Filamentous Proteins from Plant Sieve Tubes

PROTEIN filaments are characteristic structural components of the assimilatory conducting elements of angiosperm plants (“P protein” of Cronshaw and Esau¹). We have isolated filamentous structures from the phloem exudate of cut cucurbit stems². The presence of the filaments could be clearly demonstrated after negative staining with the electron microscope.     

Activation of methotrexate-alpha-alanine by carboxypeptidase A-monoclonal antibody conjugate.

Carboxypeptidase A (CP-A) and monoclonal antibody KS1/4 directed against an antigen on human lung adenocarcinoma cells (UCLA-P3) were derivatized by treatment with succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate and N-succinimidyl 3-(2-pyridyldithio)propionate, respectively. The derivatized proteins were reacted to produce thioether-linked enzyme-antibody conjugates. Sequential HPLC size-exclusion and DEAE chromatography separated the conjugate preparation from unreacted enzyme and antibody. On the basis of SDS-PAGE analysis and measurement of catalytic activity, the preparation contained approximately equal amounts of 1:1 and 2:1 (enzyme:antibody) conjugates; binding activity of the conjugate (1.8 x 10(5) molecules/cell) was similar to that of unreacted antibody. In vitro cytotoxicity studies with UCLA-P3 cells demonstrated the ability of cell-bound conjugate to convert the prodrug methotrexate-alpha-alanine (MTX-Ala) to methotrexate (MTX). In the absence of conjugate, ID50 values for MTX-Ala and MTX were 8.9 x 10(-6) and 5.2 x 10(-8) M, respectively. ID50 for the prodrug improved to 1.5 x 10(-6) M with cells containing bound conjugate. This potentiation of MTX-Ala cytotoxicity by conjugate-bound CP-A, which was at least 30-fold greater than that produced by a comparable amount of free enzyme, is attributed to enhanced effectiveness of MTX generated at the cell surface as opposed to the surrounding medium. Examination of the time course of cytotoxicity over a 96-h period showed that the conjugate-prodrug combination (at 2.5 x 10(-6) M) was nearly as effective as MTX in preventing cell replication. These results demonstrate the chemotherapeutic potential of carboxypeptidase-monoclonal antibody conjugates used in conjunction with MTX peptide prodrugs.     

Characterization of monoclonal antibody 155.8 and partial characterization of its proteoglycan antigen on human melanoma cells

Immunization of mice with a plasma membrane-enriched fraction from human malignant melanoma cells and subsequent generation of hybridomas resulted in the isolation of an IgG1 monoclonal antibody, 155.8, that recognizes chondroitin sulfate proteoglycans. By cell binding analysis, 155.8 was shown to react with seven of eight cultured melanoma cell lines, but not with a variety of lymphoblastoid cell lines or cultured tumor cells derived from other solid tumor types. Indirect immunoprecipitation of the 155.8 antigen from intrinsically labeled melanoma cells revealed a glycoprotein of Mr = 250,000 and a sulfated molecule of Mr greater than 400,000. The antigen was identified as a chondroitin sulfate type A/C proteoglycan synthesized by melanoma cells on the basis of its sensitivity to chondroitinase ABC digestion and the identification of sulfated glycosaminoglycans released from the antigen immunoprecipitated by 155.8. The determinants recognized by antibodies 155.8 and 9.2.27, another anti-chondroitin sulfate proteoglycan, immunoprecipitate only a proteoglycan from high density cesium chloride gradient fractions, (1.487 g/liter); however, they immunoprecipitate a free glycoprotein of Mr = 250,000 from low density fractions (1.317 g/liter). This demonstrated that the 155.8 and 9.2.27 determinants, both of which reside on the glycoprotein of Mr = 250,000, are also present in the proteoglycan, suggesting that this glycoprotein is the proteoglycan core protein. Monoclonal antibody 155.8 reacts with a determinant on the core protein distinct from that recognized by 9.2.27. Proteoglycans bearing 155.8 determinants are distributed on the surface of cultured melanoma cells in a punctated fashion that apparently resolves to short, filamentous structures at high magnification. Immunohistochemical analysis demonstrated that 155.8-defined proteoglycans are found in freshly biopsied melanoma tissue, suggesting that these antigens are also synthesized in vivo by melanoma cells.     

Nonradioactive hybridization probes prepared by the reaction of biotin hydrazide with DNA

A novel one-step chemical method has been developed for the introduction of biotin into nucleic acids for non-isotopic hybridization. The method is based on the interaction of biotin hydrazide with unpaired cytosine residues. The interaction is catalyzed by sodium bisulfite with an optimum at a buffered pH of about 4.5. The reaction reached its maximum after 24 h incubation at a biotin hydrazide concentration of 10 mg/ml. Using streptavidin-alkaline phosphatase conjugates, the limits for detecting the biotinylated probe, either adsorbed directly to nitrocellulose or hybridized to filter-bound target DNA, were 0.3 and 0.9 pg, respectively. The salience of the approach described here over previously used biotin derivatives is that it is quick (one-step), simple and does not involve any enzymatic or instrument-mediated step to introduce the reporter moiety. In addition, other low- and high-molecular-weight hydrazides (e.g. fluorescent or enzyme hydrazides) can serve as the reporter group. The same procedure may be employed for the single-step biotinylation of free cytidine.     

Post-translational addition of chondroitin sulfate glycosaminoglycans. Role of N-linked oligosaccharide addition, trimming, and processing

A melanoma proteoglycan model system has been used to examine the role of core protein asparagine-linked (N-linked) oligosaccharides in the transport and assembly of proteoglycan molecules. The use of agents which block discrete steps in the trimming and processing of core oligosaccharides (castanospermine, 1-deoxynojirimycin, N-methyldeoxynojirimycin, 1-deoxymannojirimycin, and swainsonine) demonstrates that removal of glucose residues from the N-linked oligosaccharides is required for the cell surface expression of a melanoma proteoglycan core protein and for the conversion of the core protein to a chondroitin sulfate proteoglycan. However, complete maturation of the oligosaccharides to a "complex" form is not required for these events. Treatment of M21 human melanoma cells with the glucosidase inhibitors castanospermine, 1-deoxynojirimycin, or N-methyldeoxynojirimycin results in a dose-dependent inhibition of glycosaminoglycan (GAG) addition to the melanoma antigen recognized by monoclonal antibody 9.2.27. In contrast, treatment with the mannosidase inhibitors 1-deoxymannojirimycin and swainsonine does not effect GAG addition. Identical results are obtained when the major histocompatibility complex class II antigen gamma chain proteoglycan is examined in inhibitor-treated melanoma and B-lymphoblastoid cells. These data, in conjunction with the known effects of the glucosidase and mannosidase inhibitors on the transport and secretion of other glycoproteins support the hypothesis that the addition, trimming, and processing of N-linked oligosaccharides is involved in the transport of certain proteoglycan core proteins to the site of GAG addition and to the cell surface.