Primary structure of bovine calpactin I heavy chain (p36), a major cellular substrate for retroviral protein-tyrosine kinases: homology with the human phospholipase A2 inhibitor lipocortin

Effect of serine phosphorylation and Ser25 phospho-mimicking mutations on nuclear localisation and ligand interactions of annexin A2

Annexin A2 (AnxA2) interacts with numerous ligands, including calcium, lipids, mRNAs and intracellular and extracellular proteins. Different post-translational modifications participate in the discrimination of the functions of AnxA2 by modulating its ligand interactions. Here, phospho-mimicking mutants (AnxA2-S25E and AnxA2-S25D) were employed to investigate the effects of Ser25 phosphorylation on the structure and function of AnxA2 by using AnxA2-S25A as a control. The overall α-helical structure of AnxA2 is not affected by the mutations, since the thermal stabilities and aggregation tendencies of the mutants differ only slightly from the wild-type (wt) protein. Unlike wt AnxA2, all mutants bind the anxA2 3' untranslated region and β-γ-G-actin with high affinity in a Ca(2+)-independent manner. AnxA2-S25E is not targeted to the nucleus in transfected PC12 cells. In vitro phosphorylation of AnxA2 by protein kinase C increases its affinity to mRNA and inhibits its nuclear localisation, in accordance with the data obtained with the phospho-mimicking mutants. Ca(2+)-dependent binding of wt AnxA2 to phosphatidylinositol, phosphatidylinositol-3-phosphate, phosphatidylinositol-4-phosphate and phosphatidylinositol-5-phosphate, as well as weaker but still Ca(2+)-dependent binding to phosphatidylserine and phosphatidylinositol-3,5-bisphosphate, was demonstrated by a protein-lipid overlay assay, whereas binding of AnxA2 to these lipids, as well as its binding to liposomes, is inhibited by the Ser25 mutations. Thus, introduction of a modification (mutation or phosphorylation) at Ser25 appears to induce a conformational change leading to increased accessibility of the mRNA- and G-actin-binding sites in domain IV independent of Ca(2+) levels, while the Ca(2+)-dependent binding of AnxA2 to phospholipids is attenuated.

The mRNA-binding site of annexin A2 resides in helices C-D of its domain IV

Annexin A2 (AnxA2) is a Ca(2+)-binding and phospholipid-binding protein involved in different intracellular processes including exocytosis, endocytosis and membrane-cytoskeleton movements. We have previously identified AnxA2 as an mRNA-binding protein present in cytoskeleton-bound polysomes, that binds to a specific approximately 100 nucleotide region in the 3'-untranslated region of c-myc and its cognate mRNA. In the present study, we show by UV cross-linking assays and surface plasmon resonance analyses that the mRNA-binding site of AnxA2 resides in its domain IV. Furthermore, the interaction of full-length AnxA2 with the 3'-untranslated region of anxA2 mRNA is Ca(2+)-dependent. By contrast, the interaction is Ca(2+)-independent for the isolated domain IV of AnxA2, suggesting that the mRNA-binding site is masked in Apo-AnxA2 and gains exposure through a Ca(2+)-induced conformational change of AnxA2 generating a favourable mRNA-binding site. The AnxA2-mRNA interaction is specific and involves helices C and D in domain IV of AnxA2, since point mutagenesis of several charged and polar exposed residues of these helices in the full-length protein strongly reduce RNA binding. The interaction appears to be sequential involving an initial phase of recognition dominated by electrostatic interactions, most likely between lysine residues and the phosphate backbone of RNA, followed by a second phase contributing to the specificity of the interaction.

Engineering, Biophysical Characterisation and Binding Properties of a Soluble Mutant form of Annexin A2 Domain IV that Adopts a Partially Folded Conformation

The four approximately 75-residue domains (repeats) that constitute the annexin core structure all possess an identical five-alpha-helix bundle topology, but the physico-chemical properties of the isolated domains are different. Domain IV of the annexins has previously been expressed only as inclusion bodies, resistant to solubilisation. Analysis of the conserved, exposed hydrophobic residues of the four annexin domains reveals that domain IV contains the largest number of hydrophobic residues involved in interfacial contacts with the other domains. We designed five constructs of domain IV of annexin A2 in which several interfacial hydrophobic residues were substituted by hydrophilic residues. The mutant domain, in which all fully exposed hydrophobic interfacial residues were substituted, was isolated as a soluble protein. Circular dichroism measurements indicate that it harbours a high content of alpha-helical secondary structure and some tertiary structure. The CD-monitored (lambda=222 nm) thermal melting profile suggests a weak cooperative transition. Nuclear magnetic resonance (1H-15N) correlation spectroscopy reveals heterogeneous line broadening and an intermediate spectral dispersion. These properties are indicative of a partially folded protein in which some residues are in a fairly structured conformation, whereas others are in an unfolded state. This conclusion is corroborated by 1-anilinonaphthalene-8-sulfonate fluorescence (ANS) analyses. Surface plasmon resonance measurements also indicate that this domain binds heparin, a known ligand of domain IV in the full-length annexin A2, although with lower affinity.

Annexin A2 recognises a specific region in the 3'-UTR of its cognate messenger RNA

Annexin A2 is a multifunctional Ca(2+)- and lipid-binding protein. We previously showed that a distinct pool of cellular Annexin A2 associates with mRNP complexes or polysomes associated with the cytoskeleton. Here we report in vitro and in vivo experiments showing that Annexin A2 present in this subset of mRNP complexes interacts with its cognate mRNA and c-myc mRNA, but not with beta(2)-microglobulin mRNA translated on membrane-bound polysomes. The protein recognises sequence elements within the untranslated regions, but not within the coding region, of its cognate mRNA. Alignment of the Annexin A2-binding 3'-untranslated regions of annexin A2 mRNA from several species reveals a five nucleotide consensus sequence 5'-AA(C/G)(A/U)G. The Annexin A2-interacting region of the 3'-untranslated region can be mapped to a sequence of about 100 nucleotides containing two repeats of the consensus sequence. The binding elements appear to involve both single and double stranded regions, indicating that a specific higher order mRNA structure is required for binding to Annexin A2. We suggest that this type of interaction is representative for a group of mRNAs translated on cytoskeleton-bound polysomes.

