Although [Cl؊ ] i regulates many cellular functions including cell secretion, the mechanisms governing these actions are not known. We have previously shown that the apical membrane of airway epithelium contains a 37-kDa phosphoprotein (p37) whose phosphorylation is regulated by chloride concentration. Using metal affinity (chelating Fe 3؉ -Sepharose) and anion exchange (PO-ROS HQ 20) chromatography, we have purified p37 from ovine tracheal epithelia to electrophoretic homogeneity. Sequence analysis and immunoprecipitation using monoclonal and specific polyclonal antibodies identified p37 as annexin I, a member of a family of Ca We have previously shown that [Cl Ϫ ]/[anions]/ATP/GTP differentially determine the profile of phosphorylation of a number of apical membrane proteins in airway epithelia (3, 4). We identified two of the phosphoproteins, a 19/21-kDa doublet that showed chloride concentration-dependent phosphorylation, as two isoforms of nucleoside-diphosphate kinase (3, 5). We also observed that phosphorylation of a 37-kDa protein (p37) from both human nasal and sheep tracheal epithelia was differentially modulated by nucleotides (GTP, ATP, and GDP), [Cl Ϫ ], and other anion species (3, 4). We found that GTP was the principal phosphate donor for p37 from both species; and in the presence of ATP/GDP, phosphorylation of p37 was enhanced 20-fold. However, this phosphorylation could not be inhibited by known serine/threonine and tyrosine protein kinase inhibitors, suggesting that phosphorylation was not due to the activity of these enzymes.In this study, our objective was to establish the identity of the ion-sensitive phosphoprotein p37 and to characterize its phosphorylated amino acid residue. By selectively enhancing phosphorylation of p37 (3) and by applying metal affinity chromatography, we show that p37 is identical to annexin I and provide evidence for novel annexin phosphorylation on histidine residues. Our data suggest that annexin is a component of an intracellular signaling system involving histidine phosphorylation, which is regulated by chloride concentration. EXPERIMENTAL PROCEDURESSample Preparation-A membrane fraction was prepared from ovine airway epithelia as described previously (3). Briefly, tracheal epithelia were scraped and dislodged into homogenization buffer (250 mM sucrose, 10 mM triethanolamine; one protease inhibitor tablet from Roche Molecular Biochemicals per 50 ml buffer). Pooled scrapings were homogenized and spun at 600 ϫ g for 15 min. The post-nuclear supernatant was re-spun at 100,000 ϫ g for 2 h. The pellet was resuspended in homogenization buffer and spun for 30 min at 16,000 ϫ g (this procedure was repeated three times). All procedures were conducted at 4°C. Use of lactate dehydrogenase as a cytosol marker showed that the pellet contained no cytosol (3). Aliquots of the cytosol and membrane pellet were stored in liquid nitrogen.Phosphorylation and Electronic Autoradiography-Phosphorylation and detection of phosphoproteins were performed as described previously (3). Briefl...

We have previously shown that nucleotide species (adenosine triphosphate [ATP] or guanosine triphosphate [GTP]), [Cl-], and anion species determine the steady-state phosphorylation of apical membrane proteins within human airway epithelium in vitro. We found that a Cl(-)-regulated 37-kD protein (p37) principally phosphorylated with GTP but not ATP as substrate. Here we show that apical membranes from sheep tracheal epithelium also contain a Cl(-)-regulated 37-kD phosphoprotein (p37s) and characterize one of the kinases involved in the regulation of p37s. Analysis of phosphorylation of apical membrane proteins with gamma[32P]GTP in the presence of MgCl2 showed that two proteins circa 19 and 21 kD (p19s and p21s) were transiently phosphorylated before p37s. Renaturation of apical membrane proteins within polyacrylamide gels showed that p19s and p21s autophosphorylated with either gamma[32P]GTP or gamma[32P]ATP as substrates, suggesting that the two proteins were kinases. Immunoblotting and immunoprecipitation with a specific polyclonal antibody showed that p21s was a membrane-bound isoform of nucleoside diphosphate kinase (NDPK, EC 2.7.4.6), a protein kinase which catalyzes transfer of terminal phosphate from ATP to diphosphate nucleotides and is, among other functions, essential for cell secretion. Incubation of apical membrane proteins in the presence of gamma[32P]ATP and guanosine diphosphate (GDP) (but not GDPbetaS) resulted in enhancement of phosphorylation of p37s. Dephosphorylation of NDPK was stimulated by the addition of Mg2+, Mn2+, and Co2+ (but not Zn2+ or Ca2+). Our data show that ovine trachea is a good model for further characterization of the chloride-dependent cascade in airway epithelium.

