Mast cell tryptases: examination of unusual characteristics by multiple sequence alignment and molecular modeling

Mast cell-restricted tetramer-forming tryptases and their beneficial roles in hemostasis and blood coagulation

Tetramer-forming tryptase (hTryptase-β) was recently discovered to have a prominent role in preventing the internal accumulation of life-threatening fibrin deposits and fibrin-platelet clots. The anticoagulant activity of hTryptase-β is an explanation for the presence of hemorrhagic disorders in some patients with anaphylaxis or mastocytosis. The fragments of hFibrinogen formed by the proteolysis of this prominent protein by hTryptase-β could be used as biomarkers in the blood and/or urine for the identification and monitoring of patients with mast cell-dependent disorders. Recombinant hTryptase-β has potential to be used in clinical settings where it is desirable to inhibit blood coagulation.

Mast cell tryptase and asthma

Recent physiological and pharmacological studies have indicated the potential importance of tryptase, the major protein component in mast cells, in inflammatory diseases (especially asthma). Being released at inflammatory sites after the activation of mast cells, tryptase is capable of causing bronchohyperresponsiveness and infiltration of eosinophils, neutrophils, etc. in animal airways. The mechanisms by which tryptase causes bronchoconstriction involve probably the potentiation of other chemical mediators such as histamine, production of bradykinin via the hydrolysis of kininogen, and cleavage of the bronchodilating peptides VIP (vasoactive intestinal peptide) and PHM (peptide histidine-methionine). Tryptase has also been found to be a potent mitogen in vitro for airway smooth muscle cells and epithelial cells, implying its role in the hyperplasia of the asthmatic airways. The experimental data providing evidence for the above roles of tryptase are summarized in the present review, as well as the effects of tryptase inhibition in animal asthma models. The potential strategies for the development of anti-asthmatic agents based on the inhibition of tryptase are discussed.

Pru p 3-sensitised Italian peach-allergic patients are less likely to develop severe symptoms when also presenting IgE antibodies to Pru p 1 and Pru p 4

BACKGROUND

The roles played by different peach allergens with respect to symptom severity have not been completely ascertained. We have evaluated the diagnostic efficacy of peach recombinant allergens ImmunoCAP compared to peach in the identification of subjects at an increased risk for severe reactions to peaches.

METHODS

148 peach-allergic patients were divided based on their symptom severity into 2 groups: mild oral allergy syndrome (OAS) and severe OAS. Anti-rPru p 1, 3 and 4 IgE levels were measured. Statistical analyses were carried out using parametric and non-parametric tests.

RESULTS

anti-rPru p 1 and anti-rPru p 4 IgE levels were significantly higher in patients with mild OAS than in patients with severe OAS (p = 0.0001); in contrast, anti-rPru p 3 IgE levels were significantly higher in patients with severe OAS than in patients with mild OAS (p < 0.00005). Moreover, we found that any unitary increase in anti-rPru p 1 IgE values corresponded to a 2.48% reduction in the odds of having severe OAS (p = 0.048), whereas any unitary increase in anti-rPru p 3 IgE values corresponded to a 9.02% increase in the probability of having severe OAS (p = 0.001). Unexpectedly, we found that patients positive to rPru p 3 as well as rPru p 1 and 4 demonstrated a significant reduction of the odds of developing severe symptoms than those positive to rPru p 3 alone. Anti-rPru p 3 IgE levels were a significantly better indicator than anti-peach IgE values (p = 0.016) of patients with the highest risk for severe OAS. A cutoff of 2.69 kUA/l for anti-rPru p 3 IgE values better discriminated peach-allergic patients at a higher risk for symptoms.

CONCLUSIONS

Italian patients with positive anti-rPru p 1, 4 and 3 IgE levels seemed less likely to experience the clinical effects of high anti-rPru p 3 IgE values.

Protease-proteoglycan complexes of mouse and human mast cells and importance of their beta-tryptase-heparin complexes in inflammation and innate immunity

Approximately 50% of the weight of a mature mast cell (MC) consists of varied neutral proteases stored in the cell's secretory granules ionically bound to serglycin proteoglycans that contain heparin and/or chondroitin sulfate E/diB chains. Mouse MCs express the exopeptidase carboxypeptidase A3 and at least 15 serine proteases [designated as mouse MC protease (mMCP) 1-11, transmembrane tryptase/tryptase gamma/protease serine member S (Prss) 31, cathepsin G, granzyme B, and neuropsin/Prss19]. mMCP-6, mMCP-7, mMCP-11/Prss34, and Prss31 are the four members of the chromosome 17A3.3 family of tryptases that are preferentially expressed in MCs. One of the challenges ahead is to understand why MCs express so many different protease-proteoglycan macromolecular complexes. MC-like cells that contain tryptase-heparin complexes in their secretory granules have been identified in the Ciona intestinalis and Styela plicata urochordates that appeared approximately 500 million years ago. Because sea squirts lack B cells and T cells, it is likely that MCs and their tryptase-proteoglycan granule mediators initially appeared in lower organisms as part of their innate immune system. The conservation of MCs throughout evolution suggests that some of these protease-proteoglycan complexes are essential to our survival. In support of this conclusion, no human has been identified that lacks MCs. Moreover, transgenic mice lacking the beta-tryptase mMCP-6 are unable to combat a Klebsiella pneumoniae infection effectively. Here we summarize the nature and function of some of the tryptase-serglycin proteoglycan complexes found in mouse and human MCs.

Mast cell-restricted tryptases: structure and function in inflammation and pathogen defense

Mast cells (MCs) are highly specialized immune cells present in mammals and in lower organisms that predate the development of adaptive immunity. The strong evolutionary pressure to retain MCs for >500 million years suggests critical roles for these cells in our survival. In support of this conclusion, no human has been identified to date that lacks MCs, despite the adverse roles of MCs in systemic anaphylaxis and varied inflammatory disorders. MCs express numerous lineage-restricted neutral proteases, and four members of the chromosome 17A3.3 family of tryptases are preferentially expressed in mouse MCs. The anatomical location of MCs at host-environment interfaces has raised the possibility that some of these enzymes are evolutionally conserved because they are needed for combating infectious organisms. Here we review recent insights into the structure and function of MC tryptases in inflammation and host defense against bacteria and other infectious organisms.

Biology of mast cell tryptase. An inflammatory mediator

In 1960, a trypsin-like activity was found in mast cells [Glenner GG & Cohen LA (1960) Nature 185, 846-847] and this activity is now commonly referred to as 'tryptase'. Over the years, much knowledge about mast cell tryptase has been gathered, and a recent (18 January 2006) PubMed search for the keywords 'tryptase + mast cell*' retrieved 1661 articles. However, still very little is known about its true biological function. For example, the true physiological substrate(s) for mast cell tryptase has not been identified, and the potential role of tryptase in mast cell-related disease is not understood. Mast cell tryptase has several unique features, with perhaps the most remarkable being its organization into a tetrameric state with all of the active sites oriented towards a narrow central pore and its consequent complete resistance towards endogenous macromolecular protease inhibitors. Much effort has been invested to elucidate these properties of tryptase. In this review we summarize the current knowledge of mast cell tryptase, including novel insights into its possible biological functions and mechanisms of regulation.

