intercellular communication and full cell transformation are independent events

The E8 open reading frame of bovine papillomavirus type 4 encodes a small hydrophobic polypeptide that contributes to primary cell transformation by conferring to cells the ability to form foci and to grow in low serum and in suspension. Wild-type E8 binds in vitro to ductin, a component of gap junctions, and this binding is accompanied by a loss of gap junction intercellular communication in transformed bovine fibroblasts. However, through the analysis of a panel of E8 mutants, we show here that binding of E8 to ductin is not sufficient for down-regulation of gap junction communication and that there is no absolute correlation between down-regulation of gap junction communication and the transformed phenotype.

Membrane assembly of the 16-kDa proteolipid channel from Nephrops norvegicus studied by relaxation enhancements in spin-label ESR

The 16-kDa proteolipid from the hepatopancreas of Nephrops norvegicus belongs to the class of channel proteins that includes the proton-translocation subunit of the vacuolar ATPases. The membranous 16-kDa protein from Nephrops was covalently spin-labeled on the unique cysteine Cys54, with a nitroxyl maleimide, or on the functionally essential glutamate Glu140, with a nitroxyl analogue of dicyclohexylcarbodiimide (DCCD). The intensities of the saturation transfer ESR spectra are a sensitive indicator of spin-spin interactions that were used to probe the intramembranous structure and assembly of the spin-labeled 16-kDa protein. Spin-lattice relaxation enhancements by aqueous Ni(2+) ions revealed that the spin label on Glu140 is located deeper within the membrane (around C9-C10 of the lipid chains) than is that on Cys54 (located around C5-C6). In double labeling experiments, alleviation of saturation by spin-spin interactions with spin-labeled lipids indicates that spin labels both on Cys54 and on Glu140 are at least partially exposed to the lipid chains. The decrease in saturation transfer ESR intensity observed with increasing spin-labeling level is evidence of oligomeric assembly of the 16-kDa monomers and is consistent with a protein hexamer. These results determine the locations and orientations of transmembrane segments 2 and 4 of the 16-kDa putative 4-helix bundle and put constraints on molecular models for the hexameric assembly in the membrane. In particular, the crucial DCCD-binding site that is essential for proton translocation appears to contact lipid.

Helical interactions and membrane disposition of the 16-kDa proteolipid subunit of the vacuolar H(+)-ATPase analyzed by cysteine replacement mutagenesis

Theoretical mechanisms of proton translocation by the vacuolar H(+)-ATPase require that a transmembrane acidic residue of the multicopy 16-kDa proteolipid subunit be exposed at the exterior surface of the membrane sector of the enzyme, contacting the lipid phase. However, structural support for this theoretical mechanism is lacking. To address this, we have used cysteine mutagenesis to produce a molecular model of the 16-kDa proteolipid complex. Transmembrane helical contacts were determined using oxidative cysteine cross-linking, and accessibility of cysteines to the lipid phase was determined by their reactivity to the lipid-soluble probe N-(1-pyrenyl)maleimide. A single model for organization of the four helices of each monomeric proteolipid was the best fit to the experimental data, with helix 1 lining a central pore and helix 2 and helix 3 immediately external to it and forming the principal intermolecular contacts. Helix 4, containing the crucial acidic residue, is peripheral to the complex. The model is consistent not only with theoretical proton transport mechanisms, but has structural similarity to the dodecameric ring complex formed by the related 8-kDa proteolipid of the F(1)F(0)-ATPase. This suggests some commonality between the proton translocating mechanisms of the vacuolar and F(1)F(0)-ATPases.

Structure of the ductin channel

Gap junctions appear to be essential components of metazoan animals providing a means of direct means of communication between neighboring cells. They are sieve-like structures which allow cell-cell movement of cytosolic solutes below 1000 MW. The major role of gap junctions would appear to be homeostatic giving rise to groups of cells which act as functional units. Ductin is the major core component of gap junctions and recent structural data shows it to be a four alpha-helical bundle which fits particularly well into a low resolution model of the gap junction channel. Ductin is also the main membrane component of the vacuolar H+-ATPase that is found in all eukaryotes and it seems likely that the gap junction channel first evolved as a housing for the rotating spindle of these proton pumps. Because ductin protrudes little from the membrane, other proteins are required to bring cell surfaces close enough together to form gap junctions. Such proteins may include connexins, a large family of proteins found in vertebrates.

