Identification of the AMP binding sites of rabbit phosphofructo-1-kinase isozymes B and C.

Rabbit liver phosphofructo-1-kinase, designated isozyme B, and rabbit brain phosphofructokinase, which contains all three isozymes as heteropolymers, have been modified by [14C]fluorosulfonylbenzoyladenosine (FSBAdo). Several lines of evidence supported modification at the binding site for AMP. The modification proceeded to the extent of 2 to 4 mol of reagent incorporated per mol of tetramer, and AMP protected against the reaction. The kinetic properties of modified isozymes A and B and of modified brain phosphofructokinase were examined and compared to their unmodified forms. It was observed that modification greatly diminished ATP inhibition of all of the isozymes. Furthermore, equilibrium binding studies of modified phosphofructokinase B showed a greatly diminished capacity and affinity for cyclic AMP. Cyclic AMP had little or no influence on the properties of modified A isozyme or brain phosphofructokinase, but was capable of further deinhibiting modified B isozyme, apparently at sites remaining unmodified by FSBAdo. Phosphofructokinase B, modified by radiolabeled FSBAdo, was digested by trypsin, and the digest separated by high-pressure liquid chromatography. The labeled peptide was isolated and sequenced to provide the sequence: Asn-Tyr-Gly-Thr-Lys-Leu-Gly-Val-Lys, with the lysine in the fifth position being the site of modification. To isolate isozyme C, a monoclonal antibody to this isozyme was produced by injecting purified rabbit brain phosphofructokinase into mice, and subsequently selecting for those clones that recognized brain phosphofructokinase but not purified phosphofructokinases A and B. The selected monoclonal was specific for native rabbit isozyme C and would not recognize mouse or rat brain phosphofructokinases. Linking the antibody to an inert phase provided an efficient means of purifying rabbit isozyme C from rabbit brain. The enzyme so recovered retained little of its original activity, but the method provided a simple technique for the preparation of enzyme for protein chemistry studies. The modified C isozyme was isolated on the immuno-affinity column and digested with trypsin. A tryptic peptide bearing the label was isolated and sequenced to provide the structure: Asn-Phe-Gly-Thr-Lys-Ile-Ser-Ala-Arg, with position 5 being the site of modification. The sequences of isozymes B and C are homologous to the site of modification of the A isozyme by FSBAdo.

The sites of phosphorylation of rabbit brain phosphofructo-1-kinase by cyclic AMP-dependent protein kinase.

The three isozymic subunits of phosphofructo-1-kinase present in rabbit brain and designated A, B and C were phosphorylated in vitro by cyclic AMP-dependent protein kinase with 32P-labeled ATP. Limited digestion of the labeled enzymes with trypsin or with Staphylococcus aureus V8 proteinase led to the solubilization of radiolabeled peptides derived from the three isozymic subunits. Limited digestion by V8 proteinase was accompanied by a slight reduction in the apparent sizes of the subunits, indicating that the phosphorylated sites are located near either the amino or carboxyl termini of the protein. V8 proteinase digestion led to no change in the maximal activity of the enzyme but did abolish sensitivity to ATP inhibition. The phosphopeptides of the tryptic and the V8 digests were purified by chromatography and their amino acid sequences were determined and compared to the previously established sequence from rabbit muscle isozyme A. PFK-A E H I S R K R S G E A T V PFK-B H V T R R S L S M A K G F PFK-C V S A S P R G S Y R K F L In each instance, the phosphorylated serine, underlined in the above sequences, was found to be one or two residues toward the C-terminus of one or more basic residues. No other similarities in structure were noted.

Gypsy moth midgut proteinases: purification and characterization of luminal trypsin, elastase and the brush border membrane leucine aminopeptidase.