Urokinase-type plasminogen activator is a preferred substrate of the human epithelium serine protease tryptase epsilon/PRSS22

Tryptase epsilon is a member of the chromosome 16p13.3 family of human serine proteases that is preferentially expressed by epithelial cells. Recombinant pro-tryptase epsilon was generated to understand how the exocytosed zymogen might be activated outside of the epithelial cell, as well as to address its possible role in normal and diseased states. Using expression/site-directed mutagenesis approaches, we now show that Lys20, Cys90, and Asp92 in the protease's substrate-binding cleft regulate its enzymatic activity. We also show that Arg(-1) in the propeptide domain controls its ability to autoactivate. In vitro studies revealed that recombinant tryptase epsilon possesses a restricted substrate specificity. Once activated, tryptase epsilon cannot be inhibited effectively by the diverse array of protease inhibitors present in normal human plasma. Moreover, this epithelium protease is not highly susceptible to alpha1-antitrypsin or secretory leukocyte protease inhibitor, which are present in the lung. Recombinant tryptase epsilon could not cleave fibronectin, vitronectin, laminin, single-chain tissue-type plasminogen activator, plasminogen, or any prominent serum protein. Nevertheless, tryptase epsilon readily converted single-chain pro-urokinase-type plasminogen activator (pro-uPA/scuPA) into its mature, enzymatically active protease. Tryptase epsilon also was able to induce pro-uPA-expressing smooth muscle cells to increase their migration through a basement membrane-like extracellular matrix. The ability to activate uPA in the presence of varied protease inhibitors suggests that tryptase epsilon plays a prominent role in fibrinolysis and other uPA-dependent reactions in the lung.

Histidines are critical for heparin-dependent activation of mast cell tryptase

Mast cell tryptase is a tetrameric serine protease that is stored in complex with negatively charged heparin proteoglycans in the secretory granule. Tryptase has potent proinflammatory properties and has been implicated in diverse pathological conditions such as asthma and fibrosis. Previous studies have shown that tryptase binds tightly to heparin, and that heparin is required in the assembly of the tryptase tetramer as well as for stabilization of the active tetramer. Because the interaction of tryptase with heparin is optimal at acidic pH, we investigated in this study whether His residues are of importance for the heparin binding, tetramerization, and activation of the tryptase mouse mast cell protease 6. Molecular modeling of mouse mast cell protease 6 identified four His residues, H35, H106, H108, and H238, that are conserved among pH-dependent tryptases and are exposed on the molecular surface, and these four His residues were mutated to Ala. In addition, combinations of different mutations were prepared. Generally, the single His-Ala mutations did not cause any major defects in heparin binding, activation, or tetramerization, although some effect of the H106A mutation was observed. However, when several mutations were combined, large defects in all of these parameters were observed. Of the mutants, the triple mutant H106A/H108A/H238A was the most affected with an almost complete inability to bind to heparin and to form active tryptase tetramers. Taken together, this study shows that surface-exposed histidines mediate the interaction of mast cell tryptase with heparin and are of critical importance in the formation of active tryptase tetramers.

Uterine secretion of ISP1 & 2 tryptases is regulated by progesterone and estrogen during pregnancy and the endometrial cycle

We have described two novel implantation serine proteinase (ISP) genes that are expressed during the implantation period. The ISP1 gene may encode the embryo-derived enzyme strypsin, which is necessary for blastocyst hatching in vitro and the initiation of invasion. The ISP2 gene, which encodes a related tryptase, is expressed in endometrial glands and is regulated by progesterone during the peri-implantation period. Based on similarities between ISP2 gene expression and that of a progesterone-regulated lumenal serine proteinase activity associated with lysis of the zona pellucida, we have suggested that the strypsin related protein, ISP2, may encode a zona lysin proteinase. Recently strypsin has also been found within uterine fluid, suggesting a second potential role in hatching. Consistently, we have discovered that ISP1 is also expressed in the uterine secretory gland at the time of hatching. In this study we demonstrate that both ISP1 and ISP2 are secreted together into the uterine lumen at peri-implantation, and that the appearance of ISP protein is regulated positively at the transcriptional level by progesterone and negatively at the posttranscriptional level by estrogen. This negative regulation by estrogen may be overridden in pregnancy as ISP protein expression is restored during oil-induced decidualization. ISP1 and ISP2 proteins are also expressed in proestrous suggesting additional roles in the endometrial cycle.

Mouse chromosome 17A3.3 contains 13 genes that encode functional tryptic-like serine proteases with distinct tissue and cell expression patterns

Probing of the mouse EST data base at GenBank trade mark with known tryptase cDNAs resulted in the identification of undiscovered serine protease transcripts whose genes reside at a 1.5-Mb complex on mouse chromosome 17A3.3. Mouse tryptase-5 (mT5), tryptase-6 (mT6), and mast cell protease-11 (mMCP-11) are new members of this serine protease superfamily whose amino acid sequences are 36-54% identical to each other and to their other 10 family members. The 13 functional mouse proteases can be subdivided into two subgroups based on conserved features in their propeptides. Of the three new serine proteases, mT6 is most widely expressed in tissues. mT5 is preferentially expressed in smooth muscle, whereas mMCP-11 is preferentially expressed in the spleen and bone marrow. In contrast to mT5 and mT6, mMCP-11 is also expressed in mast cells. Although mT6 and mMCP-11 are constitutively secreted when expressed in mammalian and insect cells, mT5 remains membrane-associated. The fact that recombinant mT5, mT6, and mMCP-11 possess non-identical expression patterns and substrate specificities suggests that each protease has a unique function in vivo. Of the 13 functional mouse tryptase genes identified at the complex, 12 have orthologs that reside in the syntenic region of human chromosome 16p13.3. The establishment of these ortholog pairs helps clarify the evolutionary relationship of the serine protease locus in the two species. This information provides a useful framework for the functional analysis of each protease using gene targeting and other molecular approaches.

Human tryptase epsilon (PRSS22), a new member of the chromosome 16p13.3 family of human serine proteases expressed in airway epithelial cells

Probing of the GenBank expressed sequence tag (EST) data base with varied human tryptase cDNAs identified two truncated ESTs that subsequently were found to encode overlapping portions of a novel human serine protease (designated tryptase epsilon or protease, serine S1 family member 22 (PRSS22)). The tryptase epsilon gene resides on chromosome 16p13.3 within a 2.5-Mb complex of serine protease genes. Although at least 7 of the 14 genes in this complex encode enzymatically active proteases, only one tryptase epsilon-like gene was identified. The trachea and esophagus were found to contain the highest steady-state levels of the tryptase epsilon transcript in adult humans. Although the tryptase epsilon transcript was scarce in adult human lung, it was present in abundance in fetal lung. Thus, the tryptase epsilon gene is expressed in the airways in a developmentally regulated manner that is different from that of other human tryptase genes. At the cellular level, tryptase epsilon is a major product of normal pulmonary epithelial cells, as well as varied transformed epithelial cell lines. Enzymatically active tryptase epsilon is also constitutively secreted from these cells. The amino acid sequence of human tryptase epsilon is 38-44% identical to those of human tryptase alpha, tryptase beta I, tryptase beta II, tryptase beta III, transmembrane tryptase/tryptase gamma, marapsin, and Esp-1/testisin. Nevertheless, comparative protein structure modeling and functional studies using recombinant material revealed that tryptase epsilon has a substrate preference distinct from that of its other family members. These data indicate that the products of the chromosome 16p13.3 complex of tryptase genes evolved to carry out varied functions in humans.

