A 45-kDa human T-cell membrane glycoprotein functions in the regulation of cell proliferative responses

A 45-kDa human T cell surface glycoprotein which is tightly bound in the membrane of the resting T cell is released into the cell medium in soluble form after cell growth activation by phytohemagglutinin or neuraminidase/galactose oxidase treatments. In limited proteolysis by Staphylococcus aureus V8 protease, two major 35-kDa and 27-kDa peptide fragments of the surface-iodinated 45-kDa protein are common to the membrane-bound and the released forms, but a third 18-kDa fragment is observed exclusively with the released protein. The apparent molecular masses of the deglycosylated peptide backbones of the membrane-bound and the released molecule are 30 +/- 1 kDa, although a small size difference cannot be excluded. A polyclonal rabbit anti-(T cell membrane protein) antiserum precipitates the 45-kDa protein. A monoclonal anti-(45-kDa protein) antibody precipitates the membrane-bound 45-kDa protein solubilized with octyl glucoside, but does not precipitate the released protein. In cell culture assays, the monoclonal anti-(45-kDa protein) antibody specifically enhances the cell proliferative responses in phytohemagglutinin-treated and mixed lymphocyte cultures. These observations suggest that the 45-kDa protein has a specific receptor function in the regulation of cell proliferative responses.     

A new method for the rapid purification of both the membrane-bound and released forms of the variant surface glycoprotein from Trypanosoma brucei.

A simple new technique was developed for the rapid purification of either the membrane-bound or the released forms of the variant surface glycoprotein of Trypanosoma brucei in high yield. Whole cells were used as the source of the membrane-bound form, and the supernatant of benzyl alcohol-treated cells was used as the source of the released form. The technique was based on extraction of the acid-treated protein into chloroform/methanol, followed by selective re-partition into aqueous salt solution. The yield of purified protein was found to be dependent critically on a low pH during the extraction/re-partition stages. This finding and the ability to cycle the protein repeatedly through organic and aqueous phases in a strictly pH-dependent manner suggested that the protein could undergo fully reversible denaturation/renaturation only while in an extensively protonated form. The yield was independent of the polarity of the organic phase and the protein concentration over a wide range. After purification, both forms retain their ability to react with specific antibody raised against the authentic native protein purified by conventional means. The amino acid composition and the identity of the N-terminal amino acid was the same for both forms of the protein. In addition, both forms had blocked C-terminal residues. There were determined to be 1.13 X 10(7) copies of the variant surface glycoprotein per cell.     

Purification of 5'-nucleotidase from human placenta after release from plasma membranes by phosphatidylinositol-specific phospholipase C

5'-Nucleotidase was purified greater than 1000-fold from human placenta by treatment of plasma membranes with S. aureus phosphatidylinositol-specific phospholipase C and affinity chromatography on Con A Sepharose and AMP-Sepharose. The resulting enzyme had a specific activity of greater than 5000 mumol/hr/mg protein and a subunit molecular weight of 73,000. Goat antibodies against 5'-nucleotidase inhibited enzyme activity and detected 5'-nucleotidase after Western blotting. These antibodies also recognized a soluble form of 5'-nucleotidase and residual membrane-bound 5'-nucleotidase which could not be released by phosphatidylinositol-specific phospholipase C treatment, suggesting that the three forms of the enzyme are structurally related. The soluble 5'-nucleotidase may be derived from the membrane-bound form by the action of an endogenous phospholipase C. The structural basis for the inability of some of the membrane-bound 5'-nucleotidase to be released by phosphatidylinositol-specific phospholipase C is unknown.     