Atomic force microscopy of arthropod gap junctions

Atomic force microscopy has been used to characterize gap junctions isolated from the hepatopancreas of Nephrops norvegicus. The major polypeptide of these gap junctions is ductin, a highly conserved 16- to 18-kDa protein. The hydrated gap junctions, imaged in phosphate-buffered saline, appeared as membrane plaques with a thickness of 14 nm, consistent with their being a pair of apposing membranes. The upper membrane was removed by force dissection using an increased imaging force. The thickness of the lower membrane was 6 nm, giving a separation or gap between the two membranes of 2 nm. High-resolution images show fine details of the force-dissected extracellular surfaces, as previously reported for vertebrate and heart gap junctions. In addition high-resolution AFM images show for the first time detailed substructure on the cytoplasmic face of hydrated gap junctions of either vertebrate or invertebrate. The plaques had particles on their exposed and force-dissected faces. These particles were packed in a hexagonal lattice (a = b = 8.9 nm on both faces) and had a diameter of approximately 6.5 nm, with a central, pore-like depression. Fourier maps calculated from the AFM data suggested that each particle was composed of six subunits. These images show a marked similarity to the widely accepted structure of the connexon channel of vertebrate gap junctions.

The vacuolar H+-ATPase: a universal proton pump of eukaryotes

The vacuolar H+-ATPase (V-ATPase) is a universal component of eukaryotic organisms. It is present in the membranes of many organelles, where its proton-pumping action creates the low intra-vacuolar pH found, for example, in lysosomes. In addition, there are a number of differentiated cell types that have V-ATPases on their surface that contribute to the physiological functions of these cells. The V-ATPase is a multi-subunit enzyme composed of a membrane sector and a cytosolic catalytic sector. It is related to the familiar FoF1 ATP synthase (F-ATPase), having the same basic architectural construction, and many of the subunits from the two display identity with one another. All the core subunits of the V-ATPase have now been identified and much is known about the assembly, regulation and pharmacology of the enzyme. Recent genetic analysis has shown the V-ATPase to be a vital component of higher eukaryotes. At least one of the subunits, i.e. subunit c (ductin), may have multifunctional roles in membrane transport, providing a possible pathway of communication between cells. The structure of the membrane sector is known in some detail, and it is possible to begin to suggest how proton pumping is coupled to ATP hydrolysis.

Molecular genetic analysis of V-ATPase function in Drosophila melanogaster

V-ATPases are phylogenetically widespread, highly conserved, multisubunit proton pumps. Originally characterised in endomembranes, they have been found to energise transport across plasma membranes in a range of animal cells and particularly in certain epithelia. While yeast is the model of choice for the rapid generation and identification of V-ATPase mutants, it does not allow their analysis in a plasma membrane context. For such purposes, Drosophila melanogaster is a uniquely suitable model. Accordingly, we have cloned and characterised genes encoding several V-ATPase subunits in D. melanogaster and, using P-element technology, we have succeeded in generating multiple new alleles. Reporter gene constructs reveal ubiquitous expression, but at particularly high levels in those epithelial thought to be energised by V-ATPases, and several of the alleles have lethal recessive phenotypes characterised by epithelial dysfunction. These results, while providing the first gene knockouts of V-ATPases in animals, also illustrate the general utility of D. melanogaster as a model for the genetic analysis of ion transport and its control in epithelia.

The bovine papillomavirus type 4 E8 protein binds to ductin and causes loss of gap junctional intercellular communication in primary fibroblasts

The E8 open reading frame of bovine papillomavirus type 4 encodes a small hydrophobic polypeptide which contributes to cell transformation by conferring anchorage-independent growth. Using an in vitro translation system, we show that the E8 polypeptide binds to ductin, the 16-kDa proteolipid that forms transmembrane channels in both gap junctions and vacuolar H+-ATPase. This association is not due to nonspecific hydrophobic interactions. PPA1, a Saccharomyces cerevisiae polypeptide homologous (with 25% identity) to ductin, does not complex with E8. Furthermore, E5B, structurally similar to E8 but with no transforming activity, does not form a complex with ductin. Primary bovine fibroblasts expressing E8 show a loss of gap junctional intercellular communication, and it is suggested that this results from the interaction between E8 and ductin.