Tryptase 4, a new member of the chromosome 17 family of mouse serine proteases

Genomic blot analysis raised the possibility that uncharacterized tryptase genes reside on chromosome 17 at the complex containing the three genes that encode mouse mast cell protease (mMCP) 6, mMCP-7, and transmembrane tryptase (mTMT). Probing of GenBank's expressed sequence tag data base with these three tryptase cDNAs resulted in the identification of an expressed sequence tag that encodes a portion of a novel mouse serine protease (now designated mouse tryptase 4 (mT4) because it is the fourth member of this family). 5'- and 3'-rapid amplification of cDNA ends approaches were carried out to deduce the nucleotide sequence of the full-length mT4 transcript. This information was then used to clone its approximately 5.0-kilobase pair gene. Chromosome mapping analysis of its gene, sequence analysis of its transcript, and comparative protein structure modeling of its translated product revealed that mT4 is a new member of the chromosome 17 family of mouse tryptases. mT4 is 40-44% identical to mMCP-6, mMCP-7, and mTMT, and this new serine protease has all of the structural features of a functional tryptase. Moreover, mT4 is enzymatically active when expressed in insect cells. Due to its 17-mer hydrophobic domain at its C terminus, mT4 is a membrane-anchored tryptase more analogous to mTMT than the other members of its family. As assessed by RNA blot, reverse transcriptase-polymerase chain reaction, and/or in situ hybridization analysis, mT4 is expressed in interleukin-5-dependent mouse eosinophils, as well as in ovaries and testes. The observation that recombinant mT4 is preferentially retained in the endoplasmic reticulum of transiently transfected COS-7 cells suggests a convertase-like role for this integral membrane serine protease.

Bivalent inhibition of human beta-tryptase

BACKGROUND

Human beta-tryptase is a mast cell specific trypsin-like serine protease that is thought to play a key role in the pathogenesis of diverse allergic and inflammatory disorders like asthma and psoriasis. The recently resolved crystal structure revealed that the enzymatically active tetramer consists of four quasi-identical monomers. The spatial display of the four identical active sites represents an ideal basis for the rational design of bivalent inhibitors.

RESULTS

Based on modeling experiments homobivalent inhibitors were constructed using (i) 6A,6D-dideoxy-6A,6D-diamino-beta-cyclodextrin as a rigid template to bridge the space between the two pairs of identical active sites and (ii) 3-(aminomethyl)benzene as a headgroup to occupy the arginine/lysine specific S1 subsites. A comparative analysis of the inhibitory potencies of synthetic constructs that differ in size and type of the spacer between headgroup and template revealed that the construct contained two 3-(aminomethyl)benzenesulfonyl-glycine groups linked to the 6A,6D-diamino groups of beta-cyclodextrin as an almost ideal bivalent inhibitor with a cooperativity factor of 1.9 vs. the ideal value of 2. The bivalent binding mode is supported by the inhibitor/tetramer ratio of 2:1 required for inactivation of tryptase and by X-ray analysis of the inhibitor/tryptase complex.

CONCLUSION

The results obtained with the rigid cyclodextrin template underlined the importance of a minimal loss of conformational entropy in bivalent binding, but also showed the limitations imposed by such rigid core molecules in terms of optimal occupancy of binding sites and thus of enthalpic strains in bidentate binding modes. The main advantage of bivalent inhibitors is their high selectivity for the target enzyme that can be achieved utilizing the principle of multivalency.

Characteristics of monkey tryptase purified from cheek pouch vascular tissues

Tryptase purified from rat and dog tissues has been reported, although the characteristics of these enzymes are different from human tryptase. For pathophysiological studies of human tryptase, studies on species that have a similar tryptase to humans is needed. In this study, we purified monkey tryptase from cheek pouch vascular tissues using heparin affinity and gel filtration columns. The monkey tryptase, which had a molecular weight of 130 kDa by gel filtration, consisted of a tetramer of 33 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The N-terminal sequence showed high homology with tryptases from other species. The optimum pH and temperature were 7.5-9.0 and 25-40 degrees C, respectively. The enzyme was labile in high-KCl buffer, and the optimum KCl concentration was 0.1 M. The enzyme activity was completely inhibited by diisopropyl phosphorofluoridate and leupeptin but not by soybean trypsin inhibitor and alpha-antitrypsin. The enzyme hydrolyzed vasoactive intestinal peptide but did not affect angiotensin I, somatostatin and bradykinin. In the present study, we first isolated monkey tryptase from cheek pouch vascular tissues and showed that the characteristics of monkey tryptase are very similar to those of human tryptase.

Senescent jejunal mast cells and eosinophils in the mouse preferentially translocate to the spleen and draining lymph node, respectively, during the recovery phase of helminth infection

Because mice infected with Trichinella spiralis experience a pronounced, but transient, mastocytosis and eosinophilia in their intestine, this disease model was used to follow the fate of senescent T cell-dependent mast cells (MCs) and eosinophils. Very few MCs or eosinophils undergoing apoptosis were found in the jejunum during the resolution phase of the infection, even though apoptotic MCs were common in the large intestine. Although the mesenteric draining lymph nodes contained large numbers of apoptotic eosinophils, MCs were rarely found at this location. During the recovery phase, large numbers of MCs were present in the spleen, and many of these cells possessed segmented nuclei. These splenic MCs were not proliferating. Although MCs from the jejunum and spleen of noninfected mice failed to express mouse MC protease (mMCP) 9, essentially all of the MCs in the jejunal submucosa and spleen of T. spiralis-infected mice expressed this serine protease during the recovery phase. The MCs in the jejunum expressed mMCP-9 before any mMCP-9-containing cells could be detected in the spleen. The fact that mMCP-9-containing MCs were detected in splenic blood vessels as these cells began to disappear from the jejunum supports the view that many jejunal MCs translocate to the spleen during the recovery phase of the infection. During this translocation process, some senescent jejunal MCs undergo nuclear segmentation. These studies reveal for the first time different exit and disposal pathways for T cell-dependent eosinophils and MCs after their expansion in the jejunum during a helminth infection.

The human mast cell tryptase tetramer: a fascinating riddle solved by structure

Tryptases, the predominant proteins of human mast cells, have been implicated as pathogenetic mediators of allergic and inflammatory conditions, most notably asthma. Until recently, the fascinating properties that distinguish tryptases among the serine proteinases, particularly their activity as a heparin-stabilized tetramer, resistance to most proteinaceous inhibitors, and preference for peptidergic over macromolecular substrates presented a riddle. This review solves this riddle with the help of the crystal structure of the human beta(2)-tryptase tetramer, but also indicates controversies between the unique quaternary architecture and some experimental data.

Formation of enzymatically active, homotypic, and heterotypic tetramers of mouse mast cell tryptases. Dependence on a conserved Trp-rich domain on the surface

Mouse mast cell protease (mMCP) 6 and mMCP-7 are homologous tryptases stored in granules as macromolecular complexes with heparin and/or chondroitin sulfate E containing serglycin proteoglycans. When pro-mMCP-7 and pseudozymogen forms of this tryptase and mMCP-6 were separately expressed in insect cells, all three recombinant proteins were secreted into the conditioned medium as properly folded, enzymatically inactive 33-kDa monomers. However, when their propeptides were removed, mMCP-6 and mMCP-7 became enzymatically active and spontaneously assumed an approximately 150-kDa tetramer structure. Heparin was not required for this structural change. When incubated at 37 degrees C, recombinant mMCP-7 progressively lost its enzymatic activity in a time-dependent manner. Its N-linked glycans helped regulate the thermal stability of mMCP-7. However, the ability of this tryptase to form the enzymatically active tetramer was more dependent on a highly conserved Trp-rich domain on its surface. Although recombinant mMCP-6 and mMCP-7 preferred to form homotypic tetramers, these tryptases readily formed heterotypic tetramers in vitro. This latter finding indicates that the tetramer structural unit is a novel way the mast cell uses to assemble varied combinations of tryptases.

