Peptidoglycan inducible expression of a serine proteinase homologue from kuruma shrimp (Marsupenaeus japonicus)

A cDNA encoding a serine proteinase homologue of kuruma shrimp (Marsupenaeus japonicus) was cloned. The 1257 bp cDNA encodes a 339 amino acid putative peptide, with a signal sequence of 16 amino acid residues. The deduced amino acid sequence is 42-67% similar to the immune-related serine proteinases and serine proteinase homologues of arthropods. It contains catalytic triad residues in the putative catalytic domain except for one substitution of Ser by a Gly residue. The six cysteine residues that form three disulphide bridges in most serine proteinases were conserved. The M. japonicus serine proteinase homologue was mainly expressed in haemocytes, in which expression dramatically increased after 3 days feeding with peptidoglycan at 0.2 mg kg(-1) shrimp body weight per day.     

Structural relationship between lipases and peptidases of the prolyl oligopeptidase family.

In prolyl oligopeptidase and its homologues, which constitute a new serine protease family, the order of the catalytic Ser and His residues in the amino acid sequence is the reverse of what is found in the trypsin and subtilisin families. The exact position of the third member of the catalytic triad, an Asp residue, has not yet been identified in the new family. Recent determination of the three-dimensional structures of pancreatic and microbial lipases has shown that the order of their catalytic residues is Ser, Asp, His, and this fits the order Ser, His of prolyl oligopeptidase. However, there is no sequence homology between lipases and peptidases, except for a 10-residue segment, which encompasses the essential Ser, and for the immediate vicinity of the catalytic Asp and His residues. This comparison identifies the catalytic Asp residue in the prolyl oligopeptidase family. The relative positions of the three catalytic residues in peptidases and microbial lipases were the same and this indicated structural and possibly evolutionary relationship between the two families.     

A theoretical study of the active sites of papain and S195C rat trypsin: implications for the low reactivity of mutant serine proteinases.

The serine and cysteine proteinases represent two important classes of enzymes that use a catalytic triad to hydrolyze peptides and esters. The active site of the serine proteinases consists of three key residues, Asp...His...Ser. The hydroxyl group of serine functions as a nucleophile and the imidazole ring of histidine functions as a general acid/general base during catalysis. Similarly, the active site of the cysteine proteinases also involves three key residues: Asn, His, and Cys. The active site of the cysteine proteinases is generally believed to exist as a zwitterion (Asn...His+...Cys-) with the thiolate anion of the cysteine functioning as a nucleophile during the initial stages of catalysis. Curiously, the mutant serine proteinases, thiol subtilisin and thiol trypsin, which have the hybrid Asp...His...Cys triad, are almost catalytically inert. In this study, ab initio Hartree-Fock calculations have been performed on the active sites of papain and the mutant serine proteinase S195C rat trypsin. These calculations predict that the active site of papain exists predominately as a zwitterion (Cys-...His+...Asn). However, similar calculations on S195C rat trypsin demonstrate that the thiol mutant is unable to form a reactive thiolate anion prior to catalysis. Furthermore, structural comparisons between native papain and S195C rat trypsin have demonstrated that the spatial juxtapositions of the triad residues have been inverted in the serine and cysteine proteinases and, on this basis, I argue that it is impossible to convert a serine proteinase to a cysteine proteinase by site-directed mutagenesis.     

Homology modeling and identification of serine 160 as nucleophile of the active site in a thermostable carboxylesterase from the archaeon Archaeoglobus fulgidus

The hyperthermophilic Archaeon Archaeoglobus fulgidus has a gene (AF1763) which encodes a thermostable carboxylesterase belonging to the hormone-sensitive lipase (HSL)-like group of the esterase/lipase family. Based on secondary structure predictions and a secondary structure-driven multiple sequence alignment with remote homologous proteins of known three-dimensional structure, we previously hypothesized for this enzyme the alpha/beta-hydrolase fold typical of several lipases and esterases and identified Ser160, Asp 255 and His285 as the putative members of the catalytic triad. In this paper we report the building of a 3D model for this enzyme based on the structure of the homologous brefeldin A esterase from Bacillus subtilis whose structure has been recently elucidated. The model reveals the topological organization of the fold corroborating our predictions. As regarding the active-site residues, Ser160, Asp255 and His285 are located close each other at hydrogen bond distances. The catalytic role of Ser160 as the nucleophilic member of the triad is demonstrated by the [(3)H]diisopropylphosphofluoridate (DFP) active-site labeling and sequencing of a radioactive peptide containing the signature sequence GDSAGG.     

