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

Discovery and Biological Characterization of PRMT5:MEP50 Protein-Protein Interaction Inhibitors

Protein arginine methyltransferase 5 (PRMT5) is a master epigenetic regulator and an extensively validated therapeutic target in multiple cancers. Notably, PRMT5 is the only PRMT that requires an obligate cofactor, methylosome protein 50 (MEP50), to function. We developed compound 17, a novel small-molecule PRMT5:MEP50 protein-protein interaction (PPI) inhibitor, after initial virtual screen hit identification and analogue refinement. Molecular docking indicated that compound 17 targets PRMT5:MEP50 PPI by displacing the MEP50 W54 burial into a hydrophobic pocket of the PRMT5 TIM barrel. In vitro analysis indicates IC50 < 500 nM for prostate and lung cancer cells with selective, specific inhibition of PRMT5:MEP50 substrate methylation and target gene expression, and RNA-seq analysis suggests that compound 17 may dysregulate TGF-β signaling. Compound 17 provides a proof of concept in targeting PRMT5:MEP50 PPI, as opposed to catalytic targeting, as a novel mechanism of action and supports further preclinical development of inhibitors in this class.

The PRMT5/HURP axis retards Golgi repositioning by stabilizing acetyl-tubulin and Golgi apparatus during cell migration

The Golgi apparatus (GA) translocates to the cell leading end during directional migration, thereby determining cell polarity and transporting essential factors to the migration apparatus. The study provides mechanistic insights into how GA repositioning (GR) is regulated. We show that the methyltransferase PRMT5 methylates the microtubule regulator HURP at R122. The HURP methylation mimicking mutant 122F impairs GR and cell migration. Mechanistic studies revealed that HURP 122F or endogenous methylated HURP, that is, HURP m122, interacts with acetyl-tubulin. Overexpression of HURP 122F stabilizes the bundling pattern of acetyl-tubulin by decreasing the sensitivity of the latter to a microtubule disrupting agent nocodazole. HURP 122F also rigidifies GA via desensitizing the organelle to several GA disrupting chemicals. Similarly, the acetyl-tubulin mimicking mutant 40Q or tubulin acetyltransferase αTAT1 can rigidify GA, impair GR, and retard cell migration. Reversal of HURP 122F-induced GA rigidification, by knocking down GA assembly factors such as GRASP65 or GM130, attenuates 122F-triggered GR and cell migration. Remarkably, PRMT5 is found downregulated and the level of HURP m122 is decreased during the early hours of wound healing-based cell migration, collectively implying that the PRMT5-HURP-acetyl-tubulin axis plays the role of brake, preventing GR and cell migration before cells reach empty space.

PRMT5 function and targeting in cancer

Protein methyl transferases play critical roles in numerous regulatory pathways that underlie cancer development, progression and therapy-response. Here we discuss the function of PRMT5, a member of the nine-member PRMT family, in controlling oncogenic processes including tumor intrinsic, as well as extrinsic microenvironmental signaling pathways. We discuss PRMT5 effect on histone methylation and methylation of regulatory proteins including those involved in RNA splicing, cell cycle, cell death and metabolic signaling. In all, we highlight the importance of PRMT5 regulation and function in cancer, which provide the foundation for therapeutic modalities targeting PRMT5.

Molecular Identities and ATP Release Activities of Two Types of Volume-Regulatory Anion Channels, VSOR and Maxi-Cl

An elaborate volume regulation system based on interplay of ion channels and transporters was evolved to cope with constant osmotic challenges caused by intensive metabolism, transport and other physiological/pathophysiological events. In animal cells, two types of anion channels are directly activated by cell swelling and involved in the regulatory volume decrease (RVD): volume-sensitive outwardly rectifying anion channel (VSOR), also called volume-regulated anion channel (VRAC), and Maxi-Cl which is the most major type of maxi-anion channel (MAC). These two channels have very different biophysical profiles and exhibit opposite dependence on intracellular ATP. After several decades of verifying many false-positive candidates for VSOR and Maxi-Cl, LRRC8 family proteins emerged as major VSOR components, and SLCO2A1 protein as a core of Maxi-Cl. Still, neither of these proteins alone can fully reproduce the native channel phenotypes suggesting existence of missing components. Although both VSOR and Maxi-Cl have pores wide enough to accommodate bulky ATP^4- and MgATP^2- anions, evidence accumulated hitherto, based on pharmacological and gene silencing experiments, suggests that Maxi-Cl, but not VSOR, serves as one of the major pathways for the release of ATP from swollen and ischemic/hypoxic cells. Relations of VSOR and Maxi-Cl with diseases and their selective pharmacology are the topics promoted by recent advance in molecular identification of the two volume-activated, volume-regulatory anion channels.

OGG1 regulates the level of symmetric dimethylation of histone H4 arginine-3 by interacting with PRMT5

OGG1 is the first enzyme in the base excision repair pathway (BER) responsible for repairing 8-oxoguanine DNA lesions. Recent studies found that OGG1 may also be involved in epigenetic regulation. In this study, we focused on the roles of OGG1 in histone modification. First, to study the effects of OGG1 on histone modification, the protein levels of symmetric dimethylation of histone H4 arginine-3 (H4R3me2s) were determined by western blot analysis following the knockdown or overexpression of OGG1. Second, the molecular mechanisms by which OGG1 regulates H4R3me2s were assessed by co-immunoprecipitation (CO-IP) assays in mouse embryonic fibroblast (MEF) wild-type (WT) and Ogg^-/- cells. Finally, to verify the regulation of H4R3me2s by OGG1 on specific genes, chromatin immunoprecipitation (CHIP) was performed on MEF WT and Ogg^-/- cells. We found that OGG1 affects PRMT5 binding on histone H4 and the formation of H4R3me2s via PRMT5. The methylation level of H4R3me2s was dramatically decreased in MEF Ogg^-/- cells compared to WT cells. Knockdown of OGG1 by siRNA led to a decrease in H4R3me2s, while overexpression of OGG1 increased the level of H4R3me2s. OGG1 also interacted with PRMT5 and histone H4, and the interaction between PRMT5 and histone H4 was reduced in MEF Ogg^-/- cells. Our data not only illustrate the important roles of OGG1 in histone modification, but also reveal the mechanism by which OGG1 affects PRMT5 binding on H4R3 resulting in the symmetrical dimethylation of histone H4 arginine-3.

