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D J Kemp

Publications and source records attributed to D J Kemp.

At least 109 records · Page 6Linked to original sources
PubMed1987

Plasmodium falciparum: identification and localization of a knob protein antigen expressed by a cDNA clone.

Differential screening of cDNA libraries constructed from knobby and predominantly knobless Plasmodium falciparum isolates, identified the sequence SD17. Chromosome blotting experiments have shown that this sequence, which is located on chromosome 2 of most isolates, was deleted in the cloned parasite line E12 of the FCQ27/PNG isolate. Here we show that erythrocytes infected with the SD17-containing cloned line D10 have typical knob structures on their surfaces, whereas those infected with the line E12 lack knobs. An expression clone was constructed from SD17 and used to affinity purify antibodies from the sera of individuals living in areas of Papua New Guinea where malaria is endemic. The antibodies reacted in immunoblotting experiments with a single polypeptide that varied in Mr from 85,000 to 105,000 among different isolates. The antigen was not expressed in the knobless clone E12. Postembedding immunoelectron microscopy showed localization of the antigen over the knobs of FC27 and two other isolates, largely on the cytoplasmic side. We conclude that the parasite antigen corresponding to clone SD17 is a knob protein.

Animals↗
PubMed1987

Babesia bovis: molecular and biological characteristics of cloned parasite lines.

An in vivo limiting dilution technique was used to produce several Babesia bovis cloned lines with which to study the basis of virulence and immunogenicity in this parasite. DNA hybridization using a cloned DNA fragment from the BabR locus demonstrated that the cloned lines were a more restricted genetic population than the parent strain. Biosynthetic labeling and immunoprecipitation studies indicated that the cloned lines differed from each other and from the parentals in the expression of a small number of polypeptides and antigens. Animal trials with three of the lines demonstrated that the parental line contains both virulent and avirulent parasite populations, at least three of which are not tick transmissible, and that while the lines do provide significant protection against heterologous challenge, they may not give as effective protection as the parental line. These experiments demonstrated the existence of subpopulations with distinctive molecular and biological properties, providing evidence that the attenuation process is based on the selection of preexisting parasite subpopulations combined with the ability of these parasites to vary genetically.

Animals↗
PubMed1987

A cDNA clone expressing a rhoptry protein of Plasmodium falciparum.

Antibodies from immune humans were used to select a cDNA clone expressing an asexual blood stage antigen of Plasmodium falciparum. The expressed fused polypeptide was used as an affinity reagent to purify human antibodies specific for the corresponding parasite antigen. Western blotting and immunoelectronmicroscopy demonstrated that the antigen was a 105 kDa protein located in the rhoptries of merozoites. The cDNA encodes the carboxy terminus of the rhoptry antigen, a sequence rich both in charged and hydroxy amino acids.

Amino Acid Sequence↗
PubMed1987

Putative glycophorin-binding protein is secreted from schizonts of Plasmodium falciparum.

A cDNA clone expressing an antigen of Plasmodium falciparum, selected by screening an expression library cloned in Escherichia coli, encodes a portion of the protein identified as a glycophorin-binding protein [Kochan et al. (1986) Cell 44, 689-696]. Human antibodies affinity-purified on extracts from this clone were used to characterize the antigen by immunoblotting. This protein was present in all isolates tested, restricted to mature trophozoites and schizonts. It was abundant in culture supernatants at the time of merozoite release but present in minor amounts if at all in merozoites. The pattern of antigen distribution over schizont-infected cells observed by immunoelectron microscopy differed from that of the precursor of the major merozoite surface antigens in that most of the antigen appeared to be located over the erythrocyte cytoplasm without any obvious association with organelles. It thus appears unlikely that this antigen is present on the merozoite surface prior to schizont rupture.

Amino Acid Sequence↗
PubMed1987

Molecular karyotype of Plasmodium falciparum: conserved linkage groups and expendable histidine-rich protein genes.

We describe fractionation of the Plasmodium falciparum genome into 14 chromosomal DNA molecules by pulsed-field gel electrophoresis. This number agrees with the number of chromosomes observed by electron microscopic visualization of kinetochores. The assignment of 25 markers to 12 of the 14 chromosomes in three cloned parasite lines demonstrates that chromosomal size variation can greatly change the relative migration of genetically equivalent chromosomes. Deletions that include genes for three different histidine-rich proteins, located on chromosomes 2, 8, and 13, contribute to size differences in some clones. Other karyotypic differences result from chromosome segregation and/or recombination during meiosis.

Animals↗
PubMed1987

Changes in repeat number, sequence, and reading frame in S-antigen genes of Plasmodium falciparum.

The S antigens from different isolates of Plasmodium falciparum exhibit extensive size, charge, and serological diversity. We show here that the S-antigen genes behave as multiple alleles of a single locus. The size heterogeneity results from different numbers, lengths, and/or sequences of tandem repeat units encoded within the S-antigen genes. Two genes studied here encode antigenically different S antigens but nevertheless have closely related tandem repeat sequences. We show that antigenic differences can arise because repeats are translated in different reading frames.

