Evolution of the trappin multigene family in the Suidae

Trappins are a group of secretory proteins containing a WAP motif with an anchoring domain. Previous studies showed that their genes, especially those of pig, have undergone rapid evolution, which produced trappins with a broad spectrum of actions. To understand the evolution of such a rapidly evolving multigene family, we isolated trappin genes of the Artiodactyla, including pig, wart hog, collared peccary, hippopotamus, and cow, by means of polymerase chain reaction (PCR). Two genes newly isolated from wart hog are orthologs of trappin-1 (SPAI) and trappin-2 (elafin), the others are novel members of the trappin family and named trappins-6 to 11. The divergence of the sequences is greatest in the region that encodes the reactive site, and intron sequences appear to be more highly conserved than the protein-coding sequences, especially among the pig paralogs. Phylogenetic analysis showed that the trappin multigene family members of pig were generated through gene duplication after the divergence of the Suidae (pig and wart hog) and Tayassuidae (collared peccary). Similarities in the gene structure with seminal vesicle clotting proteins (REST) and WAP motif-containing proteins suggest that trappins are naturally occurring fusion proteins created through exon shuffling between ancestral REST and WAP motif-coding genes.