A single exon codes for the enzyme domain of a protozoan cysteine protease

A single exon-encoded Theileria parva strain Muguga cysteine protease (ThpCP): Molecular modelling and characterisation

The tick-transmitted apicomplexan Theileria parva causes East Coast fever, a bovine disease of great economic and veterinary importance in Africa. Papain-like cysteine proteases play important roles in protozoan parasite host cell entry and egress, nutrition and host immune evasion. This study reports the identification and characterisation of a T. parva strain Muguga cathepsin L-like (C1A subfamily) cysteine protease (ThpCP). Molecular modelling confirmed the papain-like fold of ThpCP, hydrophobic character of the S2 substrate binding pocket and non-covalent interaction between the pro- and catalytic domains preceding low pH autoactivation. ThpCP was recombinantly expressed in a protease deficient E. coli (Rosetta (DE3)pLysS strain) expression host as a 46 kDa proenzyme. Following Ni-chelate affinity chromatography and acidification, the 27 kDa mature ThpCP was purified by cation-exchange chromatography. Purified ThpCP hydrolysed typical cathepsin L substrates N-α-benzyloxycarbonyl (Z)-Phe-Arg-7-amino-4-methyl-coumarin (AMC) (k_cat/K_m = 4.49 × 10⁵ s^-1M^-1) and Z-Leu-Arg-AMC (k_cat/K_m = 4.20 × 10⁵ s^-1M^-1), but showed no activity against the cathepsin B-selective substrate Z-Arg-Arg-AMC. Recombinant ThpCP was active over a broad pH range from pH 4.5 to 7.5, thereby showing potential activity in the acidic parasite food vacuole and close to neutral pH of the host lymphocyte cytoplasm. Recombinant ThpCP was inhibited by the cysteine protease inhibitors E64, iodoacetate, leupeptin, chymostatin, Z-Phe-Ala-diazomethylketone (DMK) and Z-Phe-Phe-DMK and hydrolysed bovine proteins: haemoglobin, immunoglobulin G, serum albumin and fibrinogen as well as goat IgG at pH 6 and 7. Functional expression and characterisation of Theileria cysteine proteases should enable high throughput screening of cysteine protease inhibitor libraries against these proteases.

Screening and identification of host proteins interacting with Theileria annulata cysteine proteinase (TaCP) by yeast-two-hybrid system

BACKGROUND

Theileria annulata can infect monocytes/macrophages and B lymphocytes and causes severe lymphoproliferative disease in ruminants. Meanwhile, infection by T. annulata leads to the permanent proliferation of cell population through regulating signaling pathways of host cells. Cysteine proteinases (CPs) are one kind of protein hydrolase and usually play critical roles in parasite virulence, host invasion, nutrition and host immune response. However, the biological function of T. annulata CP (TaCP) is still unclear. In this study, a yeast-two-hybrid assay was performed to screen host proteins interacting with TaCP, to provide information to help our understanding of the molecular mechanisms between T. annulata and host cells.

METHODS

The cDNA from purified bovine B cells was inserted into pGADT7-SfiI vector (pGADT7-SfiI-BcDNA, Prey plasmid) for constructing the yeast two-hybrid cDNA library. TaCP was cloned into the pGBKT7 vector (pGBKT7-TaCP) and was considered as bait plasmid after evaluating the expression, auto-activation and toxicity tests in the yeast strain Y2HGold. The yeast two-hybrid screening was carried out via co-transforming bait and prey plasmids into yeast strain Y2HGold. Sequences of positive preys were analyzed using BLAST, Gene Ontology, UniProt and STRING.

RESULTS

Two host proteins, CRBN (Bos taurus cereblon transcript variant X2) and Ppp4C (Bos indicus protein phosphatase 4 catalytic subunit) were identified to interact with TaCP. The results of functional analysis showed that the two proteins were involved in many cellular processes, such as ubiquitylation regulation, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs.

CONCLUSIONS

This study is the first to screen the host proteins of bovine B cells interacting with TaCP, and 2 proteins, CRBN and Ppp4C, were identified using yeast two-hybrid technique. The results of functional analysis suggest that the two proteins are involved in many cellular processes, such as ubiquitylation regulating, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs. The interaction with CRBN and Ppp4C indicate that TaCP possibly is involved in regulating signaling pathways and cell proliferation, which is helpful for understanding the interaction between T. annulata and host cells.