Identification of the catalytic residues of alpha-amino acid ester hydrolase from Acetobacter turbidans by labeling and site-directed mutagenesis

The alpha-amino acid ester hydrolase from Acetobacter turbidans ATCC 9325 is capable of hydrolyzing and synthesizing the side chain peptide bond in beta-lactam antibiotics. Data base searches revealed that the enzyme contains an active site serine consensus sequence Gly-X-Ser-Tyr-X-Gly that is also found in X-prolyl dipeptidyl aminopeptidase. The serine hydrolase inhibitor p-nitrophenyl-p'-guanidino-benzoate appeared to be an active site titrant and was used to label the alpha-amino acid ester hydrolase. Electrospray mass spectrometry and tandem mass spectrometry analysis of peptides from a CNBr digest of the labeled protein showed that Ser(205), situated in the consensus sequence, becomes covalently modified by reaction with the inhibitor. Extended sequence analysis showed alignment of this Ser(205) with the catalytic nucleophile of some alpha/beta-hydrolase fold enzymes, which posses a catalytic triad composed of a nucleophile, an acid, and a base. Based on the alignments, 10 amino acids were selected for site-directed mutagenesis (Arg(85), Asp(86), Tyr(143), Ser(156), Ser(205), Tyr(206), Asp(338), His(370), Asp(509), and His(610)). Mutation of Ser(205), Asp(338,) or His(370) to an alanine almost fully inactivated the enzyme, whereas mutation of the other residues did not seriously affect the enzyme activity. Circular dichroism measurements showed that the inactivation was not caused by drastic changes in the tertiary structure. Therefore, we conclude that the catalytic domain of the alpha-amino acid ester hydrolase has an alpha/beta-hydrolase fold structure with a catalytic triad of Ser(205), Asp(338), and His(370). This distinguishes the alpha-amino acid ester hydrolase from the Ntn-hydrolase family of beta-lactam antibiotic acylases.     

Molecular cloning of trypsin-like cDNAs and comparison of proteinase activities in the salivary glands and gut of the tarnished plant bug Lygus lineolaris (Heteroptera: Miridae)

Using specific proteinase inhibitors, we demonstrated that serine proteinases in the tarnished plant bug, Lygus lineolaris, are major proteinases in both salivary glands and gut tissues. Gut proteinases were less sensitive to inhibition than proteinases from the salivary glands. Up to 80% azocaseinase and 90% of BApNAse activities in the salivary glands were inhibited by aprotinin, benzamidine, and PMSF, whereas only 46% azocaseinase and 60% BApNAse activities in the gut were suppressed by benzamidine, leupeptin, and TLCK. The pH optima for azocaseinase activity in salivary glands ranged from 6.2 to 10.6, whereas the pH optima for gut proteinases was acidic for general and alkaline for tryptic proteinases. Zymogram analysis demonstrated that approximately 26-kDa proteinases from salivary glands were active against both gelatin and casein substrates. Three trypsin-like cDNAs, LlSgP2-4, and one trypsin-like cDNA, L1GtP1, were cloned from salivary glands and gut, respectively. Putative trypsin precursors from all cloned cDNAs contained a signal peptide, activation peptide, and conserved N-termini (IVGG). Other structural features included His, Asp, and Ser residues for the catalytic amino acid triad of serine proteinase active sites, residues for the binding pocket, and four pairs of cysteine residues for disulfide bridges. Deduced trypsin-like proteins from LlSgP2, LlSgP3, and LlGtP1 cDNAs shared 98-99% sequence identity with a previously reported trypsin-like precursor, whereas the trypsin-like protein of LlSgP4 shared only 44% sequence identity with all other trypsin-like proteins, indicating multi-trypsin forms are present in L. lineolaris.     

