Nucleotide sequence of cloned cDNA for human pancreatic kallikrein

Cloned cDNA sequences for human pancreatic kallikrein have been isolated and determined by molecular cloning and sequence analysis. The identity between human pancreatic and urinary kallikreins is indicated by the complete coincidence between the amino acid sequence deduced from the cloned cDNA sequence and that reported partially for urinary kallikrein. The active enzyme form of the human pancreatic kallikrein consists of 238 amino acids and is preceded by a signal peptide and a profragment of 24 amino acids. A sequence comparison of this with other mammalian kallikreins indicates that key amino acid residues required for both serine protease activity and kallikrein-like cleavage specificity are retained in the human sequence, and residues corresponding to some external loops of the kallikrein diverge from other kallikreins. Analyses by RNA blot hybridization, primer extension, and S1 nuclease mapping indicate that the pancreatic kallikrein mRNA is also expressed in the kidney and sublingual gland, suggesting the active synthesis of urinary kallikrein in these tissues. Furthermore, the tissue-specific regulation of the expression of the members of the human kallikrein gene family has been discussed.     

The crystal structure of the mouse glandular kallikrein-13 (prorenin converting enzyme).

A crystal structure of the serine protease, mouse glandular kallikrein 13 (mGK-13) has been determined at 2.6-A resolution. This enzyme, isolated from the mouse submandibular gland, is also known as prorenin-converting enzyme and cleaves submandibular gland Ren-2 prorenin to yield active renin. The mGK-13 structure is similar to other members of the mammalian serine protease family, having five conserved disulfide bonds and an active site located in the cleft between two beta-barrel domains. The mGK-13 structure reveals for the first time an ordered kallikrein loop conformation containing a short 3(10) helix. This loop is disordered in the related porcine pancreatic kallikrein and rat submandibular tonin structures. The kallikrein loop is in close spatial proximity to the active site and is also involved in a dimeric arrangement of mGK-13. The catalytic specificity of mGK-13 for Ren-2 prorenin was studied by modeling a prorenin-derived peptide into the active site of mGK-13. This model emphasizes two electronegative substrate specificity pockets on the mGK-13 surface, which could accommodate the dibasic P2 and P1 residues at the site of prorenin cleavage by mGK-13.     

Human kallikrein 6 activity is regulated via an autoproteolytic mechanism of activation/inactivation

Human kallikrein 6 (protease M/zyme/neurosin) is a serine protease that has been suggested to be a serum biomarker for ovarian cancer and may also be involved in pathologies of the CNS. The precursor form of human kallikrein 6 (pro-hK6) was overexpressed in Pichia pastoris and found to be autoprocessed to an active but unstable mature enzyme that subsequently yielded the inactive, self-cleavage product, hK6 (D81-K244). Site-directed mutagenesis was used to investigate the basis for the intrinsic catalytic activity and the activation mechanism of pro-hK6. A single substitution R80 --> Q stabilized the activity of the mature enzyme, while substitution of the active site serine (S197 --> A) resulted in complete loss of hK6 proteolytic activity and facilitated protein production. Our data suggest that the enzymatic activity of hK6 is regulated by an autoactivation/autoinactivation mechanism. Mature hK6 displayed a trypsin-like activity against synthetic substrates and human plasminogen was identified as a putative physiological substrate for hK6, as specific cleavage at the plasminogen internal bond S460-V461 resulted in the generation of angiostatin, an endogenous inhibitor of angiogenesis and metastatic growth.     