The principal digestive proteinases of the gypsy moth, Lymantria dispar, larval midgut were identified, and the subcellular distribution of the enzyme activities was determined. Proteinase activities of fifth-instar larvae were largely attributed to two luminal serine proteinases, a trypsin-like enzyme (TLE) and an elastase 2-like enzyme (ELA). TLE was purified to homegeneity by benzamidine-Sepharose affinity chromatography. With respect to size (M(r) = 25 kDa), substrate specificity, and interaction with trypsin inhibitors, the gypsy moth enzyme resembled mammalian pancreatic trypsin and trypsin-like enzymes from other insects. Gypsy moth elastase (ELA) was purified from the benzamidine-Sepharose flow-through by mono-Q FPLC. ELA exhibited a slightly smaller size (M(r) = 24 kDa) than TLE. The insect enzyme was inhibited by DFP and chymostatin but was unaffected by TPCK. ELA exhibited little esterolytic activity with BTEE. Succinyl-Ala-Ala-Pro-Leu p-nitroanilide was one of the best substrates for ELA, which is characteristic of elastase 2. TLE and ELA constituted c. 6% of the total soluble protein in midgut lumen of actively feeding fifth-instar larvae. Chymotrypsin and carboxypeptidase activities were not detected in any midgut fraction examined. The brush border membrane (BBM) leucine aminopeptidase (LAP) was isolated from CHAPS-solubilized BBM by FPLC. SDS-PAGE results indicated that the aminopeptidase has an apparent molecular size of c. 100 kDa. The aminopeptidase was inhibited by bestatin and was unaffected by serine proteinase inhibitors.

Purification and properties of the soluble midgut trehalase from the gypsy moth, Lymantria dispar.

The midgut trehalase (THA) from fifth instar Lymantria dispar (gypsy moth) larvae was purified to homogeneity by two separate methods: gel filtration followed by Rotofor preparative IEF, and affinity chromatography on trehalose coupled to Sepharose 6B followed by preparative polyacrylamide gel electrophoresis. Midgut THA from the last stadium L. dispar larvae existed mainly in soluble form and displayed a single band of activity in nondenaturing polyacrylamide gels when stained by a THA-specific staining procedure. Analytical IEF of purified midgut THA revealed a single protein band with an apparent pI of 4.6. SDS-PAGE and gel permeation studies indicated that the smallest active form of THA in the late fifth instar larval midgut was a monomeric protein with an approximate size of 60 kDa. A specific activity of 67 units/mg of protein at 30 degrees C and at pH 6.4 was determined for the enzyme purified by affinity chromatography and preparative gel electrophoresis. The midgut enzyme exhibited a very high substrate specificity with a Km of 0.4 mM for trehalose. The enzyme was maximally active at pH 5.4-6.0 and was thermally stable at temperatures up to 65 degrees C. The midgut THA was insensitive to inhibition by a high concentration of Tris, sucrose, p-nitrophenyl-beta-D-glucoside or phloridzin. Divalent cations metal ions, hypertrehalosaemic hormone and octopamine had no significant effect on the activity of the purified enzyme in vitro. The purified enzyme was inactivated by modification with DEP and was competitively inhibited by castanospermine with an apparent Ki of 0.8 x 10(-6)M at pH 6.4.

Purification and characterization of the western spruce budworm larval midgut proteinases and comparison of gut activities of laboratory-reared and field-collected insects.

Three proteolytic enzymes, trypsin, chymotrypsin, and aminopeptidase-N (APN), were purified from laboratory-reared western spruce budworm, Choristoneura occidentalis [Freeman], larvae. Budworm trypsin exhibited a high degree of substrate specificity, was inactivated by DFP and TLCK, and was inhibited by trypsin inhibitors. The western spruce budworm chymotrypsin hydrolyzed SAAPFpNA and SAAPLpNA, but not SFpNA, SGGFpNA, SGGLpNA or BTpNA. The chymotrypsin was inactivated by DFP, and was inhibited by chymostatin and the chymotrypsin inhibitor, POT-1. Purified budworm chymotrypsin exhibited little BTEE esterolytic activity and was insensitive to inhibition with TPCK. The N-terminal sequence of budworm trypsin, chymotrypsin, and APN were obtained. Similar levels of trypsin and APN gut activities were found in laboratory-reared and field-collected larvae. However, in comparison to laboratory-reared insects, considerably less chymotrypsin activity, and a much higher level of gut carboxypeptidase activity were found in field-collected western spruce budworm larvae.

