Specific endotoxic lipopolysaccharide-binding proteins on murine splenocytes. I. Detection of lipopolysaccharide-binding sites on splenocytes and splenocyte subpopulations

Experiments have been carried out using a unique radio-iodinated, disulfide-reducible, photoactivatable LPS derivative (ASD-LPS) to detect specific LPS-binding proteins on murine splenocytes. Fractionation of LPS-photo-cross-linked, reduced, and solubilized splenocyte extracts on two-dimensional polyacrylamide gels has allowed the identification of an 80-kDa LPS-binding protein with approximate pI of 6.5. This LPS-binding protein is present on partially purified populations of splenic B lymphocytes, T lymphocytes, and macrophages. It is also the dominant LPS-binding protein on the murine 70Z/3 B cell line and the YAC-1 and EL4 T cell lines but is not detectable on the undifferentiated murine Sp2/0 myeloma cell line. Of potential importance is the fact that the 80-kDa protein appears to be indistinguishable when photolabeled extracts of splenocytes from the C3HeB/FeJ (lpsn) and LPS-nonresponder C3H/HeJ (lpsd) mice are compared.     

Generation and characterization of hamster-mouse hybridomas secreting monoclonal antibodies with specificity for lipopolysaccharide receptor.

Experiments are described for the partial purification of the 80-kDa LPS binding protein expressed on macrophages and lymphocytes. This partially purified Ag was used to immunize adult Armenian hamsters and splenocytes from immunized animals were fused with murine myeloma cell lines. Hybridoma cell culture supernatants containing mAb were screened by ELISA for positive binding to the immunizing Ag, murine splenocytes and the murine 70Z/3 pre B cell and for an absence of binding to sheep E. Positive clones were further screened for reciprocal competitive binding with LPS on spleen cells and ability to modulate B lymphocyte mitogenic activity. Two hybridoma cell lines secreting IgM monoclonals, termed mAb3D7 and mAb5D3, were identified that satisfied all of the selection criteria. These hybridoma cell lines were subcloned and expanded. Binding of one (mAb3D7) was abrogated by treatment of Ag with mild periodate; binding of the second (mAb5D3) was destroyed by digestion of Ag with proteinase K. Binding specificity for mAb5D3 has been confirmed by ELISA using highly purified 80-kDa protein. These mAb have been of value in establishing that the 80-kDa LPS binding protein previously identified may serve as a specific functional receptor for LPS.     

Induction of macrophage-mediated tumor cytotoxicity by a hamster monoclonal antibody with specificity for lipopolysaccharide receptor.

Experiments have been carried out to assess the immunostimulatory activity of a hamster IgM mAb (mAb5D3) with specificity for an 80-kDa LPS-binding protein expressed on murine macrophages and monocytes. The addition of mAb5D3 to cultures of murine bone marrow-derived macrophages activated these cells to become tumoricidal for mastocytoma cells in vitro. The activity of mAb5D3 was enhanced in the presence of IFN-gamma. Neither mAb5D3 nor LPS were able to activate macrophages from the LPS-hyporesponsive C3H/HeJ mouse, although these cells responded normally to heat-killed Listeria monocytogenes. The results of several experiments establish that the observed LPS-like activity of mAb5D3 was not due to contaminating endotoxin: 1) the activity of mAb5D3 but not LPS was heat labile at 100 degrees C; 2) the activity of LPS but not mAb5D3, was inhibited by addition of polymyxin B; and 3) quantitative estimates of endotoxin contamination by Limulus amoebocyte lysate reactivity. These experiments thus demonstrate that mAb5D3 can serve as an agonist for LPS-dependent macrophage responses and, when considered with those of our companion paper showing specificity of mAb5D3 for the 80-kDa LPS-binding protein, provide strong support for the concept that the 80-kDa LPS-binding protein previously identified serves as a functional receptor for LPS on murine macrophages.     

Specific endotoxic lipopolysaccharide-binding receptors on murine splenocytes. III. Binding specificity and characterization.