Isolation and characterization of annexin 2 pseudogene in Rattus norvegicus

Annexin 2 is a calcium-regulated, phospholipid-binding protein present in endothelial cells, macrophages and some tumor cells. Annexin 2 is a substrate for a variety of protein kinases, and plays roles in the regulation of endocytosis, exocytosis and thrombolysis. We have determined the nucleotide sequence of a rat genomic DNA fragment that hybridized to a rat annexin 2 DNA complementary to RNA (cDNA) probe. Sequence analysis revealed that it was an intronless rat annexin 2, consisting of a start-to-stop-codon-length copy of the processed transcript. This pseudogene contained 33 point mutations and two deletion sites in the coding region as compared with the cDNA, and thus displayed typical features of a retroposon. Transitions were more frequent than transversions, and the most frequent type of mutation was G to A transition. We isolated a phage clone that contained a functional rat annexin 2 genomic fragment including coding exons 3 and 4. Polymerase chain reaction and subsequent sequence analysis revealed an intron of approximately 4 kbp at the same site as in humans and mice. Whereas the annexin 2 gene or its cDNA homologues have been detected in various species from Xenopus to humans, its pseudogene has been reported only in humans. In the present study, we demonstrated the presence of an annexin 2 pseudogene in rats.

An immune response manifested by the common occurrence of annexins I and II autoantibodies and high circulating levels of IL-6 in lung cancer

The identification of circulating tumor antigens or their related autoantibodies provides a means for early cancer diagnosis as well as leads for therapy. The purpose of this study was to identify proteins that commonly induce a humoral response in lung cancer by using a proteomic approach and to investigate biological processes that may be associated with the development of autoantibodies. Aliquots of solubilized proteins from a lung adenocarcinoma cell line (A549) and from lung tumors were subjected to two-dimensional PAGE, followed by Western blot analysis in which individual sera were tested for primary antibodies. Sera from 54 newly diagnosed patients with lung cancer and 60 patients with other cancers and from 61 noncancer controls were analyzed. Sera from 60% of patients with lung adenocarcinoma and 33% of patients with squamous cell lung carcinoma but none of the noncancer controls exhibited IgG-based reactivity against proteins identified as glycosylated annexins I and/or II. Immunohistochemical analysis showed that annexin I was expressed diffusely in neoplastic cells in lung tumor tissues, whereas annexin II was predominant at the cell surface. Interestingly, IL-6 levels were significantly higher in sera of antibody-positive lung cancer patients compared with antibody-negative patients and controls. We conclude that an immune response manifested by annexins I and II autoantibodies occurs commonly in lung cancer and is associated with high circulating levels of an inflammatory cytokine. The proteomic approach we have implemented has utility for the development of serum-based assays for cancer diagnosis as we report in this paper on the discovery of antiannexins I and/or II in sera from patients with lung cancer.

Levels and localization of group II phospholipase A2 and annexin I in interleukin- and dexamethasone-treated rat mesangial cells: evidence against annexin mediation of the dexamethasone-induced inhibition of group II phospholipases A2

The mechanism by which glucocorticosteroids inhibit the synthesis and secretion of pro-inflammatory arachidonate metabolites is still controversial. Initially it was postulated that glucocorticoids can induce the formation of PLA2 inhibitory proteins termed annexins. We have previously shown that the cytokine-induced 14 kDa PLA2 activity and the synthesis of prostaglandin E2 in rat mesangial cells is dose-dependently blocked by pretreatment of the cells with dexamethasone (Schalkwijk et al. (1991) Biochem. Biophys. Res. Commun. 180, 46-52). Concurrently, the synthesis of 14 kDa group II PLA2 is suppressed. The regulation of PLA2 activity is complex and may well involve superimposable mechanisms. Thus, although the decrease in PLA2 protein levels could in itself explain the dexamethasone-induced decrease in PLA2 activity, a contribution of the glucocorticoid-induced anti-phospholipase A2 protein annexin cannot be ruled out a priori. To investigate this possibility we analyzed the level of annexin I by Western blotting and immunostaining in mesangial cells treated with interleukin-1 beta and/or dexamethasone. Under conditions where 14 kDa group II PLA2 activity and protein levels were dramatically affected by interleukin-1 and dexamethasone, the level of annexin I in the cells remained constant. Dexamethasone also did not induce the secretion of annexin I. In addition, no evidence for dexamethasone-induced translocation of annexin I from the cytosol to membranes, thereby possibly sequestering the substrates for PLA2, was obtained. Immunofluorescence studies localized the cytokine-induced PLA2 to the Golgi area and punctate structures in the cytoplasm. We have also studied the subcellular localization of annexin I in rat mesangial cells using confocal microscopy. These studies located annexin I mainly in the cytoplasma and the nucleus. We conclude from these experiments that the dexamethasone-induced inhibition of 14 kDa group II PLA2 in rat mesangial cells is not mediated by annexin I and is solely due to the suppression of PLA2 gene expression.

Expression and characterization of a Dictyostelium discoideum annexin

The annexins are calcium-dependent phospholipid-binding proteins. Recently the gene encoding the homologue of a mammalian annexin has been identified in Dictyostelium discoideum. Analysis of cDNA and genomic clones showed that the transcript for Dictyostelium annexin is alternatively spliced (Greenwood, M. and Tsang, A. (1991) Biochim. Biophys. Acta 1088, 429-432; Döring, V., Schleicher, M and Noegel, A. (1991) J. Biol. Chem. 266, 17509-17515). Here, we showed that the Dictyostelium annexin DNA hybridized to two populations of transcripts. We used a recombinant annexin polypeptide to raise polyclonal antibody. Immunoblot analysis revealed that the antibody recognized two polypeptides of 48 kDa and 54 kDa in developing D. discoideum cells. The molecular sizes of these polypeptides correspond well with the expected sizes of the alternatively spliced products. The 48-kDa and 54-kDa polypeptides were purified by isoelectric focusing to more than 70% homogeneity. The partially purified proteins were found to associate with phosphatidylserine vesicles in a calcium-dependent manner. These results suggest that the 48- and 54-kDa polypeptides are the products of alternative splicing of the annexin transcripts. During development the two polypeptides accumulate at different rates to about 60 times the level detected in vegetative cells. On the other hand, RNA blot analysis showed that the level of the annexin transcripts in multicellular aggregates was about 5 times that of vegetative cells.

Molecular cloning of a novel N-terminal variant of annexin II from rat basophilic leukaemia cells

Rat annexin II cDNA clones were isolated from a rat basophilic leukaemia cell plasmid library by cross-species hybridization with a mouse probe, and fully sequenced using the dideoxy-chain-termination method. Alignment of the derived amino-acid sequence with those of other mammalian annexin II species revealed a high level of conservation, characteristic of the annexin family of proteins. One of the cDNAs isolated contained an additional six nucleotides close to the N-terminus, lying in-frame and at a point corresponding to an intron/exon boundary in the human annexin II gene. As the two rat cDNAs were identical apart from the six nucleotide insert, it is likely that these represent alternatively spliced transcripts of a single gene, rather than the products of two separate genes. The six nucleotides encode serine-glutamine and therefore introduce an additional potential phosphorylation site into a region already containing one tyrosine and two serine phosphorylation sites. The discovery of this novel annexin II variant may have important implications both for p11 binding and for regulation of annexin II function by phosphorylation.

