The amino-terminal immunoglobulin-like domain of activated leukocyte cell adhesion molecule binds specifically to the membrane-proximal scavenger receptor cysteine-rich domain of CD6 with a 1:1 stoichiometry

Activated leukocyte cell adhesion molecule (ALCAM) was recently identified as a ligand for CD6, a signaling receptor expressed on T cells, a subset of B cells, and some cells in the brain. Receptor-ligand binding assays, antibody blocking experiments, and examination of the tissue distribution of these two cell surface proteins suggest that CD6-ALCAM interactions play an important role in mediating the binding of thymocytes to thymic epithelial cells and of T cells to activated leukocytes. Presently, the details of CD6-ALCAM interactions and of signaling through CD6 are unknown. A series of truncated human ALCAM and CD6 immunoglobulin fusion proteins were produced and tested in different binding assays to analyze ALCAM-CD6 interactions in more detail. In this study, we report that the amino-terminal Ig-like domain of human ALCAM specifically binds to the third membrane-proximal scavenger receptor cysteine-rich (SRCR) domain of human CD6. Using thrombin-cleaved Ig fusion proteins containing single or multiple ALCAM or CD6 domains, we were able to determine that the stoichiometry of the interaction between the amino-terminal ALCAM domains and the membrane-proximal CD6 SRCR domain is 1:1. These results provide the first example of an Ig-like domain mediating an interaction with an SRCR domain.

Regulation of CD44 binding to hyaluronan by glycosylation of variably spliced exons

The hyaluronan (HA)-binding function (lectin function) of the leukocyte homing receptor, CD44, is tightly regulated. Herein we address possible mechanisms that regulate CD44 isoform-specific HA binding. Binding studies with melanoma transfectants expressing CD44H, CD44E, or with soluble immunoglobulin fusions of CD44H and CD44E (CD44H-Rg, CD44E-Rg) showed that although both CD44 isoforms can bind HA, CD44H binds HA more efficiently than CD44E. Using CD44-Rg fusion proteins we show that the variably spliced exons in CD44E, V8-V10, specifically reduce the lectin function of CD44, while replacement of V8-V10 by an ICAM-1 immunoglobulin domain restores binding to a level comparable to that of CD44H. Conversely, CD44 bound HA very weakly when exons V8-V10 were replaced with a CD34 mucin domain, which is heavily modified by O-linked glycans. Production of CD44E-Rg or incubation of CD44E-expressing transfectants in the presence of an O-linked glycosylation inhibitor restored HA binding to CD44H-Rg and to cell surface CD44H levels, respectively. We conclude that differential splicing provides a regulatory mechanism for CD44 lectin function and that this effect is due in part to O-linked carbohydrate moieties which are added to the Ser/Thr rich regions encoded by the variably spliced CD44 exons. Alternative splicing resulting in changes in protein glycosylation provide a novel mechanism for the regulation of lectin activity.

The membrane-proximal scavenger receptor cysteine-rich domain of CD6 contains the activated leukocyte cell adhesion molecule binding site

Binding studies with a CD6 immunoglobulin fusion protein (CD6 Rg) resulted in the identification and cloning of a CD6 ligand. This ligand was found to be a member of the immunoglobulin supergene family and was named ALCAM (activated leukocyte cell adhesion molecule). Cell adhesion assays showed that CD6-ALCAM interactions mediate thymocyte-thymic epithelium cell binding. ALCAM is also expressed by activated leukocytes and neurons and may be involved in interactions between T cells and activated leukocytes and between cells of the immune and nervous systems, respectively. Herein we describe the preparation of domain-specific murine CD6 Rg fusion proteins and show that the membrane-proximal SRCR (scavenger receptor cysteine-rich) domain of CD6 contains the ALCAM binding site. We also show that mAbs which bind to this domain preferentially block CD6-ALCAM binding. These results demonstrate that the membrane-proximal SRCR domain of CD6 is necessary for CD6 binding to ALCAM and provide the first direct evidence for the interaction of an SRCR domain with a ligand.

