Train A, an RNase A-Like Protein Without RNase Activity, Is Secreted and Reabsorbed by the Same Epididymal Cells under Testicular Control

Abstract Most of the proteins secreted in the epididymis are produced by the proximal region, and several of them are secreted in abundance. Many of these major proteins have now been identified, including a new epididymis-specific RNase A-like Train A protein, which has been recently described in several mammals. This protein is expressed and secreted exclusively in the initial part of the epididymis. RNase A activity was analyzed in the fluids from the testis and from different epididymal regions, but in no case was the Train A protein found to have RNase A activity. The protein was present only in the luminal fluid of the epididymal region that secreted it. Using an in vitro/in vivo microperfusion technique and immunogold electron microscopy labeling, we demonstrated that the epithelium that secreted it specifically reabsorbed the protein that was present in the lumen of the tubule. Thus, the presence of Train A protein in epididymal fluid was the result of a steady state between secretion and absorpti...