Phosphorylation of lens intrinsic membrane proteins by protein kinase C

The role of phosphorylation in calmodulin-mediated gating of human AQP0

Aquaporin-0 (AQP0) is the main water channel in the mammalian lens and is involved in accommodation and maintaining lens transparency. AQP0 binds the Ca2+-sensing protein calmodulin (CaM) and this interaction is believed to gate its water permeability by closing the water-conducting pore. Here, we express recombinant and functional human AQP0 in Pichia pastoris and investigate how phosphorylation affects the interaction with CaM in vitro as well as the CaM-dependent water permeability of AQP0 in proteoliposomes. Using microscale thermophoresis and surface plasmon resonance technology we show that the introduction of the single phospho-mimicking mutations S229D and S235D in AQP0 reduces CaM binding. In contrast, CaM interacts with S231D with similar affinity as wild type, but in a different manner. Permeability studies of wild-type AQP0 showed that the water conductance was significantly reduced by CaM in a Ca2+-dependent manner, whereas AQP0 S229D, S231D and S235D were all locked in an open state, insensitive to CaM. We propose a model in which phosphorylation of AQP0 control CaM-mediated gating in two different ways (1) phosphorylation of S229 or S235 abolishes binding (the pore remains open) and (2) phosphorylation of S231 results in CaM binding without causing pore closure, the functional role of which remains to be elucidated. Our results suggest that site-dependent phosphorylation of AQP0 dynamically controls its CaM-mediated gating. Since the level of phosphorylation increases towards the lens inner cortex, AQP0 may become insensitive to CaM-dependent gating along this axis.

Regulation of AQP0 water permeability is enhanced by cooperativity

Aquaporin 0 (AQP0), essential for lens clarity, is a tetrameric protein composed of four identical monomers, each of which has its own water pore. The water permeability of AQP0 expressed in Xenopus laevis oocytes can be approximately doubled by changes in calcium concentration or pH. Although each monomer pore functions as a water channel, under certain conditions the pores act cooperatively. In other words, the tetramer is the functional unit. In this paper, we show that changes in external pH and calcium can induce an increase in water permeability that exhibits either a positive cooperativity switch-like increase in water permeability or an increase in water permeability in which each monomer acts independently and additively. Because the concentrations of calcium and hydrogen ions increase toward the center of the lens, a concentration signal could trigger a regulatory change in AQP0 water permeability. It thus seems plausible that the cooperative modes of water permeability regulation by AQP0 tetramers mediated by decreased pH and elevated calcium are the physiologically important ones in the living lens.

L-type calcium channels play a critical role in maintaining lens transparency by regulating phosphorylation of aquaporin-0 and myosin light chain and expression of connexins

Homeostasis of intracellular calcium is crucial for lens cytoarchitecture and transparency, however, the identity of specific channel proteins regulating calcium influx within the lens is not completely understood. Here we examined the expression and distribution profiles of L-type calcium channels (LTCCs) and explored their role in morphological integrity and transparency of the mouse lens, using cDNA microarray, RT-PCR, immunoblot, pharmacological inhibitors and immunofluorescence analyses. The results revealed that Ca (V) 1.2 and 1.3 channels are expressed and distributed in both the epithelium and cortical fiber cells in mouse lens. Inhibition of LTCCs with felodipine or nifedipine induces progressive cortical cataract formation with time, in association with decreased lens weight in ex-vivo mouse lenses. Histological analyses of felodipine treated lenses revealed extensive disorganization and swelling of cortical fiber cells resembling the phenotype reported for altered aquaporin-0 activity without detectable cytotoxic effects. Analysis of both soluble and membrane rich fractions from felodipine treated lenses by SDS-PAGE in conjunction with mass spectrometry and immunoblot analyses revealed decreases in β-B1-crystallin, Hsp-90, spectrin and filensin. Significantly, loss of transparency in the felodipine treated lenses was preceded by an increase in aquaporin-0 serine-235 phosphorylation and levels of connexin-50, together with decreases in myosin light chain phosphorylation and the levels of 14-3-3ε, a phosphoprotein-binding regulatory protein. Felodipine treatment led to a significant increase in gene expression of connexin-50 and 46 in the mouse lens. Additionally, felodipine inhibition of LTCCs in primary cultures of mouse lens epithelial cells resulted in decreased intracellular calcium, and decreased actin stress fibers and myosin light chain phosphorylation, without detectable cytotoxic response. Taken together, these observations reveal a crucial role for LTCCs in regulation of expression, activity and stability of aquaporin-0, connexins, cytoskeletal proteins, and the mechanical properties of lens, all of which have a vital role in maintaining lens function and cytoarchitecture.

