Exposed and concealed antigens as vaccine targets for controlling ticks and tick-borne diseases

Tick vaccines derived from Bm86, a midgut membrane-bound protein of the cattle tick, Boophilus microplus, are currently the only commercially available ectoparasite vaccines. Despite its introduction to the market in 1994, and the recognized need for alternatives to chemical pesticides, progress in developing effective antitick vaccines (and ectoparasite vaccines in general) is slow. The primary rate-limiting step is the identification of suitable antigenic targets for vaccine development. Two sources of candidate vaccine antigens have been identified: 'exposed' antigens that are secreted in tick saliva during attachment and feeding on a host and 'concealed' antigens that are normally hidden from the host. Recently, a third group of antigens has been distinguished that combines the properties of both exposed and concealed antigens. This latter group offers the prospect of a broad-spectrum vaccine effective against both adults and immature stages of a wide variety of tick species. It also shows transmission-blocking and protective activity against a tick-borne pathogen. With the proliferation of molecular techniques and their application to vaccine development, there are high hopes for new and effective antitick vaccines that also control tick-borne diseases.

Characterization of dog repellent factor from cuticular secretion of female yellow dog tick, Haemaphysalis leachi

During its natural life cycle, the yellow dog tick, Haemaphysalis leachi, has three hosts, and it has to spend enough time on each of them to complete a blood meal. When irritated, the females of this tick species produce a cuticular secretion that contains a dog-repelling allomone. This improves the tick's chances of survival by deterring the dog from biting the tick off its body. Employing response-guided isolation techniques in conjunction with gas chromatography-mass spectrometry, the defensive allomone of H. leachi was found to consist of the six homologous aliphatic aldehydes from hexanal to undecanal. A mixture of synthetic versions of these six aldehydes in quantities corresponding to those secreted by one tick elicited strong aversion reactions in the majority of dogs of various breeds.

Tick pheromones and their use in tick control

Tick pheromones that regulate assembly, attraction/aggregation/attachment, and mating behavior have been described. Most of the compounds regulating these behaviors are purines, substituted phenols, or cholesteryl esters. Other pheromonal compounds include organic acids, hematin, or ecdysteroids. Novel devices have been developed that combine the specific compounds comprising these pheromones with an acaricide. When applied to tick-infested vegetation or directly to the body surfaces of livestock or companion animals, these devices are effective for tick control. This review summarizes the current state of knowledge of tick pheromones. In addition, this review also presents examples illustrating how devices using tick pheromones can offer effective alternatives to conventional methods for achieving tick control.

Severe Japanese spotted fever successfully treated with fluoroquinolone

A 77 years old woman who had a bite with eschar on her left arm, was admitted to emergency ward in our hospital, because of high fever, severe malaise, skin eruption, and consciousness disturbance beginning 5 days previously. She was diagnosed as Japanese spotted fever by seropositive of Rickettsia japonica (R. japonica) antibody, and successfully treated with fluoroquinolone, after minocycline hydrochloride had been proven ineffective. R. japonica-specific DNA was detected by PCR from the tick: Haemaphysalis hystricis larvae collected from a mountainous location in Fukuoka, Japan where the patient had been bitten.

Exploring tick saliva: from biochemistry to ‘sialomes’ and functional genomics

Tick saliva, a fluid once believed to be only relevant for lubrication of mouthparts and water balance, is now well known to be a cocktail of potent anti-haemostatic, anti-inflammatory and immunomodulatory molecules that helps these arthropods obtain a blood meal from their vertebrate hosts. The repertoire of pharmacologically active components in this cocktail is impressive as well as the number of targets they specifically affect. These salivary components change the physiology of the host at the bite site and, consequently, some pathogens transmitted by ticks take advantage of this change and become more infective. Tick salivary proteins have therefore become an attractive target to control tick-borne diseases. Recent advances in molecular biology, protein chemistry and computational biology are accelerating the isolation, sequencing and analysis of a large number of transcripts and proteins from the saliva of different ticks. Many of these newly isolated genes code for proteins with homologies to known proteins allowing identification or prediction of their function. However, most of these genes code for proteins with unknown functions therefore opening the road to functional genomic approaches to identify their biological activities and roles in blood feeding and hence, vaccine development to control tick-borne diseases.