Interaction of dibutyltin-3-hydroxyflavone bromide with the 16 kDa proteolipid indicates the disposition of proton translocation sites of the vacuolar ATPase

The organotin complex dibutyltin-3-hydroxyflavone bromide [Bu2Sn(of)Br] has been shown to bind to the 16 kDa proteolipid of Nephrops norvegicus, either in the form of the native protein or after heterologous expression in Saccharomyces and assembly into a hybrid vacuolar H(+)-ATPase. Titration of Bu2Sn(of)Br against the 16 kDa proteolipid results in a marked fluorescence enhancement, consistent with binding to a single affinity site on the protein. Vacuolar ATPase-dependent ATP hydrolysis was also inhibited by Bu2Sn(of)Br, with the inhibition constant correlating well with dissociation constants determined for binding of Bu2Sn(of)Br complex to the proteolipid. The fluorescence enhancement produced by interaction of probe with proteolipid can be back-titrated by dicyclohexylcarbodiimide (DCCD), which covalently modifies Glu140 on helix-4 of the polypeptide. Expression of a mutant proteolipid in which Glu140 was changed to a glycine resulted in assembly of a vacuolar ATPase which was inactive in proton pumping and which had reduced ATPase activity. Co-expression studies with this mutant and wild-type proteolipids suggest that proton pumping can only occur in a vacuolar ATPase containing exclusively wild-type proteolipid. The fluorescent enhancement of affinity of Bu2Sn(of)Br for the mutant proteolipid was not significantly altered, with the organotin complex having no effect on residual ATPase activity. Interaction of the probe with mutant proteolipid was unaffected by DCCD. These data suggest an overlap in the binding sites of organotin and DCCD, and have implications for the organization and structure of proton-translocating pathways in the facuolar H(+)-ATPase.

The first putative transmembrane helix of the 16 kDa proteolipid lines a pore in the Vo sector of the vacuolar H(+)-ATPase

The 16 kDa proteolipid is the major component of the vacuolar H(+)-ATPase membrane sector, responsible for proton translocation. Expression of a related proteolipid from the arythropod Nephrops norvegicus in a Saccharomyces strain in which the VMA3 gene for the endogenous proteolipid has been disrupted results in restored vacuolar H(+)-ATPase function. We have used this complementation system, coupled to cysteine substitution mutagenesis and protein chemistry, to investigate structural features of the proteolipid. Consecutive cysteines were introduced individually into putative transmembrane segment 1 of the proteolipid, and at selected sites in extramembranous regions and in segment 3 and 4. Analysis of restored vacuolar H(+)-ATPase function showed that segment 1 residues sensitive to mutation to cysteine were clustered on a single face, but only if the segment was helical. Only residues insensitive to mutation could be covalently modified by the cysteine-specific reagent fluorescein 5-maleimide. A cysteine introduced into segment 3 was the only residue accessible to a relatively hydrophobic reagent, suggesting accessibility to the lipid phase. Analysis of disulphide bond formation between introduced cysteines indicates that the first transmembrane alpha-helices of each monomer are adjacent to each other at the centre of the proteolipid multimeric complex. The data are consistent with a model in which the fluorescein maleimide-accessible face of helix I lines a pore at the centre of a hexameric complex formed by the proteolipid, with the mutationally sensitive face oriented into the protein core. The implications for ion-transport function in this family of proteins are discussed in the context of this structural model.

Membrane insertion and assembly of ductin: a polytopic channel with dual orientations

Ductin is a highly conserved and polytopic transmembrane protein which is the subunit c component of the vacuolar H(+)-ATPase (V-ATPase) and a component of a connexon channel of gap junctions. Previous studies have suggested that ductin in the V-ATPase has the opposite orientation of ductin in a connexon. Using an in vitro translation system coupled to microsomes derived from the endoplasmic reticulum, we show that ductin is co-translationally inserted into the membrane bilayer, suggesting a dependency on the signal recognition particle for synthesis. By attaching a C-terminal polypeptide derived from beta-lactamase and by using cysteine replacement coupled to chemical labelling, we show that ductin is inserted into the microsomal membrane in both orientations in similar proportions. In contrast, squid rhodopsin appears to be inserted in a single orientation. Changing conserved charged residues at the N-terminus of ductin does not affect the ratio of the two orientations. Once in the microsomal membrane, ductin assembles into an oligomeric complex which contains a pore accessible to a water-soluble probe, reminiscent of the ductin complex found in the V-ATPase and a connexon.