Brush border membrane aminopeptidase-N in the midgut of the gypsy moth serves as the receptor for the CryIA(c) delta-endotoxin of Bacillus thuringiensis.

Aminopeptidase-N (AP-N) was purified from gypsy moth (Lymantria dispar, L.) brush border membrane vesicles (BBMV) proteins by mono-Q chromatography and Superdex-75 gel filtration in the presence of the zwitterionic detergent, CHAPS, using FPLC. The purified AP-N, identified by its enzymatic activity, had an apparent size of 100 kDa, and was identified as the unique Bacillus thuringiensis insecticidal toxin, CryIA(c), binding protein. AP-N clearly displayed strong binding to CryIA(c), exhibiting little or no binding to CryIA(a) or CryIA(b), and showing no binding for the coleopteran-specific toxin, CryIIIA. Protein blots of the BBMV proteins probed with biotin-labeled and 125I-labeled insecticidal proteins revealed that CryIAc binds only to 120 kDa protein which is a slightly larger size in comparison to purified AP-N. Antibodies raised against the gypsy moth AP-N demonstrated that the purified AP-N and the 120 kDa CryIA(c) binding protein of total BBMV proteins are antigenically identical.

Cloning and complete sequence characterization of two gypsy moth aminopeptidase-N cDNAs, including the receptor for Bacillus thuringiensis Cry1Ac toxin.

The complete cDNAs corresponding to two distinct gypsy moth (Lymantria dispar) larval gut aminopeptidases, APN1 and lambda APN2, were cloned and sequenced. The 3.4 kilobasepair cDNA of APN1 which encodes a 1017 amino acid prepro-protein corresponds to the previously-identified gypsy moth APN (APN-1) that specifically binds the Cry1Ac delta-endotoxin of Bacillus thuringiensis. Analysis of the primary structure of APN1 revealed a cluster of five potential N-linked glycosylation sites near the N-terminus and a C-terminal sequence characteristic of a putative glycosylphosphatidyl-inositol (GPI) anchor signal sequence. The cDNA of APN1 encodes the N-terminal peptide sequence and nine internal sequences obtained from the purified brush border membrane vesicle Cry1Ac receptor by protein sequencing. The lambda APN2 cDNA encodes a shorter protein with 51% similarity to APN1 that also appears to have a GPI anchor signal sequence. Expression of the APN1 cDNA in a baculovirus vector was confirmed by immunoblotting.

Molecular characterization of the insect immune protein hemolin and its high induction during embryonic diapause in the gypsy moth, Lymantria dispar.

During the embryonic (pharate first instar) diapause of the gypsy moth, Lymantria dispar, a 55 kDa protein is highly up-regulated in the gut. We now identify that protein as hemolin, an immune protein in the immunoglobulin superfamily. We isolated a gypsy moth hemolin cDNA and demonstrated a high degree of similarity with hemolins from three other moth species. Hemolin mRNA levels increased at the time of diapause initiation and remained high throughout the mandatory period of chilling required to terminate diapause in this species, and then dropped in late diapause. This mRNA pattern reflects the pattern of protein synthesis. These results suggest that hemolin is developmentally up-regulated in the gut during diapause. Diapause in this species can be prevented using KK-42, an imidazole derivative known to inhibit ecdysteroid biosynthesis, and gypsy moths treated in this manner failed to elevate hemolin mRNA. Conversely, this diapause appears to be initiated and maintained by the steroid hormone, 20-hydroxyecdysone, and the addition of 20-hydroxyecdysone to the culture medium elevated hemolin mRNA in the gut. Our results thus indicate a role for 20-hydroxyecdysone in the elevation of hemolin mRNA during diapause. Presumably, hemolin functions to protect the gypsy moth from microbial infection during its long, overwintering diapause.

