[A flip-flop model of the chloride channel complex explains the dysregulation of the chloride flow in the plasmalemma of cells in cystic fibrosis]

Studies on human porin XXI: gadolinium opens Up cell membrane standing porin channels making way for the osmolytes chloride or taurine-A putative approach to activate the alternate chloride channel in cystic fibrosis

We recently proposed that cell-membrane-integrated vertebrate porin/voltage-dependent anion-selective channel (VDAC) forms part of the outwardly rectifying chloride channel (ORCC) complex that may be involved in volume regulation. The results we present here support this thesis. According to light scattering measurements micromolar concentrations of Gd(3+) induce cell swelling of human healthy and cystic fibrosis (CF) B-lymphocyte cell lines in isotonic Ringer solution. In high-potassium Ringer solution additional swelling is observed. Gd(3+) induces excessive cell swelling of cell lines in hypotonic Ringer solutions, containing 70 mM NaCl or 135 mM taurine, respectively. The gadolinium effect is lost when NaCl is replaced by Na-gluconate. Using video camera monitoring we show that HeLa cells also swell in micromolar concentrations of Gd(3+) in isotonic taurine Ringer solution. The dose-dependent effect of the agonist was always blocked by extracellular application of anti-human type-1 porin antibodies. Together with data on a decreasing effect of micromolar amounts of gadolinium on the voltage dependence of reconstituted human porin the results prove the involvement of porin channels in the swelling behavior in different cell lines. As a mechanism we propose that ionic gadolinium opens up plasmalemma-integrated porin channels, chloride or taurine then following their concentration gradients into the cells. Furthermore, our data argue for a single pathway for inorganic and organic osmolytes during regulatory volume decrease after cell swelling. There is indirect evidence that porin forms part of the cystic fibrosis relevant ORCC channel. Gadolinium thus may work to open the alternate chloride channel in CF.

Channel active mammalian porin, purified from crude membrane fractions of human B lymphocytes and bovine skeletal muscle, reversibly binds adenosine triphosphate (ATP)

A new aspect of mammalian porin (mammalian VDAC = mammalian voltage-dependent anion channel) is presented: channel active VDAC binds adenosine triphosphate (ATP) in the absence of Ca2+. Channel active "Porin 31HL" or "Porin 31BM", enriched from crude membranes of human B lymphocytes or whole cell lysates of bovine skeletal muscle, respectively, was bound to a nine atoms spacer ATP-agarose at pH 7.4 or 5.0 and reeluted from the resin by 10 mM ATP disodium salt. Furthermore, channel active "Porin 31BM" was labelled by [32P]ATP in a 1:1 stoichiometric relation. Binding of ATP to human porin was confirmed by studying the interaction of the synthetic porin fragment Type-1/Ac-35, comprising the putative nucleotide binding site G Y G F G, with trinitrophenyl-ATP (TNT-ATP) by scanning fluorometry. Peptide/TNP-ATP complexes clearly show enhancement of fluorescence intensity and a spectral shift of the fluorescence maximum. In a control experiment, using a porin fragment lacking the putative nucleotide binding site, no change of fluorescence emission was observed. Further confirmation for ATP binding by human VDAC arose from an autoradiographic experimental approach: the porin fragment Type-1/Ac-35 could be labelled by [32P]ATP, while a second porin fragment ending immediately before the putative nucleotide binding site could not; nor could a synthetic non porin peptide.

Channel active mammalian porin, purified from crude membrane fractions of human B lymphocytes or bovine skeletal muscle, reversibly binds the stilbene-disulfonate group of the chloride channel blocker DIDS

Two new aspects of mammalian porin are presented. First, by affinity chromatography we show that channel active human or bovine porin reversibly bind the stilbene-disulfonate group of the chloride channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS). The procedure is suitable for further purification of porin after enrichment by ion exchange chromatography and shows a yield of 24.3%. The data support our recent proposal that VDAC forms part of the ORDIC channel complex which is affected in cystic fibrosis. Second, a purification scheme for mammalian porin is given starting with direct solubilisation of ground bovine skeletal muscle to avoid breaking up tissue. About 130 mg of channel active "Porin 31BM" are enriched from 946 g muscle tissue. Concerning its apparent molecular mass, primary structure, channel activity, channel conductance and voltage dependence the molecule shows high similarity to human porin. "Porin 31BM" is furthermore labelled by antibodies raised against human B lymphocyte derived "Porin 31HL".