Immunological analysis of rat pancreatic prokallikrein activation

The present study shows that tissue kallikrein is present in rat pancreas as a proenzyme that can be converted by autolysis to a 38 000 Da active enzyme. The activation of pancreatic prokallikrein was examined by direct radioimmunoassay, enzymatic assays, active-site labeling with immunoprecipitation, and Western blot analyses. A monoclonal antibody (V1C3), which binds only active kallikrein, was used in a direct radioimmunoassay to monitor the appearance of the active enzyme. During a 22-h autolysis of pancreatic extract, a time-dependent increase in active kallikrein concentration paralleled the increase of kallikrein activities measured by both TosArgOMe esterase and kininogenase assays. The activation process was further analyzed by labeling the pancreatic extract with [14C]diisopropylphosphorofluoridate [( 14C]DFP) followed by immunoprecipitation with sheep anti-kallikrein antiserum. Pancreatic prokallikrein was not labeled by [14C]DFP; however, upon autolysis, a 38 000 Da active kallikrein can be labeled with [14C]DFP and increase in quantity with time. Western blot analysis, using a monoclonal antibody (V4D11) which recognizes both latent and active tissue kallikreins, identified a 39 000 Da pancreatic prokallikrein prior to autolysis and a 38 000 Da active kallikrein after 7 h of autolysis. The results indicate that the pancreatic prokallikrein exists as a 39 000 Da protein which may be converted to a 38 000 Da active kallikrein, indistinguishable from purified urinary, brain, spleen or submandibular gland kallikrein.     

Activation and enzymatic characterization of recombinant human kallikrein 8

Human kallikrein 8 (hK8), whose gene was originally cloned as the human ortholog of a mouse brain protease, is known to be associated with diseases such as ovarian cancer and Alzheimer's disease. Recombinant human pro-kallikrein 8 was activated with lysyl endopeptidase-conjugated beads. Amino-terminal sequencing of the activated enzyme demonstrated the cleavage of a 9-aa propeptide from the pro-enzyme. The substrate specificity of activated hK8 was characterized using synthetic fluorescent substrates. hK8 showed trypsin-like specificity, as predicted from sequence analysis and enzymatic characterization of the mouse ortholog. All synthetic substrates tested containing either arginine or lysine at P1 position were cleaved by hK8. The highest kcat/Km value of 20x10(3)M-1 s-1 was observed with Boc-Val-Pro-Arg-7-amido-4-methylcoumarin. The activity of hK8 was inhibited by antipain, chymostatin, and leupeptin. The concentration for 50% inhibition by the best inhibitor, antipain, was 0.46 microM. The effect of different metal ions on the enzyme activity was analyzed. Whereas Na+ had no effect on hK8 activity, Ni2+ and Zn2+ decreased the activity and Ca2+, Mg2+, and K+ had a stimulatory effect. Ca2+ was the best activator, with an optimal concentration of approximately 10 microM.     

Definition of the extended substrate specificity determinants for beta-tryptases I and II.

Tryptases betaI and betaII were heterologously expressed and purified in yeast to functionally characterize the substrate specificity of each enzyme. Three positional scanning combinatorial tetrapeptide substrate libraries were used to determine the primary and extended substrate specificity of the proteases. Both enzymes have a strict primary preference for cleavage after the basic amino acids, lysine and arginine, with only a slight preference for lysine over arginine. betaI and betaII tryptase share similar extended substrate specificity, with preference for proline at P4, preference for arginine or lysine at P3, and P2 showing a slight preference for asparagine. Measurement of kinetic constants with multiple substrates designed for beta-tryptases reveal that selectivity is highly dependent on ground state substrate binding. Coupled with the functional determinants, structural determinants of tryptase substrate specificity were identified. Molecular docking of the preferred substrate sequence to the three-dimensional tetrameric tryptase structure reveals a novel extended substrate binding mode that involves interactions from two adjacent protomers, including P4 Thr-96', P3 Asp-60B' and Glu-217, and P1 Asp-189. Based on the determined substrate information, a mechanism-based tetrapeptide-chloromethylketone inhibitor was designed and shown to be a potent tryptase inhibitor. Finally, the cleavage sites of several physiologically relevant substrates of beta-tryptases show consistency with the specificity data presented here.     

Characterization and purification of an outer membrane metalloproteinase from Pseudomonas aeruginosa with fibrinogenolytic activity.

