Two protochordate genes encode pituitary adenylate cyclase-activating polypeptide and related family members

To address the origin of the glucagon superfamily, we isolated and sequenced the complementary DNA and partial gene that encode pituitary adenylate cyclase-activating polypeptide (PACAP) from a protochordate (tunicate), a sister group of the amphioxus and vertebrates, but one that evolved before the amphioxus. This is the first report of any superfamily member sequenced from an invertebrate. Transcription of the tunicate pacap1 gene results in a messenger RNA that is 507 bp. The gene contains 3 exons that encode a signal peptide, GRF-like peptide(1-27), and PACAP(1-27). The tunicate GRF-like peptide has 59% identity with human GRF, whereas the deduced amino acids of tunicate PACAP(1-27) have 96% identity with the ovine, human, and salmon PACAP(1-27) forms. Another complementary DNA clone pacap2 was isolated and shown to contain 4 exons that encode a signal peptide, a cryptic peptide, and two peptides that are clearly members of the glucagon superfamily. One of the peptides has 89% sequence identity to the tunicate PACAP encoded in pacap1. A comparison of the two structurally related PACAP clones, each encoding two peptides on separate exons, shows high inter- and intraexon nucleotide sequence identity. Sequence analysis suggests that an exon duplication followed by a gene duplication was responsible for the origin of the two genes. It is argued that the PACAP gene is derived from the protochordate ancestral genes that led to the vertebrate forms of GRF and PACAP.