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Six new members of the yeast p24 family have been identified and characterized. These six genes, named ERP1-ERP6 (for Emp24p- and Erv25p-related proteins) are not essential, but deletion of ERP1 or ERP2 causes defects in the transport of Gas1p, in the retention of BiP, and deletion of ERP1 results in the suppression of a temperature-sensitive mutation in SEC13 encoding a COPII vesicle coat protein. These phenotypes are similar to those caused by deletion of EMP24 or ERV25, two previously identified genes that encode related p24 proteins. Genetic and biochemical studies demonstrate that Erp1p and Erp2p function in a heteromeric complex with Emp24p and Erv25p.

1. Proximal enterocytes transmitted intact immunoglobulin G (IgG) preferentially in the order rat, human, sheep and bovine; the removal from the vascular compartment of these transmitted molecules occurred at about the same rate. 2. Heterologous IgGs are processed similarly to rat IgG: they are either transmitted intact or broken down to less than 1000 mol. wt. fragments. 3. All of the human transferrin removed from the intestine was broken down to less than 1000 mol. wt. fragments, but a small amount of bovine serum albumin (BSA) was transmitted intact. 4. The IgGs and BSA are relatively indigestible molecules whereas human transferrin is relatively digestible. 5. These observations are discussed in the context of receptor‐mediated transmission.

Calnexin is a lectin-like chaperone of the endoplasmic reticulum (ER) that couples temporally and spatially N-linked oligosaccharide modifications with the productive folding of newly synthesized glycoproteins. Calnexin was originally identified as a major type I integral membrane protein substrate of kinase(s) associated with the ER. Casein kinase II (CK2) was subsequently identified as an ER-associated kinase responsible for the in vitro phosphorylation of calnexin in microsomes (Ou, W-J., Thomas, D. Y., Bell, A. W., and Bergeron, J. J. M. (1992) J. Biol. Chem. 267, 23789-23796). We now report on the in vivo sites of calnexin phosphorylation. After 32PO4 labeling of HepG2 and Madin-Darby canine kidney cells, immunoprecipitated calnexin was phosphorylated exclusively on serine residues. Using nonradiolabeled cells, we subjected calnexin immunoprecipitates to in gel tryptic digestion followed by nanoelectrospray mass spectrometry employing selective scans specific for detection of phosphorylated fragments. Mass analyses identified three phosphorylated sites in calnexin from either HepG2 or Madin-Darby canine kidney cells. The three sites were localized to the more carboxyl-terminal half of the cytosolic domain: S534DAE (CK2 motif), S544QEE (CK2 motif), and S563PR. We conclude that CK2 is a kinase that phosphorylates calnexin in vivo as well as in microsomes in vitro. Another yet to be identified kinase (protein kinase C and/or proline-directed kinase) is directed toward the most COOH-terminal serine residue. Elucidation of the signaling cascade responsible for calnexin phosphorylation at these sites in vivo may define a novel regulatory function for calnexin in cargo folding and transport to the ER exit sites.

Chemokines are chemotactic proteins which play a central role in immune and inflammatory responses. Chemokine receptors are members of the seven transmembrane G-protein coupled family and have recently been shown to be involved in the entry of human immunodeficiency virus (HIV) into target cells. To study chemokine endocytosis in detail we have used novel site-specific chemistry to make a fluorescently labeled CC-chemokine agonist (rhodamine-MIP-1alpha) and antagonist (NBD-RANTES). We have also generated a CHO cell line stably expressing a hemagglutinin-tagged version of the CC-chemokine receptor 1 (CCR1), and using these reagents we have examined the receptor-mediated endocytosis of CC-chemokines by confocal microscopy. Our studies reveal that the agonist was internalized and accumulated in transferrin receptor-positive endosomes whereas the antagonist failed to internalize. However, receptor-bound antagonist could be induced to internalize by co-administration of agonist. Analysis of receptor redistribution following chemokine addition confirmed that sequestration was induced by agonists but not by antagonists.

The transepithelial transport of polymeric immunoglobulins is an essential process in the mucosal immune system. Transport across the epithelial cells of mucous or exocrine glands is affected by an integral membrane glycoprotein receptor known as membrane secretory component (SCm) or as polymeric immunoglobulin receptor (pIgR). This receptor binds polymeric immunoglobulins at the basolateral cell surface and mediates their transcellular translocation and their release from the apical plasma membrane into external secretions. Release depends on cleavage of the membrane-anchoring domain of the receptor, resulting in liberation of polymeric immunoglobulin bound to the ectoplasmic domain of the receptor (secreted SC or SCs) into extracellular secretions. Using a monoclonal antibody directed against the cytoplasmic tail of the receptor and a polyclonal antibody directed against the secreted ectoplasmic domain, we have combined cell fractionation and Western blotting techniques to examine the fate of these receptor domains in the hepatocyte. In this study, we characterize biochemically and morphologically the various subcellular components separated by our fractionation scheme, and correlate this with biochemical analysis of the receptor in each fraction.