Cell surface annexin II is a high affinity receptor for the alternatively spliced segment of tenascin-C

We have investigated the binding of soluble tenascin-C (TN-C) to several cell lines using a radioligand binding assay. Specific binding was demonstrated to U-251MG human glioma cells and to a line of bovine aortic endothelial cells, but hamster fibroblasts showed no specific binding. Recombinant proteins corresponding to specific domains of TN-C were used to map the binding site(s) in TN-C. The alternatively spliced segment (TNfnA-D) inhibited the binding of native TN-C most strongly, and itself bound to glioma and endothelial cells. Scatchard analysis of TNfnA-D binding indicated 2-5 x 10(5) binding sites per cell, with an apparent 2 nM dissociation constant. The cell surface receptor for TNfnA-D was identified as a 35-kD protein on the basis of blot binding assays and affinity chromatography of membrane extracts on native TN-C and TNfnA-D columns. Protein sequencing indicated that this 35-kD receptor was annexin II. Annexin II is well characterized as a cytoplasmic protein, so it was surprising to find it as a presumably extracellular receptor for TN-C. To confirm that it was the 35-kD receptor, we obtained purified annexin II and demonstrated its binding to TNfnA-D and TN-C at nM concentrations. Antibodies to annexin II prominently stained the external surface of live endothelial cells and blocked the binding of TNfnA-D to the cells. Thus annexin II appears to be a receptor for the alternatively spliced segment of TN-C, and may mediate cellular responses to soluble TN-C in the extracellular matrix.

Immunocytochemical detection of extracellular annexin II in cultured human skin keratinocytes and isolation of annexin II isoforms enriched in the extracellular pool

Monoclonal antibodies were raised against trypsinized human skin epidermal cells and selected for their staining of the epidermal cells in a cell periphery pattern. One antibody, CP-1, immunoprecipitated a 36 kDa protein that was identified as annexin II heavy chain by microsequencing of a CNBr-generated peptide fragment from the antigen and by cross-identification with another anti-annexin II antibody. In addition to staining a broad cell periphery band in keratinocytes, CP-1 also detected annexin II outside and in between the top layer cells before cell permeabilization. Double-labeling of annexin II and F-actin revealed a distinct topographical relationship between the two, with intercellular annexin II flanked by the submembranously located actin of the juxta-positioned cells. Annexin II was isolated from cultured keratinocytes via immunoaffinity column chromatography in one step, using the same monoclonal antibody CP-1 and was found to be resolved into multiple isoforms when analyzed by two-dimensional gel electrophoresis. The predominant components of annexin II were basic, with pI of 6.5-8.5, and some of them formed disulfide-linked monomeric multimers under non-reducing conditions. Acidic annexin II isoforms with pI 5.4-5.8 were barely detectable among the total annexin II isolated but were selectively enriched in an extracellular pool created by 0.05% ethylenediaminetetraacetic acid (EDTA) dispersion of the cultured cells into single cell suspensions. Furthermore, they can be separated from the rest of annexin II by using a different elution condition. A 46 kDa protein, the identity of which is unclear, co-eluted with the acidic isoforms in the EDTA washes. These acidic isoforms, which co-eluted with the 46 kDa protein, are suspected of corresponding to the extracellular annexin II detected immunocytochemically.

In vitro modulation of filament bundling in F-actin and keratins by annexin II and calcium

In our preliminary subcellular localization experiment we demonstrated that annexin II co-localized with submembranous actin in subpopulations of both cultured fibroblasts and keratinocytes. To investigate the physical interaction between annexin II and actin at the cell periphery, in vitro reconstitution experiments were carried out with keratins used as a control. Annexin II, isolated by immunoaffinity column chromatography, was found to exist as globular structures measuring 10 to 25 nm in diameter by rotary shadowing, similar to a previous report. We believe that these structures represent its polymeric forms. By negative staining, monomeric annexin II was detectable as tapered rods, measuring 6 nm in length and 1 to 2 nm in diameter. When annexin II was mixed with actin in 3 mM piperazine-N, N-bis-2-ethanesulfonic acid (PIPES) buffer with 10 mM NaCl2, 2 mM MgCl2 and 0.1 mM CaCl2, thick twisting actin bundles formed, confirming previous reports. This bundling was much reduced when calcium was removed. In the presence of 5 mM ethylenediamine tetra-acetic acid (EDTA) in 5 mM tris, pH 7.2, keratins were found to form a network of filaments, which began to disassemble when the chelator was removed and became fragmented when 0.1 mM CaCl2 was added. Keratins under the same conditions did not fragment when annexin II was present. These results suggest that annexin II, in conjunction with Ca2+, may be involved in a flexible system accommodating changes in the membrane cytoskeletal framework at the cell periphery in keratinocytes.

Protein kinase C-dependent phosphorylation of annexins I and II in mesangial cells

In this study we describe the phosphorylation of annexins from cultured rat mesangial cells by protein kinase C (PKC) both in vitro and in vivo. Annexins I and II were detected either by Western-blot analysis or by immunoprecipitation using specific antibodies. In the presence of [gamma-32P]ATP, cytosolic annexin I and annexin II were phosphorylated in vitro only when Ca2+ and phospholipids were added, but not in the presence of phospholipids alone. Annexin I was shown to be a better substrate than annexin II. In experiments in vivo performed on 32P-labelled mesangial cells, the addition of two well-known activators of PKC, namely angiotensin II (AII) and phorbol myristate acetate (PMA), increased preferentially the phosphorylation of annexin I. Annexin II was phosphorylated to a much lesser extent after AII treatment. Phosphoamino acid analysis of annexins, either by two-dimensional chromatography or by using a specific antiphosphotyrosine antibody, revealed only phosphoserine in these experiments in vivo. The addition of AII to mesangial cells increased serine phosphorylation of annexin I and annexin II, whereas PMA only increased serine phosphorylation of annexin I. V8-protease phosphopeptide mapping of annexin I that was phosphorylated both in vitro and in vivo by PKC from mesangial cells shows similar phosphopeptides.

Glycosylation of annexin I and annexin II

Human placental annexin I and annexin II were shown to be glycosylated by one-dimensional affinity blotting with the lectin concanavalin A, which recognizes D-mannose and D-glucose residues. Further evidence that annexin I and annexin II are glycosylated was provided by the finding that these proteins incorporated D-[2,6-3H]mannose and D-[6-3H]glucose when they were biosynthesized by the human squamous carcinoma cell line SqCC/Y1. Annexin I and annexin II could be rapidly purified from a human placental membrane extract by concanavalin A-Sepharose, which indicated that these proteins contain two biantennary mannosyl residues.