Identification of a new member of the tryptase family of mouse and human mast cell proteases which possesses a novel COOH-terminal hydrophobic extension

Mapping of the tryptase locus on chromosome 17 revealed a novel gene 2.3 kilobase 3' of the mouse mast cell protease (mMCP) 6 gene. This 3.7-kilobase gene encodes the first example of a protease in the tryptase family that contains a membrane-spanning segment located at its COOH terminus. Comparative structural studies indicated that the putative transmembrane tryptase (TMT) possesses a unique substrate-binding cleft. As assessed by RNA blot analyses, mTMT is expressed in mice in both strain- and tissue-dependent manners. Thus, different transcriptional and/or post-transcriptional mechanisms are used to control the expression of mTMT in vivo. Analysis of the corresponding tryptase locus in the human genome resulted in the isolation and characterization of the hTMT gene. The hTMT transcript is expressed in numerous tissues and is also translated. Analysis of the tryptase family of genes in mice and humans now indicates that a primordial serine protease gene duplicated early and often during the evolution of mammals to generate a panel of homologous tryptases in each species that differ in their tissue expression, substrate specificities, and physical properties.

Human tryptases alpha and beta/II are functionally distinct due, in part, to a single amino acid difference in one of the surface loops that forms the substrate-binding cleft

Tryptases alpha and beta/II were expressed in insect cells to try to ascertain why human mast cells express these two nearly identical granule proteases. In contrast to that proposed by others, residue -3 in the propeptide did not appear to be essential for the three-dimensional folding, post-translational modification, and/or activation of this family of serine proteases. Both recombinant tryptases were functional and bound the active-site inhibitor diisopropyl fluorophosphate. However, they differed in their ability to cleave varied trypsin-susceptible chromogenic substrates. Structural modeling analyses revealed that tryptase alpha differs from tryptase beta/II in that it possesses an Asp, rather than a Gly, in one of the loops that form its substrate-binding cleft. A site-directed mutagenesis approach was therefore carried out to determine the importance of this residue. Because the D215G derivative of tryptase alpha exhibited potent enzymatic activity against fibrinogen and other tryptase beta/II-susceptible substrates, Asp215 dominantly restricts the substrate specificity of tryptase alpha. These data indicate for the first time that tryptases alpha and beta/II are functionally different human proteases. Moreover, the variation of just a single amino acid in the substrate-binding cleft of a tryptase can have profound consequences in the regulation of its enzymatic activity and/or substrate preference.

Expression and characterization of recombinant mast cell tryptase

Tryptase, a serine protease, is the major protein component in mast cells. In an animal model of asthma, tryptase has been established as an important mediator of inflammation and late airway responses induced by antigen challenge. Human tryptase is notable for its tetrameric structure, requirement of heparin for stability, and resistance to endogenous inhibitors. Human protryptase was expressed as a recombinant protein in Pichia pastoris. The recombinant protein consisted of two forms of protryptase, one containing the entire propeptide and the other containing only the Val-Gly dipeptide at its amino terminus. Isolation of active recombinant tryptase required a two column purification protocol and included a heparin- and dipeptidyl peptidase I-dependent activation step. Purified recombinant tryptase migrated as a tetramer on a gel filtration column and displayed kinetic parameters identical to those of a native tryptase obtained from HMC-1 cells, a human mast cell line. Recombinant and HMC-1 tryptase exhibited comparable sensitivities to an array of protein and low-molecular-weight inhibitors, including one that is highly specific for tryptase (APC-1167). Similarly, the recombinant enzyme cleaved both alpha- and beta-chains of fibrinogen to generate fibrinogen fragments indistinguishable from those generated by HMC-1-derived tryptase. Thus, recombinant tryptase expressed in P. pastoris displays physical and enzymatic properties essentially identical to the native enzyme. This system provides a cost-effective and easy to manipulate expression system that will enable the functional characterization of this unique enzyme.

Spontaneous inactivation of human lung tryptase as probed by size-exclusion chromatography and chemical cross-linking: dissociation of active tetrameric enzyme into inactive monomers is the primary event of the entire process

A unique property of human mast cell tryptase is its spontaneous inactivation, which may be relevant to the regulation of the activity of this enzyme in vivo. We have found, using size-exclusion chromatography, that the dissociation of the tetrameric active enzyme into the inactive monomer occurred immediately from the beginning of the inactivation process and at a rate significantly faster than that of the appearance of the inactive, tetrameric form. Eventually, a relatively long-lived state of apparent equilibrium between all three forms (active tetramer, inactive monomer, inactive tetramer) was reached. When tryptase was extensively cross-linked with several heterobifunctional photoactivatable reagents, this modified enzyme exhibited a long-term stability in low-ionic-strength buffer and at elevated temperature, unlike that of the native enzyme. Its is suggested that cross-linking prevents the spontaneous inactivation and dissociation of tryptase by 'freezing' the normal association state of the enzyme and supports the hypothesis that the dissociation of native tetrameric tryptase into inactive monomer is the primary event for the entire process of spontaneous inactivation of this enzyme.

Structural and functional properties of Bos taurus tryptase: a search for a possible propeptide processing role

Some structural features of bovine tryptase were discussed based on spectroscopic analysis. The far UV-CD spectrum of the enzymatically active bovine tryptase is consistent with a structure containing very little, if any alpha-helix, as found for other serine proteases. The analysis of near UV-CD and UV absorption spectra reveals the presence of a high number of Trp residues arranged probably in strong structural motifs. At variance with other tryptases, the bovine enzyme shows an electrophoretic behaviour in native and denaturating conditions compatible with an association state larger than a tetramer (probably a dodecamer). From a biochemical point of view, the bovine tryptase shares with the human counterpart, the preference for cleaving substrates bearing dibasic cleavage sites. Thus, it is hypothesized that tryptase may be involved in some proprotein processing mechanism(s).

Reversible Expression of Tryptases and Chymases in the Jejunal Mast Cells of Mice Infected with Trichinella spiralis

It is has been established that mouse mast cells (MCs) can reversibly alter their expression of serglycin proteoglycans and the homologous granule chymases that have been designated mouse MC protease (mMCP)-1, mMCP-2, and mMCP-5 in vivo. Nevertheless, it remained to be determined whether these immune cells could modify their expression of other chymases and the granule tryptases mMCP-6 and mMCP-7. As assessed immunohistochemically, we now show that MCs reversibly change their expression of the recently described chymase mMCP-9 and both tryptases as these cells traverse the jejunum during the amplification and regression stages of the reactive MC hyperplasia. In noninfected mice, most jejunal MCs reside in the submucosa and express mMCP-6 and mMCP-7, but not mMCP-9 or the chymase mMCP-2. During the inductive phase of the helminth-induced inflammation, when the jejunal MCs move from the submucosa to the tips of the villus, the MCs briefly express mMCP-9, cease expressing mMCP-6 and mMCP-7, and then express mMCP-2. During the recovery phase of the inflammation, jejunal MCs cease expressing mMCP-2 and then express varied combinations of mMCP-6, mMCP-7, and mMCP-9 as they move from the tips of the villus back toward the submucosa. In other model systems, mMCP-6 elicits neutrophil extravasation, and mMCP-7 regulates fibrin deposition and fibrinogen-mediated signaling events. Thus, the ability of a jejunal MC to reversibly alter its tryptase expression during an inflammatory event has important functional implications.