Human telomerase reverse transcriptase immortalizes bovine lens epithelial cells and suppresses differentiation through regulation of the ERK signaling pathway

Telomerase is a specialized reverse transcriptase that extends telomeres of eukaryotic chromosomes. The functional telomerase complex contains a telomerase reverse transcriptase catalytic subunit and a telomerase template RNA. We have previously demonstrated that human telomerase reverse transcriptase (hTERT) catalytic subunit is functionally compatible with a telomerase template RNA from rabbit. In this study, we show that hTERT is also functionally compatible with a telomerase template RNA from bovine. Introduction of hTERT into bovine lens epithelial cells (BLECs) provides the transfected cells telomerase activity. The expressed hTERT in BLECs supports normal growth of the transfected cells for 108 population doublings so far, and these cells are still extremely healthy in both morphology and growth. In contrast, the vector-transfected cells display growth crisis after 20 population doublings. These cells run into cellular senescence due to shortening of the telomeres and also commit differentiation as indicated by the accumulation of the differentiation markers, beta-crystallin and filensin. hTERT prevents the occurrence of both events. By synthesizing new telomere, hTERT prevents replicative senescence, and through regulation of MEK/ERK, protein kinase C, and protein kinase A and eventual suppression of the MEK/ERK signaling pathway, hTERT inhibits differentiation of BLECs. Our finding that hTERT can suppress RAS/RAF/MEK/ERK signaling pathway to prevent differentiation provides a novel mechanism to explain how hTERT regulates cell differentiation.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

The channel architecture of aquaporin 0 at a 2.2-A resolution

We determined the x-ray structure of bovine aquaporin 0 (AQP0) to a resolution of 2.2 A. The structure of this eukaryotic, integral membrane protein suggests that the selectivity of AQP0 for water transport is based on the identity and location of signature amino acid residues that are hallmarks of the water-selective arm of the AQP family of proteins. Furthermore, the channel lumen is narrowed only by two, quasi-2-fold related tyrosine side chains that might account for reduced water conductance relative to other AQPs. The channel is functionally open to the passage of water because there are eight discreet water molecules within the channel. Comparison of this structure with the recent electron-diffraction structure of the junctional form of sheep AQP0 at pH 6.0 that was interpreted as closed shows no global change in the structure of AQP0 and only small changes in side-chain positions. We observed no structural change to the channel or the molecule as a whole at pH 10, which could be interpreted as the postulated pH-gating mechanism of AQP0-mediated water transport at pH >6.5. Contrary to the electron-diffraction structure, the comparison shows no evidence of channel gating induced by association of the extracellular domains of AQP0 at pH 6.0. Our structure aids the analysis of the interaction of the extracellular domains and the possibility of a cell-cell adhesion role for AQP0. In addition, our structure illustrates the basis for formation of certain types of cataracts that are the result of mutations.

Identification of protein phosphatase 2C and confirmation of other protein phosphatases in the ocular lenses

The reversible phosphorylation of proteins plays essential roles in regulating various cellular events, and is regulated by the opposing actions of protein kinases and protein phosphatases. Protein kinases in the lens system have been well studied, but very little is known about lens protein phosphatases. Protein phosphatases can be divided several families, such as protein phosphatase types 1, 2A, 2B and 2C (PP1, PP2A, PP2B and PP2C) and protein tyrosine phosphatases (PTP). In this study we evaluated what kinds of protein phosphatases are present in the lens by using various specific substrates and inhibitors. Samples were prepared from lenses of 17-day-old chick embryos, and fractionated by high-resolution gel permeation column chromatography, then the fractions were assayed for phosphatase activities. The results with 32P-labeled glycogen phosphorylase A, okadaic acid and inhibitor-1, which are a specific substrate and inhibitors of PP1 and/or PP2A, showed that PP1activities were present in the 500-, 115- and 45-kDa fractions of the lens protein. The 115-kDa fraction also contained PP2A activity. By using a phosphothreonine-containing peptide as a substrate, three peaks of phosphatase activities were found at around 115, 55 and 35 kDa. Based on their response to various phosphatase inhibitors and their metal dependency, the fractions of 115 and 35 kDa were concluded to contain PP2A, while the 55-kDa fraction contained PP2C. Immunoblot using specific antibodies against PP1, PP2A and PP2C confirmed that each fraction above contained corresponding protein phosphatases as proteins. When a phosphotyrosine-containing peptide substrate was examined at pH 7.4, we observed a major peak at 500 kDa, which was presumed to contain receptor-like PTP(s). On the other hand, at pH 5.5, we observed a peak of 18 kDa, which was confirmed to contain a low-molecular-weight PTP. These protein phosphatases have recently been suggested to be involved in stress response and apoptosis. Their physiological roles in the lens are of much interest.

