Male Reproductive Tract

Identification of a Member of a New RNase A Family Specifically Secreted by Epididymal Caput Epithelium

Abstract
In this study, we purified the first member of a new RNase A family from fluid of the proximal caput of the boar epididymis. This protein, named "Train A", is the most significant compound secreted in the anterior part of the boar epididymis. After 2D electrophoresis, it is characterized by more than 10 isoforms with molecular weights between 33 and 26 kDa and pI from 5 to 8.5. Several tryptic peptides were N terminal sequenced and an antiserum against one of these peptides was obtained. The protein was immunolocalized in the epididymal epithelium of the proximal caput, especially in the Golgi zone and the apical cytoplasm of the principal cells. In the lumen, spermatozoa were negative but droplets of reaction product were observed within the lumen. Full lengths of "Train A" cDNA were obtained from a gt11 boar caput epididymis library and sequenced. The deduced protein is composed of 213 amino acids, including a 23-amino acid peptide signal and a potential N-glycosylation site. The mRNA of this protein has been retrieved and partially sequenced in the bull, horse and ram and homologous cDNA is found in databanks for the rat, mouse and human. All the sequences are highly conserved between species. This protein and its mRNA are male-specific and exclusively expressed in the proximal caput of the epididymis the only site where they have been found. Train A presents a ribonuclease A (RNase A) family motif in its sequence. The RNase A family is a group of several short proteins (20-14 kDa) with greater and lesser degrees of ribonucleolytic activity and with supposed different roles in vivo. However, the presence of a long conserved N-terminal specific sequence and the absence of RNase catalytic site for Train A indicate that Train A protein is a member of a new family of RNase A.

Key words: Gamete Biology • Male Reproductive Tract • Epididymis • Sperm maturation

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