Crystal structure of bovine duodenase, a serine protease, with dual trypsin and chymotrypsin-like specificities

The three-dimensional structure of duodenase, a serine protease from bovine duodenum mucosa, has been determined at 2.4A resolution. The enzyme, which has both trypsin-like and chymotrypsin-like activities, most closely resembles human cathepsin G with which it shares 57% sequence identity and similar specificity. The catalytic Ser195 in duodenase adopts the energetically favored conformation typical of serine proteinases and unlike the strained state typical of lipase/esterases. Of several waters in the active site of duodenase, the one associated with Ser214 is found in all serine proteinases and most lipase/esterases. The conservation of the Ser214 residue in serine proteinase, its presence in the active site, and participation in a hydrogen water network involving the catalytic triad (His57, Asp107, and Ser195) argues for its having an important role in the mechanism of action. It may be referred to as a fourth member of the catalytic triad. Duodenase is one of a growing family of enzymes that possesses trypsin-like and chymotrypsin-like activity. Not long ago, these activities were considered to be mutually exclusive. Computer modeling reveals that the S1 subsite of duodenase has structural features compatible with effective accommodation of P1 residues typical of trypsin (Arg/Lys) and chymotrypsin (Tyr/Phe) substrates. The determination of structural features associated with functional variation in the enzyme family may permit design of enzymes with a specific ratio of trypsin and chymotrypsin activities.     

Homology modeling and active-site residues probing of the thermophilic Alicyclobacillus acidocaldarius esterase 2.

The moderate thermophilic eubacterium Alicyclobacillus (formerly Bacillus) acidocaldarius expresses a thermostable carboxylesterase (esterase 2) belonging to the hormone-sensitive lipase (HSL)-like group of the esterase/lipase family. Based on secondary structures predictions and a secondary structure-driven multiple sequence alignment with remote homologous protein of known three-dimensional (3D) structure, we previously hypothesized for this enzyme the alpha/beta-hydrolase fold typical of several lipases and esterases and identified Ser155, Asp252, and His282 as the putative members of the catalytic triad. In this paper we report the construction of a 3D model for this enzyme based on the structure of mouse acetylcholinesterase complexed with fasciculin. The model reveals the topological organization of the fold corroborating our predictions. As regarding the active-site residues, Ser155, Asp252, and His282 are located close to each other at hydrogen bond distances. Their catalytic role was here probed by biochemical and mutagenic studies. Moreover, on the basis of the secondary structure-driven multiple sequence alignment and the 3D structural model, a residue supposed important for catalysis, Gly84, was mutated to Ser. The activity of the mutated enzyme was drastically reduced. We propose that Gly84 is part of a putative "oxyanion hole" involved in the stabilization of the transition state similar to the C group of the esterase/lipase family.     

Functional topology of a surface loop shielding the catalytic center in lipoprotein lipase.

Lipoprotein lipase (LPL), hepatic lipase, and pancreatic lipase show high sequence homology to one another. The crystal structure of pancreatic lipase suggests that it contains a trypsin-like Asp-His-Ser catalytic triad at the active center, which is shielded by a disulfide bridge-bounded surface loop that must be repositioned before the substrate can gain access to the catalytic residues. By sequence alignment, the homologous catalytic triad in LPL corresponds to Asp156-His241-Ser132, absolutely conserved residues, and the homologous surface loop to residues 217-238, a poorly conserved region. To verify these assignments, we expressed in vitro wild-type LPL and mutant LPLs having single amino acid mutations involving residue Asp156 (to His, Ser, Asn, Ala, Glu, or Gly), His241 (to Asn, Ala, Arg, Gln, or Trp), or Ser132 (to Gly, Ala, Thu, or Asp) individually. All 15 mutant LPLs were totally devoid of enzyme activity, while wild-type LPL and other mutant LPLs containing substitutions in other positions were fully active. We further replaced the 22-residue LPL loop which shields the catalytic center either partially (replacing 6 of 22 residues) or completely with the corresponding hepatic lipase loop. The partial loop-replacement chimeric LPL was found to be fully active, and the complete loop-replacement mutant had approximately 60% activity, although the primary sequence of the hepatic lipase loop is quite different. In contrast, replacement with the pancreatic lipase loop completely inactivated the enzyme. Our results are consistent with Asp156-His241-Ser132 being the catalytic triad in lipoprotein lipase.(ABSTRACT TRUNCATED AT 250 WORDS)     

The PE-PPE domain in mycobacterium reveals a serine α/β hydrolase fold and function: an in-silico analysis