Expression of anchorin CII, a collagen-binding protein of the annexin family, in the developing chick embryo

Expression of anchorin CII, a collagen-binding protein of the annexin family, was followed in the developing chick embryo using Northern and in situ hybridization and Western blotting. During chick somite development, anchorin CII mRNA was detected by Northern blotting as early as stage 11. At stage 24, anchorin mRNA accumulated in the anterior part of the somite sclerotome near the resegmentation line, as shown by in situ hybridization. The presence of anchorin CII protein during stages 11 to 20 was confirmed by Western blotting. In situ hybridization identified anchorin CII also in the otic vesicle adjacent to the site of contact with the statoacoustic ganglion and in the mandibular mesenchyme. The level of anchorin CII mRNA in differentiated hyaline cartilage, exemplified by sternal cartilage, was lower than that in differentiating somites or cultured chondrocytes. These findings are consistent with our notion that anchorin CII may be involved in cell-matrix interactions preceding chondrogenic differentiation events in the chick embryo. A significant level of anchorin CII mRNA and protein synthesis was also found in cultured myoblasts, but less than that in chondroblasts. This distribution pattern is different from that reported for a related protein, p34, or calpactin, the major protein substrate for tyrosine kinase phosphorylation in chick chondrocytes and fibroblasts. The results confirm suggestions from previous sequencing studies that anchorin CII and p34 are different proteins of the annexin/calpactin family.

Crystal and molecular structure of human annexin V after refinement. Implications for structure, membrane binding and ion channel formation of the annexin family of proteins

Two crystal forms (P6(3) and R3) of human annexin V have been crystallographically refined at 2.3 A and 2.0 A resolution to R-values of 0.184 and 0.174, respectively, applying very tight stereochemical restraints with deviations from ideal geometry of 0.01 A and 2 degrees. The three independent molecules (2 in P6(3), 1 in R3) are similar, with deviations in C alpha positions of 0.6 A. The polypeptide chain of 320 amino acid residues is folded into a planar cyclic arrangement of four repeats. The repeats have similar structures of five alpha-helical segments wound into a right-handed compact superhelix. Three calcium ion sites in repeats I, II and IV and two lanthanum ion sites in repeat I have been found in the R3 crystals. They are located at the convex face of the molecule opposite the N terminus. Repeat III has a different conformation at this site and no calcium bound. The calcium sites are similar to the phospholipase A2 calcium-binding site, suggesting analogy also in phospholipid interaction. The center of the molecule is formed by a channel of polar charged residues, which also harbors a chain of ordered water molecules conserved in the different crystal forms. Comparison with amino acid sequences of other annexins shows a high degree of similarity between them. Long insertions are found only at the N termini. Most conserved are the residues forming the metal-binding sites and the polar channel. Annexins V and VII form voltage-gated calcium ion channels when bound to membranes in vitro. We suggest that annexins bind with their convex face to membranes, causing local disorder and permeability of the phospholipid bilayers. Annexins are Janus-faced proteins that face phospholipid and water and mediate calcium transport.

Purification of annexin I and annexin II from human placental membranes by high-performance liquid chromatography

Annexin I and annexin II were extracted from human placental membranes with ethylene glycol bis(beta-amino-ethyl ether)-N,N'-tetraacetic acid (EGTA) and purified by high-performance liquid chromatography by measuring their ability to inhibit phospholipase A2 activity in vitro. Neither protein was capable of binding to a DEAE-5PW HPLC column at neutral pH; however, they were resolved through binding to a Mono S column and passage through size-exclusion HPLC columns. Annexin I and its covalently linked dimer (36 and 66 kDa, respectively, by sodium dodecyl sulfate (SDS)-gel electrophoresis) reacted in one-dimensional immunoblots with monoclonal antibodies to annexin I and calpactin II, and with monoclonal and polyclonal antibodies to lipocortin I, confirming that annexin I, calpactin II, and lipocortin I are the same or closely related proteins. Milligram amounts of monomeric annexin I containing negligible amounts of the cross-linked dimeric annexin I were selectively isolated from placental membranes by using buffers containing the sulfhydryl reagent iodoacetic acid. Milligram amounts of cross-linked annexin I were selectively isolated when placental membranes were initially treated with buffers that did not contain iodoacetic acid and then extracted with Triton X-100, suggesting that sulfhydryl-dependent transglutaminase activity contributes to the selective isolation of this protein. A third phospholipase A2-inhibitory protein (35 kDa by SDS-gel electrophoresis) that reacted in immunoblots with monoclonal antibodies to calpactin I and annexin II, indicating their similar identity, was isolated. The procedure employed allows the rapid purification of annexins I and II in milligram amounts from placental membranes within 2 days.

Primary structure and expression of the Xenopus laevis gene encoding annexin II

Annexin II (AnxII) is one of the Ca(2+)-dependent membrane- and phospholipid-binding proteins (annexins) which are encoded by a multigene family. AnxII was originally described as a major cellular substrate for the tyrosine kinase encoded by the src oncogene, and is also phosphorylated by protein kinase C in vivo and in vitro. To obtain more information about structurally conserved regions in AnxII, which could be of structural and/or functional importance, we have identified AnxII in a nonmammalian species, the clawed toad Xenopus laevis. In a ligand overlay assay, we employed p11, the cellular protein ligand of AnxII, to show that a 36-kDa Anx capable of binding p11 is present in a cellular extract from X. laevis cells. The cDNA cloning and sequence analysis revealed that two types of AnxII mRNA are expressed in X. laevis. The transcripts are highly similar to each other, but are encoded by two different genes. The deduced amino acid sequences show a high degree of conservation when compared to the sequences of mammalian and chicken AnxII. In particular, the p11-binding domain, as well as the protein core, which harbors the binding sites for Ca2+ and phospholipid, are highly similar. However, Tyr23, which is phosphorylated by pp60src in mammalian and chicken AnxII, is replaced by a Leu residue in both X. laevis molecules. Thus, tyrosine phosphorylation is probably not a general mode of regulation of AnxII function(s).