Lipid-protein interactions and assembly of the 16-kDa channel polypeptide from Nephrops norvegicus. Studies with spin-label electron spin resonance spectroscopy and electron microscopy

The assembly of 16-kDa polypeptide channel units in membranes from the hepatopancreas of Nephrops norvegicus has been studied both by electron microscopy and by the lipid--protein interactions reported with spin-labeled lipids. Membranes prepared by extraction with N-lauroylsarcosine and Triton X-100 have a low lipid/protein ratio (ca. 4-6.5 phospholipids and 1 cholesterol per 16-kDa monomer), and those prepared by alkaline extraction have a higher lipid/protein ratio (ca. 12-16 phospholipids and 3.5-4 cholesterols per 16-kDa monomer). In the membranes extracted with detergents, the protein is assembled in membrane sheets as hexagonally packed hexameric complexes, whereas the alkali-extracted preparations consist of closed vesicles in which the channel complexes are near randomly distributed. The electron spin resonance (ESR) spectra from lipids spin-labeled at the C-14 position of the (sn-2) chain show lower mobility for the membranes extracted with N-lauroylsarcosine than for the alkaline-extracted membranes. At higher temperatures, the ESR spectra reveal a population of lipids whose mobility is restricted by direct interaction with the intramembranous sections of the channel assemblies. The population of protein-associated spin-labeled phosphatidylcholine in the alkali-extracted membranes corresponds to 4-5 phospholipid molecules plus 1 cholesterol molecule per 16-kDa polypeptide monomer.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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

The 16 kDa proteolipid (subunit c) of the eukaryotic vacuolar H(+)-ATPase (V-ATPase) is closely related to the ductin polypeptide that forms the connexon channel of gap junctions in the crustacean Nephrops norvegicus. Here we show that the major protein component of Manduca sexta gap junction preparations is a 16 kDa polypeptide whose N-terminal sequence is homologous to ductin and is identical to the deduced sequence of a previously cloned cDNA from Manduca (Dow et al., Gene, 122, 355–360, 1992). We also show that a Drosophila melanogaster cDNA, highly homologous to the Manduca cDNA, can rescue Saccharomyces cerevisiae, defective in V-ATPase function, in which the corresponding yeast gene, VMA3, has been inactivated. Evidence is presented for a single genetic locus (Vha16) in Drosophila, which in adults at least contains a single transcriptional unit. Taken together, the data suggest that in Drosophila and Manduca, the same polypeptide is both the proteolipid subunit c component of the V-ATPase and the ductin component of gap junctions. The intron/exon structure of the Drosophila Vha16 is identical to that of a human Vha16 gene, and is consistent with an ancient duplication of an 8 kDa domain. A pilot study for gene inactivation shows that transposable P-elements can be easily inserted into the Drosophila ductin Vha16 gene. Although without phenotypic consequences, these can serve as a starting point for generation of null alleles.

Functional properties of a hybrid vacuolar H(+)-ATPase in Saccharomyces cells expressing the Nephrops 16-kDa proteolipid

The hydrophobic 16-kDa polypeptide which forms gap-junction-like structures in the crustacean Nephrops norvegicus is a member of a highly conserved family of proteolipids involved in a variety of membrane transport functions in eukaryotic cells. This family also includes the product of the Saccharomyces cerevisiae VMA3 gene which encodes an integral membrane component of the vacuolar membrane H(+)-ATPase. The cDNA for the Nephrops proteolipid complements a mutation in the yeast VMA3 gene, resulting in assembly of a hybrid H(+)-ATPase comprising yeast catalytic subunits and Nephrops integral membrane components. The hybrid vacuolar ATPase was capable of ATP hydrolysis which was coupled to proton translocation and showed inhibitor binding and enzymological properties similar to those of wild-type V-ATPases (Km for ATP, 0.4 mM), suggesting that both yeast and crustacean proteolipids share conserved structure at regions of protein interaction. To facilitate isolation of the Nephrops proteolipid by affinity chromatography on a Ni(2+)-binding support, six C-terminal histidine residues were added to the proteolipid. This modification did not prohibit assembly into the hybrid H(+)-ATPase, although the resultant enzyme did have a markedly elevated Km (1.8 mM). The membrane-bound Vo sector of the ATPase was isolated by the affinity-chromatography procedure and reconstituted into synthetic vesicles. This complex was found to be impermeable to small cations in the absence of catalytic ATPase subunits either in situ in the vacuolar membrane or in the reconstituted system. The functional significance of this impermeability and the structure/function relationships between proteolipids from different sources are discussed.