Biochemical fingerprints of salivary gland for some ticks species in Egypt by SDS-polyacrylamide gel electrophresis

SDS-PAGE gel electrophoresis as a sensitive method is used for simultaneously comparing the total protein extracted from salivary glands in some ticks species (Argas percsicus, Hyalomm antolicum excavatum, H. an. antolicum, H. dromedarii, H. impeltatum and Rhipicephalus sanguineus). Results of A. persicus were characterized by 4 bands with molecular weights ranging from 82.76 to 16.076 KD. On the other hand, H. an. excavatum females were characterized by 6 bands with molecular weights ranging from 103.74 to 6.216 KD. H. an. antolicun males were characterized by 5 bands with molecular weights ranging from 79.183 to 2.092 KD. R. sanguineus was characterized by 4 and 8 bands with molecular weights ranging from 88.864 to 6.216 KD and 126.69 to 7.112 KD for male and female respectively. Female H. impeltatum was characterized by 8 bands with a range of molecular weights from 106.96 to 7.829 KD. H. dromedarii was characterized by 4 & 7 bands with ranges of molecular weights from 88.147 to 4.602 KD and 110.20 to 2.630 KD for male and female respectively.

Discovery and Pre-clinical Development of Antithrombotics from Hematophagous Invertebrates

Bloodfeeding (hematophagous) parasites have evolved effective means of inhibiting mammalian thrombosis, thereby facilitating the acquisition and digestion of a bloodmeal. To date, specific inhibitors of coagulation and platelet function have been identified from numerous invertebrate species, representing an impressive array of convergent functional strategies. These parasite antithrombotics, in addition to playing a critical role in the diseases caused or transmitted by bloodfeeding invertebrates, may also serve as potentially useful therapeutic agents for the treatment of a variety of conditions associated with activation of thrombosis. A number of naturally occurring anticoagulants and platelet inhibitors have been evaluated in vivo, with some currently in varying stages of preclinical and clinical development. Because of the unique specificity and potency of parasite antithrombotics, these invertebrate natural products hold great promise for improving the treatment of a variety of human illnesses, including heart disease, stroke, and cancer.

Tick saliva in anti-tick immunity and pathogen transmission

When feeding on vertebrate host ticks (ectoparasitic arthropods and potential vectors of bacterial, rickettsial, protozoal, and viral diseases) induce both innate and specific acquired host-immune reactions as part of anti-tick defenses. In a resistant host immune defense can lead to reduced tick viability, sometimes resulting in tick death. Tick responds to the host immune attack by secreting saliva containing pharmacologically active molecules and modulating host immune response. Tick saliva-effected immunomodulation at the attachment site facilitates both tick feeding and enhances the success of transmission of pathogens from tick into the host. On the other hand, host immunization with antigens from tick saliva can induce anti-tick resistance and is seen to be able to induce immunity against pathogens transmitted by ticks. Many pharmacological properties of saliva described in ticks are shared widely among other blood-feeding arthropods.

Crystallization and preliminary X-ray diffraction analysis of HeLp, a heme lipoprotein from the hemolymph of the cattle tickBoophilus microplus

The main protein present in the hemelymph of the cattle tick Boophilus microplus is a lipoprotein able to bind heme (HeLp). It has an apparent molecular weight of 354 000 Da and is composed of two polypeptide chains found in stoichiometric amounts. It contains 33% lipids. The protein was crystallized using the hanging-drop vapour-diffusion method in the presence of 1,6-hexanediol as a precipitant. X-ray diffraction data were collected to 2.1 A resolution using a synchrotron-radiation source. The crystal belongs to the triclinic space group P1, with unit-cell parameters a = 90.58, b = 105.50, c = 116.14 A, alpha = 112.40, beta = 111.64, gamma = 91.35 degrees. Owing to the lack of information about the amino-acid sequence, the structure of HeLp will be solved by the use of heavy atoms. Several possible derivatives have been collected and analysis is under way.

