Cloning of a swelling-induced chloride current related protein from rabbit heart

Modulation of volume regulated anion current by I(Cln)

I(Cln), a cytosolic protein associated with a nucleotide-sensitive chloride current, may be involved in the regulation of a volume-regulated anion current (VRAC) associated with hypotonic cell swelling. We have determined the nucleic acid sequences of I(Cln) from human tsA201a, colonic (T84) and myeloma (RPMI 8826) cell lines. The amino acid sequences are highly homologous (>/=99%) to each other but less homologous to I(Cln) protein from other species. Using the whole-cell patch clamp technique, we examined the effect of I(Cln) protein expression levels on VRAC properties during a hyposmotic challenge. Overexpression of T84 or RPMI 8226-derived I(Cln) protein in tsA201a cells results in a more than 9-fold increase in the rate of VRAC activation over control values, while having no effect on VRAC inactivation properties. Underexpression of endogenous I(Cln) protein in tsA201a cells using antisense oligonucleotides results in a more than 180-fold decrease in VRAC activation rate as compared to control values. These results indicate that I(Cln) protein expression modulates VRAC activation but not inactivation.

pICln inhibits snRNP biogenesis by binding core spliceosomal proteins

The U1, U2, U4, U5, and U6 small nuclear ribonucleoproteins (snRNPs) form essential components of spliceosomes, the machinery that removes introns from pre-mRNAs in eukaryotic cells. A critical initial step in the complex process of snRNP biogenesis is the assembly of a group of common core proteins (Sm proteins) on spliceosomal snRNA. In this study we show by multiple independent methods that the protein pICln associates with Sm proteins in vivo and in vitro. The binding of pICln to Sm proteins interferes with Sm protein assembly on spliceosomal snRNAs and inhibits import of snRNAs into the nucleus. Furthermore, pICln prevents the interaction of Sm proteins with the survival of motor neurons (SMN) protein, an interaction that has been shown to be critical for snRNP biogenesis. These findings lead us to propose a model in which pICln participates in the regulation of snRNP biogenesis, at least in part by interfering with Sm protein interaction with SMN protein.

pICln predominantly localizes at luminal surface membranes of distal tubules and Henle's ascending limbs

We produced a highly specific antibody to the C-terminal peptide sequence of pICln. It recognized pICln with a 38-kDa molecular mass on SDS-polyacrylamide gel electrophoresis, coinciding with that previously reported. During native polyacrylamide gel electrophoresis, three immunoreactive bands (38, 70, and 130 kDa) were detected. The isoelectric point of pICln was calculated to be 4.0. Subcellular localization study showed the presence of pICln in the soluble and microsomal fraction. pICln can be easily solubilized from the membrane fraction with Triton X-100. From immunohistochemical observations, we found pICln to be obviously located on the luminal surface membranes of the distal tubules and Henle's loop ascending limbs, and it can also be found inside proximal tubular cells. The present results suggested that pICln functions as a "cytosolic anchor = membrane insertion" model, and it plays important roles in the "urine dilution segment" cells of nephrons.

Characterization of pICln phosphorylation state and a pICln-associated protein kinase

pICln is a ubiquitous cellular protein that has been proposed to be a volume-sensitive Cl- channel or a channel regulator. Detailed biochemical, cellular and molecular characterization of pICln is required to understand its function. Our goal in the present investigation was to define further the biochemical properties of pICln and the proteins that associate with it. Immunoprecipitation of pICln from 32P-orthophosphoric acid-labeled C6 glioma cells revealed that the protein is phosphorylated constitutively, primarily on serine residues. Protein kinase activity was detected in pICln immunoprecipitates, revealing that a constitutively active protein kinase co-precipitates with pICln. A specific association between pICln and a protein kinase was also observed in affinity assays using a recombinant GST-pICln fusion protein. The pICln-associated kinase displayed broad substrate specificity and was inhibited in a concentration-dependent manner by heparin, zinc and 5,6-dichloro-1-beta-D-ribofuranosylbenose (DRB). These characteristics resembled those of casein kinase I and II. The pICln-associated kinase was not recognized, however, by antibodies against these two enzymes. Association of the kinase with pICln was disrupted by increasing concentrations of NaCl in the washing buffer, suggesting that electrostatic interactions are involved in kinase binding. Mutagenesis experiments corroborated this observation. Truncation of pICln demonstrated that two highly charged clusters of acidic amino acid residues are both necessary and sufficient for kinase binding. Phosphopeptide mapping demonstrated that pICln contains at least two phosphorylated serine residues that are located on trypsin cleavage fragments rich in acidic amino acid residues. We propose that the kinase or a kinase binding protein binds to acidic amino acids located between D101 and Y156 and phosphorylates nearby serine residues.

