Proteolytic Activation of Human Cathepsin A

Background: The human lysosomal protease cathepsin A requires proteolytic activation. Results: The crystal structure of mature and active cathepsin A reveals its mechanism of activation. Conclusion: Removal of a 3.3-kDa peptide (by unidentified proteases) allows substrate access to the active site. Significance: The results revise a 2-decades-old model of cathepsin A activation by proteolysis and subsequent conformational change. Galactosialidosis is a human lysosomal storage disease caused by deficiency in the multifunctional lysosomal protease cathepsin A (also known as protective protein/cathepsin A, PPCA, catA, HPP, and CTSA; EC 3.4.16.5). Previous structural work on the inactive precursor human cathepsin A (zymogen) led to a two-stage model for activation, where proteolysis of a 1.6-kDa excision peptide is followed by a conformational change in a blocking peptide occluding the active site. Here we present evidence for an alternate model of activation of human cathepsin A, needing only cleavage of a 3.3-kDa excision peptide to yield full enzymatic activity, with no conformational change required. We present x-ray crystallographic, mass spectrometric, amino acid sequencing, enzymatic, and cellular data to support the cleavage-only activation model. The results clarify a longstanding question about the mechanism of cathepsin A activation and point to new avenues for the design of mechanism-based inhibitors of the enzyme.

shown to play a role in chaperone-mediated autophagy by regulating degradation of the lysosome-associated membrane protein type 2 (LAMP2) receptor (10). Cathepsin A is expressed ubiquitously and acts on multiple peptide hormones, including endothelin-1 (11) and angiotensin I, highlighting its importance in endocrine regulation (12). Inhibitors against cathepsin A are currently in phase I clinical trials for hypertension because of its central role in vasoregulation (13,14). Additionally, cathepsin A is a major hydrolase for converting peptidic prodrugs to active compounds (15). Understanding the structure and function of the catalytically active mature protease is thus important for understanding cellular regulation and for the design and optimization of peptidic compounds with improved bioavailability via decreased proteolytic susceptibility.
Cathepsin A is synthesized as a zymogen, a single-chain precursor of 54 kDa (after signal peptide cleavage and addition of two N-linked glycans) that self-associates into a homodimer (Fig. 1a). In 1995, the crystal structure of the precursor cathepsin A showed each monomer with two domains, a core domain (1-182 and 303-452) interrupted by a cap domain insertion (183-302) (20 -22). The core domain has a ␣/␤-hydrolase fold containing the active site, as in other serine carboxypeptidases. The cap domain is further divided into a helical subdomain (183-253) important for dimerization and a mixed threestranded ␤-sheet maturation subdomain (254 -302). In the structure of the enzymatically inactive precursor cathepsin A, the catalytic triad residues (Ser 150 , His 429 , and Asp 372 ) have a catalytically competent arrangement at the base of the substrate-binding pocket. However, in that structure, catalysis is prevented by the occlusion of the substrate-binding pocket by two segments of the maturation subdomain originally described (20) as the blocking peptide (272-277) and the excision peptide (285-298) (Fig. 1).
The precursor cathepsin A structure led to a novel proposed activation mechanism, distinct from known activation mechanisms of serine-, cysteine-, metallo-, and aspartyl-proteases (20). Cathepsin A was proposed to mature by a two-step pro-cess involving cleavage of the 1.6-kDa excision peptide, followed by conformational changes in the blocking peptide of the maturation subdomain, allowing substrate access to the active site. In the proposed activation model, the 54-kDa monomer was processed into a large (32-kDa) and a small (20-kDa) subunit, covalently linked by two disulfide bonds (20,23). Nearly two decades later, the structure of mature cathepsin A bound to small-molecule inhibitors suggested a similar activation mechanism and described the removal of a 2-kDa excision peptide, although electron density was absent for a longer sequence (13). The maturation subdomain (254 -302) of precursor cathepsin A has multiple surface-exposed potential protease cleavage sites, lending support to the possibility that it is activated by proteolytic removal of a larger segment. To date, the longstanding question of the activation mechanism of human cathepsin A remains unclear.