A membrane proteinase from Pseudomonas aeruginosa, called insulin-cleaving membrane proteinase (ICMP), was located in the outer membrane leaflet of the cell envelope. The enzyme is expressed early in the logarithmic phase parallel to the bacterial growth during growth on peptide rich media. It is located with its active center facing to the outermost side of the cell, because its whole activity could be measured in intact cells. The very labile membrane proteinase was solubilized by non-ionic detergents (Nonidet P-40, Triton X-100) and purified in its amphiphilic form to apparent homogeneity in SDS-PAGE by copper chelate chromatography and two subsequent chromatographic steps on Red-Sepharose CL-4B (yield 58.3%, purification factor 776.3). It consisted of a single polypeptide chain with a molecular mass of 44.6 kDa, determined by mass spectrometry. ICMP was characterized to be a metalloprotease with pH-optimum in the neutral range. The ICMP readily hydrolyzed Glu(13)-Ala(14) and Tyr(16)-Leu(17) bonds in the insulin B-chain. Phe(25)-Tyr(26) and His(10)-Leu(11) were secondary cleavage sites suggesting a primary specificity of the enzyme for hydrophobic or aromatic residues at P'(1)-position. The ICMP differed from elastase, alkaline protease and LasA in its cleavage specificity, inhibition behavior and was immunologically diverse from elastase. The amino acid sequence of internal peptides showed no homologies with the known proteinases. This outer membrane proteinase was capable of specific cleavage of alpha and beta fibrinogen chains. Among the p-nitroanilide substrates tested, substrates of plasminogen activator, complement convertase and kallikrein with arginine residues in the P(1)-subsite were the substrates best accepted, but they were only cleaved at a very low rate.     

Characterization of a cholecystokinin 8-generating endoprotease purified from rat brain synaptosomes.

An endoproteolytic activity that specifically cleaves CCK 33, producing CCK 8, has been purified from a rat brain synaptosome preparation. The purification, which included anion exchange, chromatofocusing, hydroxyapatite, and gel filtration chromatography, resulted in a greater than 3000-fold increase in specific activity. This neutral endoprotease (pH optimum 8) exists as a 90-kDa species, which can be dissociated into active 40-kDa species. The enzyme is a non-trypsin serine protease, which is inhibited by diisopropyl-fluorophosphate and p-aminobenzamidine but not by soybean trypsin inhibitor, phenylmethylsulfonyl fluoride, aprotinin, or a number of thiol or metalloprotease inhibitors. It is highly substrate-specific and cleaves neither trypsin, enteropeptidase, kallikrein substrates, nor analogues of mono- or dibasic cleavage sites of prohormones other than pro-CCK. The endoprotease will not cleave CCK 12 desulfate or CCK (20-29), although these peptides contain common sequences with CCK-33. The protease does cleave [Glu27]CCK (20-29), a peptide in which the glutamate mimics the negative charge normally present on tyrosine sulfate. This suggests that the negative charge at position 27 is important in substrate recognition. The enzyme will also cleave CCK 33 and CCK (1-21) on the carboxyl-terminal side of a single lysine residue in position 11. The subcellular location and specificity of this endoprotease make it a good candidate for a CCK-processing protease.     

Enzymatic profiling of human kallikrein 14 using phage-display substrate technology

The human KLK14 gene is one of the newly identified serine protease genes belonging to the human kallikrein family, which contains 15 members. KLK14 , like all other members of the human kallikrein family, is predicted to encode for a secreted serine protease already found in various biological fluids. This new kallikrein is mainly expressed in prostate and endocrine tissues, but its function is still unknown. Recent studies have demonstrated that KLK14 gene expression is up-regulated in prostate and breast cancer tissues, and that higher expression levels correlate with more aggressive tumors. In this work, we used phage-display substrate technology to study the substrate specificity of hK14. A phage-displayed random pentapeptide library with exhaustive diversity was screened with purified recombinant hK14. Highly specific and sensitive substrates were selected from the library. We show that hK14 has dual activity, trypsin- and chymotrypsin-like, with a preference for cleavage after arginine residues. A SwissProt database search with selected sequences identified six potential human protein substrates for hK14. Two of them, laminin alpha-5 and collagen IV, which are major components of the extracellular matrix, have been demonstrated to be hydrolyzed efficiently by hK14.     

