Studies on externally disposed plasma membrane proteins. Trinitrobenzene sulfonic acid derivatization and immune precipitation

Monomorphic molecules function as additional recognition structures on haptenated target cells for HLA-A1-restricted, hapten-specific CTL

Hapten-specific T cells have been shown to recognize haptenated peptides with high avidity and, in some instances, with promiscuous MHC restriction. In this study, the impact of Ag density on MHC restriction of a CTL response specific to the trinitrophenyl (TNP) hapten was investigated. In this study, we demonstrate a novel recognition mechanism used by TNP-specific CD8(+) CTL in the presence of high Ag doses. Although low levels of TNP epitopes on target cells allowed for HLA-A1-restricted CTL activity only, entirely MHC-independent target cell recognition became operative at high TNP loading. In both cases, recognition was mediated by the TCR. This MHC-independent recognition is target cell type restricted and critically involves in our model direct recognition of the ectonucleotidase family surface molecule CD39 by the CTL.

Quantitation and topography of membrane proteins in highly water-permeable vesicles from ADH-stimulated toad bladder

Antidiuretic hormone (ADH) stimulation of toad bladder granular cells rapidly increases the osmotic water permeability (Pf) of their apical membranes by insertion of highly selective water channels. Before ADH stimulation, these water channels are stored in large cytoplasmic vesicles called aggrephores. ADH causes aggrephores to fuse with the apical membrane. Termination of ADH stimulation results in prompt endocytosis of water channel-containing membranes via retrieval of these specialized regions of apical membrane. Protein components of the ADH water channel contained within these retrieved vesicles would be expected to be integral membrane protein(s) that span the vesicle's lipid bilayer to create narrow aqueous channels. Our previous work has identified proteins of 55 (actually a 55/53-kDa doublet), 17, 15, and 7 kDa as candidate ADH water channel components. We now have investigated these candidate ADH water channel proteins in purified retrieved vesicles. These vesicles do not contain a functional proton pump as assayed by Western blots of purified vesicle protein probed with anti-H(+)-ATPase antisera. Approximately 60% of vesicle protein is accounted for by three protein bands of 55, 53, and 46 kDa. Smaller contributions to vesicle protein are made by the 17- and 15-kDa proteins. Triton X-114-partitioning analysis shows that the 55, 53, 46, and 17 kDa are integral membrane proteins. Vectorial labeling analysis with two membrane-impermeant reagents shows that the 55-, 53-, and 46-kDa protein species span the lipid bilayer of these vesicles. Thus the 55-, 53-, and 46-kDa proteins possess characteristics expected for ADH water channel components. These data show that the 55- and 53- and perhaps the 46-, 17-, and 15-kDa proteins are likely components of aqueous transmembrane pores that constitute ADH water channels contained within these vesicles.

Cell surface glycoproteins of CHO cells. I. Internalization and rapid recycling

The major cell surface proteins of Chinese hamster ovary (CHO) cells have been investigated after reacting cells at 4 degrees C with the membrane-impermeant reagent, trinitrobenzenesulfonate (TNBS). Immunoprecipitation and subsequent two-dimensional, sodium-dodecyl sulfate, polyacrylamide gel electrophoresis (SDS-PAGE) of proteins from derivatized cells that had been labelled previously with [3H]D-glucosamine or [3H]L-leucine showed that TNBS reacted with most of the high molecular weight (HMW) acidic glycoproteins that became labelled with iodine by the lactoperoxidase technique and that bind the lectin, wheat germ agglutinin (WGA). After warming the cells to allow endocytosis to proceed, molecules haptenized with trinitrophenol (TNP) groups were followed radiochemically by means of [125I]anti-DNP antibodies. The half-life for internalization of proteins tagged with either [125I]anti-DNP IgG or Fab averaged about 5 min. A similar result was obtained when a monoclonal antibody directed against a single plasma membrane glycoprotein was used, or when the rate of surface loss of TNP groups unoccupied by antibodies was measured. Within 15 min at 37 degrees C, a steady-state between surface and cytoplasmic label was reached, with about 65% of the hapten located internally. Recycling of internalized TNP groups back to the cell surface also occurred rapidly (t 1/2 approximately 5 min). Most of the intracellular radioactivity was associated with a membrane fraction of density similar to that of the plasma membrane. Over a 4-h period, there was no significant entry of labeled molecules into lysosomes. By contrast, the fluid-phase marker, horseradish peroxidase, became associated with the lysosomes within 1 h. Our results are consistent with the view that the majority of plasma membrane glycoproteins are continuously being internalized and recycled at a high rate.