Developmental changes in plasma transferrin concentrations related to red cell ferritin.

Transferrin, the plasma iron transport protein, has two iron-binding sites but is usually only partly saturated. (Essentially all plasma iron is bound to transferrin.) Thus, changes in transferrin saturation reflect differences in the concentrations of plasma iron and/or transferrin. Developmental changes in red cell ferritin content coincide with a 2.5 times increase in plasma transferrin and a proportional decrease in saturation, in the bullfrog model system. The possible relationship of the degree of transferrin saturation and structure on the distribution of iron between heme and ferritin was examined in suspensions of reticulocytes, which synthesize both ferritin and heme. The extra transferrin in adult plasma was indistinguishable from transferrin in tadpole plasma in terms of the ability to donate iron to red cell heme and ferritin in vitro and in terms of surface charge (pI 6.55, 6.34), molecular weight (73,000), carbohydrate content (2%), amino acid composition, and immunological reactivity. Only the saturation in vivo appeared to differ. When the saturation of transferrin was manipulated in vitro, an effect on the relative distribution of iron between heme and ferritin was observed. The heme-synthesizing system consumed a disproportionately large amount of the delivered iron until it was saturated, a point which coincided with transferrin saturation; as the degree of transferrin saturation decreases, iron delivered to red cell iron stores (ferritin) decreases disproportionately. Thus, the developmental increase in plasma transferrin and consequent decrease in saturation minimize the amount of iron available for storage in red cells. The effect is further enhanced by the decreased ability of adult erythrocytes to incorporate iron from transferrin, a property which may be related to quantitative changes observed in iodination of a Mr = 168,000 membrane protein.

Desensitization of muscle phosphofructokinase to ATP inhibition by removal of a carboxyl-terminal heptadecapeptide.

Brief exposure of rabbit skeletal muscle phosphofructokinase to Staphylococcus aureus V8 protease results in the release from the enzyme of two carboxyl-terminal peptides from the enzyme that together comprise 17 amino acids. The rate of proteolysis was increased in the presence of activators of the enzyme, ammonium sulfate and AMP, and was decreased in the presence of allosteric inhibitors, MgATP and citrate. No change was observed in the maximal velocity of the modified enzyme or in its affinity for substrates when assayed under noninhibitory conditions. Equilibrium binding studies indicated no change in the affinity of the modified enzyme for its allosteric activator, AMP. On the other hand, the proteolyzed enzyme exhibited markedly reduced inhibition by ATP and by citrate. ATP inhibition was observed only at very high concentrations of ATP. Fructose-6-P saturation curves of the modified enzyme were nearly hyperbolic. The interaction coefficient deduced from the slope of a Hill-type plot was 1.2 under conditions that yielded a coefficient of 3.0 with native phosphofructokinase. Binding studies verified a decrease in affinity for ATP for at least one of the ATP binding sites. Because kinetic studies showed no effect on the Km for ATP, it was concluded that the affinity was decreased at the MgATP inhibitory site only. Proteolytic removal of the terminal 8 residues from the enzyme produced no striking change in regulatory properties, thus focusing the critical region to the sequence His-Ala-His-Leu-Glu-His-Ile-Ser-Arg. It is suggested that the three histidine residues clustered in the carboxyl terminus may contribute to the binding of MgATP to the inhibitory site.

Further evidence that diapause in the gypsy moth, Lymantria dispar, is regulated by ecdysteroids: a comparison of diapause and nondiapause strains.