The 89-kDa STAT3 protein is a latent transcription factor which is activated in response to cytokines (interleukin (IL)-5 and -6) and growth factors (epidermal growth factor). Binding of IL-5 to its specific receptor activates JAK2 which leads to the tyrosine phosphorylation of STAT3 proteins. Here we report the cloning of a cDNA encoding a variant of the transcription factor STAT3 (named STAT3beta) which was isolated by screening an eosinophil cDNA library. Compared to wild-type STAT3, STAT3beta lacks an internal domain of 50 base pairs located near the C terminus. This splice product is a naturally occurring isoform of STAT3 and encodes a 80-kDa protein. We found by reconstitution of the human IL-5R in COS cells that like STAT3, STAT3beta is phosphorylated on tyrosine and binds to the pIRE from the ICAM-1 promoter after IL-5 stimulation. However, STAT3beta fails to activate a pIRE containing promoter in transient transfection assays. Instead, co-expression of STAT3beta inhibits the transactivation potential of STAT3. These results suggests that STAT3beta functions as a negative regulator of transcription.

We set out to identify potential key regulators of exocytotic fusion in the eosinophil, in the knowledge that granule exocytosis can be stimulated in these cells by intracellular application of nonhydrolyzable analogues of guanosine triphosphate, with Ca2+ acting as a modulator of guanine nucleotide-dependent secretion. To screen for GTP-binding proteins, guinea pig eosinophils were purified from peritoneal washings and subjected to western blotting analysis using specific immune sera raised against recombinant proteins or consensus peptide sequences within proteins of interest. We found a number of heterotrimeric G proteins (G alpha i3, G alpha o, G alpha q11, G alpha s and G beta subunits) and members of the small GTP-binding proteins expressed in eosinophils. Two subtypes of G-protein alpha subunits (G alpha i1 and G alpha z) could not be detected. Separation of subcellular organelles from homogenized eosinophils by density gradient centrifugation revealed that all of the detected GTP-binding proteins were mainly expressed in fractions containing peak plasma membrane and Golgi marker enzyme activities, while G beta subunits were also detected in secretory granule fractions. However, isoforms of Rab3, a putative GTP-binding regulator of exocytotic fusion, were undetectable in eosinophils. Neither, with the exception of syntaxin-3, could we detect any of the proteins belonging to the proposed synaptic vesicle fusion complex (SNAP-25; synaptobrevin (VAMP) and its non-neuronal homologue, cellubrevin; synaptophysin; synaptotagmin). The results from this study, based on western blotting, suggest that eosinophils express a different class of exocytotic fusion complex proteins from those found in neuronal tissues, although a number of potential candidates fulfilling the role of GE were identified in this important inflammatory cell.

In this study we demonstrate that 125I-labelled interleukin (IL) 1 alpha binds specifically to its receptor on the surface of EL4 6.1 cells and is subsequently endocytosed and translocated from the cell membrane to the nucleus, where it progressively accumulates. Two-dimensional polyacrylamide-gel electrophoresis revealed that the internalized 125I-IL1 alpha associated with the nucleus was intact, with negligible breakdown products present. Specific and saturable binding of 125I-IL1 alpha was demonstrated on purified nuclei isolated from these cells. Binding of the radiolabelled ligand showed similar kinetics to that of the plasma-membrane receptor, and was inhibited by both unlabelled IL1 alpha and IL1 beta. Equilibrium binding studies on isolated nuclei revealed a single high-affinity binding site, with a Kd of 17 +/- 2 pM, and 79 +/- 12 binding sites per nucleus. These studies demonstrate that receptor-mediated endocytosis of IL1 results in its accumulation in the nucleus, and this mechanism may play an important role in mediating some of the actions of IL1.

This work presents experimental data on gas holdup in slurry bubble columns with a foaming liquid. The effects of solids concentration, solid particle size, superficial phase velocities and column dimensions on the gas holdup are analyzed. At low superficial gas velocities (less than 4cm/s), for which the liquid does not foam, the presence of solids with small particle size does not affect the gas holdup whereas solids with large particle size induce foam formation and thus their presence increases the gas holdup. In the foaming regime, an increase of solids concentration decreases the gas holdup. The operating mode has a strong effect on the gas holdup: the semi-batch operating mode (stagnant liquid-solid suspension) increases the ability of the liquid to foam with respect to the continuous mode. Regarding the effect of column dimensions, the results presented show that the height of the bubble column does not affect at an appreciable extent the gas holdup in the range 6 < LID < 12. At high gas velocities (greater than 6 cm/s) the gas holdups obtained in a 30 cm-internal diameter column are the same as those measured in a 10 cm-internal diameter column.