Amino acid sequence analysis of the annexin super-gene family of proteins

The annexins are a widespread family of calcium-dependent membrane-binding proteins. No common function has been identified for the family and, until recently, no crystallographic data existed for an annexin. In this paper we draw together 22 available annexin sequences consisting of 88 similar repeat units, and apply the techniques of multiple sequence alignment, pattern matching, secondary structure prediction and conservation analysis to the characterisation of the molecules. The analysis clearly shows that the repeats cluster into four distinct families and that greatest variation occurs within the repeat 3 units. Multiple alignment of the 88 repeats shows amino acids with conserved physicochemical properties at 22 positions, with only Gly at position 23 being absolutely conserved in all repeats. Secondary structure prediction techniques identify five conserved helices in each repeat unit and patterns of conserved hydrophobic amino acids are consistent with one face of a helix packing against the protein core in predicted helices a, c, d, e. Helix b is generally hydrophobic in all repeats, but contains a striking pattern of repeat-specific residue conservation at position 31, with Arg in repeats 4 and Glu in repeats 2, but unconserved amino acids in repeats 1 and 3. This suggests repeats 2 and 4 may interact via a buried saltbridge. The loop between predicted helices a and b of repeat 3 shows features distinct from the equivalent loop in repeats 1, 2 and 4, suggesting an important structural and/or functional role for this region. No compelling evidence emerges from this study for uteroglobin and the annexins sharing similar tertiary structures, or for uteroglobin representing a derivative of a primordial one-repeat structure that underwent duplication to give the present day annexins. The analyses performed in this paper are re-evaluated in the Appendix, in the light of the recently published X-ray structure for human annexin V. The structure confirms most of the predictions and shows the power of techniques for the determination of tertiary structural information from the amino acid sequences of an aligned protein family.

Biochemical characterization of annexins I and II isolated from pig nervous tissue

Five proteins having molecular masses of 90, 67, 37, 36, and 32 kDa (p90, p67, p37, p36, and p32, respectively) were identified in the particulate fractions of pig brain cortex and pig spinal cord prepared in the presence of 0.2 mM Ca2+ and further purified using a protocol previously described for the purification of calpactins. Proteins p90, p37, and p36 are related to annexins I and II. Annexin II, represented by p90, is found as an heterotetramer, composed of two heavy chains of 36 kDa and two light chains of 11 kDa, and as a monomer of 36 kDa. Protein p37, which differs immunologically from p36, is a monomer and could be related to annexin I. All three proteins are Ca(2+)-dependent phospholipid- and F-actin-binding proteins; they are phosphorylated on a serine and on a tyrosine residue by protein kinases associated with synaptic plasma membranes. Purified p36 monomer and p36 heterotetramer proteins bind to actin at millimolar Ca2+ concentrations. The stoichiometry of p36 binding to F-actin at saturation is 1:2, corresponding to one tetramer or monomer of calpactin for two actin monomers (KD, 3 x 10(-6) M). Synaptic plasma membranes supplemented with the monomeric or tetrameric forms of p36 phosphorylate the proteins on a serine residue. The monomer is phosphorylated on a serine residue by a Ca(2+)-independent protein kinase, whereas the heterotetramer is phosphorylated on a serine residue and a tyrosine residue by Ca(2+)-dependent protein kinases. Antibodies to brain p37 and p36 together with antibodies to lymphocytes lipocortins 1 and 2 were used to follow the distribution of these proteins in nervous tissues. Polypeptides of 37, 34, and 36 kDa cross-react with these antibodies. Anti-p37 and antilipocortin 1 cross-react on the same 37- and 34-kDa polypeptides; anti-p36 and antilipocortin 2 cross-react only on the 36-kDa polypeptides.

Lung calcium-dependent phospholipid-binding proteins: structure and function

Distinct peptide maps of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins (PLBPs), 36,000 and 33,000, were generated by cyanogen bromide (CNBr) cleavage, trypsin or Staphylococcus aureus V8 proteinase digestion. The amino acid sequence of a CNBr-cleaved peptide of the 36,000 PLBP was aligned to the amino terminus of human lipocortin I with more than 77% identity, but had no identity with the known amino terminal sequence of other known annexins. Partial amino acid sequence of a 33,000 PLBP peptide demonstrated a close (56%) relationship to endonexin II, human placental anticoagulant protein, and porcine intestine protein II, but shared only 32% identity with lipocortin I, 30% with lipocortin II. Antiserum generated against purified 36,000 PLBP reacted strongly with the 33,000 PLBP, but did not react with any other rabbit lung cytosolic proteins. Both PLBPs inhibited the phospholipase A2 reaction when dioleoyl phosphatidylcholine and phosphatidylglycerol vesicles or monolayers were used as substrates. In the vesicle assay, the phospholipase A2 reaction was inhibited at lower substrate phospholipid concentrations but not at nearly saturating substrate concentrations. In the monolayer assay, the phospholipid-binding proteins did not inhibit phospholipase A2 at a low phospholipid surface concentration of 3.8.10(-3) molecules/A2, but they did at higher surface concentrations between 1.1 x 10(-2) and 3.8 x 10(-2) molecules/A2. The inhibition of phospholipase A2 by rabbit lung phospholipid-binding proteins is most likely due to the prevention of penetration by phospholipase A2 into the interface, a requirement for the enzyme to act on the substrate.

Calpactin I in the differentiating embryonic chicken lens: mRNA levels and protein distribution

Calpactin I, one of the EDTA-extractable proteins of the lens membrane, binds phospholipid and actin in a calcium-dependent manner. It is also known substrate of the pp60arc kinase. Analysis of embryonic chicken lens RNA with a bovine calpactin I-specific cDNA probe revealed the presence of a approximately 1.8 Kb calpactin mRNA in the lens cells. Six-day embryonic chicken lenses were microdissected into central epithelium, equatorial epithelium, and fiber cells. Total cytoplasmic RNA was isolated from these samples and calpactin I mRNA levels were determined by the polymerase chain reaction (PCR) following reverse transcription (RT). Quantitative PCR indicates that the calpactin I mRNA levels in the equatorial epithelium are greater than in the central epithelium by a factor of 12.7 +/- 2.7. Calpactin I mRNA in fiber cells is an additional 3.5 +/- 1.5 times greater than in the equatorial epithelium. Whole mounts of embryonic chicken lens epithelia and histological sections of whole lenses were also examined with an antibody directed against chicken calpactin I. Calpactin I was predominantly localized in a punctate distribution in equatorial epithelial cells and near the plasma membrane of elongate fiber cells. The elevated levels of calpactin I mRNA observed in the equatorial epithelium and fiber cells and the immunological localization of the protein suggest a possible role of calpactin I in the elongation of fiber cells during lens differentiation.