A new type of antimicrobial protein with multiple histidines from the hard tick, Amblyomma hebraeum

A novel 11 kDa antimicrobial protein, named as hebraein, and having a unique amino acid sequence, was purified from the hemolymph of fed female Amblyomma hebraeum ticks. A full-length cDNA clone encoding hebraein was isolated from a cDNA library made from tick synganglia. Hebraein consists of 102 amino acids, including 6 cysteine residues; has 9 histidines in its C-terminal domain that are mainly present as HX repeats; and has no significant similarity to any known protein. The secondary structure prediction is very clearly all alpha-helical (4-6 helices) except for a very short extension at the C terminus. Such high alpha-helical content is quite different from known antimicrobial proteins. Recombinant hebraein and a mutant lacking the histidine residues in the C-terminal domain were constructed and expressed. Assayed at the slightly acidic pH equivalent of fed female tick hemolymph, the wild-type and the histidine-rich recombinant hebraein had stronger antimicrobial activities than the histidine-deficient mutant. The pH-dependent properties of histidine-rich antimicrobial proteins may allow the design of agents that would function selectively in specific pH environments. The results from protein profiling of hemolymph, analyzed by surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry combined with ProteinChip technology and RT-PCR analysis suggested that this antimicrobial protein was up-regulated by blood feeding. Our findings describe a new type of antimicrobial protein with multiple cysteine and histidine residues, and with unique secondary structure.

Exon-intron structure of outlier tick lipocalins indicate a monophyletic origin within the larger lipocalin family

All tick proteins assigned to the lipocalin family lack the structural conserved regions (SCRs) that are characteristic of the kernel lipocalins and can thus be classified as outliers. These tick proteins have been assigned to the tick lipocalin family based on database searches that indicated homology between tick sequences and the fact that the histamine binding protein (HBP2) from the hard tick Rhipicephalus appendiculatus (Ixodidae) shows structural similarity to the lipocalin fold. Sequence identity between kernel and outlier lipocalins falls below 20% and the question raised is whether the outlier and kernel lipocalins are truly homologous. More specifically in the case of the tick lipocalins, whether their structural fold is derived from the lipocalin fold or whether convergent evolution resulted in the generation of the basic lipocalin-like fold which consists of an eight stranded continuous anti-parallel beta-barrel terminated by a C-terminal alpha-helix that lies parallel to the barrel. The current study determined the gene structure for HBP2 and TSGP1, TSGP2 and TSGP4, lipocalins identified from the soft tick Ornithodoros savignyi (Argasidae). All tick lipocalins have four introns (A-D) with conserved positions and phases within the tick lipocalin sequence alignment. The positions and phase information are also conserved with regard to the rest of the lipocalin family. Phylogenetic analysis using this information shows conclusively that tick lipocalins are evolutionary related to the rest of the lipocalin family. Tick lipocalins are grouped within a monophyletic clade that indicates a monophyletic origin within the tick lineage and also group with the other arthropod lipocalins in a larger clade. Phylogenetic analysis of sequence alignments based on conserved secondary structure of the lipocalin fold support the conclusions from the gene structure trees. These results indicate that exon-intron arrangement can be useful for the inclusion of outlier lipocalins within the larger lipocalin family.

Distribution of E and NS1 proteins of TBE virus in mammalian and tick cells

Four monoclonal antibodies recognizing TBEV proteins were prepared. Three of them (2/H6, 8/A10, 5/F9) specifically recognize the structural E protein and the fourth binds to the nonstructural NS1 protein. These antibodies were used for an immunofluorescence study of TBEV protein distribution in infected mammalian host and tick vector tissue culture cells. Any differences in the distribution of the E and NS1 proteins were revealed. In porcine PS cell line the proteins were localized in the cytoplasm with a distinct increase of the signal in the perinuclear area 16 h post-infection (p.i.). The area gradually expanded and at 24 h p.i. covered the major part of the cytoplasm. The localization of the proteins in tick RA-257 cells revealed a uniform spread of the proteins in whole cytoplasm of the infected cells. The signal was very weak at 16 h p.i. and gradually increased during progressing time p.i., although, the distribution of the proteins did not exhibit any changes. Difference in viral protein distribution in the two cell types points to the possibility that the maturation process of TBEV exhibits different features in mammalian and tick cells.

Quantitative ecology of ticks as a basis for transmission models of tick-borne pathogens

Tick population ecology is the basis of spatiotemporal variation in the risk of infection by tick-borne pathogens. The continental distribution of tick-borne encephalitis (TBE) in Europe is statistically associated with a specific pattern of the seasonal dynamics of Ixodes ricinus, and a particular characteristic of the seasonal land surface temperature profile. A tick population model would allow us to move from this statistical analysis to a biological, process-based model for TBE virus and the many other pathogens vectored by Ixodes spp. ticks in Europe and the United States. Long-term field data on I. ricinus are analyzed to provide empirical estimates of rates of the major demographic processes, development, questing, attachment to hosts, and mortality.