Histone phosphorylation by TRPM6's cleaved kinase attenuates adjacent arginine methylation to regulate gene expression

TRPM6 and TRPM7 are members of the melastatin-related transient receptor potential (TRPM) subfamily of ion channels. Deletion of either gene in mice is embryonically lethal. TRPM6/7 are the only known examples of single polypeptides containing both an ion channel pore and a serine/threonine kinase (chanzyme). Here we show that the C-terminal kinase domain of TRPM6 is cleaved from the channel domain in a cell type-specific fashion and is active. Cleavage requires that the channel conductance is functional. The cleaved kinase translocates to the nucleus, where it is strictly localized and phosphorylates specific histone serine and threonine (S/T) residues. TRPM6-cleaved kinases (M6CKs) bind subunits of the protein arginine methyltransferase 5 (PRMT5) molecular complex that make important epigenetic modifications by methylating histone arginine residues. Histone phosphorylation by M6CK results in a dramatic decrease in methylation of arginines adjacent to M6CK-phosphorylated amino acids. Knockout of TRPM6 or inactivation of its kinase results in global changes in histone S/T phosphorylation and changes the transcription of hundreds of genes. We hypothesize that M6CK associates with the PRMT5 molecular complex in the nucleus, directing M6CK to a specific genomic location and providing site-specific histone phosphorylation. M6CK histone phosphorylation, in turn, regulates transcription by attenuating the effect of local arginine methylation.

Chondrocyte channel transcriptomics: do microarray data fit with expression and functional data?

To date, a range of ion channels have been identified in chondrocytes using a number of different techniques, predominantly electrophysiological and/or biomolecular; each of these has its advantages and disadvantages. Here we aim to compare and contrast the data available from biophysical and microarray experiments. This letter analyses recent transcriptomics datasets from chondrocytes, accessible from the European Bioinformatics Institute (EBI). We discuss whether such bioinformatic analysis of microarray datasets can potentially accelerate identification and discovery of ion channels in chondrocytes. The ion channels which appear most frequently across these microarray datasets are discussed, along with their possible functions. We discuss whether functional or protein data exist which support the microarray data. A microarray experiment comparing gene expression in osteoarthritis and healthy cartilage is also discussed and we verify the differential expression of 2 of these genes, namely the genes encoding large calcium-activated potassium (BK) and aquaporin channels.

Protein arginine methyltransferase 5 is a potential oncoprotein that upregulates G1 cyclins/cyclin-dependent kinases and the phosphoinositide 3-kinase/AKT signaling cascade

Increasing evidence suggests that PRMT5, a protein arginine methyltransferase, is involved in tumorigenesis. However, no systematic research has demonstrated the cell-transforming activity of PRMT5. We investigated the involvement of PRMT5 in tumor formation. First, we showed that PRMT5 was associated with many human cancers, through statistical analysis of microarray data in the NCBI GEO database. Overexpression of ectopic PRMT5 per se or its specific shRNA enhanced or reduced cell growth under conditions of normal or low concentrations of serum, low cell density, and poor cell attachment. A stable clone that expressed exogenous PRMT5 formed tumors in nude mice, which demonstrated that PRMT5 is a potential oncoprotein. PRMT5 accelerated cell cycle progression through G1 phase and modulated regulators of G1; for example, it upregulated cyclin-dependent kinase (CDK) 4, CDK6, and cyclins D1, D2 and E1, and inactivated retinoblastoma protein (Rb). Moreover, PRMT5 activated phosphoinositide 3-kinase (PI3K)/AKT and suppressed c-Jun N-terminal kinase (JNK)/c-Jun signaling cascades. However, only inhibition of PI3K activity, and not overexpression of JNK, blocked PRMT5-induced cell proliferation. Further analysis of PRMT5 expression in 64 samples of human lung cancer tissues by microarray and western blot analysis revealed a tight association of PRMT5 with lung cancer. Knockdown of PRMT5 retarded cell growth of lung cancer cell lines A549 and H1299. In conclusion, to the best of our knowledge, we have characterized the cell-transforming activity of PRMT5 and delineated its underlying mechanisms for the first time.

The C-terminus of ICln is natively disordered but displays local structural preformation

ICln is a vital, ubiquitously expressed protein with roles in cell volume regulation, angiogenesis, cell morphology, activation of platelets and RNA processing. In previous work we have determined the 3D structure of the N-terminus of ICln (residues 1-159), which folds into a PH-like domain followed by an unstructured region (residues H134 - Q159) containing protein-protein interaction sites. Here we present sequence-specific resonance assignments of the C-terminus (residues Q159 - H235) of ICln by NMR, and show that this region of the protein is intrinsically unstructured. By applying (13)Cα- (13)Cβ secondary chemical shifts to detect possible preferences for secondary structure elements we show that the C-terminus of ICln adopts a preferred α-helical organization between residues E170 and E187, and exists preferentially in extended conformations (β-strands) between residues D161 to Y168 and E217 to T223.

EGF stimulates IClswell by a redistribution of proteins involved in cell volume regulation

BACKGROUND

ICln is a multifunctional protein involved in the generation of chloride currents activated during regulatory volume decrease (RVD) after cell swelling (ICl(swell)). Growth factor receptors play a key role in different cellular processes and epidermal growth factor (EGF) regulates swelling-activated chloride permeability.

AIM

We set out to investigate if the EGF-induced upregulation of ICl(swell) could be explained by a rearrangement of ICln subcellular distribution and interaction with its molecular partners.

METHODS

NIH-3T3 fibroblasts were serum-deprived for 24 hours and stimulated with EGF (40 ng/ml) for 30 minutes. ICl(swell) activation, ICln distribution and interaction with its molecular partner HSPC038 were assessed by whole cell patch clamp and fluorescence resonance energy transfer (FRET).

RESULTS

EGF treatment significantly enhanced the direct molecular interaction between ICln and HSPC038 and also resulted in an increase of ICln and HSPC038 association with the plasma membrane. Importantly, these events are associated with a significant increase of ICl(swell).

CONCLUSIONS

The present data indicate that EGF might exert its role in the modulation of volume-sensitive chloride currents in part through activation and translocation of ICln and HSPC038 to the plasma membrane.