A nondiapause strain of the gypsy moth offers an additional tool for evaluating the regulation of diapause in this species. Patterns of protein expression in the gut and gut enzyme activity distinguished the two strains. Synthesis of a 55kDa gut protein, previously linked to diapause, began 14days after oviposition in both the diapause (D) and nondiapause (ND) strains. Though synthesis of this protein persisted in the D strain, its synthesis decreased after day 18 in the ND strain. In the D strain, activity of the proteolytic enzymes (trypsin, chymotrypsin, elastase, aminopeptidase) and esterase remained low, while activity of all of these enzymes increased dramatically in the ND strain 18-20days after oviposition. By contrast, alkaline phosphatase (ALP) activity was high in both strains 15-17days after oviposition, activity remained high in the D strain but in the ND strain activity then decreased. Patterns of ALP zymograms were similar in the two strains on day 15, but later a band of high mobility appeared only in the D strain. When 20-hydroxyecdysone was added to hanging drop cultures containing ND pharate larvae 15days after oviposition, the larvae assumed the characteristics of diapause larvae: the 55kDa gut protein was synthesized, the ALP zymogram revealed the characteristic diapause pattern, and they failed to ingest culture medium. The fact that 20-hydroxyecdysone could elicit these responses in ND individuals further supports previous results indicating that ecdysteroids promote the induction and maintenance of the pharate larval diapause in this species.

Bivalent sequential binding model of a Bacillus thuringiensis toxin to gypsy moth aminopeptidase N receptor.

Specificity for target insects of Bacillus thuringiensis insecticidal Cry toxins is largely determined by toxin affinity for insect midgut receptors. The mode of binding for one such toxin-receptor complex was investigated by extensive toxin mutagenesis, followed by real-time receptor binding analysis using an optical biosensor (BIAcore). Wild-type Cry1Ac, a three-domain, lepidopteran-specific toxin, bound purified gypsy moth (Lymantria dispar) aminopeptidase N (APN) biphasically. Site 1 displayed fast association and dissociation kinetics, while site 2 possessed slower kinetics, yet tighter affinity. We empirically determined that two Cry1Ac surface regions are involved in in vivo toxicity and APN binding. Mutations within domain III affected binding rates to APN site 1, whereas mutations in domain II affected binding rates to APN site 2. Furthermore, domain III contact is completely inhibited in the presence of N-acetylgalactosamine, indicating loss of domain III binding eliminates all APN binding. Based upon these observations, the following model is proposed. A cavity in lectin-like domain III initiates docking through recognition of an N-acetylgalactosamine moiety on L. dispar APN. Following primary docking, a higher affinity domain II binding mechanism occurs, which is critical for insecticidal activity.

Production of chymotrypsin-resistant Bacillus thuringiensis Cry2Aa1 delta-endotoxin by protein engineering.

Cleavage of the Cry2Aa1 protoxin (molecular mass, 63 kDa) from Bacillus thuringiensis by midgut juice of gypsy moth (Lymantria dispar) larvae resulted in two major protein fragments: a 58-kDa fragment which was highly toxic to the insect and a 49-kDa fragment which was not toxic. In the midgut juice, the protoxin was processed into a 58-kDa toxin within 1 min, but after digestion for 1 h, the 58-kDa fragment was further cleaved within domain I, resulting in the protease-resistant 49-kDa fragment. Both the 58-kDa and nontoxic 49-kDa fragments were also found in vivo when (125)I-labeled toxin was fed to the insects. N-terminal sequencing revealed that the protease cleavage sites are at the C termini of Tyr49 and Leu144 for the active fragment and the smaller fragment, respectively. To prevent the production of the nontoxic fragment during midgut processing, five mutant proteins were constructed by replacing Leu144 of the toxin with Asp (L144D), Ala (L144A), Gly (L144G), His (L144H), or Val (L144V) by using a pair of complementary mutagenic oligonucleotides in PCR. All of the mutant proteins were highly resistant to the midgut proteases and chymotrypsin. Digestion of the mutant proteins by insect midgut extract and chymotrypsin produced only the active 58-kDa fragment, except that L144H was partially cleaved at residue 144.

Aminopeptidase N purified from gypsy moth brush border membrane vesicles is a specific receptor for Bacillus thuringiensis CryIAc toxin.