Cleavage Specificity Analysis of Six Type II Transmembrane Serine Proteases (TTSPs) Using PICS with Proteome-Derived Peptide Libraries

Background

Type II transmembrane serine proteases (TTSPs) are a family of cell membrane tethered serine proteases with unclear roles as their cleavage site specificities and substrate degradomes have not been fully elucidated. Indeed just 52 cleavage sites are annotated in MEROPS, the database of proteases, their substrates and inhibitors.

Methodology/Principal Finding

To profile the active site specificities of the TTSPs, we applied Proteomic Identification of protease Cleavage Sites (PICS). Human proteome-derived database searchable peptide libraries were assayed with six human TTSPs (matriptase, matriptase-2, matriptase-3, HAT, DESC and hepsin) to simultaneously determine sequence preferences on the N-terminal non-prime (P) and C-terminal prime (P’) sides of the scissile bond. Prime-side cleavage products were isolated following biotinylation and identified by tandem mass spectrometry. The corresponding non-prime side sequences were derived from human proteome databases using bioinformatics. Sequencing of 2,405 individual cleaved peptides allowed for the development of the family consensus protease cleavage site specificity revealing a strong specificity for arginine in the P1 position and surprisingly a lysine in P1′ position. TTSP cleavage between R↓K was confirmed using synthetic peptides. By parsing through known substrates and known structures of TTSP catalytic domains, and by modeling the remainder, structural explanations for this strong specificity were derived.

Conclusions

Degradomics analysis of 2,405 cleavage sites revealed a similar and characteristic TTSP family specificity at the P1 and P1′ positions for arginine and lysine in unfolded peptides. The prime side is important for cleavage specificity, thus making these proteases unusual within the tryptic-enzyme class that generally has overriding non-prime side specificity.     

Evaluation of the possible proteomic application of trypsin from Streptomyces griseus

Trypsin (EC 3.4.21.4) is the protease of choice for proteome analysis using mass spectrometry of peptides in sample digests. In this work, trypsin from Streptomyces griseus (SGT) was purified to homogeneity from pronase. The enzyme was evaluated in in-gel digestion of protein standards followed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) analyses of the digests. We recognized a remarkable cleavage performance of SGT. The number of produced and matching tryptic peptides was higher than in the case of commonly used bovine trypsin (BT) and allowed us to obtain higher identification scores in database searches. Interestingly, SGT was found to also generate nonspecific peptides whose sequencing by MALDI-TOF/TOF tandem mass spectrometry (MS/MS) revealed a partial F-X, Y-X, and W-X cleavage specificity. To suppress autolysis, either arginine or arginine plus lysine residues in SGT were modified by chemical reagents. In consequence, the autolytic pattern of SGT was reduced significantly, but specific activity dropped dramatically. As demonstrated by relative quantification of peptides at different times, SGT is more stable at 37 °C than is its bovine counterpart. We conclude that SGT represents a convenient alternative for proteomic applications involving protein digestion. Moreover, parallel digestions of sample aliquots by SGT and BT provide the possibility of combining partially different results (unique matching peptides) to improve protein identification.     

