Genomic organization of the Schistosoma mansoni aspartic protease gene, a platyhelminth orthologue of mammalian lysosomal cathepsin D

Schistosomes are considered the most important of the helminth parasites of humans in terms of morbidity and mortality. Schistosomes employ proteolytic enzymes to digest host hemoglobin from ingested human blood, including a cathepsin D-like, aspartic protease that is overexpressed in the gut of the adult female schistosome. Because of its key role in parasite nutrition, this enzyme represents a potential intervention target. To continue exploration of this potential, here we have determined the sequence, structure and genomic organization of the cathepsin D gene locus of Schistosoma mansoni. Using the cDNA encoding S. mansoni cathepsin D as a probe, we isolated several positive bacterial artificial chromosomes (BAC) from a BAC library that represents an approximately 8-fold coverage of the schistosome genome. Sequencing of BAC clone 25-J-24 revealed that the cathepsin D gene locus was approximately 13 kb in length, and included seven exons interrupted by six introns. The exons ranged in length from 49 to 294 bp, and the introns from 30 to 5025 bp. The genomic organization of schistosome cathepsin D was similar in sequence, structure and complexity to human cathepsin D, including to a greater or lesser extent the conservation of all six exon/intron boundaries of the schistosome gene. It was less similar to aspartic protease genes of the nematodes Caenorhabditis elegans and Haemonchus contortus, and dissimilar to those of plasmepsins from malarial parasites. Examination of the introns revealed the presence of endogenous mobile genetic elements including SR2, the ASL-associated retrotransposon, and the SINE-like element, SMalpha. Phylogenetically, schistosome cathepsin D appeared to be more closely related to mammalian cathepsin D than to other sub-families of eukaryotic aspartic proteases known from mammals. Taken together, these features indicated that schistosome cathepsin D is a platyhelminth orthologue of mammalian lysosomal cathepsin D.     

Structural organization of the human p53 gene. I. Molecular cloning of the human p53 gene

Human p53 gene was cloned from the normal human placenta DNA and DNA from the strain of human kidney carcinoma transplanted into nude mice. Representative gene library from tumor strain of human kidney carcinoma and library of 15 kb EcoRI fragments of DNA from normal human placenta were constructed. Maniatis gene library was also used. Five clones were isolated from kidney carcinoma library; they covered 27 kb and included full-length p53 gene of 19.5 kb and flanking sequences. From normal placenta libraries three overlapped clones were obtained. Restriction map of cloned sequences was constructed and polarity of the p53 gene determined. The first intron of the gene is large (10.4 kb); polymorphic BglII site was observed in this intron, which allows to discriminate between allelic genes. One of these (BglII-) is ten times more abundant that the other (BglII+). Both allelic genes are able to synthesize the 2.8 kb p53 gene.     

Molecular cloning, characterization and expression analysis of cathepsin D gene from turbot Scophthalmus maximus

Cathepsin D is a lysosomal endoproteolytic aspartic proteinase which also has been found in endosomes of macrophage. It is thought to play key roles in the developmental and physiological process of animals. The EST sequence of turbot (Scophthalmus maximus L.) cathepsin D was obtained from a subtractive cDNA library. In the present study, 5'-RACE and 3'-RACE were carried out to obtain the complete cDNA sequence of turbot cathepsin D, which contained a 91 bp 5'-UTR, a 1191 bp open reading frame encoding 396 amino acids, and a 329 bp 3'-UTR. The deduced amino acid sequence of the cathepsin D consisted of a signal peptide of 18 aa, a leader peptide extending 43 aa, and a mature peptide of 335 aa. BLAST analysis revealed that turbot cathepsin D shared high similarity with other known cathepsin D, and it showed significant homology with that of Barramundi (Lates calcarifer B., 89% aa similarity). Quantitative real-time PCR (q PCR) demonstrated that the highest expression level of the turbot cathepsin D was in liver. After turbot were challenged with Vibrio harveyi, the lowest expression levels of cathepsin D in liver, spleen and head kidney were detected at 8 h. This result was different from the expression of MHCII of which the expression lever was increased upon challenge. The expression levels of cathepsin D in liver and head kidney increased gradually after 8 h and exceeded the background level after 24 h. In spleen, the expression level was reinforced after 8 h and kept at level that was higher than the original level after 12 h. The results suggested that cathepsin D might process antigens for presentation to the immune system and have synergetic effect with apoptosis pathway until 12 h after injection.     

Sequence variant of the human cathepsin G gene

A frequent polymorphism of the human cathepsin G gene is located in exon 4.     

Molecular forms of cathepsin D in coated vesicle preparations

We have studied the polypeptide pattern of cathepsin D associated with coated vesicle fractions prepared from human placenta. In these fractions cathepsin D was about 35-fold enriched in the precursor polypeptides as compared to the unfractionated tissue extract. The enrichment was more prominent if the vesicles were fractionated in the presence of Triton X-100. Adsorption of exogenously added metabolically labelled cathepsin D precursor to the fractionated material was negligible. It is likely that the precursor and may be also the mature cathepsin D polypeptides are present in the matrix of the coated vesicles. This finding substantiates the idea that coated vesicles participate in the transport of newly synthesized cathepsin D into the lysosomes.     