The molecular and functional interaction between ICln and HSPC038 proteins modulates the regulation of cell volume

Identifying functional partners for protein/protein interactions can be a difficult challenge. We proposed the use of the operon structure of the Caenorhabditis elegans genome as a “new gene-finding tool” (Eichmüller, S., Vezzoli, V., Bazzini, C., Ritter, M., Fürst, J., Jakab, M., Ravasio, A., Chwatal, S., Dossena, S., Bottà, G., Meyer, G., Maier, B., Valenti, G., Lang, F., and Paulmichl, M. (2004) J. Biol. Chem. 279, 7136–7146) that could be functionally translated to the human system. Here we show the validity of this approach by studying the predicted functional interaction between ICln and HSPC038. In C. elegans, the gene encoding for the ICln homolog (icln-1) is embedded in an operon with two other genes, Nx (the human homolog of Nx is HSPC038) and Ny. ICln is a highly conserved, ubiquitously expressed multifunctional protein that plays a critical role in the regulatory volume decrease after cell swelling. Following hypotonic stress, ICln translocates from the cytosol to the plasma membrane, where it has been proposed to participate in the activation of the swelling-induced chloride current (ICl_swell). Here we show that the interaction between human ICln and HSPC038 plays a role in volume regulation after cell swelling and that HSPC038 acts as an escort, directing ICln to the cell membrane after cell swelling and facilitating the activation of ICl_swell. Assessment of the NMR structure of HSPC038 showed the presence of a zinc finger motif. Moreover, NMR and additional biochemical techniques enabled us to identify the putative ICln/HSPC038 interacting sites, thereby explaining the functional interaction of both proteins on a molecular level.

Arginine methylation regulates IL-2 gene expression: a role for protein arginine methyltransferase 5 (PRMT5)

Arginine methylation is a post-translational modification resulting in the generation of aDMAs (asymmetrical omega-NG, NG-dimethylated arginines) and sDMAs (symmetrical omega-NG, N'G-dimethylated arginines). The role of arginine methylation in cell signalling and gene expression in T lymphocytes is not understood. In the present study, we report a role for protein arginine methylation in regulating IL-2 (interleukin 2) gene expression in T lymphocytes. Leukaemic Jurkat T-cells treated with a known methylase inhibitor, 5'-methylthioadenosine, had decreased cytokine gene expression, as measured using an NF-AT (nuclear factor of activated T-cells)-responsive promoter linked to the luciferase reporter gene. Since methylase inhibitors block all methylation events, we performed RNA interference with small interfering RNAs against the major PRMT (protein arginine methyltransferases) that generates sDMA (PRMT5). The dose-dependent decrease in PRMT5 expression resulted in the inhibition of both IL-2- and NF-AT-driven promoter activities and IL-2 secretion. By using an sDMA-specific antibody, we observed that sDMA-containing proteins are directly associated with the IL-2 promoter after T-cell activation. Since changes in protein arginine methylation were not observed after T-cell activation in Jurkat and human peripheral blood lymphocytes, our results demonstrate that it is the recruitment of methylarginine-specific protein(s) to cytokine promoter regions that regulates their gene expression.

Role of pICLn in methylation of Sm proteins by PRMT5

pICln is an essential, highly conserved 26-kDa protein whose functions include binding to Sm proteins in the cytoplasm of human cells and mediating the ordered and regulated assembly of the cell's RNA-splicing machinery by the survival motor neurons complex. pICln also interacts with PRMT5, the enzyme responsible for generating symmetric dimethylarginine modifications on the carboxyl-terminal regions of three of the canonical Sm proteins. To better understand the role of pICln in these cellular processes, we have investigated the properties of pICln and pICln.Sm complexes and the effects that pICln has on the methyltransferase activity of PRMT5. We find that pICln is a monomer in solution, binds with high affinity (K(d) approximately 160 nm) to SmD3-SmB, and forms 1:1 complexes with Sm proteins and Sm protein subcomplexes. The data support an end-capping model of pICln binding that supports current views of how pICln prevents Sm oligomerization on illicit RNA substrates. We have found that by co-expression with pICln, recombinant PRMT5 can be produced in a soluble, active form. PRMT5 alone has promiscuous activity toward a variety of known substrates. In the presence of pICln, however, PRMT5 methylation of Sm proteins is stimulated, but methylation of histones is inhibited. We have also found that mutations in pICln that do not affect Sm protein binding can still have a profound effect on the methyltransferase activity of the PRMT5 complex. Together, the data provide insights into pICln function and represent an important starting point for biochemical analyses of PRMT5.

The ICln interactome

The many different functional phenotypes described in mammalian cells can only be explained by an intense interaction of the underlying proteins, substantiated by the fact that the number of independently expressed proteins in living cells seems not to exceed 25 K, a number way too small to explain the >250 K different phenotypes on a one-protein-one-function base. Therefore, the study of the interactome of the different proteins is of utmost importance. Here, we describe the present knowledge of the ICln interactome. ICln is a protein, we cloned and whose function was reported to be as divers as (i) ion permeation, (ii) cytoskeletal organization, and (iii) RNA processing. The role of ICln in these different functional modules can be described best as being a 'connector hub' with 'date hub' function.

FBXO11/PRMT9, a new protein arginine methyltransferase, symmetrically dimethylates arginine residues

We have identified a protein, FLJ12673 or FBXO11, that contains domains characteristically present in protein arginine methyltransferases (PRMTs). Immuno-purified protein expressed from one of the four splice variants in HeLa cells and in Escherichia coli exhibited methyltransferase activity. Monomethylarginine, symmetric, and asymmetric dimethylarginine (SDMA, ADMA) were formed on arginine residues. Accordingly, we have designated the protein PRMT9. PRMT9 is the third member of the PRMT family that forms SDMA modifications in proteins. Structurally, this protein is distinct from all other known PRMTs implying that convergent evolution allowed this protein to develop the ability to methylate arginine residues and evolved elements conserved in PRMTs to accomplish this.

Diversity of Cl(-) channels

Cl(-) channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis of molecular biologic findings, ligand-gated Cl(-) channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed by bestrophin and possibly by tweety, which encode Ca(2+)-activated Cl(-) channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential, excitation, cell-volume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The molecular structure of Cl(-) channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl(-) channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or unidentified hydrophobic membrane proteins. Thus, novel Cl(-)-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic and pathophysiologic roles.