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Cystic fibrosis results from mutations in the cystic fibrosis conductance regulator protein (CFTR), a cAMP/protein kinase A (PKA) and ATP-regulated Cl(-) channel. CFTR is increasingly recognized as a component of multiprotein complexes and although several inhibitory proteins to CFTR have been identified, protein complexes that stimulate CFTR function remain less well characterized. We report that annexin 2 (anx 2)-S100A10 forms a functional cAMP/PKA/calcineurin (CaN)-dependent complex with CFTR. Cell stimulation with forskolin/3-isobutyl-1-methylxanthine significantly increases the amount of anx 2-S100A10 that reciprocally coimmunoprecipitates with cell surface CFTR and calyculin A. Preinhibition with PKA or CaN inhibitors attenuates the interaction. Furthermore, we find that the acetylated peptide (STVHEILCKLSLEG, Ac1-14), but not the nonacetylated equivalent N1-14, corresponding to the S100A10 binding site on anx 2, disrupts the anx 2-S100A10/CFTR complex. Analysis of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and CFTR(inh172)-sensitive currents, taken as indication of the outwardly rectifying Cl(-) channels (ORCC) and CFTR-mediated currents, respectively, showed that Ac1-14, but not N1-14, inhibits both the cAMP/PKA-dependent ORCC and CFTR activities. CaN inhibitors (cypermethrin, cyclosporin A) discriminated between ORCC/CFTR by inhibiting the CFTR(inh172)-, but not the DIDS-sensitive currents, by >70%. Furthermore, peptide Ac1-14 inhibited acetylcholine-induced short-circuit current measured across a sheet of intact intestinal biopsy. Our data suggests that the anx 2-S100A10/CFTR complex is important for CFTR function across epithelia.

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Protein phosphorylation is an important post-translational modification that is an integral part of cellular function. The O-phosphorylated amino-acid residues, such as phosphoserine (pSer), phosphothreonine (pThr) and phosphotyrosine (pTyr), have dominated the literature while the acid labile N-linked phosphorylated amino acids, such as phosphohistidine (pHis), have largely been historically overlooked because of the acidic conditions routinely used in amino-acid detection and analysis. This review highlights some misinterpretations that have arisen in the existing literature, pinpoints outstanding questions and potential future directions to clarify the role of pHis in mammalian signalling systems. Particular emphasis is placed on pHis isomerization and the hybrid functionality for both pHis and pTyr of the proposed τ-pHis analogue bearing the triazole residue.

Comparison of partial primary sequences of sigma-class glutathione S-transferases (GSH) of parasitic helminths and a GSH-dependent prostaglandin (PG)-H D-isomerase of rat immune accessory cells suggested that some of the helminth enzymes may also be involved in PG biosynthesis [Meyer and Thomas (1995) Biochem. J. 311, 739-742]. A soluble GSH transferase of the parasitic nematode Ascaridia galli has now been purified which shows high activity and specificity in the GSH-dependent isomerization of PGH to PGE, comparable to that of the rat spleen enzyme in its isomerization of PGH to PGD, and similarly stimulates the activity of prostaglandin H synthase. The enzyme subunit is structurally related to the rat spleen enzyme and sigma-class GSH transferases of helminths according to the partial primary sequence. The data support the hypothesis that some sigma-class GSH transferases of helminth parasites are involved in PG biosynthesis which, in the case of PGE, is likely to be associated with the subversion or suppression of host immunity. A PG-H E-isomerase of comparable specificity and activity has not previously been isolated.

Background: AMPK phosphorylates CFTR and inhibits PKA-stimulated CFTR channel gating by unclear mechanisms.Results: NDPK-A, AMPK, and CFTR exist in a membrane-associated complex. AMPK-CFTR binding and NDPK-A catalytic function are required for CFTR inhibition by AMPK.Conclusion: NDPK-A plays an integral role in the regulation of CFTR by AMPK.Significance: Targeting the AMPK-CFTR interaction and NDPK-A function could yield new therapeutic strategies for CF.