Alleles↗
PubMed1987

Structure of the FIRA gene of Plasmodium falciparum.

The falciparum interspersed repeat antigen (FIRA) plays a dominant role in the human antibody response to the malaria parasite Plasmodium falciparum. We have therefore determined the complete sequence of a genomic clone encoding FIRA. The FIRA gene contains a single intervening sequence, located immediately 3' to the putative hydrophobic core of a signal sequence in the short (100 amino acids) exon 1. The second exon largely encodes blocks of 13 hexapeptide repeats based loosely on the consensus sequence Pro-Val-Thr-Thr-Gln-Glu. The first block encoded 39 hexapeptides followed by about nine blocks of 13 hexapeptides interspersed between a conserved region of 81 amino acids, which is itself repeated along the molecule. Although deletions of repeats in this and four other independent clones make the exact number of blocks uncertain, this structure is supported by genomic blotting studies. As 31 variants of the repeat have been identified so far, we suggest that this extreme repeat variability must have important implications for the host immune response.

Amino Acid Sequence↗
PubMed1987

Sequence variation in S-antigen genes of Plasmodium falciparum.

S-antigens are soluble heat-stable antigens released into the circulation at the time of schizogony of Plasmodium falciparum. Many serologically distinct S-antigens exist and we have shown that this diversity results from repetitive sequences that vary in repeat number, length, sequence and/or reading frame among different S-antigens. We present here the complete sequence of the S-antigen of a Vietnamese isolate V1. The major repeat of 33 base-pairs can be considered to be derived by a deletion event from a 45 base-pair sequence that is located at the 3' repeat boundary and is related in sequence to all S-antigen repeats known so far. We also show that the non-repetitive coding region of the S-antigen gene of V1 is identical to that of K1 and only two amino acids different to that of NF7. In contrast, the sequences are considerably different to those of the FC27 and Wellcome isolates. We conclude that S-antigen genes are highly polymorphic in the repetitive regions but show more restricted diversity in non-repetitive regions.

Amino Acid Sequence↗
PubMed1987

Structure and function of candidate vaccine antigens in Plasmodium falciparum.

Analysis of Plasmodium falciparum antigens expressed in Escherichia coli has identified several different proteins as potential vaccine components. Fragments of one of these antigens, the ring-infected erythrocyte surface antigen (RESA) have been used to protect Aotus monkeys against overwhelming infection with P. falciparum. In this vaccine, trial protection correlated with antibody responses to two of three repetitive sequences in RESA. RESA is released from merozoites and becomes associated with the erythrocyte membrane at the time of merozoite invasion. The 3' repeat structure of RESA encodes a polyacidic sequence that has homology with the N-terminal sequence (cytoplasmic domain) of band 3, the erythrocyte anion transporter. These homologous sequences both include a recognition sequence for a protein tyrosine kinase. It is postulated that RESA disrupts the normal intermolecular interactions of the cytoplasmic domain of band 3 and that phosphorylation of RESA by an erythrocyte membrane kinase in some way regulates this function of RESA.

Amino Acid Sequence↗
PubMed1986

Structure of the RESA gene of Plasmodium falciparum.

We have determined the nucleotide sequence of the gene encoding the ring-infected erythrocyte surface antigen (RESA) of Plasmodium falciparum, an antigen that has been shown to confer protective immunity on monkeys. The sequence has enabled us to predict the structure of the RESA gene and the amino acid sequence of its protein product. The gene consists of two exons with a short intron located near the 5' end of the coding region. A hydrophobic amino acid segment predicted for the 3' end of exon 1 is consistent with the possibility that exon 1 encodes trafficking signal sequences. We show that restriction fragment length polymorphisms can be used to define two different alleles of RESA, represented by isolates FC27 and NF7, and compare the FC27 sequence with that of a long cDNA clone from NF7 described previously.

Amino Acid Sequence↗
PubMed1986

An asparagine-rich protein from blood stages of Plasmodium falciparum shares determinants with sporozoites.

We describe a cDNA clone derived from mRNA of asexual blood-stages of the malaria parasite Plasmodium falciparum. This clone, designated Ag319, expresses a P.falciparum antigen fused to beta-galactosidase in Escherichia coli. Human antibodies from Papua New Guinea were affinity-purified by adsorption to extracts of Ag319 immobilized on CNBr-Sepharose. The antibodies reacted predominantly with P. falciparum polypeptides of Mr 220,000 and 160,000, and a number of ill-defined lower molecular weight species. Antibodies reacted in indirect immunofluorescence with all asexual blood-stages although the antigen appeared to be most abundance in the schizont. Surprizingly the antibodies also reacted with sporozoites. The amino acid sequence predicted from the complete nucleotide sequence of this clone is remarkable because 40% of the residues are Asn, and so the antigen has been termed the Asparagine-Rich Protein (ARP). Like other P. falciparum antigens, ARP contains tandemly repetitive sequences, based on the tetrapeptide Asn-Asn-Asn-Met and we have confirmed that these represent natural epitopes by reaction of the corresponding synthetic peptides with human antibodies. Surprisingly, ARP is also rich in Asn outside the tandem repeats.