Identity and synthesis of prostaglandins in the lone star tick, Amblyomma americanum (L.), as assessed by radio-immunoassay and gas chromatography/mass spectrometry

High concentrations of PGE(2) and PGF(2alpha) were identified by radio-immunoassay (RIA) and/or gas chromatography/mass spectrometry (GC/MS) in the hemolymph, salivary glands and saliva of the lone star tick Amblyomma americanum (L.). Binding studies indicated that PGE(2) was free and not bound to any proteins in the hemolymph. A small amount of 6-keto-PGF(1alpha) (breakdown product of PGI(2); prostacyclin) was also found in the salivary glands but not in the hemolymph or saliva. Neither PGD(2) nor PGA(2)/B(2) was detected in any tick material investigated. Although PGE(2) was found in the gut contents, only small amounts of label crossed the gut into the hemolymph during artificial feeding with labeled PGE(2), indicating that the high amounts of PGE(2) in hemolymph and salivary glands are not sequestered from the host blood meal. Isolated salivary glands and salivary gland homogenates demonstrated robust synthesis of PGE(2) at high concentrations of exogenous arachidonic acid. Synthesis by the salivary glands was monitored by measuring increasing PGE(2) with increasing arachidonic acid by RIA, GC/MS and labeled PGE(2) in the presence of labeled arachidonic acid. Synthesis was inhibited in a dose-dependent manner by indomethacin indicating that the cyclooxygenase synthesizing prostaglandins in ticks shares similarities to the enzyme found in mammals.

Disaggregation of aggregated platelets by savignygrin, a alphaIIbeta3 antagonist from Ornithodoros savignyi

Ticks control their host's hemostatic system by secretion of bioactive components during feeding that inhibit blood coagulation and platelet aggregation. Dissolution of platelets that have already aggregated can enhance control over the hemostatic system. It has been shown that disaggregation of aggregated platelets by the enzyme apyrase was accompanied by a shape change from the aggregated spherical form back to the discoid form associated with un-activated platelets. The present study concerns the disaggregation effect of the alpha IIb/beta3 antagonist, savignygrin. Aggregated platelets that were disaggregated by savignygrin and platelets pre-incubated with savignygrin before activation with ADP, retained a spherical form similar to platelets disaggregated by the fibrinogenolytic enzyme plasmin. The number of pseudopods were however, markedly reduced suggesting a disruption of the focal adhesion points that act as a localization point of alpha IIb/beta3. These results are concurrent with targeting of alpha IIb/beta3 and dissociation of fibrinogen from its receptor, once aggregation has taken place. This is the second mediator of platelet disaggregation found in soft ticks and suggests that disaggregation of aggregated platelets might play an important part in the anti-hemostatic strategy of ticks.

HeLp, a heme lipoprotein from the hemolymph of the cattle tick, Boophilus microplus

The main protein of the hemolymph of the cattle tick Boophilus microplus has been isolated and shown to be a heme lipoprotein (HeLp). HeLp has an apparent molecular mass of 354,000 and contains two apoproteins (103 and 92 kDa) found in equal amounts. HeLp presents a pI of 5.8 and a density of 1.28 g/ml and contains 33% lipids, containing both neutral lipids and phospholipids, and 3% of sugars. A remarkable feature of HeLp is the abundance of cholesterol ester (35% of total lipids), a lipid not previously reported in invertebrate lipoproteins. Western blot analysis showed HeLp in hemolymph from adult females and males, but not in eggs. Although HeLp contains 2 heme molecules, it is capable of binding 6 additional molecules of heme. Boophilus feeds large amount of blood, and we recently showed that this tick is unable to perform de novo synthesis of heme (Braz, G. R. C., Coelho, H. S. L., Masuda, H., and Oliveira, P. L. (1999) Curr. Biol. 9, 703-706). Injection of tick females with (55)Fe-labeled heme-HeLp indicated that this protein transports heme from hemolymph to tissues. HeLp is suggested to be an essential adaptation to the loss of the heme synthesis pathway.