Cloning, mapping, and characterization of activated leukocyte-cell adhesion molecule (ALCAM), a CD6 ligand

Antibody-blocking studies have demonstrated the role of CD6 in thymocyte-thymic epithelial (TE) cell adhesion. Here we report that CD6 expressed by COS cells mediates adhesion to TE cells and that this interaction is specifically blocked with an anti-CD6 monoclonal antibody (mAb) or with a mAb (J4-81) that recognized a TE cell antigen. We isolated and expressed a cDNA clone encoding this antigen and show that COS cells transfected with this cDNA bind a CD6 immunoglobulin fusion protein (CD6-Rg). This antigen, which we named ALCAM (activated leukocyte-cell adhesion molecule) because of its expression on activated leukocytes, appears to be the human homologue of the chicken neural adhesion molecule BEN/SC-1/DM-GRASP. The gene was mapped to human chromosome 3q13.1-q13.2 by fluorescence in situ hybridization of cDNA probes to metaphase chromosomes. We prepared an ALCAM-Rg fusion protein and showed that it binds to COS cell transfectants expressing CD6, demonstrating that ALCAM is a CD6 ligand. The observations that ALCAM is also expressed by activated leukocytes and that both ALCAM and CD6 are expressed in the brain suggest that ALCAM-CD6 interactions may play a role in the binding of T and B cells to activated leukocytes, as well as in interactions between cells of the nervous system.

Developmental abnormalities in mice transgenic for bovine oncostatin M

Oncostatin M belongs to the subfamily of hematopoietin cytokines that binds a receptor complex containing gp130. To date, only the human form of oncostatin M has been identified, and its evolutionary conservation is unresolved. We have isolated a bovine gene whose open reading frame encodes a precursor protein that is 58% identical to human oncostatin M. A comparison of the bovine and human amino acid sequences predicts significant similarity, including the four-alpha-helical-bundle structure and the placement of disulfide bridges. As with the human protein, bovine oncostatin M binds specific receptors on human H2981 cells and inhibits the proliferation of human A375 tumor cells and mouse M1 leukemia cells. To identify activities regulated in vivo, we injected bovine oncostatin M fusion genes containing various tissue-specific promoters into mouse embryos. The frequencies of transgenic mice were reduced significantly, suggesting that overexpression of the bovine cytokine is detrimental to normal mouse development. In addition to deaths associated with expression in neurons and keratinized epithelia, bovine oncostatin M caused abnormalities in bone growth and spermatogenesis, stimulated fibrosis surrounding islets in the pancreas, and disrupted normal lymphoid tissue development. This work establishes the existence of a nonprimate oncostatin M gene and provides the first demonstration that this cytokine can function in a pleiotropic manner in vivo. Information regarding bovine oncostatin M may help characterize the structure and function of this cytokine in other vertebrate species.

CD44 isoforms containing exon V3 are responsible for the presentation of heparin-binding growth factor

Glycosaminoglycan-modified isoforms of CD44 have been implicated in growth factor presentation at sites of inflammation. In the present study we show that COS cell transfectants expressing CD44 isoforms containing the alternatively spliced exon V3 are modified with heparan sulfate (HS). Binding studies with three HS-binding growth factors, basic-fibroblast growth factor (b-FGF), heparin binding-epidermal growth factor (HB-EGF), and amphiregulin, showed that the HS-modified CD44 isoforms are able to bind to b-FGF and HB-EGF, but not AR. b-FGF and HB-EGF binding to HS-modified CD44 was eliminated by pretreating the protein with heparitinase or by blocking with free heparin. HS-modified CD44 immunoprecipitated from keratinocytes, which express a CD44 isoform containing V3, also bound to b-FGF. We examined whether HS-modified CD44 isoforms were expressed by activated endothelial cells where they might present HS-binding growth factors to leukocytes during an inflammatory response. PCR and antibody-binding studies showed that activated cultured endothelial cells only express the CD44H isoform which does not contain any of the variably spliced exons including V3. Immunohistological studies with antibodies directed to CD44 extracellular domains encoded by the variably spliced exons showed that vascular endothelial cells in inflamed skin tissue sections do not express CD44 spliced variants. Keratinocytes, monocytes, and dendritic cells in the same specimens were found to express variably spliced CD44. 35SO4(-2)-labeling experiments demonstrated that activated cultured endothelial cells do not express detectable levels of chondroitin sulfate or HS-modified CD44. Our results suggest that one of the functions of CD44 isoforms expressing V3 is to bind and present a subset of HS-binding proteins. Furthermore, it is probable that HS-modified CD44 is involved in the presentation of HS-binding proteins by keratinocytes in inflamed skin. However, our data suggests that CD44 is not likely to be the proteoglycan principally involved in presenting HS-binding growth factors to leukocytes on the vascular cell wall.

