Hydrophilic anchor-deficient Thy-1 is secreted by a class E mutant T lymphoma

Metabolic correction of defects in the lipid anchoring of Thy-1 in lymphoma mutants

Many plasma membrane proteins, including Thy-1, are anchored by a carboxyl terminal glycophospholipid. This unit is absent from the Thy-1 of several lymphoma mutants that synthesize the Thy-1 polypeptide but fail to express it at the cell surface. Recessive mutants of complementation groups A to C, E, and F contain Thy-1 mRNA of normal size, which suggests that their Thy-1 polypeptide is normal. To identify possible metabolic lesions, each mutant was grown with various supplements. The class F and B mutants exhibited a reversible induction of surface lipid anchored Thy-1 when grown with the aminoglycoside G418. Other aminoglycosides, sugars, and ethanolamine were inactive. These unexpected observations are discussed in the context of lipid anchor biosynthesis.

Active production and membrane anchoring of carcinoembryonic antigen observed in normal colon mucosa

Normal colon mucosa was found to produce carcinoembryonic antigen (CEA) quite actively as cancerous tissues do when maintained in an organ culture, although the fresh normal mucosae contained a very small quantity of CEA unlike cancerous tissues. This is consistent with an active expression of CEA mRNA in normal mucosa comparable to that in cancerous tissues actively producing CEA, and suggests that the normal cell product was rapidly released into the lumen of digestive tract and turned into normal fecal antigens (NFAs) previously found in feces. 3H-Labeled precursors of glycophospholipid such as ethanolamine and stearic acid were incorporated into CEA produced by both normal and cancerous tissues, suggesting that CEA in normal mucosa is anchored to the cell membrane through a glycophospholipid as in cancerous tissues.

Characterization of [3H]palmitate- and [3H]ethanolamine-labelled proteins in the multicellular parasitic trematode Schistosoma mansoni

Biosynthetic labelling experiments with cercariae and schistosomula of the multicellular parasitic trematode Schistosoma mansoni were performed to determine whether [3H]palmitate or [3H]ethanolamine was incorporated into proteins. Parasites incorporated [3H]palmitate into numerous proteins, as judged by SDS/polyacrylamide-gel electrophoresis and fluorography. The radiolabel was resistant to extraction with chloroform, but sensitive to alkaline hydrolysis, indicating the presence of an ester bond. Further investigation of the major 22 kDa [3H]palmitate-labelled species showed that the label could be recovered in a Pronase fragment which bound detergent and had an apparent molecular mass of 1200 Da as determined by gel filtration on Sephadex LH-20. Schistosomula incubated with [3H]ethanolamine for up to 24 h incorporated this precursor into several proteins; labelled Pronase fragments recovered from the three most intensely labelled proteins were hydrophilic and had a molecular mass of approx. 200 Da. Furthermore, reductive methylation of such fragments showed that the [3H]ethanolamine bears a free amino group, indicating the lack of an amide linkage. We also evaluated the effect of phosphatidylinositol-specific phospholipase C from Staphylococcus aureus: [3H]palmitate-labelled proteins of schistosomula and surface-iodinated proteins were resistant to hydrolysis with this enzyme. In conclusion, [3H]palmitate and [3H]ethanolamine are incorporated into distinct proteins of cercariae and schistosomula which do not bear glycophospholipid anchors. The [3H]ethanolamine-labelled proteins represent a novel variety of protein modification.

No glycolipid anchors are added to Thy-1 glycoprotein in Thy-1-negative mutant thymoma cells of four different complementation classes