Predominant phosphatase in the ocular lens regulated by physiological concentrations of magnesium and calcium

A magnesium-dependent phosphatase with a molecular weight of about 55 kDa was found in the lens of chicken embryo, mouse, rabbit and bovine. It appears to be unique to the lens and, when activated by magnesium, accounts for the majority of the phosphatase activity in the lens. Phosphatases in the lens were separated by using high-resolution gel permeation column chromatography, and phosphatase activity was determined with p-nitrophenyl phosphate (pNPP) as a substrate. The 55-kDa phosphatase showed very low basal activity, but the activity was increased concentration-dependently by magnesium ion (Mg(2+)), and at 1 mM Mg(2+), this enzyme accounted for over 50% of the total phosphatase activity in the lens. Calcium potently inhibited the magnesium-activated phosphatase activity in a dose-dependent (IC(50); about 50 microM), uncompetitive manner. The phosphatase activity was high in the acidic pH range, with an optimum pH value of 5.5. The characteristics of the lens Mg(2+)-dependent phosphatase, such as substrate specificity and sensitivity to various phosphatase inhibitors, showed little similarity to those of any reported phosphatase. The Mg(2+) and Ca(2+) concentrations that were found to be effective in this study are similar to physiological concentrations in the lens; therefore it is likely that these ions physiologically regulate the 55-kDa phosphatase activity in the lens.

Characterization of the ovine-lens plasma-membrane protein-kinase substrates

The cAMP-dependent protein-kinase-catalyzed phosphorylation of the two major intrinsic lens fiber cell plasma membrane proteins, MP20 and MP26, is likely restricted to the inner cortical and nuclear regions of the lens in vivo. The ovine-lens-specific connexin, MP70, that has been identified as Cx50 in mice and Cx45.6 in the chick, is also a protein kinase substrate although it does not appear to be phosphorylated by a number of protein kinases including cAMP-dependent protein kinase, calmodulin-dependent protein kinase or protein kinase C. Rather, an extrinsic lens membrane fraction was isolated which contained protein kinase activity that catalyzed the phosphorylation of MP70; this protein kinase activity was cAMP-independent, Ca(2+)-independent, Mg(2+)-dependent, phosphorylated MP70 on a serine residue(s) and migrated with a molecular mass of 35 kDa on a gel filtration column. Both MP70 phosphorylation and the endogenous protein kinase activity were restricted to the lens outer cortical region. This membrane-associated protein kinase activity represents the first reported partial characterization of an endogenous lens fiber cell protein kinase activity that catalyzes the phosphorylation of a lens connexin protein. The phosphatase-induced shift in the electrophoretic mobility of MP70 is not reversed by this protein kinase, indicating that MP70 is likely phosphorylated on different residues by two or more protein kinases.

Phosphorylation of nodulin 26 on serine 262 affects its voltage-sensitive channel activity in planar lipid bilayers

Nodulin 26 is an symbiosome membrane protein of soybean nodules that shows ion channel activity in planar lipid bilayers. Serine 262 of nodulin 26 is phosphorylated by calmodulin-like domain protein kinase. To study the effects of phosphorylation, nodulin 26 with Ser, Ala, or Asp at position 262 were expressed in Escherichia coli. The expressed protein possessed a histidine-rich leader sequence for purification by Ni2+ chelate fast protein liquid chromatography. Upon reconstitution into planar lipid bilayers, the recombinant proteins showed a large single channel conductance (3.1 nanosiemens (nS) in cis0.2M/trans1.0 M KCl and 1.6 nS in cis 0.2M/trans0.2 M KCl) and weak anion selectivity, similar to native soybean nodulin 26. Nodulin 26 with Ser- or Ala-262 occupied the maximal open conductance state greater than 97% of the time (3.1 nS in cis0.2M/trans1.0 M KCl) regardless of applied voltage. However, nodulin 26 with Asp-262 showed increased gating and preferential occupancy of lower subconductance states (1.8 and 0.6 nS in cis0.2M/trans1.0 M KCl) at high applied voltages (e.g. 70 mV). In situ phosphorylation of Ser-262 of nodulin 26 by calmodulin-like domain protein kinase also resulted in increased voltage-dependent gating and preferential occupancy of lower subconductance states. These results suggest that phosphorylation of serine 262 of nodulin 26 modulates channel activity by conferring voltage sensitivity.