Protein interfaces in signaling regulated by arginine methylation

Posttranslational modifications are well-known effectors of signal transduction. Arginine methylation is a covalent modification that results in the addition of methyl groups to the nitrogen atoms of the arginine side chains. A probable role of arginine methylation in signal transduction is emerging with the identification of new arginine-methylated proteins. However, the functional consequences of arginine methylation and its mode of regulation remain unknown. The identification of the protein arginine methyltransferase family and the development of methylarginine-specific antibodies have raised renewed interest in this modification during the last decade. Arginine methylation was mainly observed on abundant proteins such as RNA-binding proteins and histones, but recent advances have revealed a plethora of arginine-methylated proteins implicated in a variety of cellular processes, including signaling by interferon and cytokines, and in T cell signaling. We discuss these recent advances and the role of arginine methylation in signal transduction.

Identification of Gastric Cancer–Related Genes Using a cDNA Microarray Containing Novel Expressed Sequence Tags Expressed in Gastric Cancer Cells

Purpose: Gastric cancer is one of the most frequently diagnosed malignancies in the world, especially in Korea and Japan. To understand the molecular mechanism associated with gastric carcinogenesis, we attempted to identify novel gastric cancer–related genes using a novel 2K cDNA microarray.

Experimental Design: A 2K cDNA microarray was fabricated from 1,995 novel expressed sequence tags (ESTs) showing no hits or a low homology with ESTs in public databases from our 143,452 ESTs collected from gastric cancer cell lines and tissues. An analysis of the gene expression for human gastric cancer cell lines to a normal cell line was done using this cDNA microarray. Data for the different expressed genes were verified using semiquantitative reverse transcription-PCR, Western blotting, and immunohistochemical staining in the gastric cell lines and tissues.

Results: Forty genes were identified as either up-regulated or down-regulated genes in human gastric cancer cells. Among these, genes such as SKB1, NT5C3, ZNF9, p30, CDC20, and FEN1, were confirmed to be up-regulated genes in nine gastric cell lines and in 25 pairs of tissue samples from patients by semiquantitative reverse transcription-PCR. On the other hand, genes such as MT2A and CXX1 were identified as down-regulated genes. In particular, the SKB1, CDC20, and FEN1 genes were overexpressed in ≥68% of tissues and the MT2A gene was down-expressed in 72% of the tissues. Western blotting and immunohistochemical analyses for CDC20 and SKB1 showed overexpression and localization changes of the corresponding protein in human gastric cancer tissues.

Conclusions: Novel genes that are related to human gastric cancer were identified using cDNA microarray developed in our laboratory. In particular, CDC20 and MT2A represent a potential biomarker of human gastric cancer. These newly identified genes should provide a valuable resource for understanding the molecular mechanism associated with tumorigenesis of gastric carcinogenesis and for the discovery of potential diagnostic markers of gastric cancer.

Human PRMT5 expression is enhanced during in vitro tubule formation and after in vivo ischemic injury in renal epithelial cells

BACKGROUND

The interactions between cells and the extracellular matrix (ECM) are important in the regulation of cell growth and differentiation. Cells cultured in ECM differentiate and develop tubular structures. The kidney has the ability to partially recover function after an ischemic insult through repairing its tubular epithelium. The factors that contribute to tubule formation in vitro may mediate tubule regeneration in the recovery stage of acute tubular necrosis.

METHODS

RNA purified from cells grown on plastic, on Matrigel and in Matrigel matrix were subjected to differential display analysis to identify the transcripts that are differentially expressed during in vitro tubulogenesis.

RESULTS

Protein arginine methyltransferase 5 (PRMT5) expression increased in renal epithelial cells undergoing tubule formation. PRMT5 expression is developmentally regulated and ubiquitously expressed in a variety of adult tissues. We also demonstrated that expression of PRMT5 is enhanced in the renal tubular epithelium of animals subjected to ischemic reperfusion injury (IRI).

CONCLUSION

The role of PRMT5 in the regulation of mitosis, its induction in renal epithelial cells undergoing tubule formation in vitro and its expression in the tubules of the kidneys subjected to IRI suggest that it functions in the regulation of cell growth and differentiation during tubule formation and regeneration.

Cell swelling stimulates cytosol to membrane transposition of ICln

ICln is a multifunctional protein that is essential for cell volume regulation. It can be found in the cytosol and is associated with the cell membrane. Besides its role in the splicing process, ICln is critically involved in the generation of ion currents activated during regulatory volume decrease after cell swelling (RVDC). If reconstituted in artificial bilayers, ICln can form ion channels with biophysical properties related to RVDC. We investigated (i) the cytosol versus cell membrane distribution of ICln in rat kidney tubules, NIH 3T3 fibroblasts, Madin-Darby canine kidney (MDCK) cells, and LLC-PK1 epithelial cells, (ii) fluorescence resonance energy transfer (FRET) in living fibroblasts between fluorescently tagged ICln and fluorochromes in the cell membrane, and (iii) possible functional consequences of an enhanced ICln presence at the cell membrane. We demonstrate that ICln distribution in rat kidneys depends on the parenchymal localization and functional state of the tubules and that cell swelling causes ICln redistribution from the cytosol to the cell membrane in NIH 3T3 fibroblasts and LLC-PK1 cells. The addition of purified ICln protein to the extracellular solution or overexpression of farnesylated ICln leads to an increased anion permeability in NIH 3T3 fibroblasts. The swelling-induced redistribution of ICln correlates to altered kinetics of RVDC in NIH 3T3 fibroblasts, LLC-PK1 cells, and MDCK cells. In these cells, RVDC develops more rapidly, and in MDCK cells the rate of swelling-induced depolarization is accelerated if cells are swollen for a second time. This coincides with an enhanced ICln association with the cell membrane.