Recent evidence shows that the mature Thy-1 surface glycoprotein lacks the C-terminal amino acids 113 to 143 predicted from the cDNA sequence and is anchored in the plasma membrane by a complex, phosphatidylinositol-containing glycolipid attached to the alpha-carboxyl group of amino acid 112. Here we studied the biosynthesis of Thy-1 in two previously described and two newly isolated Thy-1-deficient mutant cell lines. Somatic cell hybridization indicated that their mutations affected some processing step rather than the Thy-1 structural gene. The Thy-1 made by mutants of classes C, F, and H bound detergent but, in contrast to wild-type Thy-1, their detergent-binding moieties could not be removed by phospholipase C. In addition, tryptophan, which only occurs in position 124, was incorporated into Thy-1 of these mutants but not of wild-type cells. Last, the Thy-1 of wild-type but not mutant cells could be radiolabeled with [3H]palmitic acid. Together, these findings strongly suggest that mutants of classes C, F, and H accumulate a biosynthetic intermediate of Thy-1 which retains at least part of the hydrophobic C-terminal peptide. The Thy-1 of these mutants remained endoglycosidase H sensitive, suggesting that it accumulated in the rough endoplasmic reticulum or the Cis-Golgi. A different Thy-1 intermediate was found in a class B mutant cell line: the Thy-1 of this mutant was 2 kilodaltons smaller than the Thy-1 of other cell lines, did not bind detergent, and was rapidly secreted via a normal secretory pathway.

A phosphatidylinositol-linked peanut agglutinin-binding glycoprotein in central nervous system myelin and on oligodendrocytes

Here we report the isolation and initial biochemical characterization of a 120-kD peanut agglutinin-binding glycoprotein from the adult human central nervous system (CNS), which is anchored to membranes through a phosphatidylinositol linkage. Myelin incubated with phosphatidylinositol-specific phospholipase C released the protein as a soluble polypeptide of 105 kD, which was isolated with peanut agglutinin-agarose affinity chromatography. The protein was found to be highly glycosylated. The protein appears to be confined to the CNS, where its developmental expression is region specific and parallels myelination. It is in greater quantity in white matter than in gray matter and it is in isolated human CNS myelin. Furthermore, ovine oligodendrocytes in culture contain the protein on their surfaces and release it into the supernatant as a soluble 105-kD form. We call this protein the oligodendrocyte-myelin protein.

TAP transcription and phosphatidylinositol linkage mutants are defective in activation through the T cell receptor

TAP is a phosphatidylinositol-anchored membrane protein that is involved in murine T lymphocyte activation. To determine the relationship between TAP and T cell receptor/CD3-mediated activation, we derived TAP expression mutants of a T-T hybridoma. Two phenotypically distinct classes of mutants were obtained. The first has a selective defect in the transcription of TAP, while the second has a defect in the biosynthesis of phosphatidylinositol-protein linkages. Both mutations affect antigen-stimulated, T cell receptor-mediated activation of the T-T hybrid. These variants have intact immune effector gene programs, as they are responsive to pharmacologic agents that mimic receptor signals. These findings support a role for phosphatidylinositol-linked cell-surface glycoproteins in physiologic T cell activation. Consistent with this interpretation, we observed similar defects in T cell responsiveness after enzymatic removal of phosphatidylinositol-linked proteins from normal T lymphocytes.

Aspartic acid-484 of nascent placental alkaline phosphatase condenses with a phosphatidylinositol glycan to become the carboxyl terminus of the mature enzyme

A carboxyl-terminal chymotryptic peptide from mature human placental alkaline phosphatase was purified by HPLC and monitored by a specific RIA. Sequencing and amino acid assay showed that the carboxyl terminus of the peptide was aspartic acid, representing residue 484 of the proenzyme as deduced from the corresponding cDNA. Further analysis of the peptide showed it to be a peptidoglycan containing one residue of ethanolamine, one residue of glucosamine, and two residues of neutral hexose. The inositol glycan is apparently linked to the alpha carboxyl group of the aspartic acid through the ethanolamine. Location of the inositol glycan on Asp-484 of the proenzyme indicates that a 29-residue peptide is cleaved from the nascent protein during the post-translational condensation with the phosphatidylinositol-glycan.

Glycosyl-phosphatidylinositol moiety that anchors Trypanosoma brucei variant surface glycoprotein to the membrane

Two forms of protein-membrane anchor have been described for the externally disposed glycoproteins of eukaryotic plasma membranes; namely, the hydrophobic transmembrane polypeptide and the complex glycosylphosphatidylinositol (G-PI) moiety. The chemical structures of the major species of G-PI anchors found on a single variant surface glycoprotein (VSG) of the parasitic protozoan Trypanosoma brucei were determined by a combination of nuclear magnetic resonance spectroscopy, mass spectrometry, chemical modification, and exoglycosidase digestions. The G-PI anchor was found to be heterogeneous with respect to monosaccharide sequence, and several novel glycosidic linkages were present. The results are pertinent to the mechanism of the biosynthesis of G-PI anchors.