Membrane-anchored and soluble forms of betaglycan, a polymorphic proteoglycan that binds transforming growth factor-beta

Transforming growth factors beta 1 and beta 2 bind with high affinity to the core protein of a 250-350-kD cell surface proteoglycan. This proteoglycan (formerly referred to as the type III TGF-beta receptor) coexists in many cells with the receptor implicated in TGF-beta signal transduction (type I TGF-beta receptor), but its function is not known. We report here that soluble TGF-beta-binding proteoglycans are released by several cell types into the culture media, and can be found in serum and extracellular matrices. As has been shown for the membrane-bound form, the soluble proteoglycans have a heterogeneous core protein of 100-120 kD that carries chondroitin sulfate and/or heparan sulfate glycosaminoglycan chains and a small amount of N-linked carbohydrate. The membrane-bound form of this proteoglycan is hydrophobic and associates with liposomes, whereas the soluble forms lack a membrane anchor and do not associate with liposomes. Differences in the electrophoretic migration of the soluble and membrane forms of this proteoglycan suggest additional structural differences in their core proteins and glycosaminoglycan chains. These soluble and membrane-bound proteoglycans, for which we propose the name "betaglycans," might play distinct roles in pericellular retention, delivery, or clearance of activated TGF-beta.     

Prostasin is a glycosylphosphatidylinositol-anchored active serine protease

A recombinant human prostasin serine protease was expressed in several human cell lines. Subcellular fractionation showed that this serine protease is synthesized as a membrane-bound protein while a free-form prostasin is secreted into the culture medium. Prostasin was identified in nuclear and membrane fractions. Membrane-bound prostasin can be released by phosphatidylinositol-specific phospholipase C treatment, or labeled by [(3)H]ethanolamine, indicating a glycosylphosphatidylinositol anchorage. A prostasin-binding protein was identified in mouse and human seminal vesicle fluid. Both the secreted and the membrane-bound prostasin were able to form a covalently linked 82-kDa complex when incubated with seminal vesicle fluid. The complex formation between prostasin and the prostasin-binding protein was inhibited by a prostasin antibody, heparin, and serine protease inhibitors. Prostasin's serine protease activity was inhibited when bound to the prostasin-binding protein in mouse seminal vesicle fluid. This study indicates that prostasin is an active serine protease in its membrane-bound form.     

Production of soluble matriptase by human cancer cell lines and cell surface activation of its zymogen by trypsin

The membrane-bound serine proteinase matriptase, which is often released from the plasma membrane of epithelial and carcinoma cells, has been implicated to play important roles in both physiological and pathological conditions. However, the regulatory mechanism of its activity is poorly understood. In the present study, we examined expression and activation state of soluble matriptase in 24 human cancer cell lines. Soluble matriptase was detected in the conditioned media from all of 5 colon and 4 breast carcinoma cell lines and 8 of 10 stomach carcinoma cell lines tested. Only two of five lung cancer cell lines released the matriptase protein into the culture media. Out of the five matriptase-negative cell lines, two cell lines expressed the matriptase mRNA. Among 24 cancer cell lines tested, 13 cell lines secreted trypsin in an active or latent form and all of them released matriptase. Most of the 24 cell lines released a latent, single-chain matriptase of 75 kDa as a major form, as well as low levels of complex forms of an activated two-chain enzyme with its specific inhibitor HAI-1. Thus, these soluble matriptases appeared to have little proteolytic activity. Treatment of stomach and colon cancer cell lines with epidermal growth factor stimulated the release of matripatase/HAI-1 complexes. In cancer cell lines secreting active trypsin, however, matriptase was released mostly as an inhibitor-free, two-chain active form. Trypsin seemed to activate the membrane-bound, latent matriptase on the cell surface. These results suggest that matriptase and trypsin cooperatively function for extracellular proteolysis.     

Studies on a membrane-bound and solubilized ribonucleotide reductase preparation from Escherichia coli TAU-.

Ribonucleotide reductase has been shown to be associated with the DNA-membrane complex in Escherichia coli TAU- cells. The membrane-bound enzyme has been released in a soluble form using a combined treatment of 1% sarcosyl (pH 8.0) and 1% sodium deoxycholate (pH 6.5). Allotropic differences in the modulatory effects of ATP, Mg2+, EDTA and dithiothreitol on the membrane-bound and solubilized enzyme activity are discussed.     

Phosphatidylinositol is involved in the membrane attachment of NCAM-120, the smallest component of the neural cell adhesion molecule.