Characterization and detection of sc4: a sixth gene encoded by sonchus yellow net virus

The nucleotide sequence of a sixth gene (sc4) of the plant rhabdovirus sonchus yellow net virus (SYNV) was determined from viral genomic and poly(A)+ cDNA clones. The sc4 gene is 1196 nucleotides (nt) and has an open reading frame of 972 nt that is capable of encoding a protein of 324 amino acids. Primer extension analyses of poly(A)+ RNA isolated from infected plants indicate that the 5' end of the sc4 mRNA corresponds to nucleotide 2840, relative to the 3' end of the minus-sense genomic RNA and extends to nucleotide 4035. A 43-nt untranslated leader sequence precedes the predicted first AUG codon and a 181-nt untranslated sequence follows the translational stop codon. This gene is similar to the other SYNV genes in that it is flanked on each side by a conserved gene junction sequence. Polyclonal antibodies raised to an sc4 fusion protein react with a 37-kDa protein in virus-infected plants that is close to the predicted size of the sc4 protein. Western blot analyses of cellular fractionations from infected plants show that sc4 is membrane associated and sucrose density gradient analyses demonstrate that sc4 sediments in the same fractions as SYNV virions. Analysis of the sc4 open reading frame reveals that 16% of the amino acids are serine or threonine residues and that the protein has four potential consensus casein kinase II phosphorylation sites. The deduced amino acid sequence of sc4 also contains a motif related to alpha amylases and aspartic proteases. This completes the sequence determination of the 13,720-nt SYNV genome.

Interactions between oncostatin M and the IL-6 signal transducer, gp130

Recently, gp130, the signal transducer for interleukin 6 (IL-6), leukemia inhibitory factor (LIF), and ciliary neurotrophice factor (CNTF), was identified as the low-affinity receptor for oncostatin M (OM). However, it is not yet clear if OM binding to gp130 requires accessory factor(s) and if gp130 alone can mediate OM signalling. Here we report that: (a) expressing murine gp130 in BAF-B03 cells (BAF-m130) resulted in the appearance of a single class of low-affinity OM binding sites; (b) chemical cross-linking studies with 125I-OM identified a 180 kDa labelled complex on BAF-m130 cells; (c) OM cross-linking to the H2981 cell line which expresses both low- and high-affinity OM receptor, identified a 180 kDa and an additional 280 kDa species; (d) 125I-OM was specifically cross-linked to soluble recombinant gp130 (sgp130-Rg) in solution; and (e) the cellular proliferation of BAF-m130 was unaffected by OM treatment. These data indicate that gp130 can act as the low-affinity receptor for OM, however, gp130-OM interactions alone are unable to elicit cellular proliferation. This suggests that an additional factor(s) are required to interact with the OM/gp130 complex to form the high-affinity functional receptor. We propose that the 280 kDa species detected on H2981 cells is likely a complex of OM, gp130, and the putative beta chain of the functional OM high-affinity receptor. Recently, OM has been shown to be the major growth factor for Kaposi's sarcoma derived cells.(ABSTRACT TRUNCATED AT 250 WORDS)

LIF and OM directly interact with a soluble form of gp130, the IL-6 receptor signal transducing subunit

Oncostatin M and LIF are related members of a cytokine family that also includes IL-6, CNTF and G-CSF. These proteins exhibit overlapping biological properties and with the exception of G-CSF, they all appear to utilize gp130 as a signaling component of their high affinity receptor complexes. Recently it has been demonstrated that monomeric, membrane bound gp130 can directly bind OM. To further investigate the binding properties of gp130 we generated a soluble form of gp130, sgp130-Rg, to investigate potential gp130 interactions with OM and other members of this cytokine family. Using chemical crosslinking techniques we demonstrate that OM and LIF but not CNTF or IL-6 directly interact with sgp130-Rg. Since OM signaling can be prevented by binding gp130 with anti gp130 mAbs we also investigated the potential of sgp130-Rg to prevent the biological activities of both LIF and OM. Here we demonstrate that sgp130-Rg can bind LIF and OM preventing their biological activities on the TF-1 erythroleukemia cell line. This property suggests that sgp130-Rg may have therapeutic value in the specific prevention of LIF or OM mediated pathologies.

