Cloning and expression of a cDNA encoding mouse endoglin, an endothelial cell TGF-beta ligand.

The rat monoclonal antibody, MJ7/18, which reacts selectively with the endothelium of blood vessels in mouse was used to screen a cDNA library derived from a transformed mouse brain endothelial cell line. The sequence of a cDNA encoding the cell surface MJ7/18 antigen revealed homology to human endoglin, a homodimeric transforming growth factor-beta (TGF-beta)-binding cell-surface glycoprotein expressed predominantly on vascular endothelial cells. Northern blot analysis shows a 3.4-kb single transcript of the mouse endoglin. The mouse endoglin is a type-I integral membrane protein of 653 amino acids (aa). The human and mouse sequences display 71% aa sequence identity with almost identical transmembrane and cytoplasmic domains. Like its human counterpart, mouse endoglin displays significant sequence homology to the type-III TGF-beta receptor in two extracellular domains, as well as striking similarity in the transmembrane and cytoplasmic regions. One of the extracellular regions of homology with TGF-beta receptor III represents a truncated version of a homology unit defining a novel gene family including uromodulin, the pancreatic granule protein gp2, and zona pellucida receptors for sperm. However, unlike its human counterpart, mouse endoglin does not contain an RGD tripeptide which has been suggested as a ligand of integrins.

Location of a Bombyx mori receptor binding region on a Bacillus thuringiensis delta-endotoxin.

Receptor binding studies were performed with 125I-labeled trypsin-activated insecticidal toxins, CryIA(a) and CryIA(c), from Bacillus thuringiensis on brush-border membrane vesicles (BBMV) prepared from Bombyx mori larval midgut. Bioassays were performed by gently force feeding B. mori with diluted toxins. CryIA(a) toxin (LD50; 0.002 micrograms) was 200 times more active against B. mori larvae than CryIA(c) toxin (LD50; 0.421 micrograms) and showed high-affinity saturable binding. The Kd and the binding site concentration for CryIA(a) toxin were 3.5 nM and 7.95 pmol/mg, respectively. CryIA(c) toxin (Kd, 50.35 nM; Bmax, 2.85 pmol/mg) did not demonstrate high-affinity binding to B. mori BBMV. Control experiments with CryIA(a) and CryIA(c) toxins revealed no binding to mouse small intestine BBMV and nonspecific binding to pig kidney BBMV. These data provide evidence that binding to a specific receptor on the membrane of midgut epithelial cells is an important determinant with respect to differences in insecticidal spectrum of insecticidal crystal proteins. To locate a B. mori receptor binding region on the CryIA(a) toxin, homologous and heterologous competition binding studies were performed with a set of mutant proteins which had previously been used to define the B. mori "specificity domain" on this toxin (Ge, A. Z., Shivarova, N. I., and Dean, D. H. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 4037-4041). These mutant proteins have had regions of their genes reciprocally exchanged with the cryIA(c) gene. A B. mori receptor binding region on CryIA(a) toxin includes the amino-terminal portion of the hypervariable region, amino acids 332-450, which is identical to the previously described B. mori specificity determining region. These data provide direct evidence that delta-endotoxins contain a tract of amino acids that comprise a binding region and as a results determines the specificity of a toxin.

Functional domains of Bacillus thuringiensis insecticidal crystal proteins. Refinement of Heliothis virescens and Trichoplusia ni specificity domains on CryIA(c).

Insecticidal crystal proteins (delta-endotoxins), CryIA(a) and CryIA(c), from Bacillus thuringiensis are 82% homologous. Despite this homology, CryIA(c) was determined to have 10-fold more insecticidal activity toward Heliothis virescens and Trichoplusia ni than CryIA(a). Reciprocal recombinations between these two genes were performed by the homolog-scanning technique. The resultant mutants had different segments of their primary sequences exchanged. Bioassays with toxin proteins from these mutants revealed that amino acids 335-450 on CryIA(c) are associated with the activity against T. ni, whereas amino acids 335-615 on the same toxin are required to exchange full H. virescens specificity. One chimeric protein toxin, involving residues 450-612 from CryIA(c), demonstrated 30 times more activity against H. virescens than the native parental toxin, indicating that this region plays an important role in H. virescens specificity. The structural integrity of mutant toxin proteins was assessed by treatment with bovine trypsin. All actively toxic proteins formed a 65-kDA trypsin-resistant active toxic core, similar to the parental CryIA(c) toxin, indicating that toxin protein structure was not altered significantly. Contrarily, certain inactive mutant proteins were susceptible to complete protease hydrolysis, indicating that their lack of toxicity may have been due to structural alterations.

Hyperexpression of a Bacillus thuringiensis delta-endotoxin-encoding gene in Escherichia coli: properties of the product.

Conditions for hyperexpression, in Escherichia coli, of the Bacillus thuringiensis var, kurstaki gene, cryIA9(c)73, encoding an insecticidal crystal protein, CryIA(c)73, were investigated by varying the promoter type, host cell, plasmid copy number, the second codon and number of terminators. The cryIA(c)73 gene was cloned into three E. coli expression vectors, pKK223-3 (Ptac promoter), pET-3a (P phi 10 promoter), and pUC19 (Ptac promoter). The level of cryIA(c)73 expression was measured by ELISA and compared to total cellular protein over growth periods of 24 and 48 h. Maximum expression levels of 284 microgram CryIA(C)73/ml (48% of cellular protein) were obtained in shake flasks with the Ptac promoter in E. coli JM103. Optimal conditions were found to be low-copy-number plasmid (pBR322 ori), 48 h of growth, in lon+ cells. A change of the gene's second codon to AAA can improve expression by two to three fold but is undetectable in the presence of a strong E. coli promoter. The cryIA(c)73 gene product, in E. coli, formed crystals with the same lattice structure as the native crystals formed in B. thuringiensis (as visualized by electron microscopy). Bioassay results (insect toxicity and specificity) of the crystal produced in E. coli were similar to that produced in B. thuringiensis.

Location of the Bombyx mori specificity domain on a Bacillus thuringiensis delta-endotoxin protein.

Bacillus thuringiensis produces different types of insecticidal crystal proteins (ICPs) or delta-endotoxins. In an effort to identify the insect specificity of ICP toxins, two icp genes were cloned into the Escherichia coli expression vector pKK223-3, and bioassays were performed with purified crystals. The type A protein [from an icpA1, or 4.5-kilobase (kb) gene, from B. thuringiensis var. kurstaki HD-1] was found to be 400 times more active against Bombyx mori than type C protein (from an icpC73, or 6.6-kb gene, from B. thuringiensis var. kurstaki HD-244). The type C protein was 9 times more active against Trichoplusia ni than the type A protein, while both have similar activity against Manduca sexta. To locate the specificity domain of the type A protein for B. mori, site-directed mutagenesis was used to introduce or remove restriction enzyme sites, facilitating the exchange of regions of the two genes. The hybrid genes were overexpressed, and purified ICP was used in bioassays. The B. mori specificity domain for the ICP A toxin is located in the amino-terminal portion of the hypervariable region between amino acids 332 and 450.