Amazing chloride channels: an overview

AIM

This review describes molecular and functional properties of the following Cl- channels: the ClC family of voltage-dependent Cl- channels, the cAMP-activated transmembrane conductance regulator (CFTR), Ca2+ activated Cl- channels (CaCC) and volume-regulated anion channels (VRAC). If structural data are available, their relationship with the function of Cl- channels will be discussed. We also describe shortly some recently discovered channels, including high conductance Cl- channels and the family of bestrophins. We illustrate the growing physiological importance of these channels in the plasma membrane and in intracellular membranes, including their involvement in transepithelial transport, pH regulation of intracellular organelles, regulation of excitability and volume regulation. Finally, we discuss the role of Cl- channels in various diseases and describe the pathological phenotypes observed in knockout mice models.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

A novel WD repeat protein component of the methylosome binds Sm proteins

We have recently described a large (20 S) protein arginine methyltransferase complex, termed the methylosome, that contains the methyltransferase JBP1 (PRMT5) and the pICln protein. The methylosome functions to modify specific arginines to dimethylarginines in the arginine- and glycine-rich domains of several spliceosomal Sm proteins, and this modification targets these proteins to the survival of motor neurons (SMN) complex for assembly into small nuclear ribonucleoprotein (snRNP) core particles. Here, we describe a novel component of the methylosome, a 50-kilodalton WD repeat protein termed methylosome protein 50 (MEP50). We show that MEP50 is important for methylosome activity and binds to JBP1 and to a subset of Sm proteins. Because WD repeat proteins provide a platform for multiple protein interactions, MEP50 may function to mediate the interaction of multiple substrates with the methylosome. Interestingly, all of the known components of the methylosome bind Sm proteins, suggesting that in addition to producing properly methylated substrates for the SMN complex, the methylosome may be involved in Sm protein rearrangements or pre-assembly required for snRNP biogenesis.

The novel human protein arginine N-methyltransferase PRMT6 is a nuclear enzyme displaying unique substrate specificity

Protein arginine methylation is a prevalent posttranslational modification in eukaryotic cells that has been implicated in signal transduction, the metabolism of nascent pre-RNA, and the transcriptional activation processes. In searching the human genome for protein arginine N-methyltransferase (PRMT) family members, a novel gene has been found on chromosome 1 that encodes for an apparent methyltransferase, PRMT6. The polypeptide chain of PRMT6 is 41.9 kDa consisting of a catalytic core sequence common to other PRMT enzymes. Expressed as a glutathione S-transferase fusion protein, PRMT6 demonstrates type I PRMT activity, capable of forming both omega-N(G)-monomethylarginine and asymmetric omega-N(G),N(G)-dimethylarginine derivatives on the recombinant glycine- and arginine-rich substrate in a processive manner with a specific activity of 144 pmol methyl groups transferred min(-1) mg(-1) enzyme. A comparison of substrate specificity reveals that PRMT6 is functionally distinct from two previously characterized type I enzymes, PRMT1 and PRMT4. In addition, PRMT6 displays automethylation activity; it is the first PRMT to do so. This novel human PRMT, which resides solely in the nucleus when fused to the green fluorescent protein, joins a family of enzymes with diverse functions within cells.

The methylosome, a 20S complex containing JBP1 and pICln, produces dimethylarginine-modified Sm proteins

snRNPs, integral components of the pre-mRNA splicing machinery, consist of seven Sm proteins which assemble in the cytoplasm as a ring structure on the snRNAs U1, U2, U4, and U5. The survival motor neuron (SMN) protein, the spinal muscular atrophy disease gene product, is crucial for snRNP core particle assembly in vivo. SMN binds preferentially and directly to the symmetrical dimethylarginine (sDMA)-modified arginine- and glycine-rich (RG-rich) domains of SmD1 and SmD3. We found that the unmodified, but not the sDMA-modified, RG domains of SmD1 and SmD3 associate with a 20S methyltransferase complex, termed the methylosome, that contains the methyltransferase JBP1 and a JBP1-interacting protein, pICln. JBP1 binds SmD1 and SmD3 via their RG domains, while pICln binds the Sm domains. JBP1 produces sDMAs in the RG domain-containing Sm proteins. We further demonstrate the existence of a 6S complex that contains pICln, SmD1, and SmD3 but not JBP1. SmD3 from the methylosome, but not that from the 6S complex, can be transferred to the SMN complex in vitro. Together with previous results, these data indicate that methylation of Sm proteins by the methylosome directs Sm proteins to the SMN complex for assembly into snRNP core particles and suggest that the methylosome can regulate snRNP assembly.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Mass spectrometry for the study of protein-protein interactions

The identification of subpicomolar amounts of protein by mass spectrometry (MS) coupled with two-dimensional methods to separate complex protein mixtures is fueling the field of proteomics, and making feasible the notion of cataloging and comparing all of the expressed proteins in a biological sample. Functional proteomics is a complementary effort aimed at the characterization of functional features of proteins, such as their interactions with other proteins. Proteins comprise modular domains, many of which are noncatalytic modules that direct protein-protein interactions. Capturing proteins of interest and their interacting proteins by using high-affinity antibodies presents a simple method to prepare relatively simple protein mixtures easily resolved in one-dimensional formats. Individual or mixtures of proteins identified as stained bands in polyacrylamide gels are subjected to in situ digestion with the protease trypsin, and the extracted peptide fragments are analyzed by MS. The quality, quantity, and complexity of the tryptic digest, the species origin of the proteins, and the quality of the corresponding databases of genomic and protein information greatly influence the subsequent MS analysis in terms of degree of difficulty and methodological approach required to make an unambiguous protein identification. In this article we report the isolation of associated proteins from a complex cell-derived lysate by using an epitope-directed antibody. The protein pICLn engineered to carry an epitope tag was purified from cultured human embryonic kidney cells, and found to associate with a variety of proteins including the spliceosomal proteins smE and smG. By application of this general approach, the systematic identification of protein complexes and assignment of protein function are possible.

The highly conserved protein methyltransferase, Skb1, is a mediator of hyperosmotic stress response in the fission yeast Schizosaccharomyces pombe

The p21-activated kinase, Shk1, is required for cell viability, establishment and maintenance of cell polarity, and proper mating response in the fission yeast, Schizosaccharomyces pombe. Previous genetic studies suggested that a presumptive protein methyltransferase, Skb1, functions as a positive modulator of Shk1. However, unlike Shk1, Skb1 is not required for viability or mating of S. pombe cells and contributes only modestly to the regulation of cell morphology under normal growth conditions. Here we demonstrate that Skb1 plays a more significant role in regulating cell growth and polarity under conditions of hyperosmotic stress. We provide evidence that the inability of skb1Delta cells to properly maintain cell polarity in hyperosmotic conditions results from inefficient subcellular targeting of F-actin. We show that Skb1 localizes to cell ends, sites of septation, and nuclei of S. pombe cells. Hyperosmotic shock results in substantial delocalization of Skb1 from cell ends and nuclei, as well as stimulation of Skb1 protein methyltransferase activity. Taken together, our results demonstrate a new role for Skb1 as a mediator of hyperosmotic stress response in fission yeast. We show that the protein methyltransferase activity of the human Skb1 homolog, Skb1Hs, is also stimulated by hyperosmotic stress in fission yeast, providing evidence for evolutionary conservation of a role for Skb1-related proteins as mediators of hyperosmotic stress response, as well as mechanisms involved in regulating this novel class of protein methyltransferases.

