Protease trafficking in two primitive eukaryotes is mediated by a prodomain protein motif

17-AAG-Induced Activation of the Autophagic Pathway in Leishmania Is Associated with Parasite Death

The heat shock protein 90 (Hsp90) is thought to be an excellent drug target against parasitic diseases. The leishmanicidal effect of an Hsp90 inhibitor, 17-N-allylamino-17-demethoxygeldanamycin (17-AAG), was previously demonstrated in both in vitro and in vivo models of cutaneous leishmaniasis. Parasite death was shown to occur in association with severe ultrastructural alterations in Leishmania, suggestive of autophagic activation. We hypothesized that 17-AAG treatment results in the abnormal activation of the autophagic pathway, leading to parasite death. To elucidate this process, experiments were performed using transgenic parasites with GFP-ATG8-labelled autophagosomes. Mutant parasites treated with 17-AAG exhibited autophagosomes that did not entrap cargo, such as glycosomes, or fuse with lysosomes. ATG5-knockout (Δatg5) parasites, which are incapable of forming autophagosomes, demonstrated lower sensitivity to 17-AAG-induced cell death when compared to wild-type (WT) Leishmania, further supporting the role of autophagy in 17-AAG-induced cell death. In addition, Hsp90 inhibition resulted in greater accumulation of ubiquitylated proteins in both WT- and Δatg5-treated parasites compared to controls, in the absence of proteasome overload. In conjunction with previously described ultrastructural alterations, herein we present evidence that treatment with 17-AAG causes abnormal activation of the autophagic pathway, resulting in the formation of immature autophagosomes and, consequently, incidental parasite death.

Induction of autophagy increases the proteolytic activity of reservosomes during Trypanosoma cruzi metacyclogenesis

Cruzipain, the major cysteine protease of the pathogenic protozoa Trypanosoma cruzi, is an important virulence factor that plays a key role in the parasite nutrition, differentiation and host cell infection. Cruzipain is synthesized as a zymogen, matured, and delivered to reservosomes. These organelles that store proteins and lipids ingested by endocytosis undergo a dramatic decrease in number during the metacyclogenesis of T. cruzi. Autophagy is a process that digests the own cell components to supply energy under starvation or different stress situations. This pathway is important during cell growth, differentiation and death. Previously, we showed that the autophagy pathway of T. cruzi is induced during metacyclogenesis. This work aimed to evaluate the participation of macroautophagy/autophagy in the distribution and function of reservosomes and cruzipain during this process. We found that parasite starvation promotes the cruzipain delivery to reservosomes. Enhanced autophagy increases acidity and hydrolytic activity in these compartments resulting in cruzipain enzymatic activation and self- processing. Inhibition of autophagy similarly impairs cruzipain traffic and activity than protease inhibitors, whereas mutant parasites that exhibit increased basal autophagy, also display increased cruzipain processing under control conditions. Further experiments showed that autophagy induced cruzipain activation and self-processing promote T. cruzi differentiation and host cell infection. These findings highlight the key role of T. cruzi autophagy in these processes and reveal a potential new target for Chagas disease therapy.Abbreviations: Baf: bafilomycin A1; CTE: C-terminal extension; Cz: cruzipain; IIF: indirect immunofluorescence; K777: vinyl sulfone with specific Cz inhibitory activity; Prot Inh: broad-spectrum protease inhibitor; Spa1: spautin-1; Wort: wortmannin.

Structure, interdomain dynamics, and pH-dependent autoactivation of pro-rhodesain, the main lysosomal cysteine protease from African trypanosomes

Rhodesain is the lysosomal cathepsin L-like cysteine protease of Trypanosoma brucei rhodesiense, the causative agent of Human African Trypanosomiasis. The enzyme is essential for the proliferation and pathogenicity of the parasite as well as its ability to overcome the blood-brain barrier of the host. Lysosomal cathepsins are expressed as zymogens with an inactivating prodomain that is cleaved under acidic conditions. A structure of the uncleaved maturation intermediate from a trypanosomal cathepsin L-like protease is currently not available. We thus established the heterologous expression of T. brucei rhodesiense pro-rhodesain in Escherichia coli and determined its crystal structure. The trypanosomal prodomain differs from nonparasitic pro-cathepsins by a unique, extended α-helix that blocks the active site and whose side-chain interactions resemble those of the antiprotozoal inhibitor K11777. Interdomain dynamics between pro- and core protease domain as observed by photoinduced electron transfer fluorescence correlation spectroscopy increase at low pH, where pro-rhodesain also undergoes autocleavage. Using the crystal structure, molecular dynamics simulations, and mutagenesis, we identify a conserved interdomain salt bridge that prevents premature intramolecular cleavage at higher pH values and may thus present a control switch for the observed pH sensitivity of proenzyme cleavage in (trypanosomal) CathL-like proteases.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Recruited lysosomal enzymes as major digestive enzymes in insects

The mass recruitment to the midgut contents of lysosomal proteolytic enzymes occurred in insects under three major selective pressures. Hemipteran (true bugs, aphids, and cicadas) ancestors lost their serine peptidases (SP) on adapting to feed on protein-free plant sap. When they returned to protein diets, their cathepsins L and B were recruited to replace their lost SP. Among beetles of the series Cucujiformia, cathepsins L were recruited to hydrolyze ingested plant inhibitors that affect their major SP and/or to deal with special seed proteins, such as prolamins. Larval flies have a very acid middle midgut and use cathepsin D to digest bacteria from their infected food. All the recruited enzymes originated from duplicated genes. The recruited digestive enzymes differ from their lysosomal counterparts in critical regions of their amino acid sequences that resulted in changes in substrate specificities and other kinetic properties. The discharge of digestive cathepsins in the midgut contents, instead of lysosomes, seems to be a consequence of their overexpression or the existence of new targeting signals. Their activation at the midgut contents occurs by an autoactivation mechanism or with the help of other enzymes or by a combination of both. The targeting to lysosomes of the insect lysosomal enzymes does not follow the mammalian mannose 6-phosphate route, but an incompletely known mechanism.