Intracellular transport of membrane glycoproteins: two closely related histocompatibility antigens differ in their rates of transit to the cell surface

The intracellular transport of two closely related membrane glycoproteins was studied in the murine B cell lymphoma line, AKTB-1b. Using pulse-chase radiolabeling, the kinetics of appearance of the class I histocompatibility antigens, H-2Kk and H-2Dk, at the cell surface were compared and found to be remarkably different. Newly synthesized H-2Kk is transported rapidly such that all radiolabeled molecules reach the surface within 1 h. In contrast, the H-2Dk antigen is transported slowly with a half-time of 4-5 h. The rates of surface appearance for the two antigens closely resemble the rates at which their Asn-linked oligosaccharides mature from endoglucosaminidase H (endo H)-sensitive to endo H-resistant forms, a process that occurs in the Golgi apparatus. This suggests that the rate-limiting step in the transport of H-2Dk to the cell surface occurs before the formation of endo H-resistant oligosaccharides in the Golgi apparatus. Subcellular fractionation experiments confirmed this conclusion by identifying the endoplasmic reticulum (ER) as the site where the H-2Dk antigen accumulates. The retention of this glycoprotein in the ER does not appear to be due to a lack of solubility or an inability of the H-2Dk heavy chain to associate with beta 2-microglobulin. Our data is inconsistent with a passive membrane flow mechanism for the intracellular transport of membrane glycoproteins. Rather, it suggests that one or more receptors localized to the ER membrane may mediate the selective transport of membrane glycoproteins out of the ER to the Golgi apparatus. The fact that H-2Kk and H-2Dk are highly homologous (greater than or equal to 80%) indicates that this process can be strongly influenced by limited alterations in protein structure.

Reassortment of cells according to position in mouse morulae

Sixteen-cell mouse morulae were disaggregated and blastomeres originally occupying outer or inner positions were separated. Outer, inner, or unsorted populations of blastomeres were labeled with either trinitrobenzene sulphonic acid (TNP) or fluorescein isothiocyanate (FITC) and individual blastomeres aggregated to unlabelled partially decompact eight- to ten-cell morulae. After up to 6 h in culture, the positions of the labelled blastomeres within the aggregates were examined. The combined results demonstrated that between 86 and 92% of outer cells remained on the surface of the aggregate and flattened into extensive polygonal shapes, whereas 76-77% of the inner cells had become engulfed by the host morula cells and retained their initial spherical shape. Using unsorted cells, 33-37% were internalised, which is compatible with the most recent estimates of the presence of six to eight inner cells at the 16-cell stage. The possibility that differential adhesiveness of the outer and inner cells is involved in the allocation of cells to the trophectoderm and inner cell mass of the blastocyst is discussed.

Quantitation of proteins and internal antigen pools by a monoclonal sandwich radioimmune assay

We describe a monoclonal sandwich radioimmune assay (MSRA) that is capable of measuring 0.5 fmoles of protein antigens. The basis of the assay is the ability of most rabbit heteroantisera directed against protein antigens to precipitate labeled monoclonal Fab-antigen complexes. The assay offers the advantages of specificity, sensitivity, linearity and rapidity. The MSRA circumvents the need for preparation of either highly purified protein antigens for use in radioimmune inhibition assays or affinity purified antisera for sandwich immunoassays. Utilizing variations of MSRA protocols, we determined that 48% of FcR gamma 2b/gamma 1, 34% of an 90,000 Mr glycoprotein identified by monoclonal antibody 2D2C, and 24% of an 82,000 Mr glycoprotein identified by monoclonal antibody 2E2A were accessible to labeled Fab probes at the plasma membrane. The total amount of antigen per cell was determined by dividing the amount of antigen at the cell surface (determined from binding saturation assays) by the percentage of surface/total antigen. This value of total antigen agrees well with the total cellular antigen pool determined by MSRA after appropriate corrections for probe saturation and recovery of pre-formed Fab-antigen complexes were made.

A simple method for electrophoretic analysis of cell surface glycoproteins based on concanavalin A binding

A simple and sensitive method is described for the detection of cell surface proteins. This method consists of reacting concanavalin A (Con A) with 35S-labeled methionine, detergent solubilized whole cell extracts and immunoprecipitating the Con A-protein complexes with anti-Con A antibody bound to Staphylococcus aureus Cowan I strain (SACI). These reagents are commercially available, stable and easy to standardize. Mouse lymphocytes cell surface proteins purified by this procedure show great similarities to those purified with goat anti-cell surface antibodies.