Collagen-binding proteins of mammary epithelial cells are related to Ca2(+)- and phospholipid-binding annexins

Three major proteins of 34, 36, and 38 kDa were isolated from membrane preparations of chemically induced mammary tumors of the rat by collagen type I affinity chromatography and therefore were termed collagen-binding proteins (CBP). Three proteins in the same molecular weight range isolated from cell extracts by precipitation with calcium, solubilization of the precipitate with EGTA, and chromatography on hydroxylapatite were demonstrated to be immunologically related to CBP. As shown by immunoblot analysis, an antiserum directed against the cluster of the 34-38 kDa proteins reacted strongly with porcine intestinal protein I, weakly with porcine lipocortin I, and very weakly with porcine intestinal protein II. Antiserum against the 34 kDa protein reacted weakly with protein I but strongly with protein II. All three CBP reacted with protein I/calpactin I-specific antiserum of immunoblots and in immunoprecipitation experiments. However, antisera directed against CBP failed to show cross-reaction with collagen-binding protein anchorin II from chicken chondrocytes. Conversely, antisera against anchorin II did not react with CBP. Antiserum AS/87 immunoprecipitated CBP of 38 kDa that was labeled in a lactoperoxydase-catalyzed iodination, suggesting that this polypeptide is associated with the cell surface. Further, all three CBP were found to be phosphorylated by incubating mammary cells with 32P-orthophosphate. CBP bound to epithelial cell membranes in a Ca2+ dependent manner (= Triton X 100 insoluble form). Fractionated extraction and immunofluorescence microscopy also show that another form of CBP (= Triton X 100 soluble form) exists in these cells and is associated with a granular fraction. We therefore conclude that mammary collagen-binding proteins represent members of a family of Ca2(+)-binding membrane proteins. The 38 kDa CBP seems closely related to the pp60src kinase substrate protein I/calpactin I monomer, the 34 kDa CBP seems to be related or equivalent to protein II, while the relationship of the 36 kDa CBP to other defined proteins is still unclear.

Monoclonal antibodies specific to a Ca2(+)-bound form of lipocortin I distinguish its Ca2(+)-dependent phospholipid-binding ability from its ability to inhibit phospholipase A2

Lipocortin I, a Ca2(+)-and phospholipid-binding protein without EF-hand structures, has many biological effects in vitro. Its actual role in vivo, however is unknown. We obtained and characterized five monoclonal antibodies to lipocortin I. Two of these monoclonal antibodies (L2 and L4-MAbs) reacted with the Ca(+)-bound form of lipocortin I, but not with the Ca2(+)-free form, both in vivo and in vitro. Lipocortin I required greater than or equal to 10 microM-Ca2+ to bind the two antibodies, and this Ca2+ requirement was not affected by phosphatidylserine. L2-MAb abolished the phospholipase A2 inhibitory activity of lipocortin I and inhibited its binding to Escherichia coli membranes and to phosphatidylserine in vitro. L4-MAb abolished the phospholipase A2 inhibitory activity of lipocortin I, but did not affect its binding to E. coli membranes or to phosphatidylserine. These findings indicated that the inhibition of phospholipase A2 by lipocortin I was not simply due to removal or capping of the substrates in E. coli membranes. Furthermore, an immunofluorescence study using L2-MAb showed the actual existence of Ca2(+)-bound form of lipocortin I in vivo.

Phospholipases: old enzymes with new meaning

Phospholipases are enzymes that hydrolyze specific portions of phospholipid molecules. Their role in the digestion of exogenous phospholipids and as the active principle in snake and bee venoms has long been appreciated. Interest has increased in phospholipases recently because of new data implicating them in the inflammatory response. The ability of phospholipases to hydrolyze bacterial phospholipids has also received considerable attention. These new data have brought pertinence to studies of the physicochemical nature of potential substrates that greatly influence enzyme activity. Interest in the regulation of enzyme activity, both by physiological and pharmacological means, has increased as the importance of the phospholipases in response to various stimuli has become better appreciated. Finally, considerable interest has focused on the role of the phospholipases in response to hormones in a variety of cell systems. Data pertinent to all of these areas of interest will be discussed in this review with a view toward stimulating those with an interest in gastrointestinal physiology to apply them to their own areas of research in the gastrointestinal tract or liver.

The growth-regulated gene 1B6 is identified as the heavy chain of calpactin I

The expression of 1B6, a growth-regulated sequence isolated from a Syrian hamster fibroblast cDNA library, was studied in BALB/c 3T3 cells. The level of cytoplasmic 1B6 mRNA (1600 bases) was low in quiescent cells and plateaued in mid/late G1 after the cells were stimulated with 15% fetal calf serum (FCS). Protein synthesis was not required for the induction of 1B6 mRNA; therefore, the expression of 1B6 is a primary response to serum stimulation. The induction of 1B6 mRNA was also observed after stimulation with insulin, epidermal growth factor, and fibroblast growth factor but not with platelet-derived growth factor. When quiescent cells were serum-stimulated, the percentage of cells that became committed to enter DNA synthesis was proportional to the length of their incubation with serum. To determine if 1B6 expression was also correlated with the time of exposure to serum, quiescent cells were stimulated with a pulse of 15% FCS and the abundance level of 1B6 induced by that pulse was determined. The amount of 1B6 mRNA increased with increasing time of exposure to serum and paralleled the increase in the percentage of nuclei that were induced into DNA synthesis by the serum pulse. Comparison of the nucleotide sequence of the p1B6 cDNA to the GenBank database revealed a striking identity of 1B6 to the 3' end of p36, the heavy chain of calpactin I. The previous characterization of p36 as a substrate for tyrosine kinases suggests a possible role for 1B6/p36 in cell proliferation.

The functional site of placental anticoagulant protein: essential histidine residue of placental anticoagulant protein

Placental anticoagulant protein (PAP) rapidly lost its anticoagulant effect due to photooxidation in the presence of methylene blue at pH 7.9 and 8 degrees C. Photooxidized PAP failed to bind the phospholipid vesicle. It seemed unlikely that the protein underwent a change in molecular size during the photooxidation on the basis of its behavior in electrophoresis and gel filtration. Photooxidized PAP had significantly decreased histidine contents, whereas the contents of other amino acids remained essentially unchanged. The peptide, SHLRKV, was included in the functional site of PAP and still showed an anticoagulant activity. On the other hand, the peptide which substituted histidine by alanine, SALRKV, no longer showed the activity. It was shown that the histidine residue is involved in Ca2+ or the phospholipid binding site of the protein.

Identification and characterization of phospholipase A2 inhibitory proteins in human mononuclear cells

Calcium-dependent phospholipid binding and phospholipase A2 inhibitory proteins were isolated from human mononuclear cells. Lipocortins I and II were present whereas lipocortin IV (endonexin I) was not. The other proteins were purified to homogeneity and shown to have molecular masses of 35, 36, 32 and 73 kDa. The 36-kDa and 73-kDa proteins are related, the smaller appears to be part of the larger. The 73-kDa protein is related to the 67-kDa calelectrin and to lipocortin VI; the 32-kDa protein is different from endonexin I but related to chromobindin 7 and to lipocortin V. The 35-kDa protein has been identified by tryptic peptide sequencing as lipocortin III. All these proteins inhibit phospholipase A2 activity in vitro and the three smaller ones inhibit the [3H]arachidonic acid release from prelabelled monocytes induced by the calcium ionophore A23187 in a dose-dependent manner.

