Isolation of cDNAs Encoding Secreted and Transmembrane Proteins fromSchistosoma mansoniby a Signal Sequence Trap Method

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Abstract
Surface and secreted proteins of schistosomes orchestrate the basic physiologic requirements of a parasitic existence. These proteins are often exposed to host tissues during penetration, migration, feeding, and immune evasion, and they are obvious targets for control strategies. Signal sequence trap (SST) represents a novel approach that selects for cDNAs encoding secreted and surface proteins with N-terminal signal peptides, so we constructed a randomly primed adultSchistosoma mansonicDNA library fused to a signalless reporter gene encoding placental alkaline phosphatase. The library was used to transfect COS-7 cells, which were then assayed for the presence of reporter at the cell surface. EighteenS. mansonicDNA fragments were isolated and sequenced. Expression profiles of the novel clones were determined for different developmental stages; some transcripts were restricted to single-sex adult worms, while others were ubiquitously distributed. Most clones contained signal peptides or signal anchors as determined by the SignalP algorithm. Open reading frames (ORFs) were categorized as follows: (i) previously identifiedS. mansonicDNAs encoding proteins of known function; (ii) cDNAs encoding proteins of known function in other organisms but novel forSchistosoma; (iii)S. mansoniexpressed sequence tags (ESTs) of unknown function; and (iv) completely novel ORFs without homologues (including ESTs) from any phylum. Clones of particular interest included tetraspanins similar to human cell surface antigens, a protein kinase, and ORFs transcribed in the antisense orientation to previously characterizedS. mansonicDNAs. This is the first report describing the use of SST as a tool for identifying secreted proteins from any pathogenic organism.