Human beta-tryptase is a ring-like tetramer with active sites facing a central pore

Human tryptase, a mast-cell-specific serine proteinase that may be involved in causing asthma and other allergic and inflammatory disorders, is unique in two respects: it is enzymatically active only as a heparin-stabilized tetramer, and it is resistant to all known endogenous proteinase inhibitors. The 3-A crystal structure of human beta-tryptase in a complex with 4-amidinophenyl pyruvic acid shows four quasi-equivalent monomers arranged in a square flat ring of pseudo 222 symmetry. Each monomer contacts its neighbours at two different interfaces through six loop segments. These loops are located around the active site of beta-tryptase and differ considerably in length and conformation from loops of other trypsin-like proteinases. The four active centres of the tetramer are directed towards an oval central pore, restricting access for macromolecular substrates and enzyme inhibitors. Heparin chains might stabilize the complex by binding to an elongated patch of positively charged residues spanning two adjacent monomers. The nature of this unique tetrameric architecture explains many of tryptase's biochemical properties and provides a basis for the rational design of monofunctional and bifunctional tryptase inhibitors.

Induction of a Selective and Persistent Extravasation of Neutrophils into the Peritoneal Cavity by Tryptase Mouse Mast Cell Protease 6

Recombinant mouse mast cell protease 6 (mMCP-6) was generated to study the role of this tryptase in inflammatory reactions. Seven to forty-eight hours after the i.p. injection of recombinant mMCP-6 into BALB/c, mast cell-deficient WCB6F1-Sl/Sld, C5-deficient, or mMCP-5-null mice, the number of neutrophils in the peritoneal cavity of each animal increased significantly by &gt;50-fold. The failure of the closely related recombinant tryptase mMCP-7 to induce a comparable peritonitis indicates that the substrate specificities of the two tryptases are very different. Unlike most forms of acute inflammation, the mMCP-6-mediated peritonitis was relatively long lasting and neutrophil specific. Mouse MCP-6 did not induce neutrophil chemotaxis directly in an in vitro assay, but did promote chemotaxis of the leukocyte in the presence of endothelial cells. Mouse MCP-6 did not induce cultured human endothelial cells to express TNF-α, RANTES, IL-1α, or IL-6. However, the tryptase induced endothelial cells to express large amounts of IL-8 continually over a 40-h period. Neither enzymatically active mMCP-7 nor enzymatically inactive pro-mMCP-6 was able to induce endothelial cells to increase their expression of IL-8. Although the mechanism by which mMCP-6 induces neutrophil accumulation in tissues remains to be determined, the finding that mMCP-6 induces cultured human endothelial cells to selectively release large amounts of IL-8 raises the possibility that this tryptase regulates the steady state levels of neutrophil-specific chemokines in vivo during mast cell-mediated inflammatory events.

Bovine mast cell tryptase inactivation: effect of temperature

The thermal stability of bovine tryptase, a serine proteinase present in the bovine mast cell secretory granules, has been studied by circular dichroism and catalytic activity measurements. Bovine tryptase shows a peculiar dichroic negative band centered at 230 nm. The decrease of this band in a temperature dependent fashion represents a good marker to monitor the native conformation of the enzyme. Bovine tryptase inactivation has been followed in the temperature range between 10 degrees C and 80 degrees C, and reversibility of the process has been also studied. The results obtained show that the temperature dependent loss of activity and the conformational change, as monitored by circular dichroism, are both fully reversible between 10 degrees C and 40 degrees C, while only the CD change displays reversibility in going from 60 degrees C to 10 degrees C. Moreover, a functional analysis of the temperature-dependent enzymatic activity of bovine tryptase toward peptide substrates in the 10 degrees C - 40 degrees C range is reported and compared with the temperature dependence of the enzymatic activity of trypsin.

The tryptase, mouse mast cell protease 7, exhibits anticoagulant activity in vivo and in vitro due to its ability to degrade fibrinogen in the presence of the diverse array of protease inhibitors in plasma

Mouse mast cell protease (mMCP) 7 is a tryptase of unknown function expressed by a subpopulation of mast cells that reside in numerous connective tissue sites. Because enzymatically active mMCP-7 is selectively released into the plasma of V3 mastocytosis mice undergoing passive systemic anaphylaxis, we used this in vivo model system to identify a physiologic substrate of the tryptase. Plasma samples taken from V3 mastocytosis mice that had been sensitized with immunoglobulin (Ig) E and challenged with antigen were found to contain substantial amounts of four 34-55-kDa peptides, all of which were derived from fibrinogen. To confirm the substrate specificity of mMCP-7, a pseudozymogen form of the recombinant tryptase was generated that could be activated after its purification. The resulting recombinant mMCP-7 exhibited potent anticoagulant activity in the presence of normal plasma and selectively cleaved the alpha-chain of fibrinogen to fragments of similar size as that seen in the plasma of the IgE/antigen-treated V3 mastocytosis mouse. Subsequent analysis of a tryptase-specific, phage display peptide library revealed that recombinant mMCP-7 preferentially cleaves an amino acid sequence that is nearly identical to that in the middle of the alpha-chain of rat fibrinogen. Because fibrinogen is a physiologic substrate of mMCP-7, this tryptase can regulate clot formation and fibrinogen/integrin-dependent cellular responses during mast cell-mediated inflammatory reactions.

Structure of the complex of leech-derived tryptase inhibitor (LDTI) with trypsin and modeling of the LDTI-tryptase system

BACKGROUND

Tryptase is a trypsin-like serine proteinase stored in the cytoplasmic granules of mast cells, which has been implicated in a number of mast cell related disorders such as asthma and rheumatoid arthritis. Unlike almost all other serine proteinases, tryptase is fully active in plasma and in the extracellular space, as there are no known natural inhibitors of tryptase in humans. Leech-derived tryptase inhibitor (LDTI), a protein of 46 amino acids, is the first molecule found to bind tightly to and specifically inhibit human tryptase in the nanomolar range. LDTI also inhibits trypsin and chymotrypsin with similar affinities. The structure of LDTI in complex with an inhibited proteinase could be used as a template for the development of low molecular weight tryptase inhibitors.

RESULTS

The crystal structure of the complex between trypsin and LDTI was solved at 2.0 A resolution and a model of the LDTI-tryptase complex was created, based on this X-ray structure. LDTI has a very similar fold to the third domain of the turkey ovomucoid inhibitor. LDTI interacts with trypsin almost exclusively through its binding loop (residues 3-10) and especially through the sidechain of the specificity residue Lys8. Our modeling studies indicate that these interactions are maintained in the LDTI-tryptase complex.

CONCLUSIONS

The insertion of nine residues after residue 174 in tryptase, relative to trypsin and chymotrypsin, prevents inhibition by other trypsin inhibitors and is certainly responsible for the higher specificity of tryptase relative to trypsin. In LDTI, the disulfide bond between residues 4 and 25 causes a sharp turn from the binding loop towards the N terminus, holding the N terminus away from the 174 loop of tryptase.