Active subsite properties, subsite residues and targeting to lysosomes or midgut lumen of cathepsins L from the beetle Tenebrio molitor

Cathepsins L are the major digestive peptidases in the beetle Tenebrio molitor. Two digestive cathepsins L (TmCAL2 and TmCAL3) from it had their 3D structures solved. The aim of this paper was to study in details TmCAL3 specificity and properties and relate them to its 3D structure. Recombinant TmCAL3 was assayed with 64 oligopeptides with different amino acid replacements in positions P2, P1, P1' and P2'. Results showed that TmCAL3 S2 specificity differs from the human enzyme and that its specificities also explain why on autoactivation two propeptide residues remain in the enzyme. Data on free energy of binding and of activation showed that S1 and S2' are mainly involved in substrate binding, S1' acts in substrate binding and catalysis, whereas S2 is implied mainly in catalysis. Enzyme subsite residues were identified by docking with the same oligopeptide used for kinetics. The subsite hydrophobicities were calculated from the efficiency of hydrolysis of different amino acid replacements in the peptide and from docking data. The results were closer for S1 and S2' than for S1' and S2, indicating that the residue subsites that were more involved in transition state binding are different from those binding the substrate seen in docking. Besides TmCAL1-3, there are nine other cathepsins L, most of them more expressed at midgut. They are supposed to be directed to lysosomes by a Drosophila-like Lerp receptor and/or motifs in their prodomains. The mannose 6-phosphate lysosomal sorting machinery is absent from T. molitor transcriptome. Cathepsin L direction to midgut contents seems to depend on overexpression.

The Lys-Asp-Tyr Triad within the Mite Allergen Der p 1 Propeptide Is a Critical Structural Element for the pH-Dependent Initiation of the Protease Maturation

The major house dust mite allergen, Der p 1, is a papain-like cysteine protease expressed as an inactive precursor, proDer p 1, carrying an N-terminal propeptide with a unique structure. The maturation of the zymogen into an enzymatically-active form of Der p 1 is a multistep autocatalytic process initiated under acidic conditions through conformational changes of the propeptide, leading to the loss of its inhibitory ability and its subsequent gradual cleavage. The aims of this study were to characterize the residues present in the Der p 1 propeptide involved in the initiation of the zymogen maturation process, but also to assess the impact of acidic pH on the propeptide structure, the activity of Der p 1 and the fate of the propeptide. Using various complementary enzymatic and structural approaches, we demonstrated that a structural triad K17p-D51p-Y19p within the N-terminal domain of the propeptide is essential for its stabilization and the sensing of pH changes. Particularly, the protonation of D51p under acidic conditions unfolds the propeptide through disruption of the K17p-D51p salt bridge, reduces its inhibition capacity and unmasks the buried residues K17p and Y19p constituting the first maturation cleavage site of the zymogen. Our results also evidenced that this triad acts in a cooperative manner with other propeptide pH-responsive elements, including residues E56p and E80p, to promote the propeptide unfolding and/or to facilitate its proteolysis. Furthermore, we showed that acidic conditions modify Der p 1 proteolytic specificity and confirmed that the formation of the first intermediate represents the limiting step of the in vitro Der p 1 maturation process. Altogether, our results provide new insights into the early events of the mechanism of proDer p 1 maturation and identify a unique structural triad acting as a stabilizing and a pH-sensing regulatory element.

Glycosome turnover in Leishmania major is mediated by autophagy

Autophagy is a central process behind the cellular remodeling that occurs during differentiation of Leishmania, yet the cargo of the protozoan parasite's autophagosome is unknown. We have identified glycosomes, peroxisome-like organelles that uniquely compartmentalize glycolytic and other metabolic enzymes in Leishmania and other kinetoplastid parasitic protozoa, as autophagosome cargo. It has been proposed that the number of glycosomes and their content change during the Leishmania life cycle as a key adaptation to the different environments encountered. Quantification of RFP-SQL-labeled glycosomes showed that promastigotes of L. major possess ~20 glycosomes per cell, whereas amastigotes contain ~10. Glycosome numbers were significantly greater in promastigotes and amastigotes of autophagy-defective L. major Δatg5 mutants, implicating autophagy in glycosome homeostasis and providing a partial explanation for the previously observed growth and virulence defects of these mutants. Use of GFP-ATG8 to label autophagosomes showed glycosomes to be cargo in ~15% of them; glycosome-containing autophagosomes were trafficked to the lysosome for degradation. The number of autophagosomes increased 10-fold during differentiation, yet the percentage of glycosome-containing autophagosomes remained constant. This indicates that increased turnover of glycosomes was due to an overall increase in autophagy, rather than an upregulation of autophagosomes containing this cargo. Mitophagy of the single mitochondrion was not observed in L. major during normal growth or differentiation; however, mitochondrial remnants resulting from stress-induced fragmentation colocalized with autophagosomes and lysosomes, indicating that autophagy is used to recycle these damaged organelles. These data show that autophagy in Leishmania has a central role not only in maintaining cellular homeostasis and recycling damaged organelles but crucially in the adaptation to environmental change through the turnover of glycosomes.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Development of novel prime-boost strategies based on a tri-gene fusion recombinant L. tarentolae vaccine against experimental murine visceral leishmaniasis

Visceral leishmaniasis (VL) is a vector-borne disease affecting humans and domestic animals that constitutes a serious public health problem in many countries. Although many antigens have been examined so far as protein- or DNA-based vaccines, none of them conferred complete long-term protection. The use of the lizard non-pathogenic to humans Leishmania (L.) tarentolae species as a live vaccine vector to deliver specific Leishmania antigens is a recent approach that needs to be explored further. In this study, we evaluated the effectiveness of live vaccination in protecting BALB/c mice against L. infantum infection using prime-boost regimens, namely Live/Live and DNA/Live. As a live vaccine, we used recombinant L. tarentolae expressing the L. donovani A2 antigen along with cysteine proteinases (CPA and CPB without its unusual C-terminal extension (CPB^-CTE)) as a tri-fusion gene. For DNA priming, the tri-fusion gene was encoded in pcDNA formulated with cationic solid lipid nanoparticles (cSLN) acting as an adjuvant. At different time points post-challenge, parasite burden and histopathological changes as well as humoral and cellular immune responses were assessed. Our results showed that immunization with both prime-boost A2-CPA-CPB^-CTE-recombinant L. tarentolae protects BALB/c mice against L. infantum challenge. This protective immunity is associated with a Th1-type immune response due to high levels of IFN-γ production prior and after challenge and with lower levels of IL-10 production after challenge, leading to a significantly higher IFN-γ/IL-10 ratio compared to the control groups. Moreover, this immunization elicited high IgG1 and IgG2a humoral immune responses. Protection in mice was also correlated with a high nitric oxide production and low parasite burden. Altogether, these results indicate the promise of the A2-CPA-CPB^-CTE-recombinant L. tarentolae as a safe live vaccine candidate against VL.