Here we report the crystal structure of mature and catalytically active cathepsin A, suggesting an alternate model for activation. Using x-ray crystallography, mass spectrometry, amino acid sequencing, and cellular experiments, we show the activation of precursor cathepsin A is achieved by proteolytic removal of a larger 3.3-kDa peptide that includes the blocking peptide, bypassing the requirement for conformational changes. Our results suggest an activation mechanism of cathepsin A consistent with that of homologous carboxypeptidases such as CPW. Alanine scanning of residues in the processing region reveals alternative sites for proteolytic activation of cathepsin A. In addition, we show the importance of removal of the blocking peptide on carboxypeptidase activity of human cathepsin A, using a N-blocked dipeptide carbobenzoxycarbonyl-phenylalanine-leucine (Z-Phe-Leu) as substrate. The structure and enzymatic studies presented here clarify the regulatory mechanisms of cathepsin A and suggest new mechanisms for its control in therapeutic uses.

EXPERIMENTAL PROCEDURES
Molecular Biology-Human cathepsin A (CTSA) cDNA (National Center for Biotechnology Information Sequence ID: NM_000308.2) was purchased from Harvard Institute of Proteomics, DNA Resource Core. The complete cDNA including the native human signal sequence was cloned into the pIB/V5-HIS-TOPO-TA vector by gateway cloning methods. A His 6 tag was engineered after the C terminus of the gene, replacing the vector-encoded V5 and His 6 epitopes. Mutations in the CTSA gene were generated according to the Phusion site-directed mutagenesis kit (Thermo Scientific) by PCR amplification using primers coding for the corresponding base changes.
For expression in human cells, the CTSA gene was cloned into the p3ϫFLAG-CMV14 vector to encode C-terminally FLAG-tagged protein. Mutations in the CTSA gene were generated as above. CTSA constructs for human cell expression contained the nucleophile knock-out S150A.
Protein Expression and Purification-We expressed and purified cathepsin A in Tn5 (Trichoplusia ni) insect cells as a secreted protein. Adherent cultures were transfected with plasmid DNA in T25 flasks. After 2 days, treatment with 100 g/ml blasticidin for 10 days generated stable cell lines. For protein purification, cells were resuspended into 25 ml of homemade Disulfide bonds are depicted as brackets. b and c, each monomer of cathepsin A is composed of a core hydrolase domain containing the catalytic residues (spheres) and the cap domain. In the precursor, the active site is occluded by the blocking peptide. d, structure of mature cathepsin A dimer (purple) superimposes on the precursor cathepsin A (green) without conformational changes. Removal of the blocking peptide, excision peptide, and connecting sequence in the mature protease renders the active site accessible for substrate binding (surface representation). In the inset, 2F o Ϫ F c density from a A -weighted map contoured at 1 shows the 253-303 disulfide flanking the region processed during maturation.
Sfx-equivalent medium and then scaled up to 6 -10 liters in 3-liter Fernbach flasks (Corning). The cells were harvested at 3 ϫ 10 6 cells/ml and clarified by centrifugation. The spent medium was concentrated and buffer exchanged into Ni 2ϩ wash buffer (50 mM phosphate and 250 mM NaCl) by tangential flow filtration.
Cathepsin A was purified by affinity-and ion-exchange chromatography using an FPLC (Bio-Rad). Cathepsin A was partially purified by a Ni 2ϩ -Sepharose FF column (GE Healthcare) with elution buffer (50 mM phosphate, 250 mM NaCl, 250 mM imidazole). The fractions containing cathepsin A were pooled and concentrated, and buffer was exchanged into 50 mM sodium acetate, pH 5.1, and further purified on a SOURCE15 S anion-exchange column by a gradient of 0 -500 mM NaCl. The fractions containing pure cathepsin A were buffer-exchanged into 20 mM Tris-HCl, 150 mM NaCl and concentrated to 1 mg/ml for storage. The purified precursor cathepsin A was observed to undergo limited proteolysis over 1-3 weeks, presumably by small amounts of co-purified endogenous proteases.
Mammalian Cell Expression and Immunoprecipitation-Cathepsin A plasmid DNA in the p3ϫFLAG-CMV14 vector was transfected into HEK293T cells using Lipofectamine (Invitrogen) according to the manufacturer's protocol. Cells were harvested 36 h after transfection and rinsed in cell lysis buffer. Whole cell lysates were immunoprecipitated with anti-FLAG M2 magnetic beads (Sigma-Aldrich) for 1 h at 4°C. Beads were washed in buffer (50 mM Tris, 150 mM NaCl) and acid-eluted (100 mM sodium citrate, pH 3). Eluates were analyzed for proteolytic processing by Western blotting using anti-cathepsin A antibody (Rockland Immunochemicals) to detect precursor and large subunit cathepsin A. The small subunit of cathepsin A was detected by analyzing the whole cell lysate by Western blotting using anti-FLAG antibody.