The PE and PPE proteins first reported in the genome sequence of Mycobacterium tuberculosis strain H37Rv are now identified in all mycobacterial species. The PE-PPE domain (Pfam ID: PF08237) is a 225 amino acid residue conserved region located towards the C-terminus of some PE and PPE proteins and hypothetical proteins. Our in-silico sequence analysis revealed that this domain is present in all Mycobacteria, some Rhodococcus and Nocardia farcinica genomes. This domain comprises a pentapeptide sequence motif GxSxG/S at the N-terminus and conserved amino acid residues Ser, Asp and His that constitute a catalytic triad characteristic of lipase, esterase and cutinase activity. The fold prediction and comparative modeling of the 3-D structure of the PE-PPE domain revealed a "serine α/β hydrolase" structure with a central β-sheet flanked by α-helices on either side. The structure comprises a lid insertion with a closed structure conformation and has a solvent inaccessible active site. The oxyanion hole that stabilizes the negative charge on the tetrahedral intermediate has been identified. Our findings add to the growing list of serine hydrolases in mycobacterium, which are essential for the maintenance of their impermeable cell wall and virulence. These results provide the directions for the design of experiments to establish the function of PE and PPE proteins.     

Molecular cloning and partial characterization of a trypsin-like protein in salivary glands of Lygus hesperus (Hemiptera: Miridae)

Trypsin-like enzymes from the salivary gland complex (SGC) of Lygus hesperus Knight were partially purified by preparative isoelectric focusing (IEF). Enzyme active against Nalpha-benzoyl-L-arginine-p-nitroanilide (BApNA) focused at approximately pH 10 during IEF. This alkaline fraction gave a single activity band when analyzed with casein zymograms. The serine proteinase inhibitors, phenylmethylsulfonyl fluoride (PMSF) and lima bean trypsin inhibitor, completely inhibited or suppressed the caseinolytic activity in the crude salivary gland extract as well as the IEF-purified sample. Chicken egg white trypsin inhibitor also inhibited the IEF-purified sample but was not effective against a major caseinolytic band in the crude salivary gland extract. These data indicated the presence of serine proteinases in the SGC of L. hesperus. Cloning and sequencing of a trypsin-like precursor cDNA provided additional direct evidence for serine proteinases in L. hesperus. The encoded trypsin-like protein included amino acid sequence motifs, which are conserved with five homologous serine proteinases from other insects. Typical features of the putative trypsin-like protein from L. hesperus included residues in the serine proteinase active site (His(89), Asp(139), Ser(229)), conserved cysteine residues for disulfide bridges, residues (Asp(223), Gly(252), Gly(262)) that determine trypsin specificity, and both zymogen signal and activation peptides.     

Molecular basis of formaldehyde detoxification. Characterization of two S-formylglutathione hydrolases from Escherichia coli, FrmB and YeiG

The Escherichia coli genes frmB (yaiM) and yeiG encode two uncharacterized proteins that share 54% sequence identity and contain a serine esterase motif. We demonstrated that purified FrmB and YeiG have high carboxylesterase activity against the model substrates, p-nitrophenyl esters of fatty acids (C2-C6) and alpha-naphthyl acetate. However, both proteins had the highest hydrolytic activity toward S-formylglutathione, an intermediate of the glutathione-dependent pathway of formaldehyde detoxification. With this substrate, both proteins had similar affinity (Km = 0.41-0.43 mM), but FrmB was almost 5 times more active. Alanine replacement mutagenesis of YeiG demonstrated that Ser145, Asp233, and His256 are absolutely required for activity, indicating that these residues represent a serine hydrolase catalytic triad in this protein and in other S-formylglutathione hydrolases. This was confirmed by inspecting the crystal structure of the Saccharomyces cerevisiae S-formylglutathione hydrolase YJG8 (Protein Data Bank code 1pv1), which has 45% sequence identity to YeiG. The structure revealed a canonical alpha/beta-hydrolase fold and a classical serine hydrolase catalytic triad (Ser161, His276, Asp241). In E. coli cells, the expression of frmB was stimulated 45-75 times by the addition of formaldehyde to the growth medium, whereas YeiG was found to be a constitutive enzyme. The simultaneous deletion of both frmB and yeiG genes was required to increase the sensitivity of the growth of E. coli cells to formaldehyde, suggesting that both FrmB and YeiG contribute to the detoxification of formaldehyde. Thus, FrmB and YeiG are S-formylglutathione hydrolases with a Ser-His-Asp catalytic triad involved in the detoxification of formaldehyde in E. coli.     

