Molecular cloning and partial characterization of a nematode-specific 24 kDa protein from Ascaris suum

The cloning and molecular characterization of a cDNA encoding Ascaris suum 24 kDa antigen (As24) are described. The cDNA sequence consists of 853 bp with an open reading frame coding for a protein of 147 amino acids with an inferred signal peptide of 19 amino acids. The predicted molecular mass and pI were 16 kDa and 8.35 respectively. The endogenous protein in adult A. suum was 24 kDa with the expected pI. A search of the public databases revealed over 50% homology with proteins from filarial parasites but not to other known proteins, suggesting that As24 is a nematode-specific protein. Immunohistochemical studies using polyclonal antibodies raised against Escherichia coli-expressed recombinant As24 demonstrated that the endogenous As24 proteins were intensely localized in unembryonated eggs within the uterus, uterine and gut epithelium, muscle tissues and in the hypodermis of an adult female A. suum. Endogenous As24 was expressed throughout A. suum development and was detected in the excretory/secretory products by immunoblot analysis. Importantly, a homologous protein(s) was detected in Ascaris from human and Toxocara canis from dog, suggesting that As24 is a nematode-specific protein.