Interleukin-6 signal transducer gp130 mediates oncostatin M signaling

Oncostatin M (OM) is a multifunctional cytokine that is structurally and functionally related to interleukin 6 (IL-6) and leukemia inhibitory factor (LIF). The specific receptor for OM has been demonstrated (by chemical cross-linking) to be a 150-kDa protein in a number of cell lines. The IL-6 signal transducer, gp130, is also an affinity converter for the LIF receptor. It does not bind to either IL-6 or LIF, but associates with the alpha subunits of the receptors and transduces the signals. We examined the possible involvement of gp130 in OM binding and signaling. We demonstrate that: (a) anti-gp130 monoclonal antibodies (mAbs) block the inhibitory effect of OM on A375 cell growth, (b) the binding and cross-linking of 125I-OM to H2981 cells are completely abolished by anti-gp130 mAbs, (c) the cross-linked OM-receptor complex is immunoprecipitated by anti-gp130 mAbs, and (d) COS-7 cells transfected with the full-length cDNA encoding gp130 exhibit increased OM binding and cross-linking, which are also blocked by anti-gp130 mAbs. Therefore, we conclude that the 150-kDa OM binding protein previously characterized in a variety of cell lines is gp130. OM is the natural ligand for gp130 and gp130 mediates the biological responses of OM.

The interaction of amphiregulin with nuclei and putative nuclear localization sequence binding proteins

Amphiregulin (AR) is a 23 kDa, bifunctional growth modulating glycoprotein belonging to the epidermal growth factor (EGF) family of polypeptide growth regulators. AR possesses two putative nuclear localization sequences (NLS), binds to DNA sepharose, and localizes to the nucleoli of human ovarian surface epithelial carcinoma cells suggesting that AR has a direct nuclear role. We have found that 125I-labeled AR, when exogenously applied to several carcinoma cell lines, associated with nuclei in a time, temperature, and concentration dependent fashion. The control peptide, EGF, also associated with these fractions but at approximately 20% of the efficiency of AR. Cross-linking experiments with 125I-labeled AR and nuclear fractions derived from various carcinoma and normal cell lines demonstrated that AR binds two proteins of molecular mass 205 and 120 kDa. AR binding to these nuclear fraction proteins was specific and saturable as shown by competition experiments utilizing both SV-40 large T antigen NLS and an AR derived peptide encompassing both putative AR NLS. The combined results suggest that nuclear interactions may play a significant role in AR induced growth responses.

Structure of the glycoprotein gene of sonchus yellow net virus, a plant rhabdovirus

The nucleotide sequence of the glycoprotein (G) gene of sonchus yellow net virus (SYNV), a plant rhabdovirus, was determined from viral genomic and mRNA cDNA clones. The G cistron is 2045 nucleotides (nt) long and the G protein mRNA open reading frame (ORF), as determined from the cDNA sequence, contains 1896 nt and encodes a protein of 632 amino acids. Immunoblots with antibodies elecited against the purified glycoprotein from virus particles reacted with a fusion protein produced in Escherichia coli, indicating that the cloned ORF encodes the G protein. The 5' end of the G protein mRNA corresponds to nt 5111, relative to the 3' end of the viral (minus sense) genome, as determined by primer extension from mRNA isolated from infected plants, and extends to nt position 7155 on the genomic RNA. A 34-nt untranslated 5' leader sequence and a 115-nt untranslated 3' end flank the ORF on the mRNA. The gene junctions on either side of the G gene on the genomic RNA are identical to those previously described for other SYNV genes and are similar to sequences separating genes of animal rhabdoviruses. The predicted molecular weight of the G protein is 70,215 Da, a value less than the 77,000 Da estimated for the glycosylated G protein from virus particles. The deduced amino acid sequence of the SYNV G protein shares little direct relatedness with the G proteins of other rhabdoviruses, but appears to contain a similar signal sequence, a transmembrane anchor domain, and glycosylation signals. In addition, the SYNV G protein contains a putative nuclear targeting site near the carboxy terminus, which may be involved in transit to the nuclear membrane prior to morphogenesis.