Visceral leishmaniasis (VL) is the most severe form of leishmaniasis and has emerged as an opportunistic infection in HIV-1 infected patients in many parts of the world. Drug-resistant forms have developed so emergence and increased the need for advanced preventive strategies. Using live avirulent organisms as a vaccine has been proven to be more effective than other regimens. The lizard protozoan parasite Leishmania tarentolae is considered as nonpathogenic to humans. In our previous work, a recombinant L. tarentolae strain expressing the amastigote-specific L. donovani A2 antigen as a vaccine candidate elicited protection against L. infantum challenge in mice. Furthermore, combinations of CPA/CPB cysteine proteinases were more protective against visceral and cutaneous Leishmania infections than the individual forms. Herein, we used DNA/Live and Live/Live prime-boost vaccination strategies against visceral leishmaniasis in BALB/c mice consisting of the A2-CPA-CPB^-CTE tri-fusion genes formulated with cationic solid lipid nanoparticles and a recombinant L. tarentolae expressing the tri-fusion. Assessments of cytokine production, humoral responses, parasite burden and histopathological studies support that the recombinant L. tarentolae A2-CPA-CPB^-CTE candidate vaccine elicits a protective response against visceral leishmaniasis in mice and represents an important step forward in the development of new vaccine combinations against Leishmania infections.

The 3D structure and function of digestive cathepsin L-like proteinases of Tenebrio molitor larval midgut

Cathepsin L-like proteinases (CAL) are major digestive proteinases in the beetle Tenebrio molitor. Procathepsin Ls 2 (pCAL2) and 3 (pCAL3) were expressed as recombinant proteins in Escherichia coli, purified and activated under acidic conditions. Immunoblot analyses of different T. molitor larval tissues demonstrated that a polyclonal antibody to pCAL3 recognized pCAL3 and cathepsin L 3 (CAL3) only in the anterior two-thirds of midgut tissue and midgut luminal contents of T. molitor larvae. Furthermore, immunocytolocalization data indicated that pCAL3 occurs in secretory vesicles and microvilli in anterior midgut. Therefore CAL3, like cathepsin L 2 (CAL2), is a digestive enzyme secreted by T. molitor anterior midgut. CAL3 hydrolyses Z-FR-MCA and Z-RR-MCA (typical cathepsin substrates), whereas CAL2 hydrolyses only Z-FR-MCA. Active site mutants (pCAL2C25S and pCAL3C26S) were constructed by replacing the catalytic cysteine with serine to prevent autocatalytic processing. Recombinant pCAL2 and pCAL3 mutants (pCAL2C25S and pCAL3C26S) were prepared, crystallized and their 3D structures determined at 1.85 and 2.1 Å, respectively. While the overall structure of these enzymes is similar to other members of the papain superfamily, structural differences in the S2 subsite explain their substrate specificities. The data also supported models for CAL trafficking to lysosomes and to secretory vesicles to be discharged into midgut contents.

Form and function in the trypanosomal secretory pathway

Recent advances in secretory biology of African trypanosomes reveal both similarities and striking differences with other model eukaryotic organisms. Secretion is streamlined for rapid and selective transport of the major cargo, VSG. Selectivity in the early and post-Golgi compartments is dependent on glycosylphosphatidyl inositol anchors. Streamlining includes reduced organellar abundance, and close association of ER exit sites with Golgi and with unique flagellar cytoskeletal elements that govern organellar replication and segregation. These elements include a novel centrin containing bilobe structure. Innate signals for post-Golgi sorting of biosynthetic lysosomal cargo trafficking have been defined, as have pathways for both biosynthetic and endocytic trafficking to the lysosome. Less well-defined secretory organelles such as the multivesicular body and acidocalcisomes are receiving closer scrutiny.

Late endosomal Rab7 regulates lysosomal trafficking of endocytic but not biosynthetic cargo in Trypanosoma brucei

We present the first functional analysis of the small GTPase, TbRab7, in Trypanosoma brucei. TbRab7 defines discrete late endosomes closely juxtaposed to the terminal p67(+) lysosome. RNAi indicates that TbRab7 is essential in bloodstream trypanosomes. Initial rates of endocytosis were unaffected, but lysosomal delivery of cargo, including tomato lectin (TL) and trypanolytic factor (TLF) were blocked. These accumulate in a dispersed internal compartment of elevated pH, likely derived from the late endosome. Surface binding of TL but not TLF was reduced, suggesting that cellular distribution of flagellar pocket receptors is differentially regulated by TbRab7. TLF activity was reduced approximately threefold confirming that lysosomal delivery is critical for trypanotoxicity. Unexpectedly, delivery of endogenous proteins, p67 and TbCatL, were unaffected indicating that TbRab7 does not regulate biosynthetic lysosomal trafficking. Thus, unlike mammalian cells and yeast, lysosomal trafficking of endocytosed and endogenous proteins occur via different routes and/or are regulated differentially. TbRab7 silencing had no effect on a cryptic default pathway to the lysosome, suggesting that the default lysosomal reporters p67ΔTM, p67ΔCD and VSGΔGPI do not utilize the endocytic pathway as previously proposed. Surprisingly, conditional knockout indicates that TbRab7 may be non-essential in procyclic insect form trypanosomes.

Cysteine peptidases of kinetoplastid parasites

We review Clan CA Family C1 peptidases of kinetoplastid parasites (Trypanosoma and Leishmania) with respect to biochemical and genetic diversity, genomic organization and stage-specificity and control of expression. We discuss their contributions to parasite metabolism, virulence and pathogenesis and modulation of the host's immune response. Their applications as vaccine candidates and diagnostic markers as well as their chemical and genetic validation as drug targets are also summarized.

Cruzain : the path from target validation to the clinic

Cruzain is the major papain-like cysteine protease of Trypanosoma cruzi, the etiological agent causing Chagas' disease in humans in South America. Cruzain is indispensable for the survival and propagation of this protozoan parasite and therefore, it has attracted considerable interest as a potential drug target. This chapter charts the path from the initial identification of this proteases activity and its validation as a bone fide drug target to the arduous task of the discovery of an inhibitor targeting this protease and finally the path towards the clinic.