In previously published studies, we employed a photoreactive radioiodinated derivative of LPS from Escherichia coli 0111:B4 to identify and characterize a membrane-localized specific LPS binding protein of approximately 80-kDa molecular mass. Our more recent studies demonstrating that mAb with specificity for this 80-kDa protein will act as an agonist in mediating macrophage activation have established that this protein serves as a specific receptor for LPS. In the experiments reported here, we have more accurately determined the apparent molecular mass of this protein to be 73 kDa (p73). We have also extended the sources of LPS-derivatized photo-cross-linking preparations (including Re-LPS) to determine generality of LPS binding to this receptor. Binding to the p73 LPS receptor is demonstrated with all of the LPS derivatives synthesized in our laboratory, as well as probes synthesized by other investigators. Binding of S-LPS is readily inhibited by Re chemotype LPS, and we have shown that this competitive inhibition is most likely not the result of formation of LPS aggregates. These results confirm and extend our earlier studies suggesting that the binding of LPS to the p73 receptor is lipid A specific. We further demonstrate that, in contrast to results published in a recent report, the p73 LPS receptor has no significant binding specificity for a variety peptidoglycan polymer preparations. Finally, we show that this LPS receptor can be detected on murine fibroblast, macrophage, and mastocytoma cell lines. Differences have been observed in the level of expression of LPS receptors on the various cell lines studied.     

Isolation of a lipopolysaccharide (LPS)-resistant mutant, with defective LPS binding, of cultured macrophage-like cells.

Lipopolysaccharide (LPS)-resistant mutants which did not respond to LPS were isolated from a macrophage-like mouse cell line, J774.1. Unlike the parental J774.1 cells, these mutants grew even in LPS added medium as well as in normal growth medium without any morphological changes. Assay of 125I-LPS binding to the cell monolayers revealed that one of these LPS-resistant mutants (LR-9) was strikingly defective in LPS-binding activity. Scatchard plot showed that LR-9 cells lacked the high affinity binding sites which were present in J774.1. The high affinity binding was inhibited by addition of excess unlabeled LPS, lipid A, lipid IVA (tetraacyl-beta(1'-6)-linked D-glucosamine disaccharide-1,4'-bisphosphate), and lipid X (2,3-diacylglucosamine 1-phosphate) and sensitive to proteinase K. LPS enhanced O2- generation and the release of arachidonic acid in J774.1 cells but not in LR-9 cells. Other stimulants such as zymosan and 12-O-tetradecanoylphorbol 13-acetate, however, induced the release of arachidonic acid in LR-9 cells as well as in J774.1 cells. LPS-photocross-linked assay allowed the identification of 65- and 55-kDa LPS-binding proteins in the membrane fraction of J774.1 cells. Both of the bands were not detectable in that of LR-9 cells and disappeared by competing with unlabeled LPS or lipid X. These results show that one or both of the two LPS-binding proteins might relate to the specific membrane receptor for LPS.     

Identification of Re lipopolysaccharide-binding protein on murine erythrocyte membrane

Our recent studies have suggested that bacterial lipopolysaccharide (LPS) attaches to Pronase-sensitive proteins on the murine erythrocyte membrane. In the present study, in order to identify the LPS-binding protein on the murine erythrocyte membrane, a unique method to detect LPS-binding protein on a nitrocellulose membrane was developed. Murine erythrocyte membrane proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, then transferred electrophoretically onto a nitrocellulose membrane. The membrane was incubated with LPS of Salmonella minnesota R595 (Re LPS) in phosphate-buffered saline (PBS), after the remaining sites were blocked with gelatin in PBS. We were able to obtain a non-background stain by adding the nonionic detergent octylglucoside at the low concentration of 0.1% to the Re LPS solution. The Re LPS bound to the protein on the nitrocellulose membrane was exposed to affinity purified anti-Re LPS antibodies (IgG) and then to alkaline phosphatase-conjugated anti-IgG. The alkaline phosphatase was detected on the membrane by an enzymatic reaction. This method demonstrated that Re LPS was bound to an erythrocyte protein of 96 kDa. Treatment of erythrocytes with Pronase led to disappearance of the Re LPS-binding protein on the erythrocyte membrane. There was no difference between LPS-responder and LPS-nonresponder murine erythrocyte membranes in amount and molecular weight of the Re LPS-binding protein.     