Molecular organization of the human CD3 gene family on chromosome 11q23

The genes encoding three invariant components of the human T-cell antigen receptor, the CD3 , , and chains, are located on human chromosome 11 at band q23. We isolated cosmid clones containing the human CD3 and chain genes in vectors designed for rapid and efficient chromosome walking. The human CD3 gene was located in the region immediately downstream of the CD3 and genes using synthetic oligonucleotide probes and the localization of this gene confirmed by DNA sequencing. Detailed restriction mapping of the CD3 locus demonstrated that all three CD3 subunits are encoded within 60 kb of DNA with the CD3 gene located 26 kb downstream of the CD3 and genes. Analysis of genomic DNA on pulsed field gels using probes isolated from these cosmid clones defined a physical map of 750 kb spanning the CD3 locus on human chromosome 11g23. The CD3 genes thus comprise a multigene family encoding cell surface components important for transmembrane signaling on T lymphocytes. The arrangement of these genes suggest that they may share common regulatory elements for the control of gene expression during T-cell ontogeny.     

Purification and crystallization of human cathepsin D.

The two-chain form of human cathepsin D was purified from human spleen with a method utilizing an ion exchange chromatography step prior to the pepstatin affinity column normally used to purify aspartic proteases. The protein was crystallized from 21% polyethylene glycol 8000 at pH 4.0 using the hanging drop vapour diffusion method. Small crystals were used as seeds to grow crystals suitable for X-ray data collection. The crystals diffract to a resolution of 3.2 A and have space group P2(1)2(1)2(1) with unit cell dimensions a = 59.9 A, b = 99.6 A, c = 133.6 A. There are two molecules in the asymmetric unit.     

Interaction of human cathepsin D with the inhibitor pepstatin.

1. Because of the proposed role of cathepsin D in a variety of biological and pathological processes, the characteristics of inhibition by the potentially useful agent, pepstatin, were determined. 2. The beta and gamma forms of human cathepsin D, separated by isoelectric focusing, have identical specific extinction coefficients and specific activity in the degradation of haemoglobin. 3. Cathepsin D showed tight binding of 1 mol of pepstatin per 43000 g of protein, indicating that titration with the inhibitor represents a useful method for determination of absolute concentrations of the enzyme. 4. The titration curves were used to determine apparent dissociation constants (KD) for the binding of pepstatin and pepstatin methyl ester at pH3.5; values of approx. 5 X 10(-10)M were obtained. 5. Pepstatinyl-[3H]glycine was synthesized and shown to have a KD similar to that of pepstatin. Gel-chromatographic experiments showed that the binding of pepstatin and its derivatives is strongly pH-dependent. 6. The effect of pH on the KD for pepstatinyl-glycine was determined by equilibrium dialysis. As the pH was raised from 5.0 to 6.4, KD rose from 5 X 10(-10)M to 2 X 10(-6)M. 7. The catalytic activity of cathepsin D declines essentially to zero on going from pH5.0 to pH7.0, and we suggest that the binding site for substrate and pepstatin is abolished by a conformational change in the enzyme molecule. 8. The data indicate that, in biological experiments near neutral pH, large molar excesses of pepstatin over cathepsin D will be required for efficient inhibition.     

Inactivation of human cystatin C and kininogen by human cathepsin D

A papain inhibitor or 22 kDa was isolated from human placenta and shown to be identical to residues Cys246-Leu373 of the third domain of human kininogen. This kininogen domain and recombinant human cystatin C were inactivated by peptide bond cleavages at hydrophobic amino acid residues due to the action of cathepsin D. These results further support the proposed role cathepsin D in the regulation of cysteine proteinase activity.     

Processing of human cathepsin D in lysosomes in vitro

The proteolytic maturation of cathepsin D polypeptides was studied in lysosomes isolated from metabolically labeled fibroblasts. In lysosomes isolated from fibroblasts labeled with [35S]methionine, 70-95% of labeled cathepsin D polypeptides were represented by a Mr = 47,000 polypeptide after a 20-min pulse and 75-min chase. When these lysosomes were incubated in vitro, up to 70% of the Mr = 47,000 polypeptide was processed to mature cathepsin D polypeptides. The processing was dependent on the integrity of the lysosomes, had an optimum between pH 6 and 7, and could be stimulated by dithiothreitol and ATP. The noncleavable ATP analogue, adenosine 5'-(beta, gamma-imido)triphosphate, and GTP, CTP, and UTP could not substitute for ATP. The ATP-dependent stimulation was associated with an acidification of lysosomes. It was inhibited by agents that dissipate the lysosomal pH gradient (carbonyl cyanide p-trifluoromethoxyphenylhydrazone, N,N'-dicyclohexylcarbodiimide, nigericin, NH4Cl). A stimulatory effect of ATP was observed also at pH 5.5. The stimulation at pH 5.5 was not associated with acidification of lysosomes and was resistant to protonophores. Inhibitors of lysosomal cysteine proteinases and N-ethylmaleimide inhibited the processing. In the presence of ATP the processing activity was partially protected from inhibition by N-ethylmaleimide. In conclusion, the maturation of cathepsin D in lysosomes depends on cysteine proteinases and is stimulated by the ATP-driven acidification of lysosomes. In addition, ATP stimulates maturation at pH 5.5 by a mechanism not involving the proton pump.