Comparative study of mature and zymogen mite cysteine protease stability and pH unfolding

BACKGROUND

Papain-like proteases (CA1) are synthesized as inactive precursors carrying an N-terminal propeptide, which is further removed under acidic conditions to generate active enzymes.

METHODS

To have a better insight into the mechanism of activation of this protease family, we compared the pH unfolding of the zymogen and the mature form of the mite cysteine protease Der p 1.

RESULTS

We showed that the presence of the propeptide does not significantly influence the pH-induced unfolding of the catalytic domain but does affect its fluorescence properties by modifying the exposure of the tryptophan 192 to the solvent. In addition, we demonstrated that the propeptide displays weaker pH stability than the protease domain confirming that the unfolding of the propeptide is the key event in the activation process of the zymogen.

GENERAL SIGNIFICANCE

Finally, we show, using thermal denaturation and enzymatic activity measurements, that whatever the pH value, the propeptide does not stabilize the structure of the catalytic domain but very interestingly, prevents its autolysis.

Role of AP-1 in developmentally regulated lysosomal trafficking in Trypanosoma brucei

African trypanosomes are the causative agents of human trypanosomiasis (sleeping sickness). The pathogenic stage of the parasite has unique adaptations to life in the bloodstream of the mammalian host, including upregulation of endocytic and lysosomal activities. We investigated stage-specific requirements for cytoplasmic adaptor/clathrin machinery in post-Golgi apparatus biosynthetic sorting to the lysosome using RNA interference silencing of the Tbmu1 subunit of adaptor complex 1 (AP-1), in conjunction with immunolocalization, kinetic analyses of reporter transport, and quantitative endocytosis assays. Tbmu1 silencing was lethal in both stages, indicating a critical function(s) for the AP-1 machinery. Transport of soluble and membrane-bound secretory cargoes was Tbmu1 independent in both stages. In procyclic parasites, trafficking of the lysosomal membrane protein, p67, was disrupted, leading to cell surface mislocalization. The lysosomal protease trypanopain was also secreted, suggesting a transmembrane-sorting receptor for this soluble hydrolase. In bloodstream trypanosomes, both p67 and trypanopain trafficking were unaffected by Tbmu1 silencing, suggesting that AP-1 is not necessary for biosynthetic lysosomal trafficking. Endocytosis in bloodstream cells was also unaffected, indicating that AP-1 does not function at the flagellar pocket. These results indicate that post-Golgi apparatus sorting to the lysosome is critically dependent on the AP-1/clathrin machinery in procyclic trypanosomes but that this machinery is not necessary in bloodstream parasites. We propose a simple model for stage-specific default secretory trafficking in trypanosomes that is consistent with the behavior of other soluble and glycosylphosphatidylinositol-anchored cargos and which is influenced by upregulation of endocytosis in bloodstream parasites as an adaptation to life in the mammalian bloodstream.

Strategies for carbohydrate recognition by the mannose 6-phosphate receptors

The two members of the P-type lectin family, the 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) and the 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR), are ubiquitously expressed throughout the animal kingdom and are distinguished from all other lectins by their ability to recognize phosphorylated mannose residues. The best-characterized function of the MPRs is their ability to direct the delivery of approximately 60 different newly synthesized soluble lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) on their N-linked oligosaccharides to the lysosome. In addition to its intracellular role in lysosome biogenesis, the CI-MPR, but not the CD-MPR, participates in a number of other biological processes by interacting with various molecules at the cell surface. The list of extracellular ligands recognized by this multifunctional receptor has grown to include a diverse spectrum of Man-6-P-containing proteins as well as several non-Man-6-P-containing ligands. Recent structural studies have given us a clearer view of how these two receptors use related, but yet distinct, approaches in the recognition of phosphomannosyl residues.

Leishmania lysosomal targeting signal is recognized by yeast and not by mammalian cells

Leishmaniasis, caused by Leishmania parasites, is an important public health problem worldwide. Leishmania, like other trypanosomatids, present unique biological features as compared to higher eukaryotes that can be exploited with the intent of finding new chemotherapeutical/vaccine candidates. Mechanisms of cellular sorting in Leishmania can be viewed as such potential targets. We have previously demonstrated a role for the pro-domain of a Leishmania cysteine proteinase in lysosomal targeting. In this paper, we show that this signal is not recognized by mammalian cells and is recognized by yeast; we also discuss here the implications of these findings related to evolution and further characterization of the Leishmania trafficking machinery.

The prodomain of Toxoplasma gondii GPI-anchored subtilase TgSUB1 mediates its targeting to micronemes

Subtilisin-like proteases have been proposed to play an important role for parasite survival in Toxoplasma gondii (Tg) and Plasmodium falciparum. The T. gondii subtilase TgSUB1 is located in the microneme, an apical secretory organelle whose contents mediate adhesion to the host during invasion. TgSUB1 is predicted to contain a glycosyl-phosphatidylinositol (GPI) anchor. This is unusual as Toxoplasma GPI-anchored proteins are targeted to the parasite's surface. In this study, we report that the subtilase TgSUB1 is indeed a GPI-anchored protein but contains dominant microneme targeting signals. Accurate targeting of TgSUB1 to the micronemes is dependent upon several factors including promoter strength and timing, accurate processing and folding. We analyzed the targeting domains of TgSUB1 using TgSUB1 deletion constructs and chimeras made between TgSUB1 and reporter proteins. The TgSUB1 prodomain is responsible for trafficking to the micronemes and is sufficient for targeting a reporter protein to the micronemes. Trafficking is dependent upon correct folding or other context-dependent conformation as the prodomain expressed alone is unable to reach the micromenes. Therefore, TgSUB1 is a novel example of a GPI-anchored protein in T. gondii that bypasses the GPI-dependent surface trafficking pathway to traffic to micronemes, specialized regulated secretory organelles.