Prmt5, which forms distinct homo-oligomers, is a member of the protein-arginine methyltransferase family

We found that JBP1, known as a human homolog (Skb1Hs) of Skb1 of fission yeast, interacts with NS3 of the hepatitis C virus in a yeast two-hybrid screen. Amino acid sequence analysis revealed that Skb1Hs/JBP1 contains conserved motifs of S-adenosyl-l-methionine-dependent protein-arginine methyltransferases (PRMTs). Here, we demonstrate that Skb1Hs/JBP1, named PRMT5, is a distinct member of the PRMT family. Recombinant PRMT5 protein purified from human cells methylated myelin basic protein, histone, and the amino terminus of fibrillarin fused to glutathione S-transferase. Myelin basic protein methylated by PRMT5 contained monomethylated and dimethylated arginine residues. Recombinant glutathione S-transferase-PRMT5 protein expressed in Escherichia coli also contained the catalytic activity. Sedimentation analysis of purified PRMT5 on a sucrose density gradient indicated that PRMT5 formed distinct homo-oligomeric complexes, including a dimer and tetramer, that comigrated with the enzyme activity. The PRMT5 homo-oligomers were dissociated into a monomer in the presence of a reducing agent, whereas a monomer, dimer, and multimer were detected in the absence or at low concentrations of a reducing agent. The results indicate that both covalent linkage by a disulfide bond and noncovalent association are involved in the formation of PRMT5 homo-oligomers. Western blot analysis of sedimentation fractions suggests that endogenous PRMT5 is present as a homo-oligomer in a 293T cell extract. PRMT5 appears to have lower specific enzyme activity than PRMT1. Although PRMT1 is known to be mainly located in the nucleus, human PRMT5 is predominantly localized in the cytoplasm.

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.

ICln is essential for cellular and early embryonic viability

pICln is a 26-kDa protein that is ubiquitously expressed and highly conserved from Xenopus laevis to Homo sapiens. The physiological functions of pICln remain to be established. To address this question, we disrupted the ICln gene in embryonic stem cells. We found that murine embryos lacking ICln die early in gestation (between stages E3.5 and E7.5). Furthermore, we found that ICln is essential for embryonic stem cell viability. Previously, we showed that pICln interacts directly with a homolog of a yeast protein that binds a PAK-like kinase and participates in the regulation of cell morphology and cell cycling. pICln also forms a complex with several core spliceosomal proteins, and this interaction may play a role in the regulation of spliceosomal biogenesis. Collectively, these data strongly suggest that pICln participates in critical cellular pathways, including regulation of the cell cycle and RNA processing.

Molecular cloning and characterization of a novel chloride intracellular channel-related protein, parchorin, expressed in water-secreting cells

We previously reported a 120-kDa phosphoprotein that translocated from cytosol to the apical membrane of gastric parietal cells in association with stimulation of HCl secretion. To determine the molecular identity of the protein, we performed molecular cloning and expression of the protein. Immunoblot analysis showed that this protein was highly enriched in tissues that secrete water, such as parietal cell, choroid plexus, salivary duct, lacrimal gland, kidney, airway epithelia, and chorioretinal epithelia. We named this protein "parchorin" based on its highest enrichment in parietal cells and choroid plexus. We obtained cDNA for parchorin from rabbit choroid plexus coding a protein consisting of 637 amino acids with a predicted molecular mass of 65 kDa. The discrepancy in size on 6% SDS-polyacrylamide gel electrophoresis is considered to be due to its highly acidic nature (pI = 4.18), because COS-7 cells transfected with parchorin cDNA produced a protein with apparent molecular mass of 120 kDa on 6% SDS-polyacrylamide gel electrophoresis. Parchorin is a novel protein that has significant homology to the family of chloride intracellular channels (CLIC), especially the chloride channel from bovine kidney, p64, in the C-terminal 235 amino acids. When expressed as a fusion protein with green fluorescent protein (GFP) in the LLC-PK1 kidney cell line, GFP-parchorin, unlike other CLIC family members, existed mainly in the cytosol. Furthermore, when Cl(-) efflux from the cell was elicited, GFP-parchorin translocated to the plasma membrane. These results suggest that parchorin generally plays a critical role in water-secreting cells, possibly through the regulation of chloride ion transport.

A peptide derived from pICln induced a strong hypotonic resistance in Escherichia coli cells

We demonstrated previously that expression of rat pICln in Escherichia coli conferred a strong resistance to hypotonic stress. To define the intramolecular functional domain responsible for the resistance, molecular dissection of pICln was performed and the obtained peptides were expressed in E. coli. The cells expressing the peptides were exposed to a hypotonic solution, and their 'survival rates' were observed. The cells expressing only the peptides including the second acidic domain of pICln exhibited significantly higher 'survival rates' after hypotonic stress. The functional domain against hypotonicity was finally narrowed down to a peptide consisting of a 46-amino acid residue, P107-152. We conclude that the expression of P107-152 in E. coli cells could enhance their resistance to a hypotonic environment.

Interaction of the somatostatin receptor subtype 1 with the human homolog of the Shk1 kinase-binding protein from yeast

Interaction between the C terminus of a G-protein-coupled receptor and intracellular constituents may represent a crucial step in regulating signal transduction. To identify potential interacting candidates the C terminus of the somatostatin receptor subtype 1 was used as bait in a yeast two hybrid screen of a human brain cDNA library. We identified the human Skb1 sequence (Skb1Hs) as interacting protein, which is homologous to the yeast protein known Skb1 to down-regulate mitosis in Schizosaccharomyces pombe via binding to the Shk1 protein kinase; the latter is a homolog to the mammalian p21(cdc42/Rac)-activated protein kinases. Interaction required almost the entire C terminus of the somatostatin receptor subtype 1 including the conserved NPXXY motif of transmembrane region seven; in the case of the Skb1Hs most of the N terminus and an S-adenosylmethionine binding domain were mandatory, whereas the C terminus was not essential. Interaction was verified by coexpression experiments in human embryonic kidney cells. As revealed by immunocytochemical analysis Skb1Hs expressed alone aggregates in large cytosolic clusters. When coexpressed, receptor subtype 1 and Skb1Hs were colocalized at the cell surface; these cells showed a strong increase in somatostatin binding compared with cells expressing the receptor only. This may suggest that Skb1Hs acts like a chaperone by correctly targeting the receptor to the cell surface.