Complete, precise, and innocuous loss of multiple introns in the currently intronless, active cathepsin L-like genes, and inference from this event

Retrotransposition typically generates pseudogenes. Here we demonstrate a different fate of the retro-processed genes through a novel mechanism in which the retro-processed genes still maintain their sequence intactness and the original functions. We show that the shrimp cathepsin L (CatL) gene MeCatL has lost all of its five introns. Also, ProEPB, the ancestor of the CatL-like barley EPBs and rice REP1, has lost all of its three introns. The multiple introns in a gene might have been eliminated simultaneously and precisely at the original locus for the CatL-like genes of shrimp, barley, rice, Drosophila, and Theileria. We reason that retrotransposition is not responsible for the generation of a processed active intronless (PAI) gene when the gene product retains its sequence intactness and its original function. We propose that double-strand-break repair (DSBR) machinery might play a role in cDNA-mediated homologous recombination (cDMHR) that causes the loss of introns. The cDMHR/DSBR pathway is probably a fundamental mechanism for intron loss in PAI genes and in some asymmetric-intron genes.

Characterization and expression of AmphiCL encoding cathepsin l proteinase from amphioxus Branchiostoma belcheri tsingtauense

An amphioxus complementary DNA, AmphiCL, encoding cathepsin L proteinase was isolated from the gut cDNA library of Branchiostoma belcheri tsingtauense. It is 1480 bp long, and its longest open reading frame codes for a precursor protein, which consists of 327 amino acid residues including a signal peptide (preregion), a propeptide, and a mature proteinase. Northern blot showed that AmphiCL was expressed in the gill, testis, hepatic cecum, and hind-gut with a molecular size of about 1480 bp. AmphiCL was also expressed at low level in the muscle, notochord, and ovary as revealed by the more sensitive reverse transcriptase polymerase chain reaction techniques. Semiquantitative RT-PCR also showed that although AmphiCL expression in the gut was significantly downregulated by feeding Arthrospira platensis powder, a protein-rich food, its expression in the same tissue was upregulated by exposure to lipopolysaccharide, an integral component of the outer membrane of gram-negative bacteria. This suggests that although the involvement of AmphiCL in food digestion remains to be confirmed, AmphiCL may play a role in inflammatory reaction in amphioxus.

Shrimp cathepsin L encoded by an intronless gene has predominant expression in hepatopancreas, and occurs in the nucleus of oocyte

We have cloned the cDNA and genomic DNA of an active intronless cathepsin L from Metapenaeus ensis. The encoded enzyme has the shortest prosequence among cathepsin L subgroup. It was predominantly expressed in hepatopancreas with an expression level of at least 10 times higher than in any other tissues. It also has expression in stomach, intestine, eye, testis, ovary and muscle. Western blots visualized the mature enzyme in hepatopancreas and a procathepsin L in ovary, intestine and stomach. Metapenaeus cathepsin L (MeCatL) is localized in the large digestive vacuole of the digestive B cell of hepatopancreas. MeCatL has a role in food digestion. An interesting finding is that it exists in the nucleus of oocyte. MeCatL might have a specified physiological role in the nucleus of oocyte. MeCatL might also have a house-keeping function as is suggested for mammalian cathepsin L.

Hemoglobin metabolism in the malaria parasite Plasmodium falciparum

Hemoglobin degradation in intraerythrocytic malaria parasites is a vast process that occurs in an acidic digestive vacuole. Proteases that participate in this catabolic pathway have been defined. Studies of protease biosynthesis have revealed unusual targeting and activation mechanisms. Oxygen radicals and heme are released during proteolysis and must be detoxified by dismutation and polymerization, respectively. The quinoline antimalarials appear to act by preventing sequestration of this toxic heme. Understanding the disposition of hemoglobin has allowed identification of essential processes and metabolic weakpoints that can be exploited to combat this scourge of mankind.