The LAMP-like protein p67 plays an essential role in the lysosome of African trypanosomes

RNAi knockdown was employed to study the function of p67, a lysosome-associated membrane protein (LAMP)-like type I transmembrane lysosomal glycoprotein in African trypanosomes. Conditional induction of p67 dsRNA resulted in specific approximately 90% reductions in de novo p67 synthesis in both mammalian bloodstream and procyclic insect-stage parasites. Bloodstream cell growth was severely retarded with extensive death after > 24 h of induction. Biosynthetic trafficking of residual p67, and of the soluble lysosomal protease trypanopain, were unimpaired. Endocytosis of tomato lectin, a surrogate receptor-mediated cargo, was only mildly impaired (approximately 20%), but proper lysosomal targeting was unaffected. p67 ablation had dramatic effects on lysosomal morphology with gross enlargement (four- to fivefold) and internal membrane profiles reminiscent of autophagic vacuoles. Ablation of p67 expression rendered bloodstream trypanosomes refractory to lysis by human trypanolytic factor (TLF), a lysosomally activated host innate immune mediator. Similar effects on lysosomal morphology and TLF sensitivity were also obtained by two pharmacological agents that neutralize lysosomal pH--chloroquine and bafilomycin A1. Surprisingly, however, lysosomal pH was not affected in ablated cells suggesting that other physiological alterations must account for increased resistance to TLF. These results indicate p67 plays an essential role in maintenance of normal lysosomal structure and physiology in bloodstream-stage African trypanosomes.

Multiple motifs regulate trafficking of the LAMP-like protein p67 in the ancient eukaryote Trypanosoma brucei

p67 is a lysosome-associated membrane protein-like lysosomal type I transmembrane glycoprotein in African trypanosomes. The p67 cytoplasmic domain (CD) is both necessary and sufficient for lysosomal targeting in procyclic insect-stage parasites. The p67CD contains two [DE]XXXL[LI]-type dileucine motifs, which function as lysosomal targeting signals in mammalian cells. Using a green fluorescent protein fusion to the p67 transmembrane and cytoplasmic domains as a reporter system, we investigated the role of these motifs in lysosomal targeting in procyclic trypanosomes. Pulse-chase turnover studies, steady-state immunolocalization and quantitative flow cytometry all gave consistent results. Mutagenesis of the membrane-distal dileucine motif impairs lysosomal trafficking leading to partial appearance of the reporter on the cell surface. Mutagenesis of the membrane-proximal motif has little effect on proper targeting. Simultaneous mutagenesis of both motifs results in quantitative delivery to the cell surface. Thus, the distal motif plays a dominant role, but both dileucine motifs are necessary for maximal lysosomal targeting. Additional studies suggest that the upstream acidic residues in each motif influence lysosomal targeting and may also affect forward trafficking in the early secretory pathway. These results strongly suggest an evolutionary conservation in lysosomal trafficking mechanisms in the ancient eukaryote Trypanosoma brucei.

Falcipain cysteine proteases require bipartite motifs for trafficking to the Plasmodium falciparum food vacuole

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 hydrolyze hemoglobin in an acidic food vacuole to provide amino acids for erythrocytic malaria parasites. Trafficking to the food vacuole has not been well characterized. To study trafficking of falcipains, which include large membrane-spanning prodomains, we utilized chimeras with portions of the proteases fused to green fluorescent protein. The prodomains of falcipain-2 and falcipain-3 were sufficient to target green fluorescent protein to the food vacuole. Using serial truncations, deletions, and point mutations, we showed that both a 20-amino acid stretch of the lumenal portion and a 10-amino acid stretch of the cytoplasmic portion of the falcipain-2 prodomain were required for efficient food vacuolar trafficking. Mutants with altered trafficking were arrested at the plasma membrane, implicating trafficking via this structure. Our results indicate that falcipains utilize a previously undescribed bipartite motif-dependent mechanism for targeting to a hydrolytic organelle, suggesting inhibition of this unique mechanism as a new means of antimalarial chemotherapy.

Protein turnover and differentiation in Leishmania

Leishmania occurs in several developmental forms and thus undergoes complex cell differentiation events during its life-cycle. Those are required to allow the parasite to adapt to the different environmental conditions. The sequencing of the genome of L. major has facilitated the identification of the parasite’s vast arsenal of proteolytic enzymes, a few of which have already been carefully studied and found to be important for the development and virulence of the parasite. This review focuses on these peptidases and their role in the cellular differentiation of Leishmania through their key involvement in a variety of degradative pathways in the lysosomal and autophagy networks.

Leishmania aethiopica: identification and characterization of cathepsin L-like cysteine protease genes

There is limited information on the biology and pathogenesis of Leishmania aethiopica, causative agent of cutaneous leishmaniasis (CL) in Ethiopia. In this study we have identified and characterized two cathepsin L-like cysteine protease genes, Laecpa and Laecpb, from L. aethiopica. The predicted amino acid sequence of Laecpa and Laecpb is more than 75% identical with homologous cathepsin L-like cysteine protease genes of other Leishmania species and less than 50% identical with human cathepsin L. Laecpa is expressed predominantly in the stationary, and to a lower level, during the amastigote stage while Laecpb is specifically expressed in the stationary stage of L. aethiopica development. Phylogenetic analysis showed that the two genes are grouped into separate clades which are the result of gene duplication. The isolation of these genes will be useful in developing Leishmania species specific diagnostics for molecular epidemiological studies and serves as a first step to study the role of cysteine proteases in L. aethiopica pathogenesis.

Novel antitrypanosomal agents

Trypanosomes are the causative agents of Chagas' disease in Central and South America and sleeping sickness in sub-Saharan Africa. The current chemotherapy of the human trypanosomiases relies on only six drugs, five of which were developed > 30 years ago. In addition, these drugs display undesirable toxic side effects and the emergence of drug-resistant trypanosomes has been reported. Therefore, the development of new drugs in the treatment of Chagas' disease and sleeping sickness is urgently required. This article summarises the recent progress in identifying novel lead compounds for antitrypanosomal chemotherapy. Particular emphasis is placed on those agents showing promising, selective antitrypanosomal activity.