Comparison of the protein profiles of salivary gland extracts derived from three species of unfed and partially fed ixodid ticks analysed by SDS-PAGE

Salivary gland extracts (SGE) from unfed and 5 days fed adult female Ixodes ricinus (Linnaeus, 1758); Haemaphysalis inermis (Birula, 1895) and Dermacentor reticulatus (Fabricius, 1794) ticks were prepared. The protein content after feeding increased by 10.6, 8.7 and 6.8 times, respectively. Extracts were equilibrated to the same protein content and submitted to SDS-polyacrylamide gel electrophoresis followed by computer analysis of the scanned gels. Relative differences in protein profiles of extracts obtained from unfed and partially fed ticks were found in all species and some of them were similar in all three species used in the study. Results demonstrate that the increase of the protein content in salivary glands during the feeding does not occur proportionally. Some proteins are synthesised preferentially (67.1 kDa, 13.5 kDa) but other bands (in range of 15-16 kDa) present in the SGE derived from unfed ticks are less discernible in that of fed ticks.

Detection and micro-scale isolation of a low molecular mass paralysis toxin from the tick, Argas (Persicargas) walkerae

This study describes the isolation of a 11 kDa paralysis toxin from crude larval extracts of Argas (Persicargas) walkerae by exploiting the cross-reactivity of a monoclonal antibody (4B12), directed against the paralysis toxin of Rhipicephalus evertsi evertsi. This low molecular mass is in contrast to previous findings of a 60-70 kDa toxin for A. (P.) walkerae, but is similar to neurotoxins isolated from venomous forms of the class Arachnida, which comprise the orders Araneae (spiders). Scorpionida (Scorpions) and Acari (ticks and mites). Since numerous antigenic bands, ranging between 11 and 115 kDa, were detected by the monoclonal antibody 4B12, the possibility of toxin-complex formation and the effect of pH on the latter were investigated by means of HPLC and ammonium sulphate precipitation. The results suggest that physiological conditions, with respect to pH and ionic strength, promote the formation of heterogeneous toxin-complexes while an acidic pH favours the formation of a more homogeneous toxin-containing complex. Furthermore, the effect of partially purified toxin on neurotransmitter release from crude rat brain synaptosomes was investigated, since tick paralysis toxins are hypothesised to inhibit neurotransmitter release from the presynaptic terminal. Both calcium-dependent, as well as calcium-independent release was inhibited by the toxin-containing sample.

Purification, cloning, and expression of a novel salivary anticomplement protein from the tick, Ixodes scapularis

The alternative pathway of complement is an important defense against pathogens and in tick rejection reactions. The tick Ixodes scapularis is able to feed repeatedly on its natural host and has a salivary anticomplement activity that presumably facilitates feeding. In this study, we purified and then obtained the amino-terminal sequence of the I. scapularis salivary anticomplement (Isac). We found a full-length clone coding for Isac by random screening of a salivary gland cDNA library. Expressing Isac cDNA in COS cells reproduced the activity found in tick saliva, namely, inhibition of rabbit erythrocyte lysis by human serum in the presence of Mg(2+) and EGTA, inhibition of C3b binding to agarose in the presence of Mg(2+) and EGTA, and acceleration of factor Bb uncoupling from the C3 convertase generated by the alternative pathway. Recombinant Isac had no effect on the recalcification time of human platelet-poor plasma or in the classical complement pathway, indicating that it is a specific inhibitor similar to the regulators of complement activation of the alternative pathway such as factor H. Isac, however, has no similarity to any protein in the GenBank(TM) data base, indicating that it is a novel and relatively small (18.5 kDa) anticomplement molecule.

Isolation and characterization of Dorin M, a lectin from plasma of the soft tick Ornithodoros moubata

A lectin with high hemagglutinating activity, which we have named Dorin M, was identified in the plasma of the soft tick Ornithodoros moubata. The activity of the plasma lectin could be efficiently inhibited by sialic acid, N-acetyl-D-hexosamines and sialoglycoproteins. Dorin M was purified to homogeneity using two different isolation systems: affinity chromatography on a column of bovine submaxillary mucin conjugated to Sepharose 4B with specific elution by N-acetyl-D-glucosamine and chromatography on Blue-Sepharose followed by anion exchange FPLC on a MonoQ column. The purified lectin is a glycoprotein which, in the native state, forms aggregates with molecular mass of about 640 kDa. Non-reducing SDS PAGE revealed that the lectin consists of two noncovalently bound subunits migrating closely around 37 kDa. Dorin M is a glycoprotein, probably modified by N-type glycosylation. After chemical deglycosylation, only one band of about 32 kDa was detected. Dorin M is the first lectin purified from ticks.