Disposition and orientation of ductin (DCCD-reactive vacuolar H(+)-ATPase subunit) in mammalian membrane complexes

The disposition and orientation of mouse ductin (the subunit c of the vacuolar H(+)-ATPase) in gap junctions has been examined. Like the Nephrops norvegicus (arthropod) form, mouse ductin in the intact junctional structure is resistant to high levels of nonspecific proteinase, suggesting that it is for the most part buried in the bilayer. Antisera to an octapeptide near the N-terminus cross-react with ductins in gap junction preparations from four different mouse tissues, from chicken and Xenopus laevis liver, and from N. norvegicus hepatopancreas. The antisera and antibodies, affinity purified against the octapeptide, agglutinate isolated gap junctions, suggesting that the N-terminus is located on the exposed surface, equivalent to the cytoplasmic face of an intercellular gap junction. The antibodies also block dye coupling when injected into cells in culture, confirming the cytoplasmic location of the epitope. The lipophylic reagent dicylohexyl carbodiimide (DCCD), which targets carboxyl groups within the membrane and selectively reacts with ductin in N. norvegicus gap junction preparations, rapidly inhibits junctional communication. Bafilomycin A1, which inhibits V-ATPase and stops vacuolar acidification, does not affect dye coupling, showing that the inhibition seen with antibodies and DCCD is not an indirect consequence of their action on the ductin of V-ATPase. Consistent with this interpretation the anti-peptide antibodies do not bind to intact chromaffin granules or inhibit their V-ATPase activity, but do bind to osmotically disrupted granule membrane. This suggests that ductin has an orientation (N-terminus pointing away from the cytoplasm) in the vacuolar membrane opposite to that in the gap junction membrane.

Evidence for a common structure for a class of membrane channels

Electron microscopic analysis of gap-junction-like structures isolated from an anthropod (Nephrops norvegicus) and composed of a 16-kDa polypeptide, show the functional unit to be a star-shaped hexamer of protein arranged around a central channel which runs perpendicular to the plane of the membrane. Estimations of the molecular volume carried out on an averaged projection are consistent with a subunit mass of 16-18 kDa. Fourier transform infrared spectroscopy indicates a high alpha-helical content for the protein, supporting secondary-structure predictions of four transmembrane alpha helices/monomer. The averaged projection shows a close resemblance to a hexamer of the 16-kDa protein built on the basis of a four alpha-helical bundle [Finbow, M. E., Eliopoulos, E. E., Jackson, P. J., Keen, J. N., Meagher, L., Thompson, P., Jones, P. C. & Findlay, J. B. C. (1992) Protein Eng. 5, 7-15]. The reconstructed image is also similar to that obtained for gap-junction-like channels isolated from a related arthropod [Homarus americanus; Sikerwar, S. S., Downing, K. H. & Glaeser, R. M. (1991) J. Struct. Biol. 106, 255-263] whose protein content was unknown but which we demonstrate may be composed of a related 16-kDa protein. Previous studies have shown a high sequence identity of the Nephrops 16-kDa protein with the 16-kDa proteolipid subunit c of the vascular H(+)-ATPase, both of which in turn bear similarity to the 8-kDa proteolipid subunit of the F1F0-ATP synthase. Expression of cDNA coding for the Nephrops 16-kDa protein in Saccharomyces cerevisiae, in which the endogenous gene coding for the V-ATPase proteolipid has been inactivated, restores V-ATPase activity and cell growth.