Amino Acid Sequence↗
PubMed1986

Affinity purification of human antibodies directed against cloned antigens of Plasmodium falciparum.

A technique has been developed for the affinity purification of antibodies recognizing cloned antigens of the malaria parasite Plasmodium falciparum expressed in bacteria. Adsorbents prepared by coupling bacterial lysates to Sepharose were used to isolate monospecific antibodies from human immune sera. Production of an abundant stable fused polypeptide by the bacteria was not a prerequisite for the success of this approach. Also the procedure permits the characterization of antigens which elicit the production of very low levels of antibodies. Affinity-purified human antibodies were used to characterized the corresponding P. falciparum antigens by immunoblotting and a number of antigens identified in this way illustrate some commonly observed features of P. falciparum antigens. Several of these antibody preparations recognized multiple bands in the electrophoretic patterns. Studies on a number of isolates of P. falciparum indicate that many antigens exhibit size polymorphisms. Production of some antigens was shown to be restricted to particular stages of the asexual blood cycle of the parasite while others appear to be specifically processed during the life cycle. Affinity-purified antibodies have also been used to locate antigens within the infected erythrocyte and to delineate subsets of antibodies recognizing different epitopes of a single antigen.

Animals↗
PubMed1986

Chromosome size polymorphisms in Plasmodium falciparum can involve deletions and are frequent in natural parasite populations.

A comparison of independent cultured isolates of Plasmodium falciparum revealed that while chromosome number was constant, the sizes of analogous chromosomes varied widely. We show here that chromosome size polymorphisms are not generated during differentiation of the asexual blood stages, as the molecular karyotype of a cloned parasite line is constant through this part of the life cycle. Experiments using whole P. falciparum chromosomes as hybridization probes to examine polymorphisms within two independent parasite populations indicate that the polymorphisms observed here are not the consequence of large-scale interchromosomal exchanges, and imply that deletions/duplications represent one mode of generating chromosome length polymorphisms. Although the deletions probably involve repetitive DNA, we show here that structural genes for P. falciparum antigens can also be lost. Furthermore, these dramatic size polymorphisms occur not only in cultured lines of P. falciparum, but with surprising frequency in natural malarial infections.

Animals↗
PubMed1986

Antigenic repeat structures in proteins of Plasmodium falciparum.

The majority of malaria antigens that have been cloned contain short sequence repeats which encode antigenic epitopes that are naturally immunogenic. Synthetic peptides have been used to show that natural antibody responses to a strain-specific Plasmodium falciparum S antigen are largely directed against epitopes encoded in an 11-amino acid sequence that is repeated approximately 100 times in the molecule. A 16-amino acid peptide conjugated to bovine serum albumin induced antibodies specific for the S antigen of the homologous isolate. Synthetic peptides have also been used to confirm the natural immunogenicity of epitopes encoded within two blocks of related repeats in the Ring-infected Erythrocyte Surface Antigen (RESA). A 16-amino acid peptide, comprising four repeats of the tetrameric sequence EENV, induced antibodies reactive with the native molecule. Detailed analyses of these anti-peptide antisera indicate that short sequence repeats express more than one epitope, some of which may cross-react with other repeat structures.

Amino Acid Sequence↗
PubMed1986

Variable antigen associated with the surface of erythrocytes infected with mature stages of Plasmodium falciparum.

Immune human sera were used to select a cDNA clone expressing an asexual blood-stage antigen of Plasmodium falciparum. Antibodies affinity-purified on extracts from this clone were used to characterize the antigen by immunoblotting and immunofluorescence. The antigen is present in mature-stage parasites as a high molecular weight protein of about 250 kDa and is apparently processed to smaller fragments in the merozoite. It varies in molecular weight and antibody reactivity in different isolates, and has been localized at the erythrocyte membrane by immunoelectronmicroscopy. Part of the protein is composed of exactly repeated hexapeptide units that constitute the strain-specific determinant. This molecule has similar characteristics to the strain-specific molecule believed to be responsible for cytoadherence.

Amino Acid Sequence↗
PubMed1986

A repetitive antigen of Plasmodium falciparum that is homologous to heat shock protein 70 of Drosophila melanogaster.

We describe an antigen of Plasmodium falciparum, defined by a cDNA clone designated Ag63. The antigen is an abundant, soluble cytoplasmic polypeptide of Mr 75,000 present in all stages of asexual development in the blood and in gametocytes, but not in sporozoites. The sequence of the cDNA clone revealed that, like many other antigens of P. falciparum, it contains tandemly repeated amino acid sequences, in this case Gly-Gly-Met-Pro. However, the rest of the sequence is 70% homologous at the amino acid level to the heat shock protein hsp70 of Drosophila melanogaster.

Amino Acid Sequence↗