The human homologue of the yeast proteins Skb1 and Hsl7p interacts with Jak kinases and contains protein methyltransferase activity

To expand our understanding of the role of Jak2 in cellular signaling, we used the yeast two-hybrid system to identify Jak2-interacting proteins. One of the clones identified represents a human homologue of the Schizosaccaromyces pombe Shk1 kinase-binding protein 1, Skb1, and the protein encoded by the Saccharomyces cerevisiae HSL7 (histone synthetic lethal 7) gene. Since no functional motifs or biochemical activities for this protein or its homologues had been reported, we sought to determine a biochemical function for this human protein. We demonstrate that this protein is a protein methyltransferase. This protein, designated JBP1 (Jak-binding protein 1), and its homologues contain motifs conserved among protein methyltransferases. JBP1 can be cross-linked to radiolabeled S-adenosylmethionine (AdoMet) and methylates histones (H2A and H4) and myelin basic protein. Mutants containing substitutions within a conserved region likely to be involved in AdoMet binding exhibit little or no activity. We mapped the JBP1 gene to chromosome 14q11.2-21. In addition, JBP1 co-immunoprecipitates with several other proteins, which serve as methyl group acceptors and which may represent physiological targets of this methyltransferase. Messenger RNA for JBP1 is widely expressed in human tissues. We have also identified and sequenced a homologue of JBP1 in Drosophila melanogaster. This report provides a clue to the biochemical function for this conserved protein and suggests that protein methyltransferases may have a role in cellular signaling.

Relationships between the actin cytoskeleton and cell volume regulation

The actin cytoskeleton mediates a variety of essential biological functions in cells, including division, shape changes, and movement. A number of studies have suggested that the abundant submembranous actin cytoskeleton present in the cortex of many cell types is involved in the regulation of cell volume. This relationship is supported by numerous works which document the changes in the structural organization of the actin cytoskeleton which accompany cell volume changes and the F-actin-dependence of the regulatory volume responses. In addition, other studies demonstrate structural and functional relationships between the actin cytoskeleton and the membrane transporters known to be involved in cell volume homeostasis. This review provides a summary of the current level of knowledge in this area and discusses the mechanisms which may underlie the linkage between the actin cytoskeleton and cell volume regulation.

Hsl7 localizes to a septin ring and serves as an adapter in a regulatory pathway that relieves tyrosine phosphorylation of Cdc28 protein kinase in Saccharomyces cerevisiae

Successful mitosis requires faithful DNA replication, spindle assembly, chromosome segregation, and cell division. In the budding yeast Saccharomyces cerevisiae, the G(2)-to-M transition requires activation of Clb-bound forms of the protein kinase, Cdc28. These complexes are held in an inactive state via phosphorylation of Tyr19 in the ATP-binding loop of Cdc28 by the Swe1 protein kinase. The HSL1 and HSL7 gene products act as negative regulators of Swe1. Hsl1 is a large (1,518-residue) protein kinase with an N-terminal catalytic domain and a very long C-terminal extension. Hsl1 localizes to the incipient site of cytokinesis in the bud neck in a septin-dependent manner; however, the function of Hsl7 was not previously known. Using both indirect immunofluorescence with anti-Hsl7 antibodies and a fusion of Hsl7 to green fluorescent protein, we found that Hsl7 also localizes to the bud neck, congruent with the septin ring that faces the daughter cell. Both Swe1 and a segment of the C terminus of Hsl1 (which has no sequence counterpart in two Hsl1-related protein kinases, Gin4 and Kcc4) were identified as gene products that interact with Hsl7 in a two-hybrid screen of a random S. cerevisiae cDNA library. Hsl7 plus Swe1 and Hsl7 plus Hsl1 can be coimmunoprecipitated from extracts of cells overexpressing these proteins, confirming that Hsl7 physically associates with both partners. Also consistent with the two-hybrid results, Hsl7 coimmunoprecipitates with full-length Hsl1 less efficiently than with a C-terminal fragment of Hsl1. Moreover, Hsl7 does not localize to the bud neck in an hsl1Delta mutant, whereas Hsl1 is localized normally in an hsl7Delta mutant. Phosphorylation and ubiquitinylation of Swe1, preludes to its destruction, are severely reduced in cells lacking either Hsl1 or Hsl7 (or both), as judged by an electrophoretic mobility shift assay. Collectively, these data suggest that formation of the septin rings provides sites for docking Hsl1, exposing its C terminus and thereby permitting recruitment of Hsl7. Hsl7, in turn, presents its cargo of bound Swe1, allowing phosphorylation by Hsl1. Thus, Hsl1 and Hsl7 promote proper timing of cell cycle progression by coupling septin ring assembly to alleviation of Swe1-dependent inhibition of Cdc28. Furthermore, like septins and Hsl1, homologs of Hsl7 are found in fission yeast, flies, worms, and humans, suggesting that its function in this control mechanism may be conserved in all eukaryotes.

The morphogenesis checkpoint in Saccharomyces cerevisiae: cell cycle control of Swe1p degradation by Hsl1p and Hsl7p

In Saccharomyces cerevisiae, the Wee1 family kinase Swe1p is normally stable during G(1) and S phases but is unstable during G(2) and M phases due to ubiquitination and subsequent degradation. However, perturbations of the actin cytoskeleton lead to a stabilization and accumulation of Swe1p. This response constitutes part of a morphogenesis checkpoint that couples cell cycle progression to proper bud formation, but the basis for the regulation of Swe1p degradation by the morphogenesis checkpoint remains unknown. Previous studies have identified a protein kinase, Hsl1p, and a phylogenetically conserved protein of unknown function, Hsl7p, as putative negative regulators of Swe1p. We report here that Hsl1p and Hsl7p act in concert to target Swe1p for degradation. Both proteins are required for Swe1p degradation during the unperturbed cell cycle, and excess Hsl1p accelerates Swe1p degradation in the G(2)-M phase. Hsl1p accumulates periodically during the cell cycle and promotes the periodic phosphorylation of Hsl7p. Hsl7p can be detected in a complex with Swe1p in cell lysates, and the overexpression of Hsl7p or Hsl1p produces an effective override of the G(2) arrest imposed by the morphogenesis checkpoint. These findings suggest that Hsl1p and Hsl7p interact directly with Swe1p to promote its recognition by the ubiquitination complex, leading ultimately to its destruction.

