Monoclonal antibodies directed against protoplasts of soybean cells: analysis of the lateral mobility of plasma membrane-bound antibody MVS-1

Polyphosphoinositides are enriched in plant membrane rafts and form microdomains in the plasma membrane

In this article, we analyzed the lipid composition of detergent-insoluble membranes (DIMs) purified from tobacco (Nicotiana tabacum) plasma membrane (PM), focusing on polyphosphoinositides, lipids known to be involved in various signal transduction events. Polyphosphoinositides were enriched in DIMs compared with whole PM, whereas all structural phospholipids were largely depleted from this fraction. Fatty acid composition analyses suggest that enrichment of polyphosphoinositides in DIMs is accompanied by their association with more saturated fatty acids. Using an immunogold-electron microscopy strategy, we were able to visualize domains of phosphatidylinositol 4,5-bisphosphate in the plane of the PM, with 60% of the epitope found in clusters of approximately 25 nm in diameter and 40% randomly distributed at the surface of the PM. Interestingly, the phosphatidylinositol 4,5-bisphosphate cluster formation was not significantly sensitive to sterol depletion induced by methyl-beta-cyclodextrin. Finally, we measured the activities of various enzymes of polyphosphoinositide metabolism in DIMs and PM and showed that these activities are present in the DIM fraction but not enriched. The putative role of plant membrane rafts as signaling membrane domains or membrane-docking platforms is discussed.

The use of antibodies to study the architecture and developmental regulation of plant cell walls

This review covers the generation and use of antibodies to defined components of plant and algal cell walls and how these have contributed to our understanding of the spatial and developmental regulation of cell walls. Particular emphasis is placed upon the generation and characterization of monoclonal antibodies to matrix polysaccharides, extensins, and arabinogalactan-proteins of higher plants, and algal polysaccharides and glycoproteins. Immunolocalization studies are discussed in relation to the identification of molecular domains within cell walls, cell adhesion, cell differentiation, and the establishment of plant interactions with other organisms.

Auxins and Cytokinins as Antipodal Modulators of Elasticity within the Actin Network of Plant Cells

The cytoskeleton of plant and animal cells serves as a transmitter, transducer, and effector of cell signaling mechanisms. In plants, pathways for proliferation, differentiation, intracellular vesicular transport, cell-wall biosynthesis, symbiosis, secretion, and membrane recycling depend on the organization and dynamic properties of actin- and tubulin-based structures that are either associated with the plasma membrane or traverse the cytoplasm. Recently, a new in vivo cytoskeletal assay (cell optical displacement assay) was introduced to measure the tension within subdomains (cortical, transvacuolar, and perinuclear) of the actin network in living plant cells. Cell optical displacement assay measurements within soybean (Glycine max [L.]) root cells previously demonstrated that lipophilic signals, e.g. linoleic acid and arachidonic acid or changes in cytoplasmic pH gradients, could induce significant reductions in the tension within the actin network of transvacuolar strands. In contrast, enhancement of cytoplasmic free Ca2+ resulted in an increase in tension. In the present communication we have used these measurements to show that a similar antipodal pattern of activity exists for auxins and cytokinins (in their ability to modify the tension within the actin network of plant cells). It is suggested that these growth substances exert their effect on the cytoskeleton through the activation of signaling cascades, which result in the production of lipophilic and ionic second messengers, both of which have been demonstrated to directly effect the tension within the actin network of soybean root cells.

Lipids trigger changes in the elasticity of the cytoskeleton in plant cells: a cell optical displacement assay for live cell measurements

An assay has been developed to quantitatively measure the tension and elasticity of the cytoskeleton in living plant cells. The cell optical displacement assay (CODA) uses a focused laser beam to optically trap and displace transvacuolar and cortical strands through a defined distance within the cell. Results from these experiments provide evidence for the classification of at least two rheologically distinct cytoskeletal assemblies, cortical and transvacuolar, that differ in their tension and response to both signaling molecules and reagents that perturb the cytoskeleton. It is further demonstrated that the tension of the transvacuolar strands can be significantly decreased by the addition of either linoleic acid, 1,2 dioctanoyl-sn-glycerol, or 1,3 dioctanoylglycerol. These decreases in tension could also be induced by lowering the cytoplasmic pH. In contrast, addition of Ca2+, Mg2+, or the ionophore A23187 to the cells caused a considerable increase in the tension of the transvacuolar strands. The data provides evidence that: (a) linoleic acid may be a signaling molecule in plant cells; (b) diacylglycerol functions as a signaling molecule through a protein kinase C-independent pathway mediated by PLA2; and (c) Ca2+ and pH have regulatory roles for controlling cytoskeleton tension and organization.