Mouse mast cell protease 9, a novel member of the chromosome 14 family of serine proteases that is selectively expressed in uterine mast cells

Mouse mast cell protease (mMCP) 1, mMCP-2, mMCP-4, and mMCP-5 are members of a family of related serine proteases whose genes reside within an approximately 850 kilobase (kb) complex on chromosome 14 that does not readily undergo crossover events. While mapping the mMCP-1 gene, we isolated a novel gene that encodes a homologous serine protease designated mMCP-9. The mMCP-9 and mMCP-1 genes are only approximately 7 kb apart on the chromosome and are oriented back to back. The proximity of the mMCP-1 and mMCP-9 genes now suggests that the low recombination frequency of the complex is due to the closeness of some of its genes. The mMCP-9 transcript and protein were observed in the jejunal submucosa of Trichinella spiralis-infected BALB/c mice. However, in normal BALB/c mice, mMCP-9 transcript and protein were found only in those mast cells that reside in the uterus. Thus, the expression of mMCP-9 differs from that of all other chymases. The observation that BALB/c mouse bone marrow-derived mast cells developed with interleukin (IL) 10 and c-kit ligand contain mMCP-9 transcript, whereas those developed with IL-3 do not, indicates that the expression of this particular chymase is regulated by the cytokine microenvironment. Comparative protein structure modeling revealed that mMCP-9 is the only known granule protease with three positively charged regions on its surface. This property may allow mMCP-9 to form multimeric complexes with serglycin proteoglycans and other negatively charged proteins inside the granule. Although mMCP-9 exhibits a >50% overall amino acid sequence identity with its homologous chymases, it has a unique substrate-binding cleft. This finding suggests that each member of the chromosome 14 family of serine proteases evolved to degrade a distinct group of proteins.

The three-dimensional structure of recombinant leech-derived tryptase inhibitor in complex with trypsin. Implications for the structure of human mast cell tryptase and its inhibition

The x-ray crystal structure of recombinant leech-derived tryptase inhibitor (rLDTI) has been solved to a resolution of 1.9 A in complex with porcine trypsin. The nonclassical Kazal-type inhibitor exhibits the same overall architecture as that observed in solution and in rhodniin. The complex reveals structural aspects of the mast cell proteinase tryptase. The conformation of the binding region of rLDTI suggests that tryptase has a restricted active site cleft. The basic amino terminus of rLDTI, apparently flexible from previous NMR measurements, approaches the 148-loop of trypsin. This loop has an acidic equivalent in tryptase, suggesting that the basic amino terminus could make favorable electrostatic interactions with the tryptase molecule. A series of rLDTI variants constructed to probe this hypothesis confirmed that the amino-terminal Lys-Lys sequence plays a role in inhibition of human lung tryptase but not of trypsin or chymotrypsin. The location of such an acidic surface patch is in accordance with the known low molecular weight inhibitors of tryptase.

Soybean Bowman-Birk protease inhibitor is a highly effective inhibitor of human mast cell chymase

Soybean Bowman-Birk protease inhibitor (BBI) is an inhibitor of serine proteases with two functional inhibitory domains of different specificities: one is specific for chymotrypsin-like proteases, the other for trypsin-like proteases. Chymase and tryptase are serine proteases which are stored in mast cell granules and released upon degranulation. This work investigated the inhibition of human chymase and tryptase by BBI. Active-site titration of human skin chymase by BBI demonstrated that BBI was a highly effective inhibitor of human chymase. Virtually stoichiometric inhibition of chymase by BBI was observed at 10 nM chymase. Kinetic studies of the inhibition reaction yielded an association rate constant of 4.0 x 10(5) M(-1) s(-1) and a dissociation rate constant of 1.7 x 10(-5) s(-1). From these two constants we estimate a K(i) of 50 pM. Chymase/BBI complexes did not dissociate in SDS-PAGE analyses under nonreducing conditions, consistent with the formation of a very tight complex with little tendency to dissociate. In contrast to chymase, human tryptase was not inhibited by BBI. These studies demonstrate that BBI is a good inhibitor of human chymase, exhibiting reaction properties better than physiological inhibitors described to date.

Mouse Mast Cells That Possess Segmented/Multi-lobular Nuclei

Because in humans mast cells and basophils tend to possess nonsegmented and segmented/multi-lobular nuclei, respectively, nuclear morphology has been a major criterion for assessing the lineage of metachromatic cells of hematopoietic origin. Immature metachromatic cells with mono- and multi-lobular nuclei were both obtained when bone marrow cells from BALB/c mice were cultured for 3 weeks in the presence of interleukin-3. Analogous to the indigenous mature mast cells that reside in the peritoneal cavity and skin, both populations of in vitro–derived cells expressed the surface receptor c-kit, the chymase mouse mast cell protease (mMCP) 5, the tryptase mMCP-6, and the exopeptidase carboxypeptidase A (mMC-CPA). Immunogold electron microscopy confirmed the granule location of mMC-CPA and mMCP-6 in both populations of cells, and cytochemical analysis confirmed the presence of chymotryptic enzymes in the granules. Because mature mast cells possessing multi-lobular nuclei also were occasionally found in the skeletal muscle and jejunum of the BALB/c mouse, the V3 mouse mast cell line was used to investigate the developmental relationship of mast cells that have very different nuclear structures. After the adoptive transfer of V3 mast cells into BALB/c mice, v-abl–immortalized mast cells with mono- and multi-lobular nuclei were detected in the lymph nodes and other tissues of the mastocytosis mice that expressed c-kit, mMCP-5, mMCP-6, and mMC-CPA. These studies indicate that mouse mast cells can exhibit varied nuclear profiles. Moreover, the nuclear morphology of this cell type gives no insight as to its protease phenotype or stage of development.

Mouse Mast Cells That Possess Segmented/Multi-lobular Nuclei

Because in humans mast cells and basophils tend to possess nonsegmented and segmented/multi-lobular nuclei, respectively, nuclear morphology has been a major criterion for assessing the lineage of metachromatic cells of hematopoietic origin. Immature metachromatic cells with mono- and multi-lobular nuclei were both obtained when bone marrow cells from BALB/c mice were cultured for 3 weeks in the presence of interleukin-3. Analogous to the indigenous mature mast cells that reside in the peritoneal cavity and skin, both populations of in vitro–derived cells expressed the surface receptor c-kit, the chymase mouse mast cell protease (mMCP) 5, the tryptase mMCP-6, and the exopeptidase carboxypeptidase A (mMC-CPA). Immunogold electron microscopy confirmed the granule location of mMC-CPA and mMCP-6 in both populations of cells, and cytochemical analysis confirmed the presence of chymotryptic enzymes in the granules. Because mature mast cells possessing multi-lobular nuclei also were occasionally found in the skeletal muscle and jejunum of the BALB/c mouse, the V3 mouse mast cell line was used to investigate the developmental relationship of mast cells that have very different nuclear structures. After the adoptive transfer of V3 mast cells into BALB/c mice, v-abl–immortalized mast cells with mono- and multi-lobular nuclei were detected in the lymph nodes and other tissues of the mastocytosis mice that expressed c-kit, mMCP-5, mMCP-6, and mMC-CPA. These studies indicate that mouse mast cells can exhibit varied nuclear profiles. Moreover, the nuclear morphology of this cell type gives no insight as to its protease phenotype or stage of development.