Preparation and characterization of truncated human lipopolysaccharide-binding protein in Escherichia coli

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria, and is the causative agent of endotoxin shock. LPS induces signal transduction in immune cells when it is recognized by the cell surface complex of toll-like receptor 4 (TLR4) and MD-2. The complex recognizes the lipid A structure in LPS, which is buried in the membrane of the outer envelope. To present the Lipid A structure to the TLR4/MD-2, processing of LPS by LPS-binding protein (LBP) and CD14 is required. In previous studies, we expressed recombinant proteins of human MD-2 and CD14 as fusion proteins with thioredoxin in Escherichia coli, and demonstrated their specific binding abilities to LPS. In this study, we prepared a recombinant fusion protein containing 212 amino terminal residues of human LBP (HLB212) by using the same expression system. The recombinant protein expressed in E. coli was purified as a complex form with host LPS. The binding was not affected by high concentrations of salt, but was prevented by low concentrations of various detergents. Both rough-type LPS lacking the O antigen and smooth-type LPS with the antigen bound to HLBP212. Therefore, oligosaccharide repeats appeared to be unnecessary for the binding. A nonpathogenic penta-acylated LPS also bound to HLBP212, but the binding was weaker than that of the wild type. The hydrophobic interaction between the LBP and acyl chains of lipid A appears to be important for the binding. The recombinant proteins of LPS-binding molecules would be useful for analyzing the defense mechanism against infections.     

Peptidoglycan and lipopolysaccharide bind to the same binding site on lymphocytes

Bacterial cell wall peptidoglycan (PGN) and lipopolysaccharide (LPS), which are both macrophage activators and polyclonal B cell mitogens, were shown to bind to the same dominant 70-kDa 6.5 pI protein on the surface of mouse B lymphocytes. This conclusion was supported by the following results: (a) the PGN- and LPS-binding proteins co-migrated following photoaffinity cross-linking and two-dimensional polyacrylamide gel electrophoresis; (b) cross-linking of PGN to this 70-kDa protein was competitively inhibited by LPS (IC50 = 7.3 microM), LPS from a deep rough mutant (IC50 = 6.9 microM), and lipid A (IC50 = 18-72 microM); (c) cross-linking of LPS to this 70-kDa protein was competitively inhibited by polymeric soluble PGN (IC50 = 0.09 microM) and sonicated high Mr PGN (IC50 = 0.6 microM); (d) cross-linking of both PGN and LPS to this 70-kDa protein was also competitively inhibited by dextran sulfate (IC50 = 115-124 microM); (e) cross-linking of both PGN and LPS to this 70-kDa protein was inhibited by a (GlcNAc)2-specific lectin; and (f) peptide maps of the 70-kDa proteins digested with chymotrypsin, subtilisin, staphylococcal protease V, or papain were identical for PGN- and LPS-binding proteins and unique for each enzyme. Based on competitive inhibition experiments, binding of PGN to the 70-kDa protein was 20-1200 times stronger than the binding of LPS or lipid A on a per mol basis. However, when aggregated micellar structures of LPS or lipid A were considered, the avidities of LPS and PGN binding were similar. These results demonstrate binding of PGN and LPS to the same 70-kDa protein on lymphocytes and suggest that the binding is specific for the (GlcNAc-MurNAc)n backbone of PGN and the (GlcNAc)2 part of lipid A.     

Identification of the 80-kDa LPS-binding protein (LMP80) as decay-accelerating factor (DAF, CD55)

The activation of immunocompetent cells by lipopolysaccharide (LPS) during severe Gram-negative infections is responsible for the pathophysiological reactions, possibly resulting in the clinical picture of sepsis. Monocytes recognize LPS mainly through the LPS receptor CD14, however, other cellular binding structures have been assumed to exist. In previous studies, we have described an 80-kDa LPS-binding membrane protein (LMP80), which is present on human monocytes as well as endothelial cells. Here we demonstrate that LMP80 is widely distributed and that it forms complexes together with LPS and sCD14. Furthermore, we report on the biochemical purification of LMP80 and its identification as decay-accelerating factor, CD55, by amino acid sequencing and cloning techniques. Our results imply a new feature of CD55 as a molecule which interacts with LPS/sCD14 complexes. However, the involvement of CD55 in LPS-induced signaling remains to be elucidated.     

Rough and smooth forms of fluorescein-labelled bacterial endotoxin exhibit CD14/LBP dependent and independent binding that is influencedby endotoxin concentration.