We have evaluated the binding of Bacillus thuringiensis Cry toxins to aminopeptidase N (APN) purified from Lymantria dispar (gypsy moth) brush border membrane vesicle (BBMV). CryIAc toxin bound strongly to APN, while either the structurally related CryIAa and CryIAb toxins or CryIC, CryIIA, and CryIIIA toxins showed weak binding to APN. An in vitro competition binding study demonstrated that the binding of CryIAc to L. dispar BBMV was inhibited by APN. Inhibition of short circuit current for CryIAc, measured by voltage clamping of whole L. dispar midgut, was substantially reduced by addition of phosphatidylinositol-specific phospholipase C, which is known to release APN from the midgut membrane. In contrast, addition of phosphatidylinositol-specific phospholipase C had only a marginal effect on the inhibition of short circuit current for CryIAa. These data suggest that APN is the major functional receptor for CryIAc in L. dispar BBMV. A ligand blotting experiment demonstrated that CryIAc recognized a 120-kDa peptide (APN), while CryIAa and CryIAb recognized a 210-kDa molecule in L. dispar BBMV. In contrast, CryIAa and CryIAb bound to both the 120- and 210-kDa molecules in Manduca sexta BBMV, while CryIAc recognized only the 120-kDa peptide. The 120-kDa peptide (APN) in L. dispar BBMV reacted with soybean agglutinin, indicating that N-acetylgalactosamine is a component of this glycoprotein.

Isolation and partial characterization of gypsy moth BTR-270, an anionic brush border membrane glycoconjugate that binds Bacillus thuringiensis Cry1A toxins with high affinity.

BTR-270, a gypsy moth (Lymantria dispar) brush border membrane molecule that binds Bacillus thuringiensis (Bt) Cry1A toxins with high affinity, was purified by preparative gel electrophoresis. Rabbit antibodies specific for the Bt toxin-binding molecule were raised. Attempts to label BTR-270 by protein-directed techniques were futile, but it was degraded by proteases with broad specificity indicating the presence of a peptide. Carbohydrate was detected by labeling with digoxigenin hydrazide following periodate oxidation. Mild alkaline hydrolysis destroyed toxin and antibody binding, suggesting O-linked glycans are involved in the activity. GC/MS composition analysis showed that the predominant sugars were galactose, glucose, and N-acetyl galactosamine with lesser amounts of N-acetyl glucosamine, glucuronic acid, xylose, and fucose. The carbohydrate moiety accounted for 73% of its total mass. Amino acid analysis showed a high content of aspartic/asparagine, threonine, and serine residues in the protein moiety. The purified glycoconjugate was not visualized using Coomassie or silver staining procedures, but stained "blue" using the cationic dye Stains-all. BTR-270 was labeled with biotin and used as a diagnostic probe for screening and identifying toxins that bind to the receptor. Toxin-binding kinetics obtained using a biosensor demonstrated that the receptor binds Cry1Aa and Cry1Ab toxins with high affinity, and displays a weaker affinity for Cry1Ac, in correlation with the toxicity of these toxins towards gypsy moth. Arch.

Structure and expression of the cDNA for the C isozyme of phosphofructo-1-kinase from rabbit brain.

Rabbit brain contains three phosphofructo-1-kinase (PFK) isozymic subunits designated A, B, and C. The primary structures of the first of these two isozyme types have been determined previously. The isozyme C of rabbit brain was isolated by immunoaffinity chromatography and subjected to proteolytic and chemical digestion. A large number of peptides were sequenced, the total number of amino acids identified being equal to about 80% of the total structure. The sequence of the cDNA derived from brain mRNA for C isozyme was determined from polymerase chain reaction fragments synthesized using oligonucleotides designed on the basis of the peptide sequences. The deduced size of the C isozyme was 86,371 Da, slightly larger than PFKs described previously. The amino acid sequence identity with the rabbit A isozyme was 68.9% and a range of identity to other sequenced mammalian PFKs was 67-69%. Using these data plus previously published data on chemical modification, assignments of the 6 organic ligand binding sites of PFK were inferred. The full-length cDNA was cloned into and expressed in Escherichia coli. Phosphofructokinase C was purified to homogeneity from the bacterial extracts.