BTLCP proteins: a novel family of bacterial transglutaminase-like cysteine proteinases

Using sequence similarity searches and top-of-the-range fold-recognition methods, we have identified a novel family of bacterial transglutaminase-like cysteine proteinases (BTLCPs) with an invariant Cys-His-Asp catalytic triad and a predicted N-terminal signal sequence. This family of previously uncharacterized hypothetical proteins encompasses sequences of unknown function from DUF920 (in the Pfam database) and COG3672. BTLCPs are predicted to possess the papain-like cysteine proteinase fold and catalyze post-translational protein modification through transamidase, acetylase or hydrolase activity. Inspection of neighboring genes encoding BTLCPs suggests a link between this predicted activity and a type-I secretion system resembling ATP-binding cassette exporters of toxins and proteases involved in bacterial pathogenicity.     

Cysteine proteases of positive strand RNA viruses and chymotrypsin-like serine proteases. A distinct protein superfamily with a common structural fold

Evidence is presented, based on sequence comparison and secondary structure prediction, of structural and evolutionary relationship between chymotrypsin-like serine proteases, cysteine proteases of positive strand RNA viruses (3C proteases of picornaviruses and related enzymes of como-, nepo- and potyviruses) and putative serine protease of a sobemovirus. These observations lead to re-identification of principal catalytic residues of viral proteases. Instead of the pair of Cys and His, both located in the C-terminal part of 3C proteases, a triad of conserved His, Asp(Glu) and Cys(Ser) has been identified, the first two residues resident in the N-terminal, and Cys in the C-terminal beta-barrel domain. These residues are suggested to form a charge-transfer system similar to that formed by the catalytic triad of chymotrypsin-like proteases. Based on the structural analogy with chymotrypsin-like proteases, the His residue previously implicated in catalysis, together with two partially conserved Gly residues, is predicted to constitute part of the substrate-binding pocket of 3C proteases. A partially conserved ThrLys/Arg dipeptide located in the loop preceding the catalytic Cys is suggested to confer the primary cleavage specificity of 3C toward Glx/Gly(Ser) sites. These observations provide the first example of relatedness between proteases belonging, by definition, to different classes.     

A CLN2-related and thermostable serine-carboxyl proteinase, kumamolysin: cloning, expression, and identification of catalytic serine residue

The gene encoding kumamolysin, a thermostable pepstatin-insensitive carboxyl proteinase, was cloned and expressed. (i) Kumamolysin was synthesized as a large precursor consisting of two regions: amino-terminal prepro (188 amino acids) and mature proteins (384 amino acids). (ii) The deduced amino acid sequence of the mature region exhibited high similarity to those of such bacterial pepstatin-insensitive enzymes as Pseudomonas carboxyl proteinase (PSCP; EC 3.4.23.37, identity = 37%), Xanthomonas carboxyl proteinase (XCP; EC 3.4.23.33, identity = 36%), and human CLN2 gene product (identity = 36%), which is related to a fatal neurodegenerative disease. (iii) The presumed catalytic triad, Glu78, Asp82, Ser278 [three-dimensional structure of PSCP: Wlodawer, A. et al. (2001) Nature Struct. Biol., 8, 442-446], was found to be conserved in the amino acid sequence of kumamolysin. (iv) Kumamolysin was inactivated by such aldehyde-type inhibitors as Ac-Ile-Pro-Phe-CHO (K(i) = 0.7 0.14 microM). In PSCP, it has been clarified that these inhibitors form a hemiacetal linkage with the catalytic serine residue and inactivate the enzyme. (v) Mutational analysis of the Ser278 residue revealed that the mutant lost both auto-processing activity and proteolytic activity. These results strongly suggest that kumamolysin has a unique catalytic triad consisting of Glu78, Asp82, and Ser278 residues, as previously observed for PSCP.     