Chemical composition of lipophylic compounds from the body surface of unfed adult Ixodes persulcatus ticks (Acari: Ixodidae)

Chemical compositions of ethereal extracts of the body surface of unfed male and female Ixodes persulcatus ticks (Acari: Ixodidae) were studied by gas chromatography using mass-spectrometric detection. More than 100 different organic compounds were detected. The predominant components were saturated fatty hydrocarbons, saturated and unsaturated fatty acids, aldehydes, squalene, cholesterol and cholesterol derivatives. A number of compounds found in I. persulcatus are known as components of pheromones or constituents of dermal gland secretions in tick species of the genus Amblyomma: nonanoic acid, saturated fatty acids having from 14 to 16 carbons, and squalene. Saturated fatty aldehydes have not been reported previously as body surface components of hard ticks. Substituted phenols were not found in the extracts, although they are known as common components of sex and attraction-aggregation-attachment pheromones in Amblyomma ticks. With a few exceptions (henicosanal, 2,4-holestadiene and two unidentified cholesterol derivatives), there was no marked difference in composition of surface components between male and female I. persulcatus. The possible role of the different chemical groups in communication between I. persulcatus ticks is discussed.

Purification, N-terminal sequencing and diagnostic value of the major antigens of Ornithodoros erraticus and O. moubata

To enhance the specificity and sensitivity of serological detection of swine exposed to Ornithodoros erraticus or O. moubata, we purified the 158, 186, 215 and 260 kDa antigens from the former species and the designated (owing to their MW and charge) 19C, 17A, 20A1 and 20A2 antigens of the latter by HPLC and gel electroelution methods. All the O. erraticus antigens share epitopes and are difficult to purify individually by reverse phase and ion-exchange chromatography due to their molecular similarity. Tested individually by ELISA, all of them give the same optical densities (OD) with anti-O. erraticus sera, and these ODs are always lower with anti-immature than with anti-adult sera. Although immature and adult specimens have the same antigens, immature forms induce more anti-carbohydrate antibodies than adults. This is the reason for the lower ODs of the anti-immature sera against purified antigens, since these latter antigens essentially react with anti-peptide antibodies (hence, increasing the specificity and sensitivity of the serology). The N-terminus of the 260 kDa antigen shows 80-90% similarity with the hemoglobin alpha-chain of many mammals. The antigens of O. moubata are proteins that are very different from one another and are, therefore, readily purified by ion exchange chromatography. The 20A1 antigen appears to be the most immunogenic and is recognized equally by anti-immature and anti-adult sera. This antigen does not give false positive reactions with the negative control sera analyzed and its N-terminus region shares 46.2% homology with the alpha-chain of the C3 component of rabbit complement.

Characterization of an alpha-macroglobulin-like glycoprotein isolated from the plasma of the soft tick Ornithodoros moubata

We report the identification of the first representative of the alpha-2-macroglobulin family identified in terrestrial invertebrates. An abundant acidic glycoprotein was isolated from the plasma of the soft tick Ornithodoros moubata. Its molecular mass is about 420 kDa in the native state, whereas in SDS/PAGE it migrates as one band of 190 kDa under nonreducing conditions and a band of 92 kDa when reduced. Chemical deglycosylation reveals that it is composed of two different subunits, designated A and B. The N-terminal amino-acid sequence of subunit A is similar to the N-terminus of invertebrate alpha-2-macroglobulin. Sequence analysis of several internal peptides confirms that the tick protein belongs to the alpha-2-macroglobulin family, and the protein is therefore referred to as tick alpha-macroglobulin (TAM). Functional analyses strengthen this assignment. TAM inhibits trypsin and thermolysin cleavage of the high-molecular-weight substrate azocoll in a manner similar to that of bovine alpha-2-macroglobulin. This effect is abolished by pre-treatment of TAM with methylamine. In the presence of TAM, trypsin is protected against active-site inhibition by soybean trypsin inhibitor. We cloned and sequenced a PCR product containing sequences from both subunits and spanning the N-terminus of subunit B and the putative 'bait region' (a segment of alpha-2-macroglobulin which serves as target for various proteases). This indicates that the two subunits are generated from a precursor polypeptide by post-translational processing.