The rodent neural cell adhesion molecule (NCAM) consists of three glycoproteins with Mr of 180,000, 140,000 and 120,000. The Mr 120,000 protein (NCAM-120) has been shown to exist in membrane-bound and soluble forms but the nature of its membrane association and release has remained obscure. We show here that phosphatidylinositol-specific phospholipase C (PI-PLC), but not a phospholipase C of different specificity, releases a substantial proportion of NCAM-120 from brain membranes and solubilizes almost quantitatively NCAM-120 present at the surface of C6 astroglial cells. The PI-PLC effect was highly selective since only one other protein species was detectably released from C6 cells. These results suggest that NCAM-120 is held in the membrane by covalently bound phosphatidylinositol or a closely related lipid in a way similar to several other surface proteins from eukaryotic cells. The presence of NCAM in a form which can be released from the cell surface by a highly selective mechanism raises additional possibilities for modulation and control of cell--cell adhesion.     

Secretion of gamma-glutamyltranspeptidase by the pancreas: evidence for a membrane shedding process during exocytosis

Gamma Glutamyltranspeptidase (GGT) is a membrane-bound enzyme involved in glutathione metabolism. It is present in rat exocrine pancreas at a level which is only exceeded by the kidney. It has been previously shown that most of the enzyme activity is located in the apical area of the acinar cell, more precisely at the level of zymogen granules and plasma membrane. The aim of the present study was to examine the secretory behavior of that enzyme. Under resting conditions, in vivo, high levels of GGT were found in pancreatic juice and its level was not related to protein concentration. Under secretin infusion, a relatively constant level of GGT was released, and again, there was no correlation between enzyme activity and protein concentration. Following a bolus injection of caerulein, an analog of cholecystokinin, marked and concomitant rises in protein and GGT levels were observed. Ultracentrifugation, as well as gel filtration on Sepharose 4B, demonstrated that the enzyme was not released in a soluble form. This observation is in agreement with in vitro determinations on isolated zymogen granules showing that GGT is totally associated with the ZG membrane and undetect-able in the content of these organelles. The present data show that 1 degree GGT is released from the rat pancreas acinar cells in a particulate form; 2 degree GGT release is elicited by hormonal stimulation coinciding with the exocytotic release of secretory proteins. Our observations lead us to propose that in rat pancreas, ZG membrane fragments are released along with secretory proteins during exocytosis.     

myo-Inositol dehydrogenase(s) from Acetomonas oxydans. Optimization of conditions for solubilization of membrane-bound enzyme

Methods are described for the first reported successful isolation of the soluble form of the membrane-bound myo-inositol dehydrogenase(s) from Acetomonas oxydans. Conditions for optimum yields of active enzyme in a crude membrane-free protein extract were established. The relevant conditions are (a) cell rupture by solid-shearing, (b) solubilization of the complete 60000g pellet with sodium deoxycholate at pH7.2, (c) rapid separation of the released protein from sodium deoxycholate by gel chromatography.     

The difficult challenge of cloning the angiotensin II receptor

The vasopressor peptide angiotensin II exerts its cellular effects through a membrane-bound receptor coupled to a G protein. Biochemical and pharmacological analyses of this receptor already identify two different membrane-bound receptors and one cytosoluble angiotensin-II-binding protein. Nevertheless, the purification of the membrane-bound form(s) appears to be difficult. In the absence of purified protein, two cloning strategies of the gene have been explored: (1) expression cloning, identifying the functions of the protein expressed from a cDNA library in COS cells or Xenopus oocytes, has been unsuccessful until now; (2) analogical cloning, trying to identify related members of the seven transmembrane segment receptor family, which could be related to angiotensin receptors, identifies the mas oncogene and two related genes. However, there are accumulating data to exclude their involvement in angiotensin binding.     

Vitamin A uptake from retinol-binding protein in a cell-free system from pigment epithelial cells of bovine retina. Retinol transfer from plasma retinol-binding protein to cytoplasmic retinol-binding protein with retinyl-ester formation as the intermediate step

