Abstract
A novel glutamate-binding protein was identified in Schistosoma mansoni. The protein (SmGBP) is related to metabotropic glutamate receptors from other species and has a predicted glutamate binding site located within a Venus Flytrap module but it lacks the heptahelical transmembrane segment that normally characterizes these receptors. The SmGBP cDNA was cloned, verified by 5′ and 3′ Rapid Amplification of cDNA Ends (RACE) and shown to be polyadenylated at the 3′end, suggesting the transcript is full-length. The cloned cDNA was subsequently expressed in bacteria and shown to encode a functional glutamate-binding protein. Other studies, using a specific peptide antibody, determined that SmGBP exists in two forms, a monomer of the expected size and a stable but non-covalent dimer. The monomer and dimer are both present in the membrane fraction of S. mansoni and are resistant to extraction with high-salt, alkaline pH and urea, suggesting SmGBP is either an integral membrane protein or a peripheral protein that is tightly associated with the membrane. Surface biotinylation experiments combined with western blot analyses and confocal immunolocalization revealed that SmGBP localized to the surface membranes of adult male schistosomes, especially the dorsal tubercles. In contrast, we detected little or no expression of SmGBP either in the females or larval stages. A comparative quantitative PCR analysis confirmed that the level of SmGBP expression is several-fold higher in male worms than cercariae, and it is barely detectable in adult females. Together, the results identify SmGBP as a new type of schistosome glutamate receptor that is both gender- and stage-specific. The high-level expression of this protein in the male tubercles suggests a possible role in host-parasite interaction.
Highlights
The parasitic flatworm, Schistosoma mansoni is the major cause of human schistosomiasis, a disease that afflicts nearly 200 million people worldwide [1]
We describe one of at least three metabotropic glutamate receptors (mGluRs)-like sequences encoded in the S. mansoni genome [13]
The predicted protein sequence was deduced from the genomic DNA sequence, which is truncated, and was verified experimentally by Rapid Amplification of cDNA Ends (RACE) and cloning of the cDNA
Summary
The parasitic flatworm, Schistosoma mansoni is the major cause of human schistosomiasis, a disease that afflicts nearly 200 million people worldwide [1]. S. mansoni has a complex life cycle that requires two hosts, a freshwater snail of the genus Biomphalaria and the definitive mammalian (human) host. Humans become infected when free-living freshwater larva of S. mansoni (cercariae) penetrate the skin and are quickly transformed into a parasitic larval stage (schistosomula). The newly transformed larvae enter the circulation and undergo a complex migration through the lungs and heart towards the hepatoportal system, where they continue to develop to adult male and female worms and egg production begins. The arsenal of drugs available for treatment of schistosomiasis is very limited. There is an urgent need to learn more about the basic biology of this organism and to identify new molecular targets for drug development
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