Echinococcus granulosus antigen 5 is closely related to proteases of the trypsin family

Abstract

Antigen 5 (Ag5) is a dominant secreted component of the larval stage of Echinococcus granulosus, and is highly immunogenic in human infections. Although the diagnostic value of Ag5 has been thoroughly evaluated, there has been little progress in its molecular characterization and the understanding of its biological role. In the present study, the Ag5 gene was cloned by reverse transcription-PCR on the basis of the amino acid sequences of tryptic fragments. The nucleotide sequence indicates that Ag5 is synthesized as a single polypeptide chain that is afterwards processed into single disulphide-bridged 22 and 38 kDa subunits. Whereas the 22 kDa component contains a highly conserved glycosaminoglycan-binding motif that may help to confine Ag5 in the host tissue surrounding the parasite, the 38 kDa subunit is closely related to serine proteases of the trypsin family. The sequences in the vicinity of the active-site histidine, aspartic acid and serine residues, and critical cysteine residues involved in disulphide formation, are well conserved, but the catalytic serine residue is replaced by threonine. Since there are no significant chemical differences between the O gamma atoms of these residues, we performed a series of enzymic assays to find out whether Ag5 is a catalytic molecule. Neither proteolytic activity nor binding to protease inhibitors could be detected using the native purified antigen. Thus it may be possible that Ag5 possesses a highly specific physiological substrate or, more likely, that trypsin-like folding has been recruited to fulfil novel functions.

Comparative Analysis of the Diagnostic Performance of Six Major Echinococcus granulosus Antigens Assessed in a Double-Blind, Randomized Multicenter Study

“…A wealth of reports on the diagnostic evaluation of immunopurified components from hydatid cyst fluid and protoscoleces, as well as that of numerous recombinant Echinococcus antigens, are available (9,10,14,15,23). However, the diagnostic performance of these antigens has been assessed in different laboratories, using different serum collections and different techniques, which makes it difficult to draw conclusions.…”

Immunoblotting with Human Native Antigen Shows Stage-Related Sensitivity in the Serodiagnosis of Hepatic Cystic Echinococcosis

“…Sera in group 2 recognized all the subunits of AgB, and bands between 34-50 kDa, which can be attributed to Ag5 subunits. 10 In 8 of 10 cases, the test allowed discrimination between CE3a, and CE3b cysts as sera from patients with CE3a cysts always recognized AgB bands but only one subunit (50 kDa) of Ag5 bands, whereas sera from subjects with CE3b cysts always recognized all bands of the two antigens. In the other two CE3a cases, sera recognized both Ag5 bands, but with weaker intensity.…”

Gene cloning, expression, and localization of antigen 5 in the life cycle of Echinococcus granulosus

“…In rats immunized with rAg5, the titer is very low in spite of two boosts in the early 6 weeks but acute increase appeared after the third boosts (data is not shown), this phenomenon indicates Ag5 is able to inhibit immune response at the initiating stage . Ag5 is highly similar to members of the trypsin family (Lorenzo et al 2003)-a serine protease plays an important role in the cestode eggs hatching and larvae invasion of helminth parasites (Dzik 2006;Young et al 1999). This early immunological unresponsiveness suggests it may play a role in immune evasion which is associated with its serine protease activity, the mechanisms deserve further studies.…”

“…The C-terminal 38-kDa subunit is closely related to trypsin of the serine protease family (Lorenzo et al 2003). Helminth serine protease plays an important role in the eggs hatching and larvae invasion and is a potential vaccine candidate (Dzik 2006;Young et al 1999).…”