Crystallography-Cathepsin A was purified and concentrated to 4 mg/ml for sitting drops. Crystals grew in 0.1 M sodium formate, pH 7.0 (or 0.1 M ammonium tartrate), 10% (w/v) polyethylene glycol 3350 in 7 days. Crystals were flashcooled in liquid nitrogen, and x-ray data were collected at the microfocus beamline NE-CAT 24-ID-C at Argonne National Laboratory. X-ray images were processed in HKL2000 (24) and phased by molecular replacement using the cathepsin A precursor (PDB ID 1IVY) in PHASER (25). TRUNCATE (26) reported statistics suggestive of nearly full merohedral twinning in crystal Native1, so crystal Native2 (untwinned but lower resolution) was used to confirm the structure independently. Model building and validation were done in Coot (27) and CCP4i (28), respectively. REFMAC5 (29) was used for refinement. For the Native1 twinned data, amplitude-based twin refinement was used during refinement in REFMAC5 (Table 1).
Mass Spectrometry-Insect cell-expressed cathepsin A glycoprotein was subjected to enzymatic deglycosylation with endoglycosidase H and peptide-N-glycosidase F. However, the carbohydrate was resistant to enzymatic removal (presumably because of fucosylation) (30,31), leading to poor results in electrospray ionization experiments. The glycosylated protein was subsequently used for Matrix-assisted laser desorption/ionization (MALDI) analysis on a Bruker OmniFlex MALDI-TOF MS in linear mode to determine masses. Protein samples at differ-ent times after purification were buffer-exchanged into 50 mM ammonium acetate, pH 7.4, and mixed with 50 mg/ml sinapic acid in 50% acetonitrile and 0.5% trifluoroacetic acid.
N-terminal Sequencing-Mature cathepsin A was concentrated and run on a reducing SDS-polyacrylamide gel. The proteins were transferred to PVDF membrane (Immobilon) and visualized by staining with GelCode Blue (Pierce). The band corresponding to the small subunit (ϳ20 kDa) was cut out, washed, dried, and sequenced by 10 cycles of Edman degradation (Harvard Microchemistry) with 13 pmol of Ser 293 detected.
Activity Assays-Carboxypeptidase activity of cathepsin A was measured using Z-Phe-Leu (Sigma) substrate in a two-step assay. Z-Phe-Leu cleavage by cathepsin A leads to a leucine primary amine, which was quantified in a modified trinitrobenzene sulfonate assay (32). Briefly, 40 nM mature cathepsin A was added to assay buffer (0.1 M NaOAc and 0.15 M NaCl, pH 4.5) containing 0.01-1.0 mM Z-Phe-Leu. Every 15 s for 2 min, 10 l of the reaction was added to 10 l of trinitrobenzene sulfonate (2 mM in 0.2 M sodium borate, pH 9.8). After 30 min at 27°C, 80 l of 3 mM NaS 2 O 3 in 0.2 M KH 2 PO 4 , pH 4.2, was added. Absorbance at 420 nm was compared with controls lacking enzyme and/or substrate and calibrated against a standard curve using leucine. The pH profile of enzyme activity was determined by varying assay buffers from pH 3.5 to pH 7.0, and the resulting plot was fit where K 1 and K 2 are the K a values for the relevant ionizable groups (33). Kinetic parameters were determined by measuring the rate of reaction using 0.01-1.0 mM Z-Phe-Leu in quadruplicate measurements. K m and k cat were extracted from fitting of Michaelis-Menten hyperbolae in KaleidaGraph.
To study the activation of cathepsin A, carboxypeptidase activity was measured at multiple time points during matura- tion. Briefly, cathepsin A purified from insect cells was concentrated and incubated at 4°C. Kinetic parameters were measured using 0.01-1.0 mM substrate concentrations of Z-Phe-Leu (as described above) at various time points (0 -21 days). The processing of cathepsin A at each time point was measured by quantifying band intensities on a reducing SDS-polyacrylamide gel using the GeneTools software (Syngene).