Protein kinase C activity and its relationship to myo-inositol uptake during hyperglycemic conditions in cultured bovine lens epithelial cells

Incubation of cultured bovine lens epithelial cells (BLECs) in minimal essential medium (MEM) containing 40 mM galactose for 20 hr results in an attenuation of 3H-myo-inositol (3H-MI) concentrating ability. Decreased MI uptake could negatively impact on normal phosphoinositide turnover and diacylglycerol production, and presumably, protein kinase C (PKC) activation. The present report examines the relationship between PKC activity, myo-inositol transport and hyperglycemic conditions. PKC activities in the cytosol and particulate fractions of bovine lens epithelial cells in culture were quantitated using a mixed micelle assay following DEAE-cellulose (DE52) and Sephadex G-25 chromatography. Protein kinase C activity was assessed as Ca2+ and phospholipid-dependent Ac-myelin basic protein substrate peptide phosphorylation and confirmed using a PKC pseudosubstrate inhibitor peptide (PKC 19-36). Total PKC activity was similar in galactose-incubated cells (871 +/- 64 pmol/mg total protein/min) and control cells (881 +/- 8 pmol/mg total protein/min) after 20 hr. In unstimulated cells, approximately 90% of the total cellular PKC activity was recovered in the cytosolic fraction. Enzyme translocation was induced with the tumor promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), resulting in a 6-fold increase in membrane-associated PKC activity. A similar PMA-induced translocation was observed in BLECs incubated with 40 mM galactose MEM-maintained cells briefly treated with PMA or the non-phorbol PKC activators, SC-10 and mezerein, displayed a rate of 3H-MI uptake similar to the untreated control cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C in galactosemic and tolrestat-treated lens epithelial cells

Using isozyme-specific anti-peptide antisera against peptides from the alpha-, beta-, gamma-, delta-, epsilon-, and zeta-isoforms of brain protein kinase C (PKC), we have identified proteins in bovine lens epithelial cells, in culture, that were reactive with these antisera. Western blots of lens epithelial cell homogenates showed that PKC-alpha antisera reacted with a major protein, and PKC-gamma antisera reacted with a minor protein. When the lens epithelial cells were cultured in media supplemented with 40 mM galactose, to model the conditions of sugar cataracts, a decrease in PKC-gamma, but not in PKC-alpha was observed. These were normalized if the cells were cultured in 40 mM galactose media supplemented with an inhibitor of aldose reductase, Tolrestat (10 microM). These results suggest that changes in PKC isoforms occur in the galactosemic diabetic state.

The MIP family of integral membrane channel proteins: sequence comparisons, evolutionary relationships, reconstructed pathway of evolution, and proposed functional differentiation of the two repeated halves of the proteins