Interleukin-5 (IL-5) is a cytokine that plays a major role in the differentiation and activation of eosinophils. In order to identify which charged residues of human IL-5 are important in binding to its receptor and subsequent cellular activation, we have systematically replaced all of the clusters of charged amino acids with alanine residues. The mutants have been expressed in Escherichia coli, renatured, and purified. They were assayed for ability to cause proliferation of the erythroleukaemic cell line TF-1 and the up-regulation of eosinophil adhesion to ICAM-1. In addition, we studied receptor binding using either immobilized recombinant IL-5 receptor alpha-chain or the alpha/beta-receptor complex expressed on TF-1 cells. The key charged residue involved in binding to the beta-chain of the receptor is Glu-12. This residue is in an identical position to those previously identified in IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) involved in binding to the receptor beta-chain. The alpha-chain binding site is shown to involve the side chains Arg-90 and Glu-109, located in the second beta sheet and after the end of the fourth helix, respectively. It is unique to IL-5 and does not occur in IL-3 or GM-CSF. Understanding the topology of the interaction of IL-5 with its receptor chains will help in the search for rationally designed antagonists of IL-5 function.

A 0.2-ml dose containing 500 μg labelled rat IgG saturated the receptor-mediated transport system of the enterocytes of the jejunum of the small intestine. At subsaturation dose levels very little intracellular digestion of IgG occurred – the intact transport system was primarily employed. The uptake of IgG by enterocytes, at doses above saturation, was assessed over periods up to 6 h. There was an initial phase of rapid intracellular digestion by the enterocytes, followed by a much longer period during which the receptor-mediated transport system discharged intact IgG into the vascular compartment. The observations are discussed in the light of recent evidence relating to the numbers of IgG receptors on jejunal enterocytes.

A synthetic gene coding for human interleukin 4 (IL-4) was cloned and expressed in Saccharomyces cerevisiae (baker's yeast) as a C-terminal fusion protein with the yeast prepro alpha-mating factor sequence, resulting in secretion of mature IL-4 into the culture medium (0.6-0.8 micrograms/ml). A protocol was developed for purification of this protein. Crude cell-free conditioned medium was passed over a concanavalin A-Sepharose affinity column; bound proteins were eluted and further purified by S-Sepharose Fast Flow cation exchange and C18 reverse-phase h.p.l.c. Highly purified IL-4 was obtained by this method (0.3-0.4 mg per litre of culture) with a recovery of 51%. Thermospray liquid chromatography-mass spectrometry showed the C-terminal N-glycosylation site to be largely unmodified, and also showed that the N-terminus of the purified recombinant IL-4 (rIL-4) was authentic. Thiol titration revealed no free cysteine residues, implying that there are three disulphide groups, the positions of which remain to be determined. We have characterized the biological activities of the purified rIL-4. This material is active in B-cell co-stimulator assays, T-cell proliferation assays and in the induction of cell-surface expression of CD23 (the low-affinity receptor for IgE) on tonsillar B-cells. Half-maximal biological activity of the rIL-4 was achieved at a concentration of 120 pM. We have radioiodinated rIL-4 without loss of biological activity and performed equilibrium binding studies on Raji cells, a human B-cell line. The 125I-rIL-4 bound specifically to a single class of binding studies on Raji cells, a human B-cell line. The 125I-rIL-4 bound specifically to a single class of binding site with high affinity (Kd = 100 pM) and revealed 1100 receptors per cell. Receptor-ligand cross-linking studies demonstrated a single cell-surface receptor with an apparent molecular mass of 124 kDa. Two monoclonal antibodies have been raised to the human rIL-4, one of which blocks both the biological activity of rIL-4 and binding to its receptor.

Transcytosis of polymeric immunoglobulin (pIg) across glandular and mucosal epithelia is mediated by a member of the immunoglobulin supergene family, the pIg receptor. During transcellular routing, the receptor is cleaved and its ectoplasmic domain, known as secretory component (SC), is released into secretions bound to pIg. Using receptor-domain-specific antibodies, we have combined cell fractionation and immunoblotting of rat liver to examine the cellular routing of the receptor, the cellular location of the cleavage event and the fate of the anchor domain. Cleavage is a late event in receptor processing. It appears to occur at the canalicular plasma membrane, since intact receptor is present in this membrane domain and no SC is detected in whole liver homogenate or in cell fractions. The membrane anchor remaining after cleavage can be recovered in bile, as well as in a low-density fraction obtained after equilibrium centrifugation of liver (microsomal fractions) on sucrose density gradients. These data suggest that the membrane-anchor domain may be internalized as well as secreted together with SC into bile.