RESULTS
The Precursor and Mature Structures of Cathepsin A Superimpose, without Need for a Conformational Change-To understand the activation of cathepsin A by conversion of precursor to mature protease, we determined the structure of recombinant insect cell-expressed mature cathepsin A by x-ray crystallography to 2.8 Å resolution (Fig. 1d). The structure of mature cathepsin A superimposes with the previously reported structure of precursor cathepsin A, with a root mean square deviation (r.m.s.d.) of 0.55 Å for all C␣ atoms. The core domains (1-182 and 303-452) of mature and precursor cathepsin A superimpose with an r.m.s.d. of 0.28 Å on 332 C␣ atoms. The cap domains (183-302) of precursor and mature cathepsin A also superimpose well, with a r.m.s.d. of 0.32 Å, indicating that no substantial conformational changes take place after maturation. As in the precursor cathepsin A structure, each monomer of the dimeric protease has four disulfide bonds and two N-linked carbohydrates per monomer (Asn 117 and Asn 305 ). The carbohydrate at Asn 117 participates in a crystal contact, making crystallization difficult.
As expected for an active mature cathepsin A, electron density for the excision peptide (285-297) is not observed in the structure of mature cathepsin A. However, a much larger region (259 -297, including the putative blocking peptide 272-277) is missing from the electron density, suggesting that the excision peptide might be larger than previously hypothesized (Fig. 1d). In the structure of mature cathepsin A, the absence of the blocking and excision peptides results in a completely solvent accessible substrate-binding pocket. No significant conformational changes are observed in the residues involved in FIGURE 2. The active site residues in the structure of mature cathepsin A are competent for catalysis. The substrate-binding residues are matched to those of wheat carboxypeptidase complex with benzylsuccinic acid (PDB 1WHS). a, superposition of precursor (green) and mature cathepsin A (purple) structures shows no conformational changes in the terminal carboxylate binding site (blue), S1Ј-binding site (orange), the S1-binding site (cyan) of the mature protease, or the residues adjoining the conserved disulfide bond (Cys 253 to Cys 303 ) proximal to the cleavage sites. b, no conformational changes are observed in the structure of mature cathepsin A for these residues on inhibitor binding (light brown, PDB 4AZ0). c and d, superimposition of yeast (gray) and wheat (pale green) carboxypeptidases on mature cathepsin A (purple) shows conservation of the substrate binding and catalytic residues across these homologous proteins. The disulfide bond corresponding to Cys 253 to Cys 303 in cathepsin A is conserved among the three proteases. Wheat carboxypeptidase undergoes cleavage at the region adjoining the disulfide bond without any conformational change, similar to mature cathepsin A. catalysis or substrate binding between the precursor and mature forms (Fig. 2) The Excision Peptide Is Larger Than 1.6 kDa-The crystal structure of mature cathepsin A suggested that the number of residues removed during maturation might be larger than the previously proposed 13 residues (285-298, 1.6 kDa). To test this possibility, we subjected cathepsin A to SDS-PAGE and mass spectrometry analysis at different stages in the maturation process (Fig. 3). Reducing SDS-PAGE analysis of cathepsin A shows the processing of the single-chain precursor polypeptide into the two-chain mature form as a function of time. MALDI mass spectrometry of cathepsin A revealed changes in mass as the protein matured. The MALDI spectrum of purified cathepsin A showed primarily a 54.4-kDa glycoprotein for the zymogen (single-chain) and a 51.1-kDa glycoprotein (large plus small subunits linked by disulfide bonds) at complete maturation, with a mixture of the two during intermediate stages. The difference in the masses of the precursor and mature cathepsin A samples in the mass spectrometer reveal the size of the peptide removed during processing is 3.3 Ϯ 0.2 kDa (Fig. 3).
The Precursor Is Cleaved at Ser 293 to Form the Small Subunit of the Mature Protease-Proteolytic processing of precursor cathepsin A into mature cathepsin A produces a new N-terminal residue in the small subunit. To identify the precise proteolytic location during maturation, the small subunit of mature cathepsin A was N-terminally sequenced by Edman degradation. The sequencing unambiguously identifies Ser 293 as the first residue of the small subunit (Fig. 4a). Because the mass spectrometry shows the excision peptide is 3.3 kDa and the first residue of the small subunit is Ser 293 , we estimate that the other cleavage reaction occurs after Arg 262 or Lys 265 (Fig. 4a). Removal of the 263-292 or 266 -292 peptide would result in a mass reduction of 3.6 kDa or 3.2 kDa, respectively, within the error estimate of the mass spectrometry measurement on the glycoprotein (3.3 Ϯ 0.2 kDa).