Chlorophyllase as a serine hydrolase: identification of a putative catalytic triad

Chlorophyllases (Chlases), cloned so far, contain a lipase motif with the active serine residue of the catalytic triad of triglyceride lipases. Inhibitors specific for the catalytic serine residue in serine hydrolases, which include lipases effectively inhibited the activity of the recombinant Chenopodium album Chlase (CaCLH). From this evidence we assumed that the catalytic mechanism of hydrolysis by Chlase might be similar to those of serine hydrolases that have a catalytic triad composed of serine, histidine and aspartic acid in their active site. Thus, we introduced mutations into the putative catalytic residue (Ser162) and conserved amino acid residues (histidine, aspartic acid and cysteine) to generate recombinant CaCLH mutants. The three amino acid residues (Ser162, Asp191 and His262) essential for Chlase activity were identified. These results indicate that Chlase is a serine hydrolase and, by analogy with a plausible catalytic mechanism of serine hydrolases, we proposed a mechanism for hydrolysis catalyzed by Chlase.     

Characterization of midgut trypsin-like enzymes and three trypsinogen cDNAs from the lesser grain borer, Rhyzopertha dominica (Coleoptera: Bostrichidae)

Protein digestion in the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), results from the action of a complex of serine proteinases present in the midgut. In this study we partially characterized trypsin-like enzyme activity against N-alpha-benzoyl-L-arginine p-nitroanilide (BApNA) in midgut preparations and cloned and sequenced three cDNAs for trypsinogen-like proteins. BApNAase activity in R. dominica midgut was significantly reduced by serine proteinase inhibitors and specific inhibitors of trypsin, whereas BApNAase activity was not sensitive to specific inhibitors of chymotrypsin or aspartic proteinases. However, trans-epoxysuccinyl-L-leucylamido-(4-guanidino) butane (E-64) inhibited BApNAase activity by about 30%. BApNAase was most active in a broad pH range from about pH 7 to 9.5. The gut of R. dominica is a tubular tract approximately 2.5 mm in length. BApNAase activity was primarily located in the midgut region with about 1.5-fold more BApNAase activity in the anterior region compared to that in the posterior region. Proteinases with apparent molecular masses of 23-24 kDa that were visualized on casein zymograms following electrophoresis were inhibited by TLCK. Three cDNAs for trypsinogen-like proteins were cloned and sequenced from mRNA of R. dominica midgut. The full cDNA sequences consisted of open reading frames encoding 249, 293, and 255 amino acid residues for RdoT1, RdoT2, and RdoT3, respectively. cDNAs RdoT1, RdoT2, and RdoT3 shared 77-81% sequence identity. The three encoded trypsinogens shared 54-62% identity in their amino acid sequences and had 16-18 residues of signal peptides and 12-15 residues of activation peptides. The three predicted mature trypsin-like enzymes had molecular masses of 23.1, 28, and 23.8 kDa for RdoT1, RdoT2, and RdoT3, respectively. Typical features of these trypsin-like enzymes included the conserved N-terminal residues IVGG62-65, the catalytic amino acid triad of serine proteinase active sites (His109, Asp156, Ser257), three pairs of conserved cysteine residues for disulfide bridges, and the three residues (Asp251, Gly274, Gly284) that determine specificity in trypsin-like enzymes. In addition, RdoT2 has both a PEST-like sequence at the C-terminus and a free Cys158 near the active site, suggesting instability of this enzyme and/or sensitivity to thiol reagents. The sequences have been deposited in GenBank database (accession numbers AF130840 for RdoT1, AF130841 for RdoT2, and AF130842 for RdoT3).     

The complete amino acid sequence of rat submaxillary gland tonin does contain the aspartic acid at the active site: confirmation by protein sequence analysis

The revised amino acid sequence of rat submaxillary gland tonin, a serine protease, does contain the active site Asp residue. The active site of this kallikrein-related enzyme is thus made up of the same catalytic triad (Asp, Ser, and His) found in all known serine proteases. The important Asp residue has now been localized in a 16 amino acid peptide previously reported as missing in the tonin sequence. The complete amino acid sequence thus contains 235 residues corresponding to a molecular weight of 25,658, more in agreement with previously reported molecular weights. Moreover, the revised structure led (a) to the assignment of Arg, Asn, and Val residues instead of His, Asp, and Gly at positions 63, 165, and 169, respectively; (b) to the assignment of residues occupying an overlapping sequence at positions 165-171, and finally (c) to the localization of two N-glycosylation sites at positions 82 and 165. These results further document the close relationship of tonin to the ever expanding kallikrein family.