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

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Abstract
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 α‐mannosidase precursor of Dictiostelium discoideum (P34098). The 135‐kDa protein could digest both p‐nitro‐phenyl‐α‐D‐mannoside and high mannose oligo saccharide (Man8‐GlcNAc2), strongly suggesting that it is an α‐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.