Organization of the human gene encoding the epididymis-specific EP2 protein variants and its relationship to defensin genes.

The EP2 gene codes for at least nine message variants that are all specifically expressed in the epididymis. These variants putatively encode small secretory proteins that differ in their N- and C-termini, resulting in proteins that can have little or no sequence similarity to each other. We have isolated and sequenced the human EP2 gene to determine the molecular origin of these variants. The EP2 gene has two promoters, eight exons, and seven introns. Exons 3 and 6 encode protein sequences homologous to beta-defensins, a family of antimicrobial peptides. This sequence homology and the arrangement of promoters and defensin-encoding exons suggest that the EP2 gene originated from two ancestral beta-defensin genes arranged in tandem, each contributing a promoter and two exons encoding a leader sequence and a defensin peptide. The proposed evolutionary relationship between the EP2 gene and defensin genes is supported by the observation that the EP2 gene is located on chromosome 8p23 near the defensin gene cluster and is separated by 100 kilobases or less from DEFB2, the gene for beta-defensin-2. While the EP2 gene transcribes beta-defensin-like message variants, most of the known message variants code for nondefensin proteins or proteins containing only a partial defensin peptide sequence. We suggest that, during its evolution, the EP2 gene has acquired new functions that may be important for sperm maturation and/or storage in the epididymis.