Solution Structure and Backbone Dynamics of the Trypanosoma cruzi Cysteine Protease Inhibitor Chagasin

A Trypanosoma cruzi cysteine protease inhibitor, termed chagasin, is the first characterized member of a new family of tight-binding cysteine protease inhibitors identified in several lower eukaryotes and prokaryotes but not present in mammals. In the protozoan parasite T.cruzi, chagasin plays a role in parasite differentiation and in mammalian host cell invasion, due to its ability to modulate the endogenous activity of cruzipain, a lysosomal-like cysteine protease. In the present work, we determined the solution structure of chagasin and studied its backbone dynamics by NMR techniques. Structured as a single immunoglobulin-like domain in solution, chagasin exerts its inhibitory activity on cruzipain through conserved residues placed in three loops in the same side of the structure. One of these three loops, L4, predicted to be of variable length among chagasin homologues, is flexible in solution as determined by measurements of (15)N relaxation. The biological implications of structural homology between chagasin and other members of the immunoglobulin super-family are discussed.

A potential role for ICP, a Leishmanial inhibitor of cysteine peptidases, in the interaction between host and parasite

The biological role of a natural inhibitor of cysteine peptidases (designated ICP) of Leishmania has been investigated by genetic manipulation of the parasite. Null mutants grew normally in vitro, were as infective to macrophages in vitro as wild-type parasites, but had reduced infectivity to mice. Mutants re-expressing ICP from a single gene gave partial restoration of virulence in vivo, whereas mutants overexpressing ICP secreted the inhibitor and showed markedly reduced virulence in mice. Promastigotes of the null mutants had similar cysteine peptidase activities as the wild-type parasites, suggesting that ICP is not required for the expression or processing of the enzymes. The only proteins found to bind to ICP in promastigote cell lysates were fully processed forms of CPA and CPB, showing that ICP does not bind in abundance either to zymogens of the cysteine peptidases or other leishmanial proteins. However, only a small proportion of ICP colocalized with CPA and CPB in the promastigote (in the endoplasmic reticulum and Golgi) and the majority of ICP resided in vesicles that are apparently distinct from endosomes and the multivesicular tubule (MVT)-lysosome. These data suggest that ICP has a role other than modulation of the activity of the parasite's own cysteine peptidases and their normal trafficking to the MVT-lysosome via the flagellar pocket. The finding that ICP partially colocalized with an endocytosed cysteine peptidase leads us to postulate that ICP has a role in protection of the parasite against the hydrolytic environment of the sandfly gut and/or the parasitophorous vacuole of host macrophages.

Chagasin, the endogenous cysteine-protease inhibitor of Trypanosoma cruzi, modulates parasite differentiation and invasion of mammalian cells

Chagasin is a Trypanosoma cruzi protein that was recently characterized as a tight-binding inhibitor of papain-like cysteine proteases (CPs). Considering that parasite virulence and morphogenesis depend on the endogenous activity of lysosomal CPs of the cruzipain family, we sought to determine whether chagasin and cruzipain interact in the living cell. Ultrastructural studies showed that chagasin and cruzipain both localize to the Golgi complex and reservosomes (lysosome-like organelles), whereas free chagasin was found in small intracellular vesicles, suggesting that chagasin trafficking pathways might intersect with those of cruzipain. Taking advantage of the fact that sodium dodecyl sulphate and beta-mercaptoethanol prevent binding between the isolated proteins but do not dismantle preformed cruzipain-chagasin complexes, we obtained direct evidence that chagasin-cruzipain complexes are indeed formed in epimastigotes. Chagasin transfectants (fourfold increase in CP inhibitory activity) displayed low rates of differentiation (metacyclogenesis) and exhibited increased resistance to a synthetic CP inhibitor. These phenotypic changes were accompanied by a drastic reduction of soluble cruzipain activity and by upregulated secretion of cruzipain-chagasin molecular complexes. Analysis of six T. cruzi strains revealed that expression levels of cruzipain and chagasin are variable, but the molar ratios are fairly stable ( approximately 50:1) in most strains, with the exception of the G strain (5:1), which is poorly infective. On the same vein, we found that trypomastigotes overexpressing chagasin are less infective than wild-type parasites in vitro. The deficiency of chagasin overexpressers is caused by lower activity of membrane-associated CPs, because membranes recovered from wild-type trypomastigotes restored infectivity and this effect was nullified by the CP inhibitor E-64. In summary, our studies suggest that chagasin regulates the endogenous activity of CP, thus indirectly modulating proteolytic functions that are essential for parasite differentiation and invasion of mammalian cells.

Structural features of the lysosomal hydrolase mannose 6-phosphate uncovering enzyme

The uncovering enzyme (UCE) removes N-acetylglucosamine from lysosomal enzymes to uncover the mannose 6-phosphate (Man-6-P) determinant necessary for targeting these enzymes to lysosomes. Failure to create the Man-6-P determinant is one cause of lysosomal storage diseases. Despite its medical importance, little structural information about UCE is available. In this report we have developed a model for the membrane proximal portion of the lumenal domain of UCE based on the structure of the EFG-3 and -4 domains of the extracellular segment of the beta chain of integrin alpha Vbeta 3. In this model the EGF-like domains of UCE (residues 285-345) are predicted to form a rod-shaped stalk region, similar to the stem region in Golgi glycosyltransferases. This stalk causes the proposed catalytic domain (residues 1-277) to be extended away from the Golgi membrane. A portion of the proposed catalytic domain (residues 85-256) resides in Cluster of Orthologous Group (COG) 4632 with four bacterial proteins but is not homologous to any known eukaryotic proteins. Thus, UCE may have evolved from the fusion of a unique catalytic domain with a common EGF-like stalk domain. We have determined by mass spectrometry that the four disulfide bonds of the proposed catalytic domain are located between Cys(2)-Cys(172), Cys(66)-Cys(99), Cys(83)-Cys(274), and Cys(258)-Cys(265). Finally, we determined that four of the six potential N-linked glycosylation sites are glycosylated (Asn 159, Asn 165, Asn 247, and Asn 317) in COS cells.

A Dictyostelium mutant with reduced lysozyme levels compensates by increased phagocytic activity

Lysozymes are bacteria-degrading enzymes and play a major role in the immune defense of animals. In free-living protozoa, lysozyme-like proteins are involved in the digestion of phagocytosed bacteria. Here, we purified a protein with lysozyme activity from Dictyostelium amoebae, which constitutes the founding member, a novel class of lysozymes. By tagging the protein with green fluorescent protein or the Myc epitope, a new type of lysozyme-containing vesicle was identified that was devoid of other known lysosomal enzymes. The most highly expressed isoform, encoded by the alyA gene, was knocked out by homologous recombination. The mutant cells had greatly reduced enzymatic activity and grew inefficiently when bacteria were the sole food source. Over time the mutant gained the ability to internalize bacteria more efficiently, so that the defect in digestion was compensated by increased uptake of food particles.