Lipopolysaccharide (LPS, or endotoxin), is a major constituent of the outer membrane of Gram-negative bacteria. Bacteria express either smooth LPS, which is composed of O-antigen (O-Ag), complete core oligosaccharides, and the lipid A, or rough LPS which lack O-Ag but possess lipid A and progressively shorter core oligosaccharides. CD14 has been described as the receptor for complexes of LPS with LPS-binding protein (LBP). Using flow cytometry we have compared the binding of Salmonella minnesota rough LPS (ReLPS) and Escherichia coli smooth LPS labelled with fluorescein isothiocyanate (FITC-LPS) to Chinese hamster ovary (CHO) cells transfected with human CD14 gene (hCD14-CHO), to MonoMac 6 cells and to endothelial cells. Our results showed that both forms of LPS display the same binding characteristics, and that the binding of FITC-LPS to cells was both CD14- and LBP-dependent for LPS concentrations up to 100 ng.mL-1. At LPS concentrations higher than 100 ng.mL-1 we observed CD14/LBP-independent binding. CD14/LBP-dependent binding was dose dependent, saturable, and enhanced in the presence of human pooled serum (HPS), and the monoclonal anti-CD14 antibody (MY4) or unlabelled LPS could outcompete it.     

Identification of a lipid A binding site in the acute phase reactant lipopolysaccharide binding protein

Lipopolysaccharide (LPS) binding protein (LBP), a recently discovered 60-kDa acute phase protein, is present in the acute phase serum of many species including human, rabbits, mice, and rats. Using either highly purified LBP from acute phase rabbit serum or unfractionated acute phase rabbit serum as a source of LBP, we examined the binding of LBP to LPS immobilized on plastic microtiter plates and to LPS electrotransferred to nitrocellulose after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The presence of LBP bound to LPS was detected with goat anti-rabbit LBP and peroxidase-conjugated rabbit anti-goat IgG. LBP was found to bind to a variety of LPS types from both rough and smooth strains of Gram-negative bacteria, to lipid A, and to the tetraacyl glucosamine disaccharide diphosphate precursor IVA, but bound very poorly to the diacyl glucosamine phosphate, lipid X. No binding to 3-deoxyoctulosonic acid was observed. Binding affinities for LPS are near 10(9) M-1. The data presented here support the concept that LBP contains a binding site for lipid A.     

Plasma lipopolysaccharide (LPS)-binding protein. A key component in macrophage recognition of gram-negative LPS.

LPS-binding protein (LBP) binds with high affinity (Kd approximately equal to 10(-9) M) to lipid A of LPS isolated from rough (R)- or smooth (S)-form Gram-negative bacteria as well as to lipid A partial structures such as precursor IVA. To define the role of LBP in regulating responses to LPS we have examined TNF release in rabbit peritoneal exudate macrophages (M phi) stimulated with LPS or with complete or partial lipid A preparations in the presence or absence of LBP. In the presence of LBP, M phi showed increased sensitivity to S- and R-form LPS as well as synthetic lipid A. Compared with LPS or lipid A, up to 1000-fold greater concentrations of partial lipid A structures were required to induce TNF production. However, consistent with our previous observations that these structures bind to LBP, TNF production was increased in the presence of LBP. In contrast, LBP did not enhance or inhibit TNF production produced by heat-killed Staphylococcus aureus, peptidoglycan isolated from S. aureus cell walls, or PMA. Potentiated M phi responsiveness to LPS was observed with as little as 1 ng LBP/ml. Heat-denatured LBP (which no longer binds LPS), BPI (an homologous LPS-binding protein isolated from neutrophils), or other serum proteins were without effect. LBP-treated M phi also showed a more rapid induction of cytokine mRNA (TNF and IL-1 beta), higher steady-state mRNA levels and increased TNF mRNA stability. These data provide additional evidence that LBP is part of a highly specific recognition system controlling M phi responses to LPS. The effects of LBP are lipid A dependent and importantly, extend to LPS preparations isolated from bacteria of R- and S-form phenotype.     

Delineation of lipopolysaccharide (LPS)-binding sites on hemoglobin: from in silico predictions to biophysical characterization

