Genomic structure of the amphioxus calcium vector protein

Calcium vector protein (CaVP) is an EF-hand Ca2+-binding protein, which is unique to the protochordate, amphioxus. CaVP is supposed to act as a Ca2+ s ignal transductor, but its exact function remains unknown. Not only its fun ction but also its exact evolutionary relationship to other Ca2+-binding pr oteins is unclear, To investigate the evolution of CaVP, we have determined the complete sequences of Ca VP cDNAs from two amphioxus species, Branchio stoma lanceolatum and B. floridae, whose open reading frame cDNA and amino acid sequences show 96.5 and 98.2% identity, respectively. We have also elu cidated the structure of the gene of B. floridae CaVP, which is made up of seven exons and six introns, The positions of four of the six introns (intr ons 1, 2, 3, and 5) are identical with those of calmodulin, troponin C, and the Spec protein of the sea urchin, These latter proteins belong to the so -called troponin C superfamily (TnC superfamily) and thus CaVP likely also belongs to this family. Intron 6 is positioned in the 3' noncoding region a nd is unique to Ca VP, so it may represent a landmark of the CaVP lineage o nly. The position of intron 1 is not conserved in the genes of the TnC supe rfamily or CaVP, and seems to result from either intron sliding or the addi tion of an intron (randomly inserted into or close to domain III) to the ge nes of the TnC superfamily during their evolution.