Gut CaVP is an innate immune protein against bacterial challenge in amphioxus Branchiostoma belcheri

The importance of calcium-binding proteins in immune response of vertebrates is determined, but whether they have the role in invertebrates is largely unknown. In the present study, phylogenetic analysis indicated that calcium vector protein (CaVP), a protein unique to amphioxus, shared 68% similarity in amino acid sequence with human and mouse calmodulin (CaM). CaVP cDNA was cloned into a bacterial vector pET-32a, and its His-tagged fusion protein was produced in Eschherichia coli cells (BL21). The recombinant CaVP was purified by Ni-NTA column and SDS–PAGE, and then utilized for antibody preparing. The prepared antibodies could recognize amphioxus CaVP with high specificity. Further analysis by Western blotting showed that CaVP was detected in muscle and humoral fluid of normal animals and appeared in gut of bacterial immunized or challenged amphioxus. Interestingly, gut CaVP was significantly higher in a healthy sub-group than a wounded sub-group post bacterial challenge. This response was detected strongly in immunization and challenge by the same Gram-negative bacterium Vibro parahaemolyticus and weakly in immunization by V. parahaemolyticus and then challenge by Gram-negative Aeromonas hydrophila, whereas no any feedback was found in immunization by V. parahaemolyticus and challenge by Gram-positive Staphylococcus aureus. These findings indicate the importance of gut CaVP in response to bacterial challenge. ► Calcium vector protein (CaVP) is unique to amphioxus with similarity of vertebrate's calmodulin. ► Gut CaVP appeared in bacterial immunized or challenged amphioxus. ► CaVP was strongly detected in immunization and challenge by the same bacterium. ► Gut CaVP is an innate immune protein in response to bacterial challenge.