Structural analysis of p36, a Ca2+/lipid-binding protein of the annexin family, by proteolysis and chemical fragmentation

Limited proteolysis of the core domain of the 36-kDa protein p36 by trypsin gives a first insight into the structural organization of the four annexin repeats. Trypsin opens only a single peptide bond, situated between residues 204 and 205. The two fragments (of 20 kDa and 15 kDa), each containing two annexin repeats, remain as a tight complex (nicked core), which binds phospholipids in a Ca2(+)-dependent manner. After denaturation by 9 M urea, the nicked core is again formed upon renaturation provided both fragments are present. If the fragments are separated by chromatography in urea prior to renaturation, they show different behaviour. The 15-kDa C-terminal repeats aggregate, while the 20-kDa N-terminal repeats stay in solution. In comparison to p36, fragments with two (20-kDa fragment) or one (N-terminal CNBr fragment) annexin repeats show a conformational alteration in CD spectroscopy and hydrodynamics and display an increased susceptibility to proteases. In line with these differences, their Ca2(+)-dependent affinity to phospholipids is more than 10-20-fold decreased. Thus the four annexin repeats form together an integrated domain with multiple contacts between the repeats. Although stable derivatives with less than four repeats can be obtained, their Ca2+/phospholipid binding affinities are noticeably reduced.

Use of an RNA folding algorithm to choose regions for amplification by the polymerase chain reaction

We have used the program FOLD, which employs the Zuker folding algorithm, to identify regions of stable secondary structure in three chicken proto-oncogene mRNAs: c-src, c-myc, and c-fos. We have found that use of reverse transcriptase to synthesize a cDNA template for amplification by the polymerase chain reaction is successful only if the region chosen for amplification does not contain stem structures with calculated free energies less than -14 kcal/mol.

Structure and chromosome assignment of the murine p36 (calpactin I heavy chain) gene

p36 is a major substrate of both viral and growth factor receptor associated protein kinases. This protein has recently been named calpactin I heavy chain since it is the large subunit of a Ca2(+)-dependent phospholipid and actin binding heterotetramer. The primary structure of p36 has been determined from analysis of cloned cDNA. The protein contains 338 amino acids, has an approximate molecular weight of 39,000, and is comprised of several distinct domains, including four 75 amino acid repeats. From two overlapping cosmid clones isolated from different mouse genomic liver libraries, the complete intron/exon structure of the p36 gene was determined and the 5' and 3' noncoding regions of the gene were analyzed. The coding and 3' untranslated region of the p36 gene contains 12 exons which range in size from 48 to 322 base pairs (bp) with an average size of 107 bp. The repeat structures found at the protein level are not delineated by single exons, but the N-terminal p11-binding domain is encoded by a single exon. Structural mapping of the gene demonstrated that the lengths of the first two introns in the coding region are together approximately 6 kilobases (kb), while the other introns range in size from 600 to 3600 bp with an average size of 1650 bp. The p36 gene is at least 22 kb in length and has a coding sequence of approximately 1 kb, representing only 4.5% of the gene.(ABSTRACT TRUNCATED AT 250 WORDS)

A fluorescence spectroscopy study of the calpactin I complex and its subunits p11 and p36: calcium-dependent conformation changes

A fluorescence study of the calpactin I complex, a heterotetramer composed of two molecules of p36 and two molecules of p11, and its subunits, was performed to clarify their conformation. The analysis of the fluorescence characteristics of the single Trp of p36, in the absence of Ca(2+), shows that: (i) in the complex, Trp is buried within the protein matrix and subjected to static quenching from nearby groups; (ii) for p36 the results are similar, but Trp seems even more shielded than in the complex. Adding Ca(2+) to the calpactin I complex, or to p36, shifts the Trp emission maximum wavelengths, and increases the quantum yields which reflect a conformational change, burying the Trp in a more hydrophobic environment. In the presence and even in the absence of Ca(2+), the binding of phosphatidylserine liposomes induces a conformational change, detected by fluorescence measurements. The Ca(2+) dissociation constants, as determined by fluorescence titrations, are similar for the complex and p36 (KD approximately 0.5 x 10(-3) M). The affinity is enhanced a 1000-times in the presence of negatively charged phospholipids. In p11, both Try residues are located in a hydrophobic environment and the protein fluorescence does not change upon Ca(2+) addition.

The stimulatory effect of calpactin (annexin II) on calcium-dependent exocytosis in chromaffin cells: requirement for both the N-terminal and core domains of p36 and ATP

Calpactin, or calpactin heavy chain (p36), reconstitute secretion in digitonin-permeabilized adrenal chromaffin cells after a reduction in their secretory potential resulting from the loss of cytosolic components. We have characterized the stimulatory effect of p36, which resulted in an increase in both the extent and the rate of exocytosis. A mixture of other annexins (p70 and p32) did not have any effect on secretion at similar or greater concentrations than p36. Controlled proteolysis of p36 using chymotrypsin was carried out, and the 33,000 molecular weight core and 3000 molecular weight tail peptide isolated. In contrast to p36, p33 had no effect on exocytosis, even at high calcium concentrations. The N-terminal tail peptide and a synthetic peptide based on the tail of p36 [Ac-calpactin-(1-15)-NH2] had no effect on endogenous secretion, or secretion stimulated by exogenous p36. The results show that both the tail and core domains are required for p36 to stimulate exocytosis. The tail domain is unlikely to be required for interaction with cellular components but probably has a regulatory effect on the core domain. Endogenous secretion and the stimulatory effect of p36 were markedly inhibited by depletion of ATP. The ATP requirement for p36 action was not due to a requirement for phosphorylation by protein kinase C (PKC), since the PKC inhibitor staurosporine partially inhibited endogenous secretion but did not affect the stimulation of exocytosis due to exogenous p36.

A 32 kDa lipocortin from human mononuclear cells appears to be identical with the placental inhibitor of blood coagulation

A 32 kDa protein isolated from human mononuclear cells is a member of the lipocortin family, a new group of Ca2+-dependent lipid-binding proteins thought to be involved in the regulation of phospholipase A2, in exocytosis and in membrane-cytoskeleton interactions. Purification of this protein was based on its ability to associate with membrane phospholipids in a Ca2+-dependent manner and its capacity to inhibit purified phospholipase A2 from pig pancreas. Using immunological detection, we show that it is present in various cells involved in the inflammatory and coagulation processes. We present extensive amino acid data that strongly suggest that this protein is identical with a recently described inhibitor of blood coagulation, with endonexin II and with lipocortin V. Sequence alignment with other known proteins show a significant degree of homology with lipocortins I and II, the substrates of the epidermal-growth-factor receptor tyrosine kinase and the oncogene pp60src tyrosine kinase respectively, and with protein II. The possible physiological role of this 32 kDa lipocortin is discussed.