We have investigated the steps by which retinol, released from plasma retinol-binding protein (RBP), enters the cells and is accumulated for the most part as a retinyl-ester, only a small fraction of it being present as a complex with cytoplasmic retinol-binding protein (CRBP). For this purpose, we have developed a cell-free system composed of plasma membrane-enriched fractions from bovine retinal pigment epithelium which selectively incorporates exogenous vitamin A when presented as a retinol-RBP complex. Upon incubation in the presence of [3H]retinol-RBP, isolated plasma membrane fractions take up and esterify retinol. A 4-fold reduction of total vitamin A incorporation is observed in conditions which specifically inhibit retinyl-ester formation, thus indicating that the two processes of retinol uptake and esterification are functionally coupled. Evidence is presented that retinol bound to a plasma membrane receptor sharing functional and structural similarities with CRBP is the actual substrate for esterification. Vitamin A accumulation seems to require retinol esterification to allow the recycling of a limited number of free, plasma membrane-associated, retinol receptors. Mobilization of retinol stored as a membrane-bound retinyl-ester is mediated by a membrane-associated hydrolase activity selectively controlled by the level of apo-CRBP which acts as a carrier for the released retinol. Up to 90% of membrane-bound vitamin A is released upon incubation in the presence of apo-CRBP (11 microM) with concomitant formation of retinol-CRBP. The overall process, in which retinol never needs to leave its binding proteins, allows the accumulation of vitamin A in the form of a membrane-bound retinyl-ester and its regulated mobilization as a retinol-CRBP complex.     

Dynamics of fd coat protein in lipid bilayers

The dynamics of backbone and side-chain sites of the membrane-bound form of fd coat protein are described with solid-state 2H and 15N NMR experiments. The samples were isotopically labeled coat protein in phospholipid bilayers in excess water. The protein itself is immobile and does not undergo rapid rotation within the bilayer. Like the structural form of the protein, the membrane-bound form has four mobile residues at the N-terminus. The membrane-bound form differs from the structural form in having several mobile residues at the C-terminus. Many of the side chains of residues with immobile backbone sites undergo large amplitude jump motions. The dynamics are generally similar in both the structural and membrane-bound forms of the protein.     

Altered localisation of the precursor of a secreted protein in E. coli by a carboxyl-deletion

The effect on protein localisation of a carboxy-terminal deletion of periplasmic UDP-glucose hydrolase (5'-nucleotidase) has been determined in vivo using immunoprecipitation. The processed form of the truncated protein was found in the periplasm; however, in contrast to the situation with the normal protein, the preprotein form of the truncated protein was also found in the periplasm. This result confirms previous semi-in vitro data and supports the suggestion that a conformational change in a membrane-bound intermediate is a normal part of the secretory pathway.     

A homologue of the axonally secreted protein axonin-1 is an integral membrane protein of nerve fiber tracts involved in neurite fasciculation

Axonin-1 is a glycoprotein that is released from axons of cultured neurons (Stoeckli, E. T., P. F. Lemkin, T. B. Kuhn, M. A. Ruegg, M. Heller, and P. Sonderegger. 1989. Eur. J. Biochem. 180:249-258). It has recently been purified from the ocular vitreous fluid of the chicken embryo (Ruegg, M. A., E. T. Stoeckli, T. B. Kuhn, M. Heller, R. Zuellig, and P. Sonderegger. 1989. EMBO (Eur. Mol. Biol. Organ.) J. 8:55-63). Immunohistochemistry localized axonin-1 prevalently in developing nerve fiber tracts. The presence of anti-axonin-1 Fab fragments during axon growth in vitro resulted in antibody binding to the axonal surfaces and in a marked perturbation of the fasciculation pattern. Hence, a fraction of axonin-1 is associated with axonal membranes and, by operational criteria, qualifies as a cell adhesion molecule. The major proportion of membrane-associated axonin-1 co-solubilized with the integral membrane proteins. By physico-chemical, immunological, and protein-chemical criteria, the integral membrane form was found to be highly similar to soluble axonin-1. In common with a number of other cell adhesion molecules, both soluble and membrane-bound axonin-1 express the L2/HNK-1 and the L3 epitopes. Radioactive pulse-chase and double-labeling experiments revealed that the released form was not derived from the membrane-bound form by shedding from the membrane surface, but directly secreted from an intracellular pool. Due to its high degree of similarity to the membrane-associated form and the presence of the L2/HNK-1 and L3 epitopes, reported to be ligands in adhesive cell interactions, adhesive properties are postulated for secreted axonin-1. As a soluble adhesive protein, it may function as a regulator of cell adhesion around its most likely site of secretion, the growth cone.     