Expression of a plasmin/trypsin Kunitz inhibitor by pig trophoblast

Under the influence of progesterone, the porcine uterus synthesizes plasmin/trypsin inhibitor (PTI), a low molecular weight protein (M(r) approximately 14,000) belonging to the Kunitz family of proteinase inhibitors. Here it is demonstrated that mRNA for the same protein is produced by the developing trophoblast during early pregnancy and by the placenta throughout gestation. The transcript for PTI was represented in approximately 0.1% of total phage plaques of a day 13-17 porcine conceptus cDNA library. It shared 99% nucleotide sequence identity with the cDNA isolated from the uterine library and encoded a 112-amino-acid protein identical to the uterine-produced PTI, which has a well-defined Kunitz domain comprised of 64 residues at its amino terminus. Northern analysis and in situ hybridization studies confirmed that expression of PTI by the conceptus begins as early as day 10 of pregnancy and is continued in placental tissues until at least day 90 of gestation. Expression was trophoblast specific at day 20 of gestation, as in situ hybridization detected no mRNA in the embryo. The pattern of PTI expression during pregnancy is consistent with a role either in controlling trophoblast invasiveness or in inhibiting proteinases with trypsin-like specificity released by immune or inflammatory cells.

Fate of two mast cell tryptases in V3 mastocytosis and normal BALB/c mice undergoing passive systemic anaphylaxis: prolonged retention of exocytosed mMCP-6 in connective tissues, and rapid accumulation of enzymatically active mMCP-7 in the blood

The mouse mast cell protease granule tryptases designated mMCP-6 and mMCP-7 are encoded by highly homologous genes that reside on chromosome 17. Because these proteases are released when mast cells are activated, we sought a basis for distinctive functions by examining their fates in mice undergoing passive systemic anaphylaxis. 10 min-1 h after antigen (Ag) was administered to immunoglobulin (Ig)E-sensitized mice, numerous protease/proteoglycan macromolecular complexes appeared in the extracellular matrix adjacent to most tongue and heart mast cells of normal BALB/c mice and most spleen and liver mast cells of V3 mastocytosis mice. These complexes could be intensively stained by anti-mMCP-6 Ig but not by anti-mMCP-7 Ig. Shortly after Ag challenge of V3 mastocytosis mice, large amounts of properly folded, enzymatically active mMCP-7 were detected in the plasma. This plasma-localized tryptase was approximately 150 kD in its multimeric state and approximately 32 kD in its monomeric state, possessed an NH2 terminus identical to that of mature mMCP-7, and was not covalently bound to any protease inhibitor. Comparative protein modeling and electrostatic calculations disclosed that mMCP-6 contains a prominent Lys/Arg-rich domain on its surface, distant from the active site. The absence of this domain in mMCP-7 provides an explanation for its selective dissociation from the exocytosed macromolecular complex. The retention of exocytosed mMCP-6 in the extracellular matrix around activated tissue mast cells suggests a local action. In contrast, the rapid dissipation of mMCP-7 from granule cores and its inability to be inactivated by circulating protease inhibitors suggests that this tryptase cleaves proteins located at more distal sites.

cDNA cloning and primary structure of tryptase from bovine mast cells, and evidence for the expression of bovine pancreatic trypsin inhibitor mRNA in the same cells

A partial cDNA encoding bovine tryptase, an oligomeric serine proteinase previously isolated from bovine mast cells, was obtained by reverse transcription/polymerase chain reaction of mast cell mRNA, using combinations of primers designed on the basis of information obtained from partial sequencing of the purified protein. The complete amino acid sequence of bovine tryptase (245 residues) was deduced from a 711-bp nucleotide sequence and from Edman degradation of the protein. Bovine tryptase primary structure has an identity of about 75% with tryptases from other species and includes all the essential residues of the active-site regions; sequence data in the region of the putative substrate binding pocket suggest a rearrangement capable of maintaining the specificity of trypsin-like proteinases. From the same mast cell mRNA, cDNA encoding bovine trypsin protease inhibitor (BPTI) was obtained and amplified with specific primers, confirming the synthesis of BPTI in these cells. Results are consistent with previous data on the presence of BPTI and bovine tryptase in the same granules of bovine mast cells and with their interaction in vitro.

Allergens in Hymenoptera venom. XXVII: bumblebee venom allergy and allergens

BACKGROUND

Allergic reactions to bumblebee stings are much less common than allergic reactions to honeybee stings. Preliminary studies suggest that there may be a high degree of cross-reactivity between honeybee and bumblebee venoms.

OBJECTIVE

This study was done to determine the immunochemical and structural relationships between bumblebee and honeybee venom allergens.

METHODS

Allergens were purified from bumblebee venom and compared immunochemically with sera from patients with allergy and hyperimmunized rabbits. The purified proteins were characterized, enzyme activities were measured, and the complete amino acid sequences of two proteins were determined.

RESULTS

The venoms wer highly cross-reactive, consistent with the degree of structural similarity in the phospholipases. Hyaluronidases and acid phosphatases were also similar. Bumblebee venom contained several proteins not found in honeybee venom, including an interesting tryptic amidase related to clotting enzymes and acrosin.

CONCLUSION

Skin testing with honeybee venom will detect almost all cases of bumblebee venom allergy, however; RAST with bumblebee venom can detect some additional cases.

Natural disruption of the mouse mast cell protease 7 gene in the C57BL/6 mouse

The C57BL/6 mouse differs from the BALB/c mouse in that its ear and skin mast cells and its progenitor bone marrow-derived mast cells (mBMMCs) do not express mouse mast cell protease (mMCP) 7. We now report that, as detected by nuclear run-on analysis, the mMCP-7 gene is transcribed in C57BL/6 mBMMCs at a rate comparable to that in BALB/c mBMMCs. Reverse transcriptase-polymerase chain reaction analysis and sequencing of the product revealed that the ears of C57BL/6 mice contain small amounts of a mMCP-7 transcript that possesses a 98-base pair deletion. The deletion begins at a normally quiescent cryptic splice site (G416TGAG), 98 base pairs upstream of the normal exon 2/intron 2 splice site (G514TGAG), and introduces a premature stop codon in the alternatively spliced transcript. Thus, even if translated, the mature protein would consist of only 18 amino acids as compared to 245 amino acids in normal mMCP-7. Sequence analysis of the mMCP-7 gene in the C57BL/6 mouse revealed that the cryptic splice site is activated due to a G514-->A point mutation at the first nucleotide of the normal exon 2/intron 2 splice site. This is the first report of a mutation of a gene that encodes a mast cell secretory granule constituent that leads to its loss of expression. Moreover, the mMCP-7 gene is the first found in any species that sequentially has undergone a splice site mutation to cause retention of an intron and then a second splice site mutation to cause activation of a cryptic splice site.

Packaging of proteases and proteoglycans in the granules of mast cells and other hematopoietic cells. A cluster of histidines on mouse mast cell protease 7 regulates its binding to heparin serglycin proteoglycans

Mouse mast cell protease 7 (mMCP-7) is a tryptase stored in the secretory granules of mast cells. At the granule pH of 5.5, mMCP-7 is fully active and is bound to heparin-containing serglycin proteoglycans. to understand the interaction of mMCP-7 with heparin inside and outside the mast cell, this trytase was first studied by comparative protein modeling. The "pro" form of mMCP-7 was then expressed in insect cells and studied by site-directed mutagenesis. Although mMCP-7 lacks known linear sequences of amino acis that interact with heparin, the three-dimensional model of mMCP-7 revealed an area on the surface of the folded protein away from the substrate-binding site that exhibits a strong positive electrostatic potential at the acidic pH of the granule. In agreement with this calculation, recombinant pro-mMCP-7 bound to a heparin-affinity column at pH 5.5 and readily dissociated from the column at pH > 6.5. Site-directed mutagenesis confirmed the prediction that the conversion of His residues 8,68, and 70 in the positively charged region into Glu prevents the binding of pro-mMCP-7 to heparin. Because the binding requires positively charged His residues, native mMCP-7 is able to dissociate from the protease/proteoglycan macromolecular complex when the complex is exocytosed from bone marrow-derived mast cells into a neutral pH environment. Many hematopoietic effector cells store positively charged proteins in granules that contain serglycin proteoglycans. The heparin/mMCP-7 interaction, which depends on the tertiary structure of the tryptase, may be representative of a general control mechanism by which hematopoietic cells maximize storage of properly folded, enzymatically active proteins in their granules.