Connexins and the vacuolar proteolipid-like 16-kDa protein are not directly associated with each other but may be components of similar or the same gap junctional complexes

Gap junction preparations made from mouse liver plasma membranes by alkali extraction contain variable proportions of connexins (Cx32 and Cx26) and the 16-kDa protein which is closely related or may be identical to the 16-kDa proteolipid (subunit c) of the vacuolar H(+)-ATPase and the mediatophore complex. The absence of a stoichiometric relationship suggests that connexins and the 16-kDa protein are not subunits of the same channel complex, but analysis of alkali preparations by isopycnic centrifugation shows both types of protein are in membrane structures of the same buoyant density. Electron microscopic analysis of alkali preparations shows a homogeneous population of gap junctions of uniform morphology and width, suggesting the proteins are in the same or similar structures. The structures containing connexins and the 16-kDa protein can be separated by treatment of the plasma membranes with Triton X-100. After such treatment, the connexins remain associated with dense cellular or extracellular material and the gap junctional structures, after further extraction with N-lauroyl sarcosine and urea, contain only the 16-kDa protein. These detergent-extracted gap junctions are thinner (14.1 nm) than those in alkali preparations (18.4 nm).

Structure of a 16 kDa integral membrane protein that has identity to the putative proton channel of the vacuolar H(+)-ATPase

A 16 kDa protein has been isolated in a homogeneous form as the major component of a paracrystalline paired membrane structure closely resembling the gap junction. The primary structure of this protein from arthropod and vertebrate species has been determined by protein and cDNA sequencing. The amino acid sequences are highly conserved and virtually identical to the amino acid sequence of the proteolipid subunit of the vacuolar H(+)-ATPases. The disposition of the protein in the membrane has been studied using proteases and the N,N'-dicyclohexylcarbodiimide reactive site identified. These data, together with secondary structure predictions, suggest that the 16 kDa protein is for the most part buried in the membrane, arranged in a bundle of four hydrophobic alpha-helices. Using computer graphics, a model has been constructed based on this arrangement and on the electron microscopic images of the paracrystalline arrays.

Bovine papillomavirus E5 oncoprotein binds to the 16K component of vacuolar H(+)-ATPases

The major transforming protein of bovine papillomavirus type 1, E5, is mainly associated with endomembranes, specifically binding to a cellular protein of relative molecular mass 16,000 (16K). At the same time as transformation, E5 causes the phosphorylation of tyrosine residues in epidermal and platelet-derived growth factor receptors. We show here that the 16K protein associated with E5 is the 16K component of vacuolar ATPases. This protein is known to be an integral membrane protein in endosomes, bovine chromaffin granules, synaptic vesicles, fungal and plant vacuoles and clathrin-coated vesicles, as well as a component of gap-junction-like membrane complexes. Because proton pumps are critical for the function of cellular compartments that process growth-factor receptors, the interaction of E5 with the 16K protein could explain the pleiomorphic features of cells transformed by E5.

The gap junction-like form of a vacuolar proton channel component appears not to be an artifact of isolation: an immunocytochemical localization study

Gap junctional structures containing a 16-kDa intrinsic membrane protein have been isolated from the hepatopancreas of the crustacean Nephrops norvegicus. These structures are double membranes 14-15 nm thick and composed of hexagonal arrays of particles which have a central pore that is penetrated by a cationic negative stain. Membrane preparations have also been isolated from the hepatopancreas and these contain similar gap junctional regions of uniform width. Affinity purified antibodies to the 16-kDa protein bind principally to these gap junctional regions. Antiserum raised against the isolated gap junctional structures binds strongly to the lateral surfaces of the columnar epithelial cells and in particular to gap junction-like regions.

Junctional communication and cellular differentiation

Gap junctions provide pathways of direct cell to cell communication in the tissues of metazoan animals. Cells joined by gap junctions share their small ions and molecules but can maintain distinctive activities through expression of different macromolecules which are too large to pass through the junctions. The junctional channels are made of a tissue invariant, evolutionarily conserved 16-18 k protein but the formation and maintenance of active coupling also requires one or more connexins, a family of tissue-specific proteins ranging in size from 21 k to 70 k. Junctions can be isolated as complexes containing both types of protein by mild procedures using high pH but the connexins can be removed by detergent, urea and protease treatment without destroying the characteristic junctional-morphology of hexagonally packed channels in the double membrane structures. There is also some evidence for the participation in the complex of tissue-specific proteoglycans which perhaps interact with the tissue-specific connexins and account for specificity of junction formation. Such specificity in mixed cultures leads to the production of communication compartments, groups of cells joined by junctions but separated by reduced trans-boundary coupling from cells in adjacent compartments. Compartmentation also occurs in vivo resulting in specific patterns of junctional communication which have been mapped in most detail in mouse skin. These mapping data and the changes which are associated with abnormal proliferation have lead to new ideas on intercellular control.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of dye-coupling in Patella (Mollusca) embryos by microinjection of antiserum against nephrops (Arthropoda) gap junctions