Studies on human porin. IX. Immunolocalization of porin and CFTR channels in human surface respiratory epithelium

The expression of the voltage-dependent anion channel (VDAC) "Porin 31 HL" and its cellular and subcellular immunocytochemical localization in the human respiratory epithelium were studied with monoclonal and polyclonal antibodies using immunofluorescence and immunogold labelling with light (LM) and transmission electron microscopy (TEM), respectively. Porin was identified in the apical domain of the ciliated cells and in the basal cells of the respiratory epithelium. Immunogold labelling was present in the apical plasma membrane and subapical vesicles of the ciliated cells. In pre-embedded freshly dissociated surface epithelial cells, porin could also be identified with TEM at the outer part of the plasma membrane of basal cells. By LM double immunolabelling, both porin and cystic fibrosis transmembrane conductance regulator (CFTR) were identified in the apical domain of ciliated cells but not in basal cells where CFTR was never identified. On Western blots of solubilized total membrane protein preparations from the same frozen surface epithelial respiratory cells, the antibodies recognized a group of 3 proteins of 31, 60 and 130-140 kDa with a strong reactivity for a 31 kDa protein, corresponding to the porin and a protein of 170 kDa which is consistent with mature CFTR. These results suggest that porin might be part of a multi-component chloride channel complex which could interact with CFTR.

Evidence for extra-mitochondrial localization of the VDAC/porin channel in eucaryotic cells

The expression of bacterial porin in outer membranes of gram-negative bacteria and of mitochondrial porin or voltage-dependent anion channel (VDAC) in outer mitochondrial membranes (OMM) of eucaryotic cells was demonstrated about 15 years ago. However, the expression of VDAC in the plasmalemma (PLM) of transformed human B lymphoblasts has recently been indicated by cytotoxicity and indirect immunofluorescence studies. New data suggest that the expression of VDAC may be even more widespread. Different cell types express porin channels in their PLM and in intracellular membranes other than OMM. The functional expression of these channels may differ in the various compartments since recent experiments have demonstrated that the voltage dependence and ion selectivity of mitochondrial VDAC may be altered by their interaction with modulators. The present paper proposes a unifying concept for the ion-selective channels of cell membranes, in particular, those whose regulation is affected in cystic fibrosis.

Studies on human porin. VI. Production and characterization of eight monoclonal mouse antibodies against the human VDAC "Porin 31HL" and their application for histotopological studies in human skeletal muscle

We report on the production and characterization of eight monoclonal mouse antibodies against the complete human VDAC "Porin 31HL". The antigen used was purified from a total membrane preparation of the transformed human B-lymphocyte cell line H2LCL. In Western blots all eight mAbs react with a single 31-kDa band in solubilized H2LCL membrane preparations thus demonstrating their specificity for the human VDAC "Porin 31HL". Concerning the epitope specificity we show that all eight mAbs equally react with the N-terminal part of human porin. Moreover, we demonstrate the expression of VDAC in the sarcolemma by indirect immunoenzyme labelling of cryosections of human skeletal muscle applying four of our mAbs. These data support our recent observations on the expression of porin channels in the plasmalemma of different normal and transformed human cell lines. VDAC in the plasmalemma is discussed as the molecular basis of the Blatz and Magleby channel.

Studies on human porin. IV. The primary structures of "Porin 31HM" purified from human skeletal muscle membranes and of "Porin 31HL" derived from human B lymphocyte membranes are identical

We report on the purification of "Porin 31HM" from the crude plasma membrane fraction of human skeletal muscle. Furthermore, all tryptic peptides of the molecule were purified and characterized by different methods. The alignment of the peptides with the complete primary structure of the human B lymphocyte plasma membrane-derived "Porin 31HL", published by us recently (Kayser, H. et al. (1989) this Journal 370, 1265-1278), proved both structures to be completely identical. Our data demonstrate that porin fractions from crude plasma membranes of different human cell types do not show any variation on the primary structure level.