Arg 292 immediately precedes Ser 293 , suggesting processing by a trypsin-like protease. In contrast, Arg 298 (which had been previously proposed to be removed during maturation) is clearly seen in the electron density maps of mature cathepsin A. Leupeptin reduced the proteolytic processing of cathepsin A, 3 consistent with processing by a trypsin-like protease. In an effort to suppress cleavage of the excision peptide, we expressed and purified a cathepsin A double mutant R262A/R292A, which also undergoes processing to form large and small subunits, suggesting alternative avenues for the maturation of cathepsin A. Additionally, to suppress cleavage of the predicted 1.6-kDa excision peptide, we generated a triple mutant form of cathepsin A, changing the two previously proposed cleavage sites (Arg 284 and Arg 298 ) and the catalytic nucleophile (Ser 150 ) to alanine. The cathepsin A triple mutant S150A/R284A/R298A also undergoes cleavage into large and small subunits, comparable with the wildtype cathepsin A (Fig. 4b), suggesting that these sites are not mandatory for the activation of cathepsin A.
To examine maturation of cathepsin A in human cells, we expressed recombinant human cathepsin A constructs in HEK293T cells. Cathepsin A expressed in human cells mimicked that expressed in insect cells, undergoing processing into large and small subunits. We replaced Arg and Lys residues in the processing region (on either side of the blocking peptide) with alanines, and the resulting proteins were analyzed for proteolytic processing. Seven cathepsin A variants expressed in HEK293T cells were processed into large and small subunits (Fig. 4c), suggesting that multiple processing sites can lead to mature cathepsin A. Amino acid sequencing results, alaninescanning mutagenesis, and protease inhibition all point to the specificity of the maturation protease as trypsin-like. 3 N. K. and S. C. G., unpublished data.

Enzymatic Activity Increases with Maturation of Cathepsin A to the 51.1-kDa Form-Cathepsin
A is a serine carboxypeptidase that cleaves terminal amino acids from oligopeptides in the lysosome (7,8). The carboxypeptidase activity has been studied previously with endogenous mature cathepsin A purified from placental tissue using the synthetic dipeptide Z-Phe-Leu (34). To test the activity of the insect cell-expressed mature cathepsin A activated by removal of blocking and excision peptides using endogenous proteases, Z-Phe-Leu was used to measure K m and k cat . The carboxypeptidase activity was measured by detecting the amount of Leu released using the amine reactive reagent trinitrobenzene sulfonate. Cathepsin A activity was highest at acidic pH (Fig. 5), as is common for lysosomal enzymes. Recombinant human cathepsin A showed a K m of 0.04 mM and a k cat of 12.1 s Ϫ1 , similar to previously reported values for endogenous human placental cathepsin A (0.07 mM K m and 35 s Ϫ1 k cat ) against a comparable substrate (Fig. 5).
We examined the activity of cathepsin A as a function of its proteolytic maturation. The maturation of full-length zymogen into cleaved mature form was followed over time and quantified by SDS-PAGE. At each maturation time point, the full kinetic profile of the carboxypeptidase activity of cathepsin A (0.01-1.0 mM substrate concentrations) was measured (Fig. 5, d  and e). The increase in carboxypeptidase activity of cathepsin A mirrors the amount of proteolytically cleaved cathepsin A. Conversion of the intermediate form to the completely pro-cessed mature protease with large (31-kDa) and small  subunits results in maximal activity.

DISCUSSION
Cathepsin A undergoes processing of the inactive zymogen to generate an active mature protease. The structure of the cathepsin A zymogen led to the hypothesis that enzymatic activity required cleavage of a 1.6-kDa peptide followed by conformational changes in the cap domain. The mass spectrometry, N-terminal sequencing, crystallographic, and cellular data presented here are inconsistent with the conformational change model of cathepsin A maturation. Instead, we propose an alternate model, where removal of a larger excision peptide (including the blocking peptide), allows direct access of substrate to the active site (Fig. 6). Maximal cathepsin A activity is attained after removal of both the blocking and excision peptides.