Cysteine peptidases as virulence factors of Leishmania

Leishmania mexicana amastigotes are particularly rich in cysteine peptidases (CPs), which play important roles in facilitating the survival and growth of the parasites in mammals. The importance of the CPs as virulence factors and their potential as drug targets and vaccine candidates has been investigated extensively. Recent years, however, have heralded advances in our knowledge and understanding of leishmanial CPs on two fronts. Firstly, genome analysis has revealed the great diversity of CPs, and, secondly, the ways in which the most widely studied CPs, designated CPB, influence the interaction between parasite and mammalian host have been elucidated. These topics are the focus of this review.

Location, Location, Location: Trafficking and Function of Secreted Proteases of Toxoplasma and Plasmodium

The Apicomplexan parasites Toxoplasma gondii and Plasmodium species are obligate intracellular parasites that rely upon unique secretory organelles for invasion and other specialized functions. Data is emerging that proteases are critical for the biogenesis of micronemes and rhoptries, regulated secretory organelles reminiscent of dense core granules and secretory lysosomes of higher eukaryotes. Proteases targeted to the Plasmodium food vacuole, a unique organelle dedicated to hemoglobin degradation, are also critical to parasite survival. Thus study of the targeting and function of the proteases of the Apicomplexa provides a fascinating model system to understand regulated secretion and secretory organelle biogenesis.

Biogenesis of the reservosomes of Trypanosoma cruzi

Reservosomes are endocytic compartments found in the posterior region of epimastigotes of Trypanosoma cruzi. In the differentiation from trypomastigotes to epimastigotes (reverse metacyclogenesis in vitro), one has the rare opportunity of following the biogenesis of an endocytic compartment. Metacyclic trypomastigotes incubated in LIT medium highly enriched with fetal calf serum differentiated directly to epimastigotes. In recently differentiated epimastigotes, acidic organelles were found in round compartments spread along the cell body, whereas in control epimastigotes they were found in reservosomes located in the posterior region. Ultrastructural analysis of intermediate forms showed that the cytostome and reservosomes appeared before differentiation to epimastigotes was completed. Many polymorphic reservosomes, with or without lipid inclusions, were observed from the anterior portion of the cell body, in close relationship with the Golgi complex, to the posterior region. Endocytic tracers were observed in the cytostome, flagellar pocket, vesicles, and newly formed reservosomes. Cruzipain, the main protease of T. cruzi, was localized in newly formed reservosomes and in vesicles budding from the trans-Golgi network that seem to fuse with reservosomes. Ingested gold-labeled albumin and cruzipain colocalized in recently formed reservosomes. Endocytosis and immunocytochemical analysis suggested that the endocytic and the secretory pathways may contribute to reservosome formation.

Are rhoptries in Apicomplexan parasites secretory granules or secretory lysosomal granules?

The club-shaped rhoptries in Apicomplexan parasites are one of the most unusual secretory organelles among the eukaryotes, containing unusual lipid and protein cargo that is specialized for intracellular parasitism. Rhoptries have traditionally been viewed strictly as regulated secretory granules. We discuss in this article recent data on the cargo, function and biogenesis of rhoptries in two parasitic model systems, Toxoplasma and Plasmodium. Current findings suggest that rhoptries receive products from both biosynthetic and endocytic pathways and, therefore, they are most analogous to secretory lysosomal granules found in mammalian cells.

Biosynthesis and alternate targeting of the lysosomal cysteine protease cathepsin L

Upregulation of cathepsin L expression, whether during development or cell transformation, or mediated by ectopic expression from a plasmid, alters the targeting of the protease and thus its physiological function. Upregulated procathepsin L is targeted to small dense core vesicles and to the dense cores of multivesicular bodies, as well as to lysosomes and to the plasma membrane for selective secretion. The multivesicular vesicles resemble secretory lysosomes characterized in specialized cell types in that they are endosomes that stably store an upregulated protein and they possess the tetraspanin CD63. Morphologically the multivesicular endosomes also resemble late endosomes, but they store procathepsin L, not the active protease, and they are not the major site for LAMP-1 accumulation. Distinction between the lysosomal proenzyme and active protease thus identifies two populations of multivesicular endosomes in fibroblasts, one a storage compartment and one an enzymatically active compartment. A distinctive targeting pathway using aggregation is utilized to enrich the storage endosomes with a particular lysosomal protease that can potentially activate and be secreted.

The intestinal protozoan parasite Entamoeba histolytica contains 20 cysteine protease genes, of which only a small subset is expressed during in vitro cultivation

Cysteine proteases are known to be important pathogenicity factors of the protozoan parasite Entamoeba histolytica. So far, a total of eight genes coding for cysteine proteases have been identified in E. histolytica, two of which are absent in the closely related nonpathogenic species E. dispar. However, present knowledge is restricted to enzymes expressed during in vitro cultivation of the parasite, which might represent only a subset of the entire repertoire. Taking advantage of the current E. histolytica genome-sequencing efforts, we analyzed databases containing more than 99% of all ameba gene sequences for the presence of cysteine protease genes. A total of 20 full-length genes was identified (including all eight genes previously reported), which show 10 to 86% sequence identity. The various genes obviously originated from two separate ancestors since they form two distinct clades. Despite cathepsin B-like substrate specificities, all of the ameba polypeptides are structurally related to cathepsin L-like enzymes. None of the previously described enzymes but 7 of the 12 newly identified proteins are unique compared to cathepsins of higher eukaryotes in that they are predicted to have transmembrane or glycosylphosphatidylinositol anchor attachment domains. Southern blot analysis revealed that orthologous sequences for all of the newly identified proteases are present in E. dispar. Interestingly, the majority of the various cysteine protease genes are not expressed in E. histolytica or E. dispar trophozoites during in vitro cultivation. Therefore, it is likely that at least some of these enzymes are required for infection of the human host and/or for completion of the parasite life cycle.