Enzymatic properties of human kallikrein-related peptidase 12 (KLK12)

Human kallikrein-related peptidase 12 (KLK12) is a new member of the human tissue kallikrein family. Preliminary studies suggest that KLK12 is differentially expressed in breast cancer and may have potential use as a cancer biomarker. It has been predicted that KLK12 is a secreted serine protease. However, the enzymatic properties of this protein have not been reported so far. Here, we report the production of recombinant KLK12 and analyses of its enzymatic characteristics, including zymogen activation, substrate specificity, and regulation of its activity. KLK12 is secreted as an inactive pro-enzyme, which is able to autoactivate to gain enzymatic activity. Through screening of a panel of fluorogenic and chromogenic peptide substrates, we establish that active KLK12 possesses trypsin-like activity, cleaving peptide bonds after both arginine and lysine. Active KLK12 quickly loses its activity due to autodegradation, and its activity can also be rapidly inhibited by zinc ions and by alpha2-antiplasmin through covalent complex formation. Furthermore, we demonstrate that KLK12 is able to activate KLK11 zymogen in vitro. Our results indicate that KLK12 may participate in enzymatic cascades involving other kallikreins.     

Replacement of lysine by arginine, phenylalanine and tryptophan in the reactive site of the bovine trypsin-kallikrein inhibitor (Kunitz) and change of the inhibitory properties.

The reactive-site sequence of a proteinase inhibitor can be written as . . . -P3-P2-P1-P'1-P'2-P'3- . . . , where-P1-P'1-denotes the reactive site. Three semisynthetic homologues have been synthesized of the bovine trypsin-kallikrein inhibitor (Kunitz) with either arginine, phenylalanine or tryptophan in place of the reactive-site residue P1, lysine-15. These homologues correspond to gene products after mutation of the lysine 15 DNA codon to an arginine, phenylalanine or tryptophan DNA codon. Starting from native (virgin) inhibitor, reactive-site hydrolyzed, still active (modified) inhibitor was prepared by chemical and enzymic reactions. Modified inhibitor was then converted into inactive des-Lys15-inhibitor by reaction with carboxypeptidase B. Inactive des-Lys15-inhibitor was reactivated by enzymic replacement of the P1 residue according to Leary and Laskowski, Jr. The introduction of arginine was catalyzed by an inverse reaction with carboxypeptidase B, while phenylalanine or tryptophan were replaced by carboxypeptidase A. The reactivated semisynthetic inhibitors were trapped by complex formation with either trypsin or chymotrypsin. The enzyme - inhibitor complexes were subjected to kinetic-control dissociation, and the semisynthetic virgin inhibitors were isolated. The inhibitory properties of the semisynthetic inhibitors have been investigated against bovine trypsin and chymotrypsin and against porcine pancreatic kallikrein and plasmin. The homologues with either lysine or arginine in the P1 position are equally good inhibitors of trypsin, plasmin and kallikrein. The Arg-15-homologue is a slightly more effective kallikrein inhibitor than the Lys15-inhibitor. The semisynthetic phenylalanine and tryptophan homologues, however, are weak inhibitors of trypsin and still weaker inhibitors of kallikrein, but are excellent inhibitors of chymotrypsin. Their association constant with chymotrypsin is at least ten times higher than that of native Lys-15-inhibitor. A dramatic specificity change is observed with the phenylalanine and tryptophan homologues, which in contrast to the native inhibitor do not at all inhibit porcine plasmin. Thus, the nature of the P1 residue strongly influences the primary inhibitory specificity of the bovine inhibitor (Kunitz).     

Induced fit activation mechanism of the exceptionally specific serine protease, complement factor D

We have investigated the mechanism by which the complement protease, Factor D, achieves its high specificity for the cleavage of Factor B in complex with C3(H2O). Kinetic experiments showed that Factor B and C3(H2O) associate with a KD of >/=2.5 microM and that Factor D acts on this complex with a second-order rate constant of kcat/KM >/= 2 x 10(6) M-1 s-1, close to the rate of a diffusion-controlled reaction for proteins of this size. In contrast, Factor D, which is a member of the trypsin family of serine proteases, was 10(3)-10(4)-fold less active than trypsin toward both thioester and p-nitroanilide substrates containing an arginine at P1. Furthermore, peptides spanning the Factor B cleavage site were not detectably cleaved by Factor D (kcat/KM /=9 kcal/mol of binding energy to stabilize the transition state for reaction. In support of this, we demonstrate that chemical modification of Factor D at a single lysine residue that is distant from the active site abolishes the activity of the enzyme toward Factor B while not affecting activity toward small synthetic substrates. We propose that Factor D may exemplify a special case of the induced fit mechanism in which the requirement for conformational activation of the enzyme results in a substantial increase in substrate specificity.     