Antiserum raised against Nephrops gap junctions was injected into single cells of the 2-, 4-, 8-, 16-, and 32-cell stage of the Patella vulgata embryos. The pattern of junctional communication by iontophoresis of Lucifer Yellow CH was tested at the 32-cell stage. The results show that the normal pattern of dye-coupling at the 32-cell stage is disrupted in greater than 65% of embryos previously injected with antisera. In contrast, less than 15% of embryos injected with preimmune serum exhibited disrupted patterns of dye-coupling. Up to the late 32-cell stage no effect of the antiserum on the pattern of cleavage was detected. This antiserum may provide a powerful tool to investigate the role of junctional communication in later stages of development of Patella embryos.

Tissue and species conservation of the vertebrate and arthropod forms of the low molecular weight (16-18000) proteins of gap junctions

Gap junctions have been isolated from four murine tissues, from rat and Xenopus laevis liver, and from Nephrops norvegicus (Norway lobster) hepatopancreas. The preparations of gap junctions from each vertebrate tissue contain a single major protein, Mr 16,000, and those from Nephrops hepatopancreas a protein, Mr 18,000. Immunocytochemical studies using affinity-purified antibodies raised against gap junctions from Nephrops show the junctional origin of the 18k protein. Immunological studies using Western blotting and biochemical studies using tryptic peptide mapping show no significant differences between the 16k junctional proteins of mouse and hence provide no evidence of tissue variation. These studies also suggest that the mouse, rat, and Xenopus 16k proteins and the Nephrops 18k protein share some common structural features.

Isolation and characterisation of arthropod gap junctions

Gap junctions have been isolated from the hepatopancreas of the crustacean arthropod, Nephrops norvegicus (Norway lobster). SDS-PAGE of these preparations shows two major protein bands, mol. wt. 18 000 (18 K) and mol. wt. 28 000 (28 K). The 18-K and 28-K proteins are interconvertible, cannot be distinguished by two dimensional tryptic and chymotryptic peptide mapping, and therefore appear to be different (most likely monomeric and dimeric) forms of the same protein. The protein can also aggregate to higher multimeric forms mol. wt. 38 000 (presumed trimer), and mol. wt. 52 000 (presumed tetramer). The buoyant density of the isolated gap junctions in continuous potassium iodide gradients is 1.260 g/cm. The junctions are progressively solubilized in increasing SDS concentrations, mostly between 0.1% and 0.2% SDS, and this is accompanied by the release of the 18-K and 28-K forms of the junctional protein. The Nephrops hepatopancreas 18-K junctional protein has antigenic determinants in common with the vertebrate 16-K junctional protein as shown by cross-reactivity with two different affinity purified antibody preparations. However, no detectable similarity can be seen between the major I-labelled tryptic and chymotrytpic peptides of the Nephrops hepatopancreas 18-K protein and the mouse liver 16-K protein.

Analysis of vertebrate gap junction protein

A new method for the purification of gap junctions is described which depends on the extraction of cell monolayers or tissue homogenates with Triton X-100. The major band on SDS-polyacrylamide gel electrophoresis (PAGE) of junctional preparations from a variety of vertebrate sources has an apparent mol. wt. of 16,000 (16 K). Further evidence for the junctional origin of the 16 K protein is provided by the results of four different experimental approaches. (i) The junctions form a sharp band in potassium iodide density gradients at 1.195 g/cm3 and the 16 K protein is the only detectable band in fractions of this bouyant density. (ii) The junctions are progressively solubilised by increasing concentrations of SDS (in the range 0.1-0.5%) and the dissolution of the junctional structure, observed by electron microscopy, parallels the release of the 16 K protein. (iii) Glutaraldehyde fixation of intact junctions cross-links the 16 K protein. (iv) The recoverable amount of the 16 K protein correlates with known changes in gap junctional area in the regenerating weanling rat liver after partial hepatectomy and in V79 cell cultures exposed to 4beta-phorbol 12-myristate 13-acetate.