Procongopain from Trypanosoma congolense is processed at basic pH: an unusual feature among cathepsin L-like cysteine proteases

Congopain, the major cysteine protease from Trypanosoma congolense, is synthesized as an inactive zymogen, and further converted into its active form after removal of the proregion, most probably via an autocatalytic mechanism. Processing of recombinant procongopain occurs via an apparent one-step or a multistep mechanism depending on the ionic strength. The auto-activation is pH-dependent, with an optimum at pH 4.0, and no activation observed at pH 6.0. After addition of dextran sulfate (10 microg/ml), an approx. 20-fold increase of processing (expressed as enzymatic activity) is observed. Furthermore, in the presence of dextran sulfate, procongopain can be processed at pH 8.0, an unusual feature among papain-like enzymes. Detection of procongopain and trypanosomal enzymatic activity in the plasma of T. congolense-infected cattle, together with the capacity of procongopain to be activated at weakly basic pH, suggest that procongopain may be extracellularly processed in the presence of blood vessel glycosaminoglycans, supporting the hypothesis that congopain acts as a pathogenic factor in host-parasite relationships.

Expression of cysteine proteinase type I and II of Leishmania infantum and their recognition by sera during canine and human visceral leishmaniasis

In this study, the mature domains of type I (CPB) and type II (CPA) cysteine proteinases (CPs) of Leishmania infantum were expressed and their immunogenic properties defined using sera from active and recovered cases of human visceral leishmaniasis and sera from infected dogs. Immunoblotting and ELISA analysis indicated that a freeze/thaw extract of parasite antigens showed similar and intensive recognition in both active cases of human and dog sera but lower recognition in recovered human individuals. The total IgG of actively infected human sera was higher than in recovered cases when rCPs were used as antigen. In contrast to dog sera, both active and recovered human cases have higher recognition toward rCPB than rCPA. Furthermore, the asymptomatic dogs in contrast to the symptomatic cases exhibited specific lymphocyte proliferation to both crude antigens and rCPs.

Trafficking of lysosomal cathepsin B-green fluorescent protein to the surface of thyroid epithelial cells involves the endosomal/lysosomal compartment

Cathepsin B, a lysosomal cysteine proteinase, is involved in limited proteolysis of thyroglobulin with thyroxine liberation at the apical surface of thyroid epithelial cells. To analyze the trafficking of lysosomal enzymes to extracellular locations of thyroid epithelial cells, we have expressed a chimeric protein consisting of rat cathepsin B and green fluorescent protein. Heterologous expression in CHO cells validated the integrity of the structural motifs of the chimeric protein for targeting to endocytic compartments. Homologous expression, colocalization and transport experiments with rat thyroid epithelial cell lines FRT or FRTL-5 demonstrated the correct sorting of the chimeric protein into the lumen of the endoplasmic reticulum, and its subsequent transport via the Golgi apparatus and the trans-Golgi network to endosomes and lysosomes. In addition, the chimeras were secreted as active enzymes from FRTL-5 cells in a thyroid-stimulating-hormone-dependent manner. Immunoprecipitation experiments after pulse-chase radiolabeling showed that secreted chimeras lacked the propeptide of cathepsin B. Thus, the results suggest that cathepsin B is first transported to endosomes/lysosomes from where its matured form is retrieved before being secreted, supporting the view that endosome/lysosome-derived cathepsin B contributes to the potential of extracellular proteolysis in the thyroid.

Biological roles of proteases in parasitic protozoa

Proteases from a variety of protozoan parasites have been characterized at the molecular and cellular levels, and the many roles that proteases play in these organisms are coming into focus. Central roles have been proposed for proteases in diverse processes such as host cell invasion and egress, encystation, excystation, catabolism of host proteins, differentiation, cell cycle progression, cytoadherence, and both stimulation and evasion of host immune responses. Detailed structural and functional characterization of parasite proteases has led to novel insights into the workings of these fascinating catalytic machines. The possibility of developing selective inhibitors of key proteases of pathogenic parasites into novel chemotherapeutic strategies is being vigorously explored.

Developmental changes in lysosome morphology and function Leishmania parasites

The endocytic pathway of Leishmania parasites has recently come under intense research focus through the development of several markers for various compartments of this pathway. Through these studies a novel multivesicular tubule lysosome has been discovered in promastigote-stage parasites. This organelle has a highly dynamic role during parasite growth and differentiation. This review discusses recent insights into the Leishmania lysosome with respect to its organisation within the endocytic pathway, stage-specific functions, and biogenesis.

Cysteine peptidases of mammals: their biological roles and potential effects in the oral cavity and other tissues in health and disease

Cysteine peptidases (CPs) are phylogenetically ubiquitous enzymes that can be classified into clans of evolutionarily independent proteins based on the structural organization of the active site. In mammals, two of the major clans represented in the genome are: the CA clan, whose members share a structure and evolutionary history with papain; and the CD clan, which includes the legumains and caspases. This review focuses on the properties of these enzymes, with an emphasis on their potential roles in the oral cavity. The human genome encodes at least (but possibly no more than) 11 distinct enzymes, called cathepsins, that are members of the papain family C1A. Ten of these are present in rodents, which also carry additional genes encoding other cathepsins and cathepsin-like proteins. Human cathepsins are best known from the ubiquitously expressed lysosomal cathepsins B, H, and L, and dipeptidyl peptidase I (DPP I), which until recently were considered to mediate primarily "housekeeping" functions in the cell. However, mutations in DPP I have now been shown to underlie Papillon-Lefevre syndrome and pre-pubertal periodontitis. Other cathepsins are involved in tissue-specific functions such as bone remodeling, but relatively little is known about the functions of several recently discovered enzymes. Collectively, CPs participate in multiple host systems that are active in health and in disease. They are involved in tissue remodeling and turnover of the extracellular matrix, immune system function, and modulation and alteration of cell function. Intracellularly, CPs function in diverse processes including normal protein turnover, antigen and proprotein processing, and apoptosis. Extracellularly, they can contribute directly to the degradation of foreign proteins and the extracellular matrix. However, CPs can also participate in proteolytic cascades that amplify the degradative capacity, potentially leading to pathological damage, and facilitating the penetration of tissues by cancer cells. We know relatively little regarding the role of human CPs in the oral cavity in health or disease. Most studies to date have focused on the potential use of the lysosomal enzymes as markers for periodontal disease activity. Human saliva contains high levels of cystatins, which are potent CP inhibitors. Although these proteins are presumed to serve a protective function, their in vivo targets are unknown, and it remains to be discovered whether they serve to control any human CP activity.