Purification and characterization of dog mast cell protease-3, an oligomeric relative of tryptases

The existence of a protein approximately 48% identical with mast cell tryptases was predicted previously from a dog mastocytoma cDNA. Antibodies raised against a peptide based on the deduced sequence suggested that the protein (dog mast cell protease-3, dMCP-3) is expressed in mast cells. In this report, characterization of the protein purified from mastocytomas reveals an N-glycosylated, high molecular weight, tryptic serine protease, which appears to be a tetramer of catalytic subunits, approximately half of which are linked by disulfide bonds. The oligomeric complex yields a single NH2-terminal sequence, which is identical with that predicted by dMCP-3 cDNA. This finding, and the lack of closely related genes on blots of genomic DNA, predict that each subunit is the product of one gene. Although dMCP-3 binds to heparin, it is active and stable at low ionic strength in heparin's absence. It resists inactivation by inhibitors in plasma but is sensitive to small inhibitors, e.g. leupeptin and bis(5-amidino-2-benzimidazolyl)methane (BABIM). dMCP-3 hydrolyzes extended peptidyl p-nitroanilides ending in basic residues, with P1 arginine preferred to lysine; it hydrolyzes the Arg18-Ser19 bond of calcitonin gene-related peptide but cleaves neither vasoactive intestinal peptide nor casein. These data suggest that dMCP-3 is a unique serine protease whose stability, formation of intersubunit disulfide bonds, inhibitor susceptibilities and substrate preferences differ from those of its closest relatives, the mast cell tryptases.

Evidence for multiple interacting binding sites in bovine tryptase

The interaction of bovine pancreatic trypsin inhibitor (BPTI) and bovine tryptase, which are co-localized in the same granules of bovine mast cells, has been analyzed at 30 degrees C in 0.1 M Tris-HCl, pH 8.0. The analysis has unravelled that the functional unit of bovine tryptase is formed of (at least) four binding sites for this inhibitor. These interaction sites display a simple binding behaviour for small inhibitors (and substrates), whereas heterogeneous properties have been observed in the binding of BPTI. Furthermore, in the presence of BPTI, a positive functional interaction can be detected among the binding sites also for a small synthetic inhibitor, like benzamidine. Such features indicate the existence of a complex functional interplay among the sites of the functional unit which is transmitted through the secondary specificity sites.

Human mast cell tryptase isoforms: separation and examination of substrate-specificity differences

Tryptases are trypsin-like enzymes found in mast cell granules that appear to exist as tetramers. These enzymes are not controlled by blood plasma proteinase inhibitors and only cleave a few physiological substrates in vitro, including high-molecular-mass kininogen (HMMK) and vasoactive intestinal peptide (VIP). Purified human lung mast cell tryptase (HLT) contained two bands of approx. molecular mass 29 and 33 kDa on SDS/PAGE. These two forms of HLT have been separated by chromatography on a cellulose phosphate column, with the high-molecular-mass form (high-HLT) being eluted with 10 microM heparin and the low-molecular-mass form (low-HLT) subsequently eluted with 1 M NaCl. Removal of asparagine-linked carbohydrate caused both isoforms to run as single sharp bands on SDS/PAGE, differing slightly in molecular mass. Separation of these two isoforms of tryptase shows that tetramers consist of four homologous subunits rather than mixtures of the two isoforms. Using HMMK and VIP as substrates, these two forms of HLT were found to differ with regard to specificity and rate of cleavage. High-HLT initially cleaved HMMK at Arg-431 within the C-terminal anionic binding region of the molecule, whereas low-HLT cleaved HMMK simultaneously at multiple sites within the C-terminal portion of the molecule. On the basis of HPLC peptide mapping, each isoform also cleaved VIP at different sites. Comparison of cleavage rates based on the active-site concentrations of titrated isoforms showed that low-HLT cleaved HMMK more rapidly than did high-HLT. These two isoforms may represent different gene products or they may result from post-translational modification.

Serine proteinases of mast cell and leukocyte granules. A league of their own

Serine proteinases are hydrolases that use serine's side chain hydroxyl group to attack and cleave internal peptide bonds in peptides and proteins. They reside in all mammalian tissues, including the lung and airway. As a group, they vary tremendously in form and target specificity and have a vast repertoire of functions, many of which are critical for life. A subset of these proteinases is expressed primarily in the cytosolic granules of leukocytes from bone marrow, including mast cells. Examples are elastase-related proteinases and cathepsin G of monocytes and neutrophils, the many "granzymes" of cytotoxic T lymphocytes and natural killer (NK) cells, and the tryptases and chymases of mast cells. The pace of discovery and characterization of these granule-associated serine proteinases, fueled by technical advances in molecular biology, has accelerated rapidly in the past few years. Progress has been made in assigning possible functions to individual proteinases. However, the burgeoning numbers of these enzymes; their cell, tissue and species-dependent differences in expression; and their variety of action in vitro (despite, in many cases, shared modes of activation and recent divergence in protein evolution) have vexed and challenged those of us who are anxious to establish their roles in mammalian biology. Certainly, much remains to be discovered and clarified. The purpose of this overview is to capture the state of the art in this field, stressing the similarities as well as the differences among individual granule-associated proteinases and focusing particularly on those enzymes likely to be important in the human lung and airways.

A Kazal-type inhibitor of human mast cell tryptase: isolation from the medical leech Hirudo medicinalis, characterization, and sequence analysis

Human tryptase, a tetrameric proteinase expressed by mast cells, is virtually unique among the serine proteinases as it is not inhibited by any proteinaceous inhibitor tested so far. We have now isolated, sequenced, and characterized an inhibitor of human tryptase from the medical leech Hirudo medicinalis. LDTI (Leech-Derived Tryptase Inhibitor) was purified to apparent homogeneity by cation exchange and affinity chromatography. Amino acid sequencing of the protein consisting of 46 residues (M(r) 4738) revealed a high degree of similarity to the non-classical Kazal-type inhibitors bdellin B-3 and rhodniin, inhibitors isolated from the medical leech and the insect Rhodnius prolixus, respectively. LDTI is a tight-binding and relatively specific inhibitor of human tryptase; it inhibits only trypsin (EC 3.4.21.4) and chymotrypsin (EC 3.4.21.1) with similar affinities. Inhibition studies using small chromogenic substrates revealed that LDTI inhibits the amidolytic activity of tryptase by approximately 50%, suggesting that most likely due to steric hindrance LDTI binds to and inhibits only 2 of 4 active sites of tryptase. LDTI appears useful as a prototype of inhibitors of human tryptase and as a pharmacological tool for the investigation of the role of tryptase in health and disease.