Auto-inactivation by cleavage within the dimer interface of Kaposi's sarcoma-associated herpesvirus protease.

An autolysis site of functional and structural significance has been mapped within the dimer interface of Kaposi's sarcoma-associated herpesvirus protease. Cleavage 27 residues from the C terminus of the 230 amino acid residue, 25 kDa protein was observed to cause a loss of dimerization and proteolytic activity, even though no active site moieties were lost. Gel-filtration chromatography and analytical ultracentrifugation were used to analyze the changes in oligomerization upon autolysis. The selective auto-disruption of this essential protein-protein interface by proteolytic cleavage resulted in a 60 % loss in mean residue ellipticity by circular dichroism as well as a 20 % weaker, 10 nm red-shifted intrinsic protein fluorescence emission spectrum. These apparent conformational changes induced a strict inhibition of enzymatic activity. An engineered substitution at the P1' position of this cleavage site attenuated autolysis by the enzyme and restored wild-type dimerization. In addition to retaining full proteolytic activity in a continuous fluorescence-based enzyme assay, this protease variant allowed the determination of the enzyme's dimerization dissociation constant of 1.7 (+/-0.9) microM. The structural perturbations observed in this enzyme may play a role in viral maturation, and offer general insight into the allosteric relationship between the dimer interface and active site of herpesviral proteases. The functional coupling between oligomerization and activity presented here may allow for a better understanding of such phenomena, and the design of an enzyme variant stabilized to autolysis should further the structural and mechanistic characterization of this viral protease.     

The expression of two kallikrein gene family members in the rat kidney

The mRNAs for two kallikrein gene family members expressed in the rat kidney have been characterized. One mRNA (PS) has previously been found in the pancreas and submaxillary gland and encodes true kallikrein. The second mRNA (K1) encodes a novel kallikrein-like enzyme expressed in the kidney and submaxillary gland that retains many of the key amino acid residues for the characteristic enzymatic cleavage specificity of kallikrein. Two oligonucleotide hybridization probes specific for the K1 mRNA demonstrate that the K1 mRNA is expressed in the kidney and submaxillary gland, but in none of the other eight tissues known to express one or more members of the rat kallikrein gene family. The K1 mRNA is the dominant kallikrein-related mRNA of the kidney, expressed at roughly 10 times the level of the true kallikrein (PS) mRNA. In the submaxillary gland the K1 mRNA is expressed at roughly one-fourth the level of true kallikrein mRNA.     

Human kallikrein 4: enzymatic activity, inhibition, and degradation of extracellular matrix proteins

Human kallikrein 4 (hK4) is a member of the expanded family of human kallikreins, a group of 15 secreted proteases. While this protein has been associated with ovarian and prostate cancer prognosis, only limited functional information exists. Therefore, we have undertaken an investigation of its enzymatic properties regarding substrate preference, degradation of extracellular matrix proteins, and its inhibition by various inhibitors. We successfully expressed and purified active recombinant hK4 from supernatants of the Pichia pastoris expression system. This enzyme seems to cleave more efficiently after Arg compared to Lys at the P1 position and exhibits modest specificity for amino acids at positions P2 and P3. hK4 forms complexes with alpha1-antitrypsin, alpha2-antiplasmin and alpha2-macroglobulin. The protease mediates limited degradation of extracellular matrix proteins such as collagen I and IV, and more efficient degradation of the alpha-chain of fibrinogen. The cleavage of extracellular matrix proteins by hK4 suggests that this enzyme may play a role in tissue remodeling and cancer metastasis.     

