Schistosoma mansoni: comparison of cloned tropomyosin antigens shared between adult parasites and Biomphalaria glabrata.

Rabbit antisera, raised against whole, homogenized hepatopancreas from uninfected Biomphalaria glabrata, were used to screen a cDNA library prepared from adult Schistosoma mansoni. Of 1.8 x 10(5) recombinants screened, 34 clones were specifically immunoreactive with the antisera. Twenty of the clones were subcloned for further analysis, and 13 of the 20 were demonstrated to hybridize to a cDNA probe encoding schistosome tropomyosin I (pSMTM) at high stringency (Xu et al. Experimental Parasitology 69, 373-392, 1989). One partial cDNA clone (pSM4), which failed to hybridize to pSMTM, was used to rescreen the adult worm cDNA library and a 1381-bp cDNA clone, SmTMII, was identified and characterized. SmTMII contained a single 284-amino acid open reading frame and was shown to be 65.8% homologous to SMTM. Computer-assisted analysis of the predicted protein structure depicted a hydrophilic molecule with the extensive alpha helical secondary structure characteristic of tropomyosins. SmTMII was expressed in Escherichia coli and the resulting 45-kDa fusion protein was recognized by both a polyclonal antiserum specific for SMTM and the antiserum of mice chronically infected with S. mansoni. Northern analysis showed the SmTMII mRNA in the adult stage to be about 1.6 kb in size. Analysis of the SmTMII gene by Southern blot revealed a complex hybridization pattern suggesting the presence of introns or multiple gene copies. The 650-bp EcoRI fragment of SmTMII was used to screen a cDNA library prepared from uninfected Biomphalaria glabrata and two clones, each encoding a novel snail tropomyosin, BgTMII, were isolated and characterized. The two clones, consisting of 2816 and 2314 bp, respectively, differed from each other only in the polyadenylation signal used and each contained a single and identical 284-amino acid open reading frame having 94.7% homology with the previously reported snail tropomyosin, BG39 (Dissous et al. Molecular and Biochemical Parasitology 43, 245-256, 1990). BgTMII was shown to have a higher amino acid homology with SMTM (68.3%) than with BG39 (65.1%) and a higher amino acid homology with SMTM than the two schistosome tropomyosins (SMTM and SmTMII) have with each other (65.8%). Implications of these observations in relation to the concept of host-parasite antigen mimicry are discussed.