Cloning of a cysteine proteinase gene of Theileria sergenti

A cDNA encoding cysteine proteinase of Theileria sergenti was isolated from a piroplasm cDNA library and its nucleotide sequence was determined. The gene encodes a polypeptide of 402 amino acids with predicted molecular mass of 46.4 kDa. Analysis of the predicted amino acid sequence revealed a number of features common to known cysteine proteinases. Southern blot analysis showed that the cysteine proteinase gene was likely to be a single copy per genome.

Proteolytic cleavage of surface proteins enhances susceptibility of lymphocytes to invasion by Theileria parva sporozoites

A flow cytometric method using anti-parasite antibodies was developed to measure binding of Theileria parva sporozoites to the target bovine lymphocyte membrane. Parasite-host cell interactions could be inhibited by monoclonal antibodies to bovine MHC class I and partially by one of two antibodies to BoCD45R. Proteolysis of the lymphocyte surface removed CD45R but not MHC class I determinants, and enhanced sporozoite binding. These observations support the hypothesis that CD45R and CD45R antibodies may nonspecifically prevent close approximation between sporozoites and lymphocytes. Interestingly, under normal conditions, sporozoites of T. parva did not attach to lymphocytes from goats, but did so when the cells were treated with the protease, suggesting that receptor(s) for T. parva sporozoites might be present on caprine cells but are not easily accessible. These and other results indicate that proteases may be involved in binding and entry of T. parva sporozoites. Electron microscopy revealed that the process of binding and entry of sporozoites into protease-treated goat lymphocytes was very similar to that of the bovine cells. However, schizonts did not develop and lymphocyte proliferation was not induced, indicating that cell entry by sporozoites and cellular transformation are separate processes.

Characterization of native falcipain, an enzyme involved in Plasmodium falciparum hemoglobin degradation

In Plasmodium falciparum, a cysteine protease known as falcipain has been implicated in the essential metabolic process of hemoglobin degradation. Parallel lines of investigation, using native or recombinant enzyme, have led to differing conclusions about the specificity and role of this protease. We have now determined that (1) Native falcipain does not cleave hemoglobin unless this substrate has first been denatured by reducing agents, acid-acetone treatment or plasmepsin action. (2) Reducing agents such as glutathione cannot denature hemoglobin in the presence of catalase, which is accumulated in the digestive vacuole. (3) The purified native enzyme has kinetics similar to those obtained with trophozoite extract, but substantially different from those of recombinant enzyme. (4) Although there are numerous cysteine protease genes in the P. falciparum genome, the falcipain gene is the only one whose transcript can be detected in the early intraerythrocytic parasites. We conclude that falcipain likely works by degrading hemoglobin fragments after initial aspartic protease attack has denatured the substrate. We propose that falcipain inhibitors block the initial steps of degradation indirectly by promoting vacuolar accumulation of osmotically active hemoglobin peptides.

Characterisation of genes encoding a nucleoside monophosphate kinase and a L35 ribosomal protein from Babesia bovis

We have sequenced a region of the Babesia bovis nuclear genome that encodes a L35 ribosomal protein homologue (bl35) and a putative nucleoside monophosphate kinase (bnmk) that is most similar to the adenylate kinase of gram-positive bacteria and the mitochondrial form of adenylate kinase in eukaryotes. BNMK appears to be unique in that it is the first eukaryotic family member to feature a putative zinc-binding domain. bnmk and bl35 are closely linked and transcribed from opposite DNA strands. Examination of the gene structures indicate that the coding regions contain small intervening sequences that obey the GT-AG rule of eukaryotic spliceosomal introns. The single intron separates the bl35 initiation codon from the remainder of the coding region and the 6-exon bnmk gene does not appear to be differentially spliced. Both genes utilise multiple polyadenylation sites and the canonical mammalian polyadenylation signal AATAAA is absent from their 3' untranslated regions. Primer extension analyses reveal that the bnmk gene utilises a cluster of transcription start points, one of which is used most frequently. The bnmk mRNA 5' end does not appear to be cis- or trans-spliced. We report here the first evidence of intronic sequences, as well as heterogeneous 5' and 3' ends for mRNA of a member of the Babesia genus.