No glycolipid anchors are added to Thy-1 glycoprotein in Thy-1-negative mutant thymoma cells of four different complementation classes

Recent evidence shows that the mature Thy-1 surface glycoprotein lacks the C-terminal amino acids 113 to 143 predicted from the cDNA sequence and is anchored in the plasma membrane by a complex, phosphatidylinositol-containing glycolipid attached to the alpha-carboxyl group of amino acid 112. Here we studied the biosynthesis of Thy-1 in two previously described and two newly isolated Thy-1-deficient mutant cell lines. Somatic cell hybridization indicated that their mutations affected some processing step rather than the Thy-1 structural gene. The Thy-1 made by mutants of classes C, F, and H bound detergent but, in contrast to wild-type Thy-1, their detergent-binding moieties could not be removed by phospholipase C. In addition, tryptophan, which only occurs in position 124, was incorporated into Thy-1 of these mutants but not of wild-type cells. Last, the Thy-1 of wild-type but not mutant cells could be radiolabeled with [3H]palmitic acid. Together, these findings strongly suggest that mutants of classes C, F, and H accumulate a biosynthetic intermediate of Thy-1 which retains at least part of the hydrophobic C-terminal peptide. The Thy-1 of these mutants remained endoglycosidase H sensitive, suggesting that it accumulated in the rough endoplasmic reticulum or the Cis-Golgi. A different Thy-1 intermediate was found in a class B mutant cell line: the Thy-1 of this mutant was 2 kilodaltons smaller than the Thy-1 of other cell lines, did not bind detergent, and was rapidly secreted via a normal secretory pathway.

Structural and functional roles of glycosyl-phosphatidylinositol in membranes

Glycosylated forms of phosphatidylinositol, which have only recently been described in eukaryotic organisms, are now known to play important roles in biological membrane function. These molecules can serve as the sole means by which particular cell-surface proteins are anchored to the membrane. Lipids with similar structures may also be involved in signal transduction mechanisms for the hormone insulin. The utilization of this novel class of lipid molecules for these two distinct functions suggests new mechanisms for the regulation of proteins in biological membranes.

Structure and expression of rat osteosarcoma (ROS 17/2.8) alkaline phosphatase: product of a single copy gene

Alkaline phosphatase [ALP; orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] is a ubiquitous enzyme of unknown function expressed at high levels in cells of mineralizing tissues. To study the structure, function, and expression of ALP, a full-length cDNA of rat ALP (2415 bases) was isolated from a ROS 17/2.8 osteosarcoma cell lambda gt10 cDNA library. The predicted amino acid sequence spans 524 residues and includes an N-terminal signal peptide of 17 amino acids, the phosphohydrolase active site, a rather hydrophilic backbone with five potential N-glycosylation sites, and a short hydrophobic C-terminal sequence. ALP negative CHO cells transfected with an expression vector containing the ALP coding sequences express ALP. The rat bone, liver, and kidney ALP shows remarkable 90% homology with the corresponding human enzyme, the most divergent region being the C-terminal hydrophobic domain through which the enzyme may be anchored to the plasma membrane. The rat ALP also shows 50% homology with the human placental and intestinal ALP and 25% homology with the Escherichia coli ALP. The amino acids involved in catalysis show nearly complete homology among all known ALP sequences, suggesting that these enzymes evolved from a common ancestral gene. The rat ALP cDNA pRAP 54, used as a hybridization probe in RNA blot analysis of several tissues that express ALP, revealed the presence of an ALP mRNA of approximately equal to 2500 bases. Furthermore, hybridization patterns derived from Southern blot analysis of rat chromosomal DNA offered molecular evidence that the ALP expressed in ROS 17/2.8 osteosarcoma and various rat tissues, excluding the intestine, is the product of the same single copy gene.