Proteases are generally synthesized as inactive zymogens that undergo proteolytic activation to form the active mature enzyme, a mechanism allowing strict control of catalytic activity. Proteases use a variety of mechanisms for controlling activation. Some (e.g. lipases and caspases) undergo significant conformational changes or dimerization to generate a viable active site. In others (e.g. chymotrypsin and trypsin), segments of the zymogen are removed either by autocatalysis or limited proteolysis, followed by conformational changes to form the functional active site. In others (e.g. wheat and metallo-car- . a, the blocking peptide is removed during maturation. Electron density is absent for the 259 -297 sequence (gray) in mature cathepsin A (purple) but appears in the precursor structure (green). N-terminal sequencing of the mature cathepsin A small subunit and MALDI analysis suggest two potential excision peptides (yellow), option 1 (266 -292, 3.2 kDa) and option 2 (266 -292, 3.6 kDa). The molecular mass of the excision peptide originally proposed (20) in 1995 (1.6 kDa, yellow) is inconsistent with mass spectrometric and amino acid sequencing of the small subunit of mature cathepsin A. b and c, cathepsin A can be activated by cleavage at multiple cleavage sites. b, suppressing cleavage at the sites predicted in a by expressing and purifying Ala variants of cathepsin A (R262A/R292A and R284A/R298A/S150A) also generates mature cathepsin A, suggesting processing at alternative sites. Wild-type and variants all undergo cleavage of the precursor to form mature large and mature small subunits. Lanes 1-3 are 12% acrylamide, and lane 4 is 10%. c, alanine scanning shows trypsin-like protease cleavage sites in human cells. Immunoprecipitation (IP) of cathepsin A variants around the blocking peptide (in the nucleophile knock-out background) in HEK293T cells with anti-FLAG antibody indicates cleavage of the precursor to matured large and small subunits. Immunoprecipitation using anti-FLAG antibody was Western blotted (WB) with anti-cathepsin A antibody (top panel), and lysates were Western blotted with anti-FLAG antibody (bottom panel).
boxypeptidases and many other cathepsins), the active site is preformed, and activation requires only limited proteolysis. Our data suggest that cathepsin A is activated in a manner similar to the wheat and metallo-carboxypeptidases and other cathepsins.
The structure of the mature protease shows no appreciable conformational changes with respect to the zymogen structure. N-terminal sequencing confirms that Ser 293 is the first residue of the small subunit, and mass spectrometry analysis indicated the cleaved peptide to be 3.3 kDa, suggesting the second cleavage site after Arg 262 or Lys 265 . These results show that residues 263/266 to 292 are not present in the mature protease, and the short segments 259 to 262/265 and 293 to 296 are unstructured. A disulfide bond (Cys 253 -Cys 303 ), conserved across family members, connects the polypeptide chain on either side of the cleaved peptide, presumably preventing further processing of the mature cathepsin A. In the mature cleaved form of cathepsin A, this disulfide bond stabilizes the interface between large and small subunits. Additionally, mature cathepsin A resists C-terminal degradation by itself or other carboxypeptidases by two proline residues, Pro 258 in the large subunit and Pro 451 in the small subunit. The mature cathepsin A is a stable protein that retains full enzymatic activity for months after purification.
Our data indicate that specific cleavage sites on cathepsin A lead to its maturation. However, alternative processing sites are also possible. In both biochemical and human cellular experiments, substitution of the proposed cleavage sites with alanines still allowed processing of cathepsin A. The maturation subdomain (254 -302) has multiple recognition sites for trypsinlike and cathepsin (B, D, K and L) proteases, as predicted by PROSPER (35) and SitePrediction (36). Cleavage at a number of these sites can potentially remove the excision and blocking peptides to activate cathepsin A. For instance, human cathepsin A can be treated in vitro with trypsin and cathepsin L to generate mature cathepsin A with catalytic activity (13,23). Additionally, other cathepsins (beyond cathepsin A) can be activated by a variety of proteases (37). For example, cathepsin D can be activated by multiple cellular proteases with cleavage at alternate sites (38). Our data suggest that cathepsin A matures via a mechanism shared by other cathepsins (and does not have a unique maturation mechanism, as proposed in 1995) (20). Because tissue distribution of cellular proteases varies widely, susceptibility to multiple proteases potentially allows for regulation of cathepsin A activity across many cell types.
Our data indicate the complete removal of the blocking peptide in the mature cathepsin A generated by endogenous insect cell proteases. The enzymatic properties of mature human cathepsin A from insect cells are comparable with those of endogenous mature cathepsin A purified from placental tissue, suggesting full activation of the insect cell-expressed cathepsin A. We monitored the activity of cathepsin A during its proteolytic maturation, revealing that the activity increases as the precursor and intermediates are processed to the final mature form. A second proteolytic cleavage, which results in the removal of the blocking and excision peptides, is required for the maximal activation of cathepsin A.
The importance of cathepsin A in prodrug conversion and cardiac regulation indicates its potential as a drug target, and inhibitors of cathepsin A are currently in clinical trials. Given longstanding uncertainty in the activation mechanism of cathepsin A and its central role in human disease, the results presented here can assist in the design of mechanism-based inhibitors for use as pharmaceuticals.