Hemoglobin (Hb) functions as a frontline defense molecule during infection by hemolytic microbes. Binding to LPS induces structural changes in cell-free Hb, which activates the redox activity of the protein for the generation of microbicidal free radicals. Although the interaction between Hb and LPS has implications for innate immune defense, the precise LPS-interaction sites on Hb remain unknown. Using surface plasmon resonance, we found that both the Hb α and β subunits possess high affinity LPS-binding sites, with K(D) in the nanomolar range. In silico analysis of Hb including phospho-group binding site prediction, structure-based sequence comparison, and docking to model the protein-ligand interactions showed that Hb possesses evolutionarily conserved surface cationic patches that could function as potential LPS-binding sites. Synthetic Hb peptides harboring predicted LPS-binding sites served to validate the computational predictions. Surface plasmon resonance analysis differentiated LPS-binding peptides from non-binders. Binding of the peptides to lipid A was further substantiated by a fluorescent probe displacement assay. The LPS-binding peptides effectively neutralized the endotoxicity of LPS in vitro. Additionally, peptide B59 spanning residues 59-95 of Hbβ attached to the surface of Gram-negative bacteria as shown by flow cytometry and visualized by immunogold-labeled scanning electron microscopy. Site-directed mutagenesis of the Hb subunits further confirmed the function of the predicted residues in binding to LPS. In summary, the integration of computational predictions and biophysical characterization has enabled delineation of multiple LPS-binding hot spots on the Hb molecule.     

Monoclonal antibodies to murine lipopolysaccharide (LPS)-binding protein (LBP) protect mice from lethal endotoxemia by blocking either the binding of LPS to LBP or the presentation of LPS/LBP complexes to CD14.

Cellular responses to LPS, the major lipid component of the outer membrane of Gram-negative bacteria, are enhanced markedly by the LPS-binding protein (LBP), a plasma protein that transfers LPS to the cell surface CD14 present on cells of the myeloid lineage. LBP has been shown previously to potentiate the host response to LPS. However, experiments performed in mice with a disruption of the LBP gene have yielded discordant results. Whereas one study showed that LBP knockout mice were resistant to endotoxemia, another study did not confirm an important role for LBP in the response of mice challenged in vivo with low doses of LPS. Consequently, we generated rat mAbs to murine LBP to investigate further the contribution of LBP in experimental endotoxemia. Three classes of mAbs were obtained. Class 1 mAbs blocked the binding of LPS to LBP; class 2 mAbs blocked the binding of LPS/LBP complexes to CD14; class 3 mAbs bound LBP but did not suppress LBP activity. In vivo, class 1 and class 2 mAbs suppressed LPS-induced TNF production and protected mice from lethal endotoxemia. These results show that the neutralization of LBP accomplished by blocking either the binding of LPS to LBP or the binding of LPS/LBP complexes to CD14 protects the host from LPS-induced toxicity, confirming that LBP is a critical component of innate immunity.     

Identification of lipopolysaccharide-binding proteins in 70Z/3 cells by photoaffinity cross-linking

A radioiodinated, photoactivatable derivative of Salmonella minnesota Re595 lipopolysaccharide (LPS) was used to label LPS-binding proteins in 70Z/3 cells. The labeled proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by autoradiography. 125I-Labeled-2-(p-azidosalycylamido)1,3'-dithiopropionamide S. minnesota Re595 LPS (125I-ASD-Re595) labeled a limited number of proteins. The most prominent of these had a apparent molecular mass of 18 kDa. Less prominent labeling of 25- and 28-kDa proteins was also seen. Labeling was saturated by 5 micrograms/ml 125I-ASD-Re595 and was inhibited by a 10-100-fold excess of unlabeled LPS or lipid A. Labeling was maximal within 30 min at 37 degrees C; much less labeling occurred at lower temperatures. The proteins labeled with 125I-ASD-Re595 appear to be on the surface of the cell, since they can be digested by trypsin and were found in the membrane fraction of the cell but not in the cytosol. Studies with competitive inhibitors suggested that the proteins bind to the lipid A region of the LPS molecule. Biologically inactive lipid A analogs were poor inhibitors of labeling, suggesting that the LPS-binding proteins could discriminate between active lipid A and inactive analogs. These studies suggest that the 18- and 25-kDa proteins bind specifically to the lipid A region of the LPS molecule and should be considered as candidates for a functional LPS receptor.     

Histones: a novel class of lipopolysaccharide-binding molecules

Unlike soluble and membrane forms of lipopolysaccharide (LPS)-binding proteins, intracellular LPS-binding molecules are poorly documented. We looked for such molecules in a murine lung epithelial cell line. Two proteins with LPS-binding activity were isolated and unambiguously identified as histones H2A.1 and H4 by mass spectrometry. Synthetic peptides representing partial structures indicated that the LPS binding site is located in the C-terminal moiety of the histones. Extending the study, we found that histones H1, H2A, H2B, H3, and H4 from calf thymus are all able to bind LPS. Bindings were specific, and affinities, determined by isothermal titration calorimetry, were (except for H4) higher than that of the LPS-binding antibiotic polymyxin B. In the presence of H2A the binding of LPS to the macrophage cell line RAW 264.7, and the LPS-induced production of TNF-alpha and nitric oxide by these cells, were markedly reduced. Histones may thus represent a new class of intracellular and extracellular LPS sensors.     