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.

Functional and molecular characterization of a volume-sensitive chloride current in rat brain endothelial cells

1. Volume-activated chloride currents in cultured rat brain endothelial cells were investigated on a functional level using the whole-cell voltage-clamp technique and on a molecular level using the reverse transcriptase-polymerase chain reaction (RT-PCR). 2. Exposure to a hypotonic solution caused the activation of a large, outward rectifying current, which exhibited a slight time-dependent decrease at strong depolarizing potentials. The anion permeability of the induced current was I- (1.7) > Br- (1.2) > Cl- (1.0) > F- (0. 7) > gluconate (0.18). 3. The chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 microM) rapidly and reversibly inhibited both inward and outward currents. The chloride transport blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS, 100 microM) also blocked the hypotonicity-induced current in a reversible manner. In this case, the outward current was more effectively suppressed than the inward current. The volume-activated current was also inhibited by the antioestrogen tamoxifen (10 microM). 4. The current was dependent on intracellular ATP and independent of intracellular Ca2+. 5. Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu, 100 nM) inhibited the increase in current normally observed following hypotonic challenge. 6. Extracellular ATP (10 mM) inhibited the current with a more pronounced effect on the outward than the inward current. 7. Verapamil (100 microM) decreased both the inward and the outward hypotonicity-activated chloride current. 8. RT-PCR analysis was used to determine possible molecular candidates for the volume-sensitive current. Expression of the ClC-2, ClC-3 and ClC-5 chloride channels, as well as pICln, could be shown at the mRNA level. 9. We conclude that rat brain endothelial cells express chloride channels which are activated by osmotic swelling. The biophysical and pharmacological properties of the current show strong similarities to those of ClC-3 channel currents as described in other cell types.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Molecular cloning and characterization of a mitogen-activated protein kinase-associated intracellular chloride channel

ERK7, a member of the mitogen-activated protein kinase family, has a carboxyl-terminal tail that is required for ERK7 activation, cellular localization, and its ability to inhibit DNA synthesis. To identify proteins that interact with ERK7, we utilized a yeast two-hybrid screen with the COOH-terminal tail of ERK7 as bait and isolated the cDNA for a novel protein termed CLIC3. The interaction between CLIC3 and ERK7 in mammalian cells was confirmed by co-immunoprecipitation. CLIC3 has significant homology to human intracellular chloride channels 1 (NCC27/CLIC1) and 2 and bovine kidney chloride channel p64. Like NCC27/CLIC1, CLIC3 is predominantly localized in the nucleus and stimulates chloride conductance when expressed in cells. Taken together, these results suggest that CLIC3 is a new member of the human CLIC family. The observed interaction between CLIC3 and ERK7 is the first demonstration of a stable complex between a protein that activates chloride ion transport and a member of the mitogen-activated protein kinase family of signal transducers. The specific association of CLIC3 with the COOH-terminal tail of ERK7 suggests that CLIC3 may play a role in the regulation of cell growth.

Negative regulation of mitosis in fission yeast by the shk1 interacting protein skb1 and its human homolog, Skb1Hs

We previously provided evidence that the protein encoded by the highly conserved skb1 gene is a putative regulator of Shk1, a p21(Cdc42/Rac)-activated kinase (PAK) homolog in the fission yeast Schizosaccharomyces pombe. skb1 null mutants are viable and competent for mating but less elongate than wild-type S. pombe cells, whereas cells that overexpress skb1 are hyperelongated. These phenotypes suggest a possible role for Skb1 as a mitotic inhibitor. Here we show genetic interactions of both skb1 and shk1 with genes encoding key mitotic regulators in S. pombe. Our results indicate that Skb1 negatively regulates mitosis by a mechanism that is independent of the Cdc2-activating phosphatase Cdc25 but that is at least partially dependent on Shk1 and the Cdc2 inhibitory kinase Wee1. We provide biochemical evidence for association of Skb1 and Shk1 with Cdc2 in S. pombe, suggesting that Skb1 and Shk1 inhibit mitosis through interaction with the Cdc2 complex, rather than by an indirect mechanism. These results provide evidence of a previously undescribed role for PAK-related protein kinases as mitotic inhibitors. We also describe the cloning of a human homolog of skb1, SKB1Hs, and show that it can functionally replace skb1 in S. pombe. Thus, the molecular functions of Skb1-related proteins have likely been substantially conserved through evolution.

Characterization of pI(Cln) binding proteins: identification of p17 and assessment of the role of acidic domains in mediating protein-protein interactions

pICln is a ubiquitous and abundant 27 kDa soluble protein that is localized primarily to the cytoplasm. The protein has been proposed to be a swelling-activated anion channel or a channel regulator. Recent studies, however, have cast significant doubt on these hypotheses, and the function of pI(Cln) therefore remains unknown. To further characterize the physiological role of pI(Cln), we have begun to identify the proteins that bind to it and the amino acid domains that mediate pICln protein-protein interactions. Using affinity assays and immunoprecipitation we have identified three proteins in C6 glioma cells with molecular masses of 17 kDa, 29 kDa and 72 kDa that bind selectively to pI(Cln). Microsequencing revealed that p17 is the non-muscle isoform of the alkali myosin light chain. pI(Cln) contains three acidic amino acid domains termed AD1, AD2 and AD3. Mutation of AD1 and/or AD2 had no effect on p17, p29 and p72 binding. However, binding of p72 was lost when four acidic amino acid residues were mutated in AD3, which is located at the carboxy terminus. A truncation peptide containing the last 29 amino acids of pI(Cln) was able to bind p72 normally. These results indicate that the carboxy terminus is necessary for p72-pI(Cln) interaction. Based on these and other findings, we propose that pI(Cln) is a protein responsible for regulating the structure and function of the cytoskeleton, and/or a protein involved in mediating interactions between components of intracellular signal transduction pathways.