Biosynthesis, processing and targeting of the G-protein of vesicular stomatitis virus in tobacco protoplasts

Leaf protoplasts of tobacco (Nicotlana tabacum L.) were employed for transfection of chimeric transcriptional gene fusions comprising the 35S promoter from cauliflower mosaic virus, the coding sequence of the G-protein from vesicular stomatitis virus (VSVG) and the transcriptional terminator from the Agrobacterium tumefaciens nopaline-synthetase gene. Transient expression of the chimeric gene was monitored through Northern analysis of total protoplast RNA using a labeled VSV cDNA probe, and through Western-blot analysis of protoplast proteins using a polyclonal and-VSV antiserum. Although a single species of mRNA was detected in the transfected protoplasts, two glycoproteins differing in mass by approx. 9 kDa were detected by the antiserum. Biosynthesis of the VSVG isoforms was not impeded by chemical inhibitors of cell-wall production or of proline hydroxylation. Transfection using mutant forms of the VSVG coding sequence in which either one or both consensus glycosylation sites were removed resulted in the production of progressively smaller VSVG proteins. Those proteins produced from the double mutant had mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoresis that were very similar to those produced from the wild-type construct in the presence of tunicamycin. Analysis of protoplast homogenates by differential centrifugation showed that the two VSVG isoforms were exclusively associated with cellular membranes. The larger protein co-localized with the plasma membrane and with the organelles of the endomembrane-secretory pathway leading to the plasma membrane. The smaller protein was associated with membranes of lower isopycnic densities which were not identical to the endoplasmic reticulum. The larger protein displayed greater sensitivity than did the smaller to degradation in vivo by exogenously added protease. Immunofluorescence microscopy demonstrated that the VSVG isoforms were present both within the protoplasts and at the surface of the plasma membrane. The intracellular distribution was either punctate or reticulate. These results are consistent with the progressive and accurate glycosylation of the newly synthesized VSVG polypeptide during its passage through the endomembrane-secretory pathway, the access of the larger isoform to the cell surface, and the conversion of the larger to the small isoform by selective proteolysis.

Lateral diffusion in the plasma membrane of maize protoplasts with implications for cell culture

Plasma-membrane dynamics in live protoplasts from maize (Zea mays L.) roots were characterized and examined for relationships as to the ability of the protoplasts to synthesize new cell walls and develop to cells capable of division. The lateral diffusion-coefficients and mobile fractions of fluorescence-labeled plasma-membrane proteins and lipids were measured by fluorescence photobleaching recovery. Small but significant effects on the diffusion of membrane proteins were observed after treatments with oryzalin or amiprophosmethyl, microtubule-disrupting drugs that increased the mobile fraction, and after treatments with cytochalasins B or D, microfilament-disrupting drugs that decreased the diffusion coefficient. A number of parameters were tested for correlative effects on membrane dynamics and protoplast performance in culture. Protoplasts isolated with a cellulase preparation from Trichoderma viride showed faster membrane-protein diffusion and a lower frequency of development to cells capable of division than did protoplasts isolated with a cellulase preparation from T. reesei. Membrane proteins in maize A632, a line less capable of plant regeneration from callus, diffused with a smaller diffusion coefficient but a greater mobile fraction than did membrane proteins in maize A634, a line with greater regeneration capacity. The plasma membranes of A632 and A634 protoplasts also differed with regard to lateral-diffusion characteristics of phospholipid and sterol probes, although the presence of both rapidly and slowly diffusing lipid components indicated the apparent existence of lipid domains in both A632 and A634. The protoplasts of the two lines did not differ significantly, however, in either wall regeneration or frequency of development to cells capable of division.