The Murine Nucleolin Protein Is an Inducible DNA and ATP Binding Protein Which Is Readily Detected in Nuclear Extracts of Lipopolysaccharide-Treated Splenocytes

A 100-kDa DNA binding protein was found to be dramatically up-regulated upon the mitogenic stimulation of murine splenocytes with bacterial lipopolysaccharide (LPS). The induced DNA binding protein was also found to exhibit moderate binding specificity for the immunoglobulin isotype switch DNA repeats. Furthermore, the induction of the 100-kDa protein by LPS was found to be mediated by both an increase in the protein's stability and an increase in the synthesis of the protein. In vitro phosphorylation experiments revealed that the 100-kDa DNA binding protein was one of the most heavily phosphorylated proteins in both lymphoid and nonlymphoid nuclear extracts. Although this in vitro phosphorylation initially appeared to be mediated by a potent nuclear kinase activity, it was later determined that a significant part of the detected labeling was due to the direct binding of ATP by the 100-kDa protein. Antibodies raised to the 100-kDa DNA binding protein were used to isolate cDNA clones from a lymphocyte cDNA λgt11 expression library. Nucleotide sequence analysis revealed that the cloned cDNAs were identical to the mouse nucleolin gene. The β-galactosidase fusion proteins (encoded by exons 3-14 of nucleolin) and a more severely truncated 45-kDa protein (encoded by exons 5-14 of nucleolin) were both found to bind strongly to DNA and ATP. Furthermore, the strength of DNA binding was found to be highly dependent on the overall dG content of the DNA probes. Our experiments also revealed that apart from binding ATP and G-rich DNA, nucleolin directly bound GTP, dATP, and dGTP, but not dCTP, dTTP, or dUTP. Computer analysis revealed that the putative ATP binding domains appear to fall within two of the phylogenetically conserved RNA binding domains of nucleolin.     

Localization of the lipopolysaccharide-binding protein in phospholipid membranes by atomic force microscopy

Lipopolysaccharides (LPS; endotoxin) activate immunocompetent cells of the host via a transmembrane signaling process. In this study, we investigated the function of the LPS-binding protein (LBP) in this process. The cytoplasmic membrane of the cells was mimicked by lipid liposomes adsorbed on mica, and the lateral organization of LBP in these membranes and its interaction with LPS aggregates were characterized by atomic force microscopy. Using cantilever tips functionalized with anti-LBP antibodies, single LBP molecules were localized in the membrane at low concentrations. At higher concentrations, LBP formed clusters of several molecules and caused cross-linking of lipid bilayers. The addition of LPS to LBP-containing liposomes led to the formation of LPS domains in the membranes, which could be inhibited by anti-LBP antibodies. Thus, LBP mediates the fusion of lipid membranes and LPS aggregates.     

Altered binding of the Cry1Ac toxin to larval membranes but not to the toxin-binding protein in Plodia interpunctella selected for resistance to different Bacillus thuringiensis isolates.

Immunoblotting and cytochemical procedures were used to determine whether toxin binding was altered in strains of the Indianmeal moth, Plodia interpunctella, selected for resistance to various strains of Bacillus thuringiensis. Each of these B. thuringiensis subspecies produces a mixture of protoxins, primarily Cry1 types, and the greatest insect resistance is to the Cry1A protoxins. In several cases, however, there was also resistance to toxins not present in the B. thuringiensis strains used for selection. The Cry1Ab and Cry1Ac toxins bound equally well over a range of toxin concentrations and times of incubation to a single protein of ca. 80-kDa in immunoblots of larval membrane extracts from all of the colonies. This binding protein is essential for toxicity since a mutant Cry1Ac toxin known to be defective in binding and thus less toxic bound poorly to the 80-kDa protein. This binding protein differed in size from the major aminopeptidase N antigens implicated in toxin binding in other insects. Binding of fluorescently labeled Cry1Ac or Cry1Ab toxin to larval sections was found at the tips of the brush border membrane prepared from the susceptible but not from any of the resistant P. interpunctella. Accessibility of a major Cry1A-binding protein appears to be altered in resistant larvae and could account for their broad resistance to several B. thuringiensis toxins.