The major intrinsic protein (MIP) of the bovine lens fiber cell membrane was the first member of the MIP family of proteins to be sequenced and characterized. It is probably a homotetramer with transmembrane channel activity that plays a role in lens biogenesis or maintenance. The polypeptide chain of each subunit may span the membrane six times, and both the N- and C-termini face the cell cytoplasm. Eighteen sequenced or partially sequenced proteins from bacteria, yeast, plants, and animals have now been shown to be members of the MIP family. These proteins appear to function in (1) metazoan development and neurogenesis (MIP and BIB), (2) water transport across the human erythrocyte membrane (ChIP), (3) communication between host plant cells and symbiotic nitrogen-fixing bacteria (NOD), (4) transport across the tonoplast membrane during plant seed development (alpha-TIP), (5) water stress-induced resistance to desiccation in plants (Wsi-TIP), (6) suppression of a genetic growth defect on fermentable sugars in yeast (FPS1), and (7) transport of glycerol across bacterial cell membranes (GlpF). One other sequenced member of the MIP family (ORF1 of Lactococcus lactis) has no known physiological function. The biochemical functions of the eukaryotic proteins are not well established. Computer analyses have revealed that the first and second halves of all MIP family proteins probably arose by a tandem, intragenic, duplication event. Thus, the primary structure of putative transmembrane helices 1 to 3 is similar to that of putative transmembrane helices 4 to 6 even though they are of opposite orientation in the membrane. Among the most conserved residues in these two repeated halves are a membrane-embedded glutamate (E) in helices 1 and 4, an asparagine-proline-alanine (NPA) sequence in the loops between helices 2 and 3 (cytoplasmically localized) and helices 5 and 6 (extracellularly localized), and a glycine within helices 3 and 6. Statistical analyses suggest that the two halves of these proteins have evolved to serve distinct functions: the first half is more important for the generalized or common functions of these proteins, while the second half of these proteins is more differentiated to provide specific or dissimilar functions of the proteins. The apparent origin of MIP family proteins by duplication of a three-spanner precursor protein suggests an evolutionary origin distinct from other transport proteins with six transmembrane spanners.(ABSTRACT TRUNCATED AT 400 WORDS)

Assignment of the lens intrinsic membrane protein MP19 structural gene to human chromosome 19

We have isolated and characterized a bovine cDNA clone encoding the bovine lens intrinsic membrane protein, MP19. This cDNA was used as a probe to analyze a panel of Southern blots of human-Chinese hamster somatic cell hybrid DNAs to assign the gene coding for MP19 to its human chromosome. Control human and Chinese hamster DNAs displayed a distinct EcoR1 restriction fragment pattern when hybridized with the bovine MP19 cDNA. When somatic cell hybrid DNAs were restricted with Eco R1 and Southern blots hybridized with the bovine MP19 cDNA, the characteristic human restriction pattern was observed only when human chromosome 19 was present in the hybrid panel. This assignment was confirmed using a human chromosome 19-specific genomic library. A clone from this human chromosome 19-specific library was identified and further characterized. This clone contained a 7.9 kilobase fragment that contained identical DNA sequences with that of the authentic bovine MP19 cDNA, and with a separate human genomic clone containing the MP19 gene.

Phosphorylation modulates the voltage dependence of channels reconstituted from the major intrinsic protein of lens fiber membranes

Major intrinsic polypeptide (MIP), a 28-kDa protein isolated from lens fiber cell membranes, forms large, nonselective channels when reconstituted into lipid bilayers. MIP channels are regulated by voltage, such that these channels close when the potential across the membrane is greater than 30 mV. We have investigated the modulation of the voltage-dependent closure of MIP channels by phosphorylation. In this report, we describe the isolation of two isomers of MIP from lens fiber cell membranes. These isomers differ by a single phosphate at a protein kinase A phosphorylation site. The phosphorylated isomer produces channels that close in response to applied voltages when reconstituted into bilayers. The nonphosphorylated isomer produces voltage-independent channels. Direct phosphorylation with protein kinase A converts voltage-independent channels to voltage-dependent channels in situ. Analyses of macroscopic and single-channel currents suggest that phosphorylation increases the voltage-dependent closure of MIP channels by increasing closed channel lifetimes and the rate of channel closure following the application of voltage.

Protein phosphorylation stimulates the rate of malate uptake across the peribacteroid membrane of soybean nodules

Incubation of intact isolated symbiosomes with [gamma-32P]ATP, followed by isolation of the peribacteroid membrane and polypeptide analysis, showed that a single major polypeptide at 26 kDa was labelled. Antibodies raised against nodulin 26 reacted with a similar sized polypeptide. Incubation of the symbiosomes with alkaline phosphatase removed the label from this polypeptide. Pre-incubation with ATP stimulated malate accumulation by isolated symbiosomes, but only slightly (10-30%). Pre-treatment of symbiosomes with alkaline phosphatase inhibited malate uptake substantially and this inhibition was completely relieved by addition of ATP. The ATP stimulation of malate uptake was not affected by ATPase inhibitors. It is suggested that the rate of malate uptake across the peribacteroid membrane is controlled by phosphorylation of nodulin 26.