Alpha interferon accelerates lateral diffusion of Daudi cell surface differentiation antigens: measurement by fluorescence redistribution after photobleaching

Lateral diffusion coefficients (D) of two surface differentiation antigens (sIgM and Bp35) were determined on interferon-sensitive (-IFs) or resistant (-IFr) Daudi cells by fluorescence photobleaching, using monospecific FITC-anti-IgM or PE-anti-Leu 16 probes. For untreated Daudi -IFs, mean (D) were 5.8 and 5.3 (x10(-10) cm2/sec). These increased, to 11 and 7.9 x 10(-10) cm2/sec (p less than 0.001) within 30 min after binding of recombinant IFN-a (80 to 800 U/10(6) cells), but decreased by up to 4-fold after Con A Mean (D) of identical surface antigens on Daudi-IFr were 8.2 and 9.4 x 10(-10) cm2/sec; and were not altered by IFN-a. Mean (D) of a lipid analog was up to 40-fold higher than for surface proteins and statistically identical in Daudi-IFs and Daudi-IFr. Rapid acceleration by IFN-a of surface protein lateral diffusion in Daudi-IFs obviously could facilitate anti-proliferative signal transduction; by contrast, a baseline increase of (D) in Daudi-IFr was evidently associated with their refractory state.

Isolation and characterization of monoclonal antibodies directed against plant plasma membrane and cell wall epitopes: identification of a monoclonal antibody that recognizes extensin and analysis of the process of epitope biosynthesis in plant tissues and cell cultures

Membranes from tobacco cell suspension cultures were used as antigens for the preparation of monoclonal antibodies. Use of solid phase and indirect immunofluorescence assays led to the identification of hybridomas producing antibodies directed against cell surface epitopes. One of these monoclonal antibodies (11.D2) was found to recognize a molecular species which on two-dimensional analysis (using nonequilibrium pH-gradient electrophoresis and SDS-PAGE) was found to have a high and polydisperse molecular mass and a very basic isoelectric point. This component was conspicuously labeled by [3H]proline in vivo. The monoclonal antibody cross-reacted with authentic tomato extensin, but not with potato lectin nor larch arabinogalactan. Use of the monoclonal antibody as an immunoaffinity reagent allowed the purification of a tobacco glycoprotein which was identical in amino acid composition to extensin. Finally, immunocytological analyses revealed tissue-specific patterns of labeling by the monoclonal antibody that were identical to those observed with a polyclonal antibody raised against purified extensin. We have concluded that monoclonal antibody 11.D2 recognizes an epitope that is carried exclusively by extensin. Analysis of cellular homogenates through differential and isopycnic gradient centrifugation revealed that biosynthesis of the extensin epitope was found on or within the membranes of the endoplasmic reticulum, Golgi region and plasma membrane. This result is consistent with the progressive glycosylation of the newly-synthesized extensin polypeptide during its passage through a typical eukaryotic endomembrane pathway of secretion. The 11.D2 epitope was not found in protoplasts freshly isolated from leaf tissues. However, on incubation of these protoplasts in appropriate culture media, biosynthesis of the epitope was initiated. This process was not impeded by the presence of chemicals that are reported to be inhibitors of cell wall production or of proline hydroxylation.

Monoclonal antibodies directed against protoplasts of soybean cells : Generation of hybridomas and characterization of a monoclonal antibody reactive with the cell surface

Splenocytes, derived from mice that had been immunized with protoplasts prepared from suspension cultures of root cells of Glycine max (L.) Merr. (SB-1 cell line), were fused with a murine myeloma cell line. The resulting hybridoma cultures were screened for the production of antibodies directed against the soybean protoplasts and were then cloned. One monoclonal antibody, designated MVS-1, was found to bind to the outer surface of the plasma membrane on the basis of several criteria: (a) agglutination of the protoplasts; (b) binding of fluorescence-labeled immunoglobulin on protoplasts yielding a ring staining pattern with prominent intensity at the edges; and (c) saturable binding by protoplasts of (125)I-labeled Antibody MVS-1. The antigenic target of Antibody MVS-1, identified by immunoblotting techniques, contained a polypeptide of relative molecular mass (Mr) approx. 400000 under both reducing and non-reducing conditions. When the antigenic target of Antibody MVS-1 was chromatographed in potassium phosphate buffer, the position of elution corresponded to that of a high-molecular-weight species (Mr 400000). These results provide the protein characterization required for the analysis of the mobility of Antibody MVS-1 bound to the plasma membrane of SB-1 cells.