Lipocortin I (p35) is abundant in a restricted number of differentiated cell types in adult organs

Lipocortin-I (p35) is a unique calcium- and phospholipid-binding protein of the lipocortin/calpactin family. Although several possibilities have been suggested, functions for the individual proteins of this family are not yet known with certainty. As an initial step in the identification of the biological function(s) of p35, we have used immunohistochemical methods to define precisely many of the cellular phenotypes that contain p35 in vivo. In all organs where p35 is found, we have observed a striking distribution of p35-positive cells. Typically it is highly enriched in a limited range of differentiated cell types while apparently totally absent from most others. Our identification of specific p35-positive cell types in vivo will now set limitations on likely possibilities for functions of this protein and thereby permit a more logical approach to the determination of its true function.

Tumor promoter-dependent phosphorylation of a Triton X-100 extractable form of lipocortin I in T51B rat liver cells

The phosphorylation of lipocortin (a substrate of EGF-receptor kinase, and a putative phospholipase A2 inhibitor) was examined in T51B cells. By using Western blot procedures and antisera specific to lipocortin I, we found that most immunoreactive lipocortin I was located in the cytosol (lipocortin(cvt] of cells extracted in Ca2+-free buffers These cells however had another pool of immunoreactive lipocortin I located in the particulate fraction that was Triton X-100 extractable (lipocortin(mem]. Increasing Ca2+ concentrations in the extraction buffer resulted in more lipocortin(mem) recovered. In vitro phosphorylation of endogenous proteins demonstrated that lipocortin I became phosphorylated in a Ca2+ and phosphatidylserine-dependent manner, suggesting an involvement of protein kinase C. Treatment of cells with 100 ng/ml 12-0-tetradecanoylphorbol-13-acetate (TPA) but not with 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) resulted in the in vitro phosphorylation of lipocortin(mem) by protein kinase C. TPA also increased the phosphorylation of lipocortin(mem) in [32P]phosphate-labeled cells.

Further characterization of four lipocortins from human peripheral blood mononuclear cells

Four calcium and phospholipid binding proteins purified from mononuclear cells were characterized for PKC and EGF phosphorylation, actin binding capacity, and partial tissue distribution. Those named 35K, 32K, and 73K are equivalent, respectively, to lipocortin III, endonexin II and the 67 kDa calelectrin; 36K is a fragment of 73K. After purification, 35K and 73K were phosphorylated by protein kinase C in vitro but 36K nor 32K were not. None were phosphorylated by the epidermal growth factor receptor kinase in vitro; 73K bound F-actin in a calcium-dependent manner, whereas 35K, 36K, and 32K did not. Using Western blotting analysis, 32K and 73K were detected in high amounts in human lymphocytes, monocytes, liver, and placenta and in rat adrenal medulla; but 32K was not detected in polymorphonuclear cells, and 36K and 35K were detected in high amounts only, respectively, in human blood lymphocytes and polymorphonuclear cells. Thus, 32K and 73K appear to have a wide tissue distribution, whereas 35K has a much more restricted distribution.

Calcium channel activity of purified human synexin and structure of the human synexin gene

Synexin is a calcium-dependent membrane binding protein that not only fuses membranes but also acts as a voltage-dependent calcium channel. We have isolated and sequenced a set of overlapping cDNA clones for human synexin. The derived amino acid sequence of synexin reveals strong homology in the C-terminal domain with a previously identified class of calcium-dependent membrane binding proteins. These include endonexin II, lipocortin I, calpactin I heavy chain (p36), protein II, and calelectrin 67K. The Mr 51,000 synexin molecule can be divided into a unique, highly hydrophobic N-terminal domain of 167 amino acids and a conserved C-terminal region of 299 amino acids. The latter domain is composed of alternating hydrophobic and hydrophilic segments. Analysis of the entire structure reveals possible insights into such diverse properties as voltage-sensitive calcium channel activity, ion selectivity, affinity for phospholipids, and membrane fusion.

Synthesis of p36 and p35 is increased when U-937 cells differentiate in culture but expression is not inducible by glucocorticoids

p36 and p35 are distinct but related proteins that share many structural and biochemical features which were first identified as major substrates for protein-tyrosine kinases. Subsequently, both proteins have been shown to be Ca2+-, phospholipid-, and F-actin-binding proteins that underlie the plasma membrane and are associated with the cortical cytoskeleton. Recent reports have claimed that these proteins function as lipocortins, i.e., phospholipase A2 inhibitors that mediate the anti-inflammatory action of glucocorticoids. To investigate this possibility and to learn more about the functions of p36 and p35, we used human-specific anti-p36 and anti-p35 monoclonal antibodies to determine whether the expression or secretion of either protein was inducible by dexamethasone in the human U-937 myeloid cell line and in other human cell types. Additionally, we examined the levels of mRNA for both proteins. No effect of dexamethasone was observed on p36 or p35 expression at either the mRNA or protein level, nor were these proteins secreted under any of the culture conditions investigated. However, it was observed that in these cells the rate of synthesis and accumulation of both proteins was increased when the U-937 cells were induced to differentiate in culture to adherent macrophagelike cells. This offers a model system with which to study the control of p36 and p35 expression.

Isolation of a new member of the S100 protein family: amino acid sequence, tissue, and subcellular distribution

A low molecular mass protein which we term S100L was isolated from bovine lung. S100L possesses many of the properties of brain S100 such as self association, Ca++-binding (2 sites per subunit) with moderate affinity, and exposure of a hydrophobic site upon Ca++-saturation. Antibodies to brain S100 proteins, however, do not cross react with S100L. Tryptic peptides derived from S100L were sequenced revealing similarity to other members of the S100 family. Oligonucleotide probes based on these sequences were used to screen a cDNA library derived from a bovine kidney cell line (MDBK). A 562-nucleotide cDNA was sequenced and found to contain the complete coding region of S100L. The predicted amino acid sequence displays striking similarity, yet is clearly distinct from other members of the S100 protein family. Polyclonal and monoclonal antibodies were raised against S100L and used to determine the tissue and subcellular distribution of this molecule. The S100L protein is expressed at high levels in bovine kidney and lung tissue, low levels in brain and intestine, with intermediate levels in muscle. The MDBK cell line was found to contain both S100L and the calpactin light chain, another member of this protein family. S100L was not found associated with a higher molecular mass subunit in MDBK cells while the calpactin light chain was tightly bound to the calpactin heavy chain. Double label immunofluorescence microscopy confirmed the observation that the calpactin light chain and S100L have a different distribution in these cells.