pICln binds to a mammalian homolog of a yeast protein involved in regulation of cell morphology

Since its cloning and tentative identification as a chloride channel, the function of the pICln protein has been debated. Although there is no consensus regarding the specific function of pICln, it was suggested to play a role, directly or indirectly, in the function of a swelling-induced chloride conductance. Previously, the protein was shown to exist in several discrete protein complexes. To determine the function of the protein, we have begun the systematic identification of all proteins to which it binds. Here we show that four proteins firmly bind to pICln and identify the 72-kDa pICln-binding protein by affinity purification and peptide microsequencing. The interaction between this protein and pICln was verified several ways, including the extraction of several pICln clones from a cDNA library using the 72-kDa protein as a bait in a yeast two-hybrid screen. The protein is homologous to the yeast Skb1 protein. Skb1 interacts with Shk1, a homolog of the p21(Cdc42/Rac)-activated protein kinases (PAKs). The known involvement of PAKs in cytoskeletal rearrangement suggests that pICln may be linked to a system regulating cell morphology.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Hypotonic stress induces translocation of the osmolyte channel protein pICln in embryonic skate (Raja eglanteria) heart

Volume expansion of cardiac cells from a wide variety of species stimulates the efflux of the beta-amino acid taurine through an osmolyte channel. Previous studies have suggested that the osmolyte channel in epithelial cells is a swelling-activated anion channel (pICln). In skate heart, a 37-kDa protein is present which is recognized by a specific antibody to a protein characterized in MDCK cells as pICln. This protein is present predominantly in the cytosol (only 10% in the membrane fraction) of heart incubated under isotonic conditions. After transfer to hypotonic medium (one-half osmolarity), the distribution of this protein is markedly altered and significant amounts of the protein are found in the membrane fraction. After hypotonic exposure, the amount of the protein in the membrane fraction rises to 38 +/- 11% (range 18-53, n = 3). The translocation to the membrane fraction suggests that this protein may play a role in the taurine efflux in this tissue stimulated by hypotonic stress.

Expression of human pICln and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

pICln is a protein that induces an outwardly rectifying, nucleotide-sensitive chloride current (ICln) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to ICln is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pICln and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO3- > I- > Br- > Cl- > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 degrees C) prior to the analysis. In addition, the ICln current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the ICln current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pICln resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pICln forms the channel that is responsible for the ICln current in Xenopus oocytes.

The chloride current induced by expression of the protein pICln in Xenopus oocytes differs from the endogenous volume-sensitive chloride current

1. Phenotypical similarities between ICl,swell, the cell-swelling-induced chloride current and ICln, the nucleotide-sensitive chloride current induced by expression of mammalian pICln in Xenopus oocytes, have led to models which identify pICln either as the volume-sensitive chloride channel or as a cytosolic regulator thereof. 2. To investigate critically the relationship between ICl,swell and pICln two-microelectrode voltage clamp experiments were performed on Xenopus oocytes in which either human pICln was expressed or endogenous ICl,swell was activated. 3. Several criteria that clearly differentiated ICln from ICl,swell were detected. Outward rectification and the discrimination between NO3- and Cl- were more pronounced for ICln. Cyclamate blocked ICln but not ICl,swell. In contrast to ICl,swell, inactivation kinetics of ICln were pH independent and extracellular cAMP blocked only the outward ICln component. Finally, ICln was readily expressed in collagenase-defolliculated oocytes and was not modulated by extracellular hypotonicity, whereas ICl,swell could only be triggered in follicle-enclosed or manually defolliculated oocytes. 4. We therefore conclude that ICln and ICl,swell are two different chloride currents. Consequently, any model which invokes a crucial role for pICln in ICl,swell should be critically reviewed.