Dissociation of G-protein alpha from rhabdomeric membranes decreases its interaction with rhodopsin and increases its degradation by calpain

Photoactivation of invertebrate rhodopsin activates a GTP-binding protein, Gq, which in turn activates a phospholipase C (PLC) enzyme. Gqalpha is a membrane-associated protein that is progressively released from the membrane by washing with buffers containing increasing concentrations of beta-mercaptoethanol (beta-ME). Isolated, soluble Gqalpha showed a decreased ability to be activated by rhodopsin but was more active in stimulating PLC when compared with the membrane-associated form of Gqalpha. The calcium-activated protease, calpain, selectively cleaved the soluble but not the membrane-bound form of Gqalpha. Calpain cleaved a small peptide from the amino-terminus of Gqalpha reducing the ability of the G-protein to bind GTP. The uncoupling of Gqalpha from rhodopsin and subsequent calcium-dependent proteolysis to further inactivate the G-protein may therefore be a regulatory mechanism of light adaptation in rhabdomeric photoreceptors.     

Protein processing mechanisms: from angiotensin-converting enzyme to Alzheimer's disease

Angiotensin-converting enzyme (ACE) and the Alzheimer's disease amyloid precursor protein are two examples of membrane-bound proteins that are released in a soluble form by a post-translational proteolytic cleavage event involving a secretase. Site-specific antibodies and matrix-assisted laser desorption ionization-time-of-flight ('MALDI-TOF') MS have been used to map the secretase cleavage site in somatic ACE to Arg-1203/Ser-1204, 24 residues proximal to the membrane-anchoring domain. Trypsin, which can solubilize ACE from the membrane, cleaves the protein at the same site. The use of structurally related hydroxamic acid-based zinc metalloproteinase inhibitors indicate that tumour necrosis factor-alpha convertase, a member of the ADAMs ('a disintegrin and metalloproteinase') family of proteins, is not involved in the proteolytic release of ACE, or in the constitutive or regulated alpha-secretase release of the amyloid precursor protein from a human neuronal cell line.     

Tetrameric alkaline phosphatase in human liver plasma membranes

Molecular weights of native membrane-bound alkaline phosphatase released by butanol and by nonionic detergents were more than twice that of the purified dimeric enzyme. Alkaline phosphatase released by phosphatidylinositol-specific phospholipase-C was of both high and low molecular weight: the former was a protomer of a single protein of the same molecular size as monomeric alkaline phosphatase. We conclude that the membrane-bound enzyme is probably a tetramer.     

Inactivation of fructose 1,6-bisphosphatase by a lysosomal proteinase is reversed by cystamine

Inactivation of rabbit liver fructose 1,6-bisphosphatase (Fru-P₂ase, EC 3.1.3.11) by a membrane-bound lysosomal proteinase, designated as cathepsin M, is reversed by incubation with cystamine, which has been shown to form a mixed disulfide derivative with protein SH groups. The native form of Fru-P₂ase is also activated by cystamine; this activated form is resistant to the action of membrane-bound cathepsin M.     

The SNARE Ykt6 is released from yeast vacuoles during an early stage of fusion

The farnesylated SNARE (N-ethylmaleimide-sensitive factor attachment protein receptor) Ykt6 mediates protein palmitoylation at the yeast vacuole by means of its amino-terminal longin domain. Ykt6 is localized equally to membranes and the cytosol, although it is unclear how this distribution is mediated. We now show that Ykt6 is released efficiently from vacuoles during an early stage of yeast vacuole fusion. This release is dependent on the disassembly of vacuolar SNAREs (priming). In recent literature, it had been demonstrated for mammalian Ykt6 that the membrane-bound form is both palmitoylated and farnesylated at its carboxy-terminal CAAX box, whereas soluble Ykt6 is only farnesylated. In agreement with this, we find that yeast Ykt6 becomes palmitoylated in vitro at its C-terminal CAAX motif. Mutagenesis of the potential palmitoylation site in yeast Ykt6 prevents stable membrane association and is lethal. On the basis of these and other findings, we speculate that Ykt6 is released from membranes by depalmitoylation. Such a mechanism could enable recycling of this lipid-anchored SNARE from the vacuole independent of retrograde transport.