Characterization of mouse glandular kallikrein 24 expressed in testicular Leydig cells

Mouse kallikrein 24 is thought to encode a functional serine protease belonging to the mouse glandular kallikrein gene family. Preliminary results suggest that this kallikrein may play a role in testis function in adult mice. In order to obtain insights into its physiological functions, we undertook molecular and biochemical analyses of this enzyme. We cloned a cDNA for kallikrein 24 from the adult mouse testis cDNA library. Kallikrein 24 was expressed in the kidney, submandibular glands, ovary, epididymis, and testis of the mouse. In the testis, kallikrein 24 mRNA was detectable at 4 weeks of postnatal development, and became more prominent thereafter. The kallikrein 24 gene was expressed exclusively in the Leydig cells of adult mice. When Leydig cells isolated from a 2-week-old mouse testis were cultured in the presence of testosterone, kallikrein 24 expression was induced. Active recombinant enzyme showed trypsin-like specificity, favorably cleaving Arg-X bonds of synthetic peptide substrates. The enzymatic activity was strongly inhibited by typical serine protease inhibitors. Mouse kallikrein 24 degraded casein, gelatin, fibronectin and laminin. These results suggest that the enzyme may play a role in the degradation of extracellular matrix proteins in the interstitial area surrounding the Leydig cells of the adult mouse testis. The present findings should contribute to future physiological studies of this mouse testis protease.     

Molecular analysis of the feline immunodeficiency virus protease: generation of a novel form of the protease by autoproteolysis and construction of cleavage-resistant proteases.

The feline immunodeficiency virus (FIV) protease is essential for virion maturation and subsequent viral replication in that it cleaves the Gag and Gag/Pol polyproteins at eight sites to release the respective structural proteins and enzymes. During purification of a recombinant FIV protease (PR), we noted that it underwent autoproteolysis (autolysis) to give discrete cleavage products. These additional PR cleavage sites were defined using N-terminal amino acid sequence analysis and mass spectrometry. Protease breakdown products were also found in FIV virions and were of the same apparent molecular weights as the in vitro autolysis products. Four primary PR autolysis sites were blocked via substitution of either the P1 amino acid with a beta-branched amino acid or the P1' amino acid with lysine. Cleavage-resistant PRs which had Km and k(cat) values similar to those of FIV PR were constructed. An autolysis time course determined that blocking all four primary autolysis sites yielded a cleavage-resistant PR which was enzymatically stable. Concomitant with autolysis is the generation of an N-terminally truncated form of the PR (Thr6/PR) which has enhanced stability with respect to that of FIV PR. A structural basis for the Thr6/PR activity is presented, as are the possible roles of autolysis in the viral replication cycle.     

Biochemical characterization of prostasin, a channel activating protease

Human prostasin was recently identified as a potential regulator of epithelial sodium channel (ENaC) function. Through the use of positional scanning combinatorial substrate libraries, prostasin was shown to have a preference for poly-basic substrates: in position P4 preference was for arginine or lysine; in P3 preference was for histidine, lysine or arginine; in P2 preference was for basic or large hydrophobic amino acids; and in P1 preference was for arginine and lysine. P1', P2', and P3' displayed broad selectivity with the exception of a lack of activity for isoleucine, and P4' had a preference for small, unbranched, amino acids such as alanine and serine. A prostasin-preferred poly-basic cleavage site was found in the extracellular domains of the ENaC alpha- and beta-subunits, and may present a mechanism for prostasin activation. The absence of activity seen with substrates containing isoleucine in position P1' explains the inability of prostasin to autoactivate and suggests that prostasin proteolytic activity is regulated by an upstream protease. Prostasin activity was highly influenced by mono- and divalent metal ions which were potent inhibitors and substrate specific modulators of enzymatic activity. In the presence of sub-inhibitory concentrations of zinc, the activity of prostasin increased several-fold and its substrate specificity was significantly altered in favor of a strong preference for histidine in positions P3 or P4 of the substrate.