Glycolipid anchors are attached to Thy-1 glycoprotein rapidly after translation

The attachment of glycolipid anchors to the Thy-1 glycoprotein during biosynthesis was followed by the change of detergent-binding properties of biosynthetically labelled Thy-1 precursors upon phospholipase C treatment in the murine thymoma lines BW5147 and S1A. In S1A, 80% of the Thy-1 molecules were phospholipase-C-sensitive after a 2 min pulse with [35S]methionine, indicating that these molecules were already anchored via a glycolipid tail. In BW5147, 47% of the Thy-1 molecules had phospholipase-C-sensitive anchors attached after a 1.5 min labelling and, with longer pulses, this percentage rose to 76%. Tunicamycin did not block the addition of glycolipid anchors, and glycolipid attachment also occurred at 21 degrees C. The findings suggest that the attachment of glycolipid anchors occurs in the rough endoplasmic reticulum.

Identification and characterization of Thy-1 homologues from bovine thymocytes

Two forms of Thy-1 homologues of apparent mol. wt of 25,000 (designated BTp25) and 45,000 (designated BTp45) were isolated from bovine thymocyte membrane by solubilization, affinity chromatography with Con A, and preparative SDS-PAGE. Both forms reacted with a rabbit antiserum to murine Thy-1 in an enzyme-linked immunosorbent assay (ELISA). BTp45 is most likely a dimer of BTp25, since the two are indistinguishable in their amino acid compositions. Comparison of amino acid compositions of BTp25 and BTp45 to that of rodent and human Thy-1 by the S delta Q index revealed significant relatedness among these molecules. BTp25 and BTp45 demonstrate more structural homology to rodent Thy-1 than to human Thy-1. Detailed chemical analyses indicate that bovine Thy-1 homologues contain neutral sugars and fatty acids covalently bound to the polypeptide chain; therefore, they are lipoglycoproteins.

Expression of active, membrane-bound human placental alkaline phosphatase by transfected simian cells

Human placental alkaline phosphatase (PALPase) has been transiently expressed in simian (COS) cells by transfection with a eukaryotic expression vector containing the corresponding cDNA. The level of expression of PALPase was high, and it was produced in an enzymatically active form. The bulk of PALPase was associated with the cell membrane as shown by immunocytochemistry and subcellular fractionation studies. The PALPase produced by transfected COS cells, like PALPase in human tissue, was specifically released from the intact cells in a hydrophilic form by phosphatidylinositol-specific phospholipase C and is, therefore, apparently attached to the outer membrane by means of a phosphatidylinositol-glycan. Transfected COS cells appear to be an excellent model for elucidating the mechanism of attachment of this phosphatidylinositol-glycan to a protein moiety.

Thy-1 solubilization from mouse T cells by phosphatidylinositol-specific phospholipase C: biochemical and antigenic characterization

Membrane anchorage of mouse and rat Thy-1 antigens results from the post-translational attachment of a non-proteic tail terminated by a phosphatidylinositol group. In order to determine the biochemical and antigenic properties of the material released by phosphatidylinositol-specific phospholipase C (PI-PLC), we studied, by one-(1-D) and two-dimensional (2-D) gel electrophoresis and by immunoprecipitation, the supernatant of surface-labelled mouse T cells treated with purified Staphylococcus aureus PI-PLC. The major protein released by this enzymatic treatment showed an apparent molecular weight (MW) and an isoelectric focusing (IEF) pattern identical to those of detergent-solubilized, immunoprecipitated Thy-1. In addition, a sandwich radioimmunoassay (RIA) utilizing two Thy-1-specific monoclonal antibodies (mAb) was used to quantitate the amounts of PI-PLC-released and spontaneously shed Thy-1. Considerable differences in susceptibility to enzymatic cleavage and in spontaneous shedding were observed for a variety of mouse T-cell populations, including thymocytes and hybridoma, helper and cytotoxic cloned T cells, even though time-course experiments demonstrated that excess enzyme was used. It might be useful to consider these differences in the cell biology of Thy-1 and the occurrence of other PI-linked proteins of the lymphocyte surface in terms of their implications in the transduction of activation signals.