MP26, a protein of intercellular junctions in the bovine lens: electrophoretic and chromatographic characterization

We have characterized the membrane protein of apparent molecular weight 26 kD from bovine lenses (MP26 or MIP) with respect to six different electrophoretic and chromatographic procedures. These include one- and two-dimensional gel electrophoretic procedures, as well as SDS-hydroxylapatite chromatography. The two-dimensional gels include isoelectric focusing with both conventional ampholytes and buffer focusing methods. With buffer focusing, the membranes are solubilized without the use of SDS and the isoelectric focusing is performed in the absence of SDS. As specific probes for MP26, a monoclonal antibody and an anti-MP26 rabbit serum were used, the latter prepared against electrophoretically purified MP26. These separation techniques were adapted to MP26 in order to permit a more detailed characterization of this protein and to search for any heterogeneity in this size range, specifically other junctional proteins or protein fragments. We have found evidence for charge heterogeneity in MP26, but no evidence for multiple membrane proteins of Mr 26,000 in urea-treated membranes. The charge heterogeneity appears to be related to a phosphorylation of MP26. The results reported here aid the interpretation of a variety of data, especially findings on the reconstitution of MP26 in artificial membranes and results from work with polyclonal MP26 antibodies. These investigations are all designed to evaluate the proposed role of MP26 as a protein of cell-to-cell channels in the lens fiber cell.

Chemical phosphorylation of proteins by zinc-ATP

During an examination of in vitro phosphorylation of the adipocyte lipid-binding protein (ALBP) by the insulin receptor, we detected insulin receptor-independent, chemical phosphorylation of ALBP. This activity was present in ALBP purified to homogeneity from murine 3T3-L1 cells and in recombinant murine ALBP purified from expressing E. coli cultures. Phosphoamino acid analysis revealed that chemical phosphorylation of ALBP occurred primarily on Ser residues. The phosphorylation activity occurred in the alkaline pH range from 8 to 11 and exhibited a broad temperature dependence. The reaction rate was linearly dependent upon the ATP concentration and exhibited a biphasic kinetic profile. Eight of twelve other proteins tested also underwent chemical phosphorylation. Zn+2, Mg+2, or Mn+2 promoted optimal phosphorylation of different proteins. We conclude that many proteins are capable of undergoing chemical phosphorylation.

Increase in pancreatic exocrine secretion during uncoupling: evidence for a protein kinase C-independent effect

It has been demonstrated that blockade of the normal communication between pancreatic acinar cells leads to an increase in amylase release. Although the physiological mechanisms that regulate the gating of gap junction channels are unknown, the involvement of protein kinase C (PKC) in the inhibition of cell coupling has been reported in various cell lines. Since the activation of PKC also stimulates amylase secretion of pancreatic acinar cells, we sought to determine whether blockers of gap junctions and activators of PKC modify basal secretion by a similar mechanism. Thus, we have studied the effects of heptanol and of 12-O-tetradecanoylphorbol-13-acetate (TPA) on the subcellular distribution of PKC, dye coupling, and amylase release of dispersed pancreatic acini. The data show that TPA activates PKC and stimulates amylase secretion without affecting the extensive dye coupling of acinar cells. By contrast, heptanol inhibits cell-to-cell coupling and increases enzyme output without altering the subcellular distribution of PKC. Heptanol also enhances significantly the secretion evoked by TPA. These results indicate that the stimulation of amylase release caused by uncoupling of acinar cells occurs by a mechanism(s) that does not involve the activation of PKC.

Inhibition of electrical coupling in pairs of murine pancreatic acinar cells by OAG and isolated protein kinase C

Gap junctional coupling was studied in pairs of murine pancreatic acinar cells using the double whole-cell patch-clamp technique. During stable electrical coupling, addition of OAG (1-oleoyl-2-acetyl-sn-glycerol) induced a progressive reduction of the junctional conductance to the detectable limit (approximately 3 pS). Prior to complete electrical uncoupling, various discrete single channel conductances between 20 and 100 pS could be observed. Polymyxin B, a potent inhibitor of the protein kinase C (PKC) system, completely suppressed OAG-stimulated electrical uncoupling. Dialysis of cell pairs with solutions containing PKC, isolated from rat brain, also caused electrical uncoupling. The presence of 0.1 mM dibutyryl cyclic AMP and 5 mM ATP in the pipette solution, which serves to stabilize the junctional conductance, did not suppress the effects of OAG or isolated PKC. We conclude that an increase of protein kinase C activity leads to the closure of gap junction channels, presumably via a PKC-dependent phosphorylation of the junctional peptide, and that this mechanism is dominant over cAMP-dependent upregulatory effects in the experimental time range (less than or equal to 1 hr). A correlation of the observed single channel conductances with the appearance of channel subconductance states or various channel populations is discussed.

