We have shown that defective spermatozoa become ubiquitinated in the caput epididymis, presumably by the enzymatic ubiquitination machinery residing within epididymal fluid. The percentage of defective spermatozoa is reduced after passage through the corpus epididymis. Since proteasomes have also been detected in the epididymal fluid, it is possible that defective spermatozoa are partially degraded intraluminally in the caput and corpus epididymis and that the resident clear cells of the corpus epididymis take up and degrade the ubiquitinated proteins released from moribund spermatozoa.
Such a mechanism of defective sperm ubiquitination in the caput and removal in the corpus epididymis is consistent with the transcriptional profiles of the individual epididymal segment from the present study (see Fig. 7). The mRNAs of Ubel, Ube2d3, and Uchll are most prominent in the caput epididymis, whereas the constitutive proteasomal core subunits prevail in the corpus epididymis. THP- and DNB-reduced expression of Psmbl, Psmb2, and Psmb5—the ones with actual protease activities— within the caput and corpus epididymis could alter the epididymal fluid composition and disposal of defective spermatozoa in several ways: by reducing the population of the assembled proteasomes in clear cells, by diminishing the release of assembled proteasomes from secretory epithelial cells into epididymal fluid, by saturating the proteolytic capacity of the remaining proteasomes, or by directly reducing the proteasomal proteolytic activities of assembled protea-somes in the epididymal fluid and/or the clear cell cytoplasm.