Schistosoma mansoni: SmLIMPETin, a member of a novel family of invertebrate-only regulatory proteins.

Eukaryotic LIM domain proteins contain zinc finger forming motifs rich in cysteine and histidine that enable them to interact with other proteins. A cDNA clone isolated from an adult schistosome cDNA library revealed a sequence that coded for a novel class of proteins bearing 6 LIM domains and an N-terminal PET domain, SmLIMPETin. Phylogeny reconstruction of SmLIMPETin and comparison of its sequence to invertebrate homologues and to the vertebrate four-and-a-half LIM domains protein family (FHLs), uncovered a novel LIM domain protein family, the invertebrate LIM and PET domain protein family (LIMPETin). Northern blots, RT-PCR and Western blot showed that SmLIMPETin gene was less expressed in sexually mature adult females compared to sexually immature adult females and sexually mature and immature adult males, and not expressed in schistosomula.

The N-terminus moiety of the cystatin SmCys from Schistosoma mansoni regulates its inhibitory activity in vitro and in vivo.

The complete sequence of SmCys, a cystatin expressed by Schistosoma mansoni, was obtained. Constructs of SmCys consisting of deletions of 10 and 20 amino acid residues from the N-terminal of the full length recombinant protein, were cloned in the pQE-30 vector, expressed in Escherichia coli and assayed for inhibitory activity against papain. Kinetic analysis showed that SmCys -10 and SmCys -20 had K(i) values of 0.7391 and 4.9154, respectively, as compared to 0.0647, displayed by the full length recombinant. Protease inhibition by SmCys was also observed in vivo. When the recombinant products were incubated during 7 days with live schistosomula in the presence of red blood cells, only the full length product could completely inhibit the formation of haemozoin, a dark pigment formed as a by-product of haemoglobin digestion. The sequence data of the recombinant SmCys proteins were used for the construction of molecular models, which were then subjected to molecular dynamics for 2ns. In comparison to the full length, the models corresponding to the truncated constructs, showed a distinctive change on the surface charge distribution. This parameter was more pronounced in SmCys -20, which also displayed a significant displacement of the inhibitory domain, a result which could explain the kinetic data in terms of the loss of attachment sites. These changes correlated well with the progressive lack of inhibition observed for the recombinant deletion constructs, in vitro and in vivo.