Analysis of dinucleotide frequency and codon usage in the phylum Apicomplexa

Dinucleotide frequency (DiF) and codon usage (cu) were analysed in gene sequences from four parasitic protozoa, Babesia bovis, Theileria parva, Toxoplasma gondii and Eimeria tenella, of the phylum Apicomplexa. In keeping with the 'genome hypothesis', cu was found to be non-random and species specific in these organisms, although cu among members of the same subclass was found to be very similar. Several low-usage (lu) codons were identified, and the usage of lu codons appears to be related to the taxonomic position of the organisms under study. A comparison of the observed/expected DiF ratios obtained from gene coding regions revealed a low frequency of the TA and CG dinucleotides in all organisms studied. A comparison of these DiF ratios with those found in rRNA-encoding genes and in introns, showed that in the parasites, B. bovis and Th. parva (representing the piroplasms), the low frequency of dinucleotides appeared to be the result of coding pressure alone. In T. gondii and E. tenella (representing the coccidia), however, coding pressure could not completely explain differences in DiF.

Theileria annulata-infected cells produce abundant proteases whose activity is reduced by long-term cell culture

Lysates of Theileria annulata-infected bovine lymphoblastoid cells and their uninfected counterparts were tested for protease activity using gelatin substrate SDS-PAGE. The infected cells produced a number of extra activities at pH 8.0 in the presence of Ca2+. Calcium was found to enhance the activities but was not an absolute requirement. Studies using inhibitors, including E64, 3,4-dichloroisocoumarin, pepstatin and 1, 10-phenanthroline suggested that the activities were metalloproteases. We analysed two vaccine lines; the Ode line from India and the Ankara Pendik line from Turkey. In the Ode line the later passage had very much reduced levels of the enzyme activities. In the case of the Ankara Pendik line both stages analysed had very low protease activities, but a reduction from the early to late passage was also observed. The reduction in the level of protease activity was also observed as a gradual process during on-going culture of lines derived from the Hissar and Ode stocks. In the Ode line we demonstrated a parallel decrease in the production of microschizonts upon temperature shift in vitro.

Isolation of putative cysteine protease genes of Ostertagia ostertagi

Recombinant phage containing putative Ostertagia ostertagi cysteine protease genes have been isolated from a lambda EMBL-3:genomic DNA library using a Haemonchus contortus cathepsin B-like cysteine protease cDNA as hybridization probe. Restriction enzyme maps of the phages suggest that they identify at least 3 genes, 2 of which appear to be linked in tandem. The complete nucleotide sequence of one gene, CP-1, was determined. The CP-1 gene appears to be organized into 12 exons than span 4.5 kb of DNA. The number and sizes of the exons are essentially identical to those in the H. contortus AC-2 cysteine protease gene. Partial nucleotide sequences obtained for a second O. ostertagi gene, CP-3, revealed a similar organization for exons 8-12 in this gene. Like other cathepsin B-like cysteine proteases, CP-1 appears to be synthesized initially as a preproprotein that is proteolytically processed to its mature form. The amino acid identity between the presumptive CP-1 and CP-3 proteins is 66%, which is similar to the level of homology between the presumed mature protein regions of CP-1 and AC-2. Amino acid identity between CP-1 and AC-2 is greatest in the mature protein region and lowest in the signal sequence and propeptide regions. The CP-3 protein appears to be most closely related to the H. contortus AC-5 protein. CP-1 and CP-3 display significantly greater homology to H. contortus cysteine proteases than they do to human cathepsin B or the Sm31 cysteine protease of Schistosoma mansoni (about 40% identity with each).

