Cloning of complementary DNA encoding a 135-kilodalton protein secreted from porcine corpus epididymis and its identification as an epididymis-specific alpha-mannosidase.

In the preceding study (Okamura et al., 1992; Biol Reprod 47:1040-1052) we suggested that a 135-kDa protein secreted by porcine epididymis is involved in the sperm maturation. In this work, we have isolated the cDNA clone coding the 135-kDa protein in an effort to investigate its structure and function. The 135-kDa protein was purified from porcine cauda epididymal fluid. Three oligonucleotide probes were synthesized according to the amino acid sequences of N-termini of the native protein and trypsin-digested peptides. A cDNA clone hybridizing with these three probes was isolated from the cDNA library derived from the porcine proximal corpus epididymis. It encodes a novel protein with 1,006 amino acid residues in an open reading frame. Its overall amino acid sequence was significantly homologous (25.7%) to the alpha-mannosidase precursor of Dictiostelium discoideum (P34098). The 135-kDa protein could digest both p-nitro-phenyl-alpha-D-mannoside and high mannose oligo saccharide (Man8-GlcNAc2), strongly suggesting that it is an alpha-mannosidase homologue. The expression of this protein was specific to porcine and was localized to the very narrow parts of epididymis: the border of the caput and corpus epididymis. This protein may serve as a good marker for the functional differentiation in porcine epididymis. A possible role of this protein in the species-specific sperm-egg interaction is discussed.