On the Three-Finger Protein Domain Fold and CD59-Like Proteins in Schistosoma mansoni

Abstract

BackgroundIt is believed that schistosomes evade complement-mediated killing by expressing regulatory proteins on their surface. Recently, six homologues of human CD59, an important inhibitor of the complement system membrane attack complex, were identified in the schistosome genome. Therefore, it is important to investigate whether these molecules could act as CD59-like complement inhibitors in schistosomes as part of an immune evasion strategy.Methodology/Principal FindingsHerein, we describe the molecular characterization of seven putative SmCD59-like genes and attempt to address the putative biological function of two isoforms. Superimposition analysis of the 3D structure of hCD59 and schistosome sequences revealed that they contain the three-fingered protein domain (TFPD). However, the conserved amino acid residues involved in complement recognition in mammals could not be identified. Real-time RT-PCR and Western blot analysis determined that most of these genes are up-regulated in the transition from free-living cercaria to adult worm stage. Immunolocalization experiments and tegument preparations confirm that at least some of the SmCD59-like proteins are surface-localized; however, significant expression was also detected in internal tissues of adult worms. Finally, the involvement of two SmCD59 proteins in complement inhibition was evaluated by three different approaches: (i) a hemolytic assay using recombinant soluble forms expressed in Pichia pastoris and E. coli; (ii) complement-resistance of CHO cells expressing the respective membrane-anchored proteins; and (iii) the complement killing of schistosomula after gene suppression by RNAi. Our data indicated that these proteins are not involved in the regulation of complement activation.ConclusionsOur results suggest that this group of proteins belongs to the TFPD superfamily. Their expression is associated to intra-host stages, present in the tegument surface, and also in intra-parasite tissues. Three distinct approaches using SmCD59 proteins to inhibit complement strongly suggested that these proteins are not complement inhibitors and their function in schistosomes remains to be determined.

New insights into the reaction of Schistosoma mansoni cercaria to the human complement system

“…The MAC is a component of the lytic pathway of the complement system and can lyse cells when further polymerization of C9 molecules occurs by generating pores in the cell membrane (Lachmann and Thompson 1970). Second, the viability of complement-exposed cercariae was determined by the DNA binding stain Hoechst 33258 dye, which has been previously used as an indirect evidence for tegument damage in schistosomula and adult worms (Couto et al 2010; Farias et al 2013; Jones et al 1988). Third, cercariae exposed to complement were stained with an antibody that reacts with the glucose transporter protein 4 (SGTP4).…”

Discovery of novel Schistosoma japonicum antigens using a targeted protein microarray approach

“…Sj-L6L-1 has significant homology to a S. mansoni antigen that was briefly investigated in a DNA vaccine study [31] and recently designated Sm-CD59.2 [32]. Finally calponin-like protein has a predicted size of 27 kDa and has some homology with a 38 kDa  S. japonicum calponin homologue (accession #AAD11976; 63% identity) previously investigated [33].…”

How schistosomes alter the human serum proteome

“…impeding the generation of the C3 convertases C4b2b3b and C3bBb3b) and thereby obstruct complement cascade progression. The host protein CD59 is able to impede membrane attach complex formation and the schistosome tegument contains CD59 homologs which could play a similar role [17], although recent experiments do not support this suggestion [18]. …”