Isolation and characterization of a cysteine proteinase gene of Plasmodium falciparum

We have previously identified a 28-kDa cysteine proteinase of Plasmodium falciparum trophozoites that appears to be an essential malarial hemoglobinase and a potential target for antimalarial chemotherapy. The trophozoite cysteine proteinase (TCP) shares a number of biochemical properties with the lysosomal cysteine proteinase cathepsin L. To isolate the gene encoding TCP, we synthesized degenerate oligonucleotides based on two amino acid sequences of cathepsin L that are well conserved among papain-family cysteine proteinases, and used the oligonucleotides to prime the polymerase chain reaction (PCR) with P. falciparum genomic DNA. A 549-bp DNA fragment was amplified by PCR. This fragment was used as a hybridization probe to screen a lambda gt11 library of P. falciparum genomic DNA and isolate a 1.8-kb genomic clone (C1.8) that encoded an intact malarial cysteine proteinase gene. The sequence of C1.8 predicted a 67-kDa protein containing a typical signal sequence, a large pro sequence, and a 26.8-kDa mature proteinase with 37% amino acid identity to cathepsin L. Antisera directed against a peptide encoded by C1.8 recognized a 28-kDa trophozoite protein on immunoblots. In a Northern analysis, C1.8 hybridized predominantly with RNA from rings, the life-cycle stage immediately preceding the trophozoite stage. Taken together, these results strongly suggest that the P. falciparum cysteine proteinase gene we have isolated and characterized encodes TCP.

Characterisation of the gene encoding a candidate vaccine antigen of Theileria parva sporozoites

We have cloned and characterised the gene encoding the 67-kilodalton stage-specific surface antigen, p67, of Theileria parva (Muguga) sporozoites. The gene which is present in a single copy, is divided into 2 exons by an intron 29 bp long and is transcribed into mRNA of about 2500 nucleotides. The gene is present in all stocks of T. parva and there is a related gene in Theileria annulata. The deduced amino acid sequence of 709 residues predicts that p67 is a membrane protein and that it lacks tandemly repeated sequences. Recombinant p67 has been expressed in Escherichia coli as a fusion protein with Sj-26, a glutathione-S-transferase of Schistosoma japonicum. Antibodies to purified recombinant proteins containing residues 9-316 or 397-709 of p67 bind to p67 in immunoblots and neutralise sporozoite infectivity in vitro. Recombinant p67 is, therefore, a candidate antigen for development of an anti-sporozoite vaccine for East Coast fever in cattle.

Characterisation of a cysteine protease from bloodstream forms of Trypanosoma congolense

A cysteine protease (trypanopain-Tc) with cathepsin-L-like properties has been purified from Trypanosoma congolense. The enzyme has an apparent molecular mass of 31-32 kDa by SDS/PAGE and 66 kDa by gel chromatography. It has a pI 7.4 and a high affinity for concanavalin A. Trypanopain-Tc catalyses the limited proteolysis of a variety of protein substrates such as fibrinogen, serum albumin and trypanosome variant-surface glycoprotein. It has minimal or no activity against casein or elastin. A variety of peptidyl amidomethylcoumarins and peptidyl diazomethanes were used to test the specificity of trypanopain-Tc. The better substrates had Arg or Lys in P1 and hydrophobic amino acids in P2 and P3. The best substrate found for trypanopain-Tc was Z-Phe-Arg-NHMec (Z, benzyloxycarbonyl; NHMec, 7-amido-4-methylcoumarin). The kinetic constants for the hydrolysis of Z-Phe-Arg-NHMec were kcat = 17.4 s-1, Km = 4.4 microM, kcat/Km = 4.0 microM-1.s-1, which are very similar to those of cathepsin L with this substrate. The specific substrates for cathepsin B (Z-Arg-Arg-NHMec) and cathepsin H (Arg-NHMec) were not hydrolysed by trypanopain-Tc under the conditions tested. The pH optimum of trypanopain-Tc against Z-Phe-Arg-NHMec was pH 6.0 but it showed a broad peak of activity extending well into the alkaline region. The enzyme was activated by low-molecular-mass thiol compounds and inhibited by cystatin, L-trans-epoxysuccinyl-4-guanidinobutane (E-64) and a variety of peptidyl diazomethanes. The most effective diazomethane inhibitors (Z-Leu-Leu-Met-CHN2, Z-Leu-Met-CHN2 and Z-Leu-Lys-CHN2, were inhibitory at nanomolar concentrations and were trypanocidal in vitro after 24-48 h incubation in greater than or equal to 20 microM [inhibitor]. However, it is not clear whether the trypanocidal activity of these inhibitors is a consequence of the inhibition of trypanopains or of some other essential proteolytic activities within the parasites.