In vitro and in vivo phosphorylation of chicken beta B3-crystallin

Incubations of chicken lens homogenates with [32P]-ATP revealed the phosphorylation of a 28 kDa protein, and phosphoamino acid analysis of the phosphorylated protein showed the presence of phosphoserine. The protein is present in the beta-crystallin fraction and after purification and partial sequence determination, by way of peptide mapping and subsequent amino acid analyses and Edman degradation, this 28 kDa protein was identified as the beta B3-crystallin subunit, based on its homology with the bovine and rat orthologue. From phosphate content determination it could be concluded that this chicken beta B3 subunit contains in vivo 2 mol phosphate/mol polypeptide.

Phosphorylation of MP26, a lens junction protein, is enhanced by activators of protein kinase C

MP26, a protein thought to form gap junctional channels in the lens, and other lens proteins were phosphorylated under conditions that activate protein kinase C. Phosphorylation was detected both in lens fiber cell fragments in an "in vivo" labeling procedure with 32P-phosphate and in cell homogenates with 32P-ATP. In these experiments, both calcium and 12-O-tetradecanoylphorbol 13-acetate (TPA) were necessary for maximal phosphorylation of MP26. Calcium stimulated the phosphorylation of MP26 approximately fourfold and TPA with calcium led to a sevenfold increase. If TPA was present, 1 microM calcium was sufficient for maximal labeling. Phosphoamino acid analysis demonstrated approximately 85% phosphoserine, 15% phosphothreonine, and no phosphotyrosine when MP26 was phosphorylated in lens homogenates in the presence of TPA and calcium and then electrophoretically purified. Phosphorylation occurred near the cytoplasmic, C-terminal of MP26. The possible involvement of other kinases was also examined. The Walsh inhibitor, which affects cAMP-dependent protein kinases, had no influence on the TPA-mediated increase in phosphorylation. In studies with isolated membranes and added kinases, MP26 was also found to not be a substrate for calcium/calmodulin-dependent protein kinase II. Thus, protein kinase C may have phosphorylated MP26 in a direct manner.

Cell uncoupling and protein kinase C: correlation in a cell line but not in a differentiated tissue

Second messengers have been implicated in the control of communication between cells of various tissues and of a number of cell lines. To assess whether protein kinase C (PKC) is involved in the regulation of gap junctions between primary differentiated cells, we studied the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on PKC translocation and junctional conductance of rat pancreatic exocrine cells. Our results show that although TPA induced the translocation of PKC from a "cytosolic" to a "microsomal" fraction within minutes, it failed to block the junctional conductance of acinar cell pairs up to 30 min after application. By contrast, analogous experiments on a liver-derived cell line (WB cells) showed that TPA-induced PKC translocation was paralleled by a marked and irreversible inhibition of intercellular coupling. These results indicate that, in contrast to the effects on transformed or dedifferentiated permanent cell lines, PKC is not involved in gating gap junctional channels between primary differentiated secretory cells of the pancreas.

Concentration/response effect of 2,2', 4,4', 5,5'-hexabromobiphenyl on cell-cell communication in vitro: assessment by fluorescence redistribution after photobleaching ("FRAP")

Inhibition of gap junction-mediated cell-cell communication might be a mechanism for several types of cellular dysfunctions, including tumor promotion. Although many different assays have been designed to measure gap junction-mediated intercellular communication, we applied a new technique, termed Fluorescence Redistribution After Photobleaching ("FRAP"), to assess the ability of a known tumor promoter, 2,2', 4,4', 5,5'-hexabromobiphenyl (245-HBB), to inhibit cell-cell communication in a concentration-dependent manner. WB-F344 (rat epithelial) cells were plated at low density, exposed to noncytotoxic concentrations of 1, 5, or 20 micrograms 245-HBB/ml medium, and stained with 6-carboxyfluorescein diacetate. Single cells in pairs or clusters of touching cells in each exposure group were examined with FRAP. The results revealed an inverse correlation between the degree of fluorescence redistribution in photobleached cells and the concentration of 245-HBB. Therefore, FRAP appears to be a sensitive and rapid technique for determining complete or partial inhibition of chemically induced intercellular communication in vitro. These results also provide further evidence for the ability of 245-HBB to inhibit gap junction-mediated cell-cell communication in a concentration-dependent manner.