Amphiregulin-associated protein: complete amino acid sequence of a protein produced by the 12-0-tetradecanoylphorbol-13-acetate-treated human breast adenocarcinoma cell line MCF-7

Amphiregulin-associated protein (ARAP) was purified from serum-free conditioned medium of MCF-7, human breast carcinoma cells, treated with 12-0-tetradecanoylphorbol-13-acetate (TPA). ARAP is a single-chain, extremely hydrophilic, heparin-binding protein. Its apparent molecular weight is approximately 21,500 as assessed by gel chromatography and approximately 15,500 as determined by polyacrylamide gel electrophoresis. The complete amino acid sequence of ARAP was determined. The larger form contains 123 amino acids, whereas a shorter form is missing two amino acids at the amino-terminal. ARAP contains 10 cysteines and 30 basic amino acids (23 lysines and 7 arginines). ARP sequence has been found to be identical to protein encoded by human MK gene.

Structure of the gene encoding the M1 protein of sonchus yellow net virus

The gene encoding the M1 protein of sonchus yellow net virus (SYNV), a plant rhabdovirus, has been sequenced and identified by Western blot analysis of SYNV proteins using antibodies directed against a fusion protein derived from a portion of the sequenced gene. The M1 gene is positioned between nucleotides 4039 and 5109 relative to the 3' end of the viral RNA and is the fourth gene from the 3' end of the genome. The 1071-nucleotide (nt) M1 gene lies between a putative nonstructural gene of unknown function and the gene encoding the glycoprotein and is bordered on either side by the same GG intergenic dinucleotide that separates other genes in the SYNV genome. The M1 mRNA (scRNA 6) consists of a 71-nt untranslated region at the 5' terminus followed by an 858-nt open reading frame (ORF) capable of encoding a protein with a calculated molecular weight of 31,779. The amino acid sequence deduced from this ORF is not highly homologous to those of other rhabdovirus matrix proteins, but has some localized regions of similarity. The UGA codon that terminates the M1 ORF is followed by a 3' untranslated region of 142 nt. The viral RNA (minus-sense) sequence corresponding to the extreme 3' end of the mRNA contains a 9-nt tract (3'-AUUGUUUUU-5') that is identical to the sequences at the termini of other SYNV genes.

Turkey gizzard caldesmon: molecular weight determination and calmodulin binding studies

Sedimentation equilibrium and sedimentation velocity measurements demonstrate that turkey gizzard caldesmon is an elongated molecule of molecular mass 75 +/- 2 kDa. The frictional ratio (2.14) is consistent with a prolate ellipsoid of axial ratio 24, corresponding to an apparent length and width of 516 and 21.5 A, respectively. As was previously determined for chicken gizzard caldesmon [Graceffa, P., Wang, C.-L.A., & Stafford, W.F. (1988) J. Biol. Chem. 263, 14196-14202], this molecular weight is appreciably smaller than the value (approximately 135,000) estimated from the results of NaDodSO4 gel electrophoresis experiments. However, a significant difference between the true molecular weights of turkey and chicken gizzard caldesmons--75,000 versus 93,000--also points to probable molecular weight variations within the subclass. Binding measurements, based on perturbation of the intrinsic tryptophan fluorescence of caldesmon in the presence of calmodulin, show that the interaction between the two proteins is strongly ionic strength and temperature dependent. Dissociation constants of 0.075 and 0.38 microM were determined in solutions containing 0.1 and 0.2 M KCl, respectively, at 24.3 degrees C. Fluorescence emission spectra and fluorescence anisotropy excitation spectra indicate that the tryptophanyl residues of caldesmon are located in solvent-accessible regions of the molecule, where they exhibit a high degree of mobility even when calmodulin is bound.