A new immunoreactive probe for the isolation and analysis of plasma membrane polypeptides. Synthesis and properties of isethionyl-3-(N-2,4-dinitrophenyl)-aminopropioimidate

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

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.

Rapid endocytosis and recycling of wheat germ agglutinin binding sites on CHO cells: evidence for two compartments in a nondegradative pathway

The internalization and recycling of CHO cell plasma membrane components have been quantified by using iodinated wheat germ agglutinin (WGA) as an adsorptive tracer. Most of these binding sites are thought to be composed of a subpopulation of plasma membrane proteins called high-molecular-weight acidic glycoproteins (HMWAG). Greater than 90% of the WGA bound on the cell surface can be removed by brief treatment with N-acetylglucosamine (GlcNAc). At 37 degrees C, endocytosis of WGA that had been allowed to bind to the surface at 4 degrees C is curvilinear with an initial rapid phase occurring with a t1/2 of 6-8 min. Within 20 min, accumulation has slowed gradually to steady-state with 65% of the cell-associated WGA located intracellularly or resistant to removal by GlcNAc. These portions are unaffected by increasing the extracellular concentration of WGA from 0.003 microM to 2.8 microM. By using pulse-chase experiments, the observed decrease in rate of endocytosis is shown to be due to return of the WGA-HMWAG complexes to the cell surface. More than 60% of the WGA that had been internalized is recycled within 30 min, with a mean t1/2 of 17 min. Recycling involved at least two intracellular populations where a significant fraction (less than 30%) of the WGA-HMWAG complexes are lost gradually from the rapidly recycling pool. Most of the WGA-HMWAG complexes that had internalized are not delivered to the lysosome. These results demonstrate the magnitude of rapid and continuous recycling of WGA binding sites between the cell surface and endosomes in fibroblasts.

Evidence for the extramembranous location of the putative amphipathic helix of acetylcholine receptor

Evidence has been obtained demonstrating that the peptides GVKYIAE and AIKYIAE found in the potential amphipathic helices of the alpha and beta subunits, respectively, of acetylcholine receptor are not buried in the membrane. The peptide KYIAE was synthesized, and polyclonal antibodies were prepared against a conjugate of bovine serum albumin and synthetic peptide. An immunoadsorbent capable of binding and subsequently releasing peptides ending with the sequence-YIAE was produced by attaching these specific antibodies to agarose. Native acetylcholine receptor was labeled with pyridoxal phosphate and Na[3H]BH4. The labeled protein was stripped of phospholipid and digested with the protease from Staphylococcus aureus strain V8. The digest was submitted to immunoadsorption to isolate the labeled indigenous peptides. As a control, alpha and beta polypeptides prepared by gel filtration of a solution of acetylcholine receptor in detergent were stripped of detergent and labeled with pyridoxal phosphate and Na[3H]BH4 in the presence of 8 M urea. The labeled alpha and beta polypeptides were digested and submitted to immunoadsorption. The specific radioactivities of the indigenous peptides from the alpha and beta subunits labeled under native and denaturing conditions were nearly equal. In similar experiments using isethionyl (2', 4'-dinitrophenyl)-3-amino-propionimidate as the labeling agent, the indigenous peptides from native and denatured receptor were also labeled to the same extent. Since these peptides are labeled to the same extent whether or not the protein is denatured, they cannot be buried in the membrane.

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.