MP17, a fiber-specific intrinsic membrane protein from mammalian eye lens

A major protein with a molecular weight of 17,000, designated as MP17, has been identified in mammalian eye lens plasma membranes. Hydrophobic photolabeling experiments revealed that MP17 is a genuine intrinsic membrane protein. By using monoclonal antibodies we demonstrated that MP17 is not detectable in liver, heart, muscle, spleen and kidney, and thus can be considered, like MP26, as a lens-specific membrane protein. Furthermore, we showed that MP17 is a substrate for cAMP-dependent protein kinase and that it is a calmodulin-binding protein.

Phosphorylation of troponin I by protein kinase C: mechanism of inhibition by calmodulin and troponin C

The mechanism by which calmodulin and troponin C influence phosphorylation of troponin I (TnI) by protein kinase C was investigated. The phosphorylation of TnI by protein kinase C requires the presence of acidic phospholipid, calcium and diacylglycerol. Light scattering intensity and fluorescence intensity experiments showed that TnI associated with the phospholipid membranes and caused extensive aggregation. In the presence of Ca2+, TnI-phospholipid interactions were prevented by approximately stoichiometric amounts of either troponin C or calmodulin. Troponin C was shown to completely inhibit phosphorylation of TnI by either protein kinase C or by phosphorylase b kinase. In contrast, calmodulin completely inhibited phosphorylation of TnI by protein kinase C, but had only little effect on TnI phosphorylation by phosphorylase b kinase. Inhibition by calmodulin did not appear to be due to interaction with PKC, since calmodulin mildly increased protein kinase C phosphorylation of histone III-S. The ratio of phosphoserine to phosphothreonine in protein kinase C-phosphorylated TnI remained approximately constant for reactions inhibited by up to 90% by calmodulin. TnI interactions with phospholipid and phosphorylation of TnI by PKC were also prevented by high salt concentrations. However, salt concentrations adequate to inhibit phosphorylation were sufficient to dissociate only TnI, but not protein kinase C from the membrane. These results suggest that the binding of TnI to phospholipid is required for phosphorylation by protein kinase C and that prevention of this binding by any means completely inhibited phosphorylation of TnI by protein kinase C.

Electron microscopic observations of reconstituted proteoliposomes with the purified major intrinsic membrane protein of eye lens fibers

The purified major intrinsic protein of the lens fiber plasma membrane (MP26) reconstituted into liposomes favored membrane-to-membrane close contacts as visualized by freeze fracture and immunoelectron microscopy. Reconstituted apposed unilamellar vesicles formed pentalaminar profiles, and multilamellar liposomes showed regions of stacked bilayers. Immunogold labeling, using antibody directed against MP26, demonstrated that this polypeptide is present in regions of membrane-to-membrane close interaction. Fracture faces displayed both randomly distributed clusters of 8-nm polygonal intramembrane particles and membrane domains where a bidimensional lattice of repeating subunits was present. The structural pleomorphism which characterized the MP26-reconstituted proteoliposomes seems quite comparable to that visualized in natural fiber plasma membrane domains.

EDTA-extractable proteins from calf lens fiber membranes are phosphorylated by Ca2+-phospholipid-dependent protein kinase

A distinct group of EDTA-extractable proteins (EEP), being a major protein component of calf lens fiber membranes, is bound to these membranes in a calcium-dependent way. Both purified and membrane-bound EEP can be phosphorylated in vitro by a Ca2+-activated, phospholipid-dependent protein kinase (protein kinase C). In general, this protein kinase preferentially phosphorylates serine and threonine residues of protein substrates. Phosphoamino-acid analysis of the two major bands of EEP phosphorylated by protein kinase C, representing the 33,000 + 34,000 EEP proteins and the 30,700-31,800 proteins, respectively, revealed differences in the phosphoamino-acid patterns. For the 33,000 + 34,000 EEP proteins, only phosphothreonine was detected whereas for the 30,700-31,800 proteins, the label was incorporated in both threonine and serine residues. No label was found on tyrosine residues. These results implicate differences in the primary structure of the individual EEP proteins. Regarding previous observations that EEP is a main protein component of lens fiber junctions and of the many covering epithelial and endothelial cells, and considering the fact that protein kinase C is involved in cell-cell communication, growth and differentiation processes we suggest that a correlation exists between phosphorylation-dephosphorylation of EEP and the regulation of a number of cellular processes.