Identification and Characterisation of the Antigenic Cpa135 protein

This chapter focuses on the identification and characterization of the antigenic Cpal35 protein. Three antigenic peptides that represent portions of larger sporozoite proteins have been identified previously. One of those peptides (SA35) was specifically recognized by human IgG and the objective of the present study was to determine the origin of that peptide. The search for homologies at the protein level reveals that Cpal35 has a composite structure with regions of similarity with different unrelated proteins. At its N-terminus, Cpal35 shows a possible leader peptide flanked by a region with homology to yeast coronins and animal tropomyosins. The coronins are the proteins involved in cytoskeleton re-modelling that promote actin polymerization; tropomyosins are also actin binding proteins. In the middle of the protein, there is a large region without homology with other proteins and two short repetitive amino acidic sequences. The C-terminus of this protein is a region with high-cystein content that shows a homology to a soybean protein of unknown function. Histidine-tagged peptides are a useful way of obtaining specific antibodies raised against antigenic proteins; thus, the monoclonal antibody specific for SA35 permitted the identification and the characterization of the Cpal35 protein. This protein is localized in the apical complex of the sporozoites, suggesting that Cpal35 can play a role in the host-cell invasion. The presence of a putative leader peptide in the protein sequence suggests that Cpal35 can be a protein secreted through specialized organelles, Tmicronemes. The composite arrangement of that protein suggests that Cpal35 is a multi-domain protein with different motifs to exert different molecular interactions. Further studies will be dedicated to ascribing a functional role to the Cpal35 putative domains and to determine the potential role as a protective immunizing antigen.