The Plasmodium falciparum serine repeat antigen (SERA) is one of the blood stage malaria vaccine candidates. The malaria genome project has revealed that SERA is a member of the SERA multigene family consisting of eight SERA homologues clustered on chromosome 2 and one SERA homologue on chromosome 9. Northern blotting and real time quantitative reverse transcription-PCR with five independent parasite strains, including three allelic representative forms of the SERA gene, have shown that all of the SERA homologues are transcribed most actively at trophozoite and schizont stages and that SERA5 (SERA/SERP) is transcribed predominantly among the family. Polyclonal antibodies were raised against recombinant proteins representing the Nterminal portions of four significantly transcribed SERA homologues (SERA3 to -6) in the center of the cluster on chromosome 2. Using these antibodies, indirect immunofluorescence microscopy detected the expression of SE-RA3 to -6, with similar localization, in all trophozoite-and schizont-infected erythrocytes. We have examined 40 sera from Ugandan adults for their antibody reactivity and found that enzyme-linked immunosorbent assay titer against SERA5 N-terminal domain, but not against other SERA proteins, is positively correlated with the inhibition of in vitro parasite growth by individual sera. Our data confirm the usefulness of the N-terminal domain of SERA5 as a promising malaria candidate vaccine.
The human malaria parasite (Plasmodium falciparum) possesses a plastid-derived organelle called the apicoplast, which is believed to employ metabolisms crucial for the parasite's survival. We cloned and studied the biochemical properties of plant-type ferredoxin (Fd) and Fd-NADP+ reductase (FNR), a redox system that potentially supplies reducing power to Fd-dependent metabolic pathways in malaria parasite apicoplasts. The recombinant P. falciparum Fd and FNR proteins were produced by synthetic genes with altered codon usages preferred in Escherichia coli. The redox potential of the Fd was shown to be considerably more positive than those of leaf-type and root-type Fds from plants, which is favourable for a presumed direction of electron flow from catabolically generated NADPH to Fd in the apicoplast. The backbone structure of P. falciparum Fd, as solved by X-ray crystallography, closely resembles those of Fds from plants, and the surface-charge distribution shows several acidic regions in common with plant Fds and some basic regions unique to this Fd. P. falciparum FNR was able to transfer electrons selectively to P. falciparum Fd in a reconstituted system of NADPH-dependent cytochrome c reduction. These results indicate that an NADPH-FNR-Fd cascade is operative in the apicoplast of human malaria parasites.
Telomerase is a ribonucleoprotein ribonucleic enzyme that is essential for cellular immortalization
via
elongation of telomere repeat sequences at the end of chromosomes. Human telomerase reverse transcriptase (
hTERT
), the catalytic subunit of telomerase holoenzyme, is a key regulator of telomerase activity. Telomerase activity, which has been detected in the majority of cancer cells, is accompanied by
hTERT
expression, suggesting that this enzyme activity contributes to an unlimited replication potential of cancer cells
via
regulation of telomere length. Thus,
hTERT
is an attractive target for cancer-specific treatments. We previously reported that pared-like homeodomain 1 (
PITX1
) is a negative regulator of
hTERT
through direct binding to the
hTERT
promoter. However, the mechanism by which the function of
PITX1
contributes to transcriptional silencing of the
hTERT
gene remains to be clarified. Here, we show that PITX1 and zinc finger CCHC-type containing 10 (ZCCHC10) proteins cooperate to facilitate the transcriptional regulation of the
hTERT
gene by functional studies
via
FLAG pull-down assay. Co-expression of
PITX1
and
ZCCHC10
resulted in inhibition of
hTERT
transcription, in melanoma cell lines, whereas mutate-deletion of homeodomain in PITX1 that interact with ZCCHC10 did not induce similar phenotypes. In addition, ZCCHC10 expression levels showed marked decrease in the majority of melanoma cell lines and tissues. Taken together, these results suggest that ZCCHC10-PITX1 complex is the functional unit that suppresses
hTERT
transcription, and may play a crucial role as a novel tumor suppressor complex.
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