Among its many functions, the ubiquitin-proteasome system regulates substrate-specific proteolysis during the cell cycle, apoptosis, and fertilization and in pathologies such as Alzheimer's disease, cancer, and liver cirrhosis. Proteasomes are present in human and boar spermatozoa, but little is known about the interactions of proteasomal subunits with other sperm proteins or structures. We have created a transgenic boar with green fluorescent protein (GFP) tagged 20S proteasomal core subunit α-type 1 (PSMA1-GFP), hypothesizing that the PSMA1-GFP fusion protein will be incorporated into functional sperm proteasomes. Using direct epifluorescence imaging and indirect immunofluorescence detection, we have confirmed the presence of PSMA1-GFP in the sperm acrosome. Western blotting revealed a protein band corresponding to the predicted mass of PSMA1-GFP fusion protein (57 kDa) in transgenic spermatozoa. Transgenic boar fertility was confirmed by in vitro fertilization, resulting in transgenic blastocysts, and by mating, resulting in healthy transgenic offspring. Immunoprecipitation and proteomic analysis revealed that PSMA1-GFP copurifies with several acrosomal membrane-associated proteins (e.g., lactadherin/milk fat globule E8 and spermadhesin alanine-tryptophan-asparagine). The interaction of MFGE8 with PSMA1-GFP was confirmed through cross-immunoprecipitation. The identified proteasome-interacting proteins may regulate sperm proteasomal activity during fertilization or may be the substrates of proteasomal proteolysis during fertilization. Proteomic analysis also confirmed the interaction/coimmunoprecipitation of PSMA1-GFP with 13/14 proteasomal core subunits. These results demonstrate that the PSMA1-GFP was incorporated in the assembled sperm proteasomes. This mammal carrying green fluorescent proteasomes will be useful for studies of fertilization and wherever the ubiquitin-proteasome system plays a role in cellular function or pathology.

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During animal and human fertilization, the fertilizing spermatozoon creates and passes through a fertilization slit in the vitelline coat (VC) of the oocyte. It has been hypothesized that the penetration of the mammalian VC, the zona pellucida (ZP), is aided by a proteolytic enzyme capable of locally degrading ZP proteins. This putative "zona lysin" is predicted to reside within the sperm head acrosome and be released or exposed by ZP-induced acrosomal exocytosis. Evidence has been accumulating in favor of the 26S proteasome, the ubiquitin-dependent multi-subunit protease acting as the putative vitelline coat/zona lysin in humans and animals. To confi rm this hypothesis, three criteria must be met: (1) the sperm receptor on the ZP must be ubiquitinated, (2) proteasomes must be present, exposed, and enzymatically active in the sperm acrosome, and (3) sperm proteasomes must be able to degrade the sperm receptor on the egg coat/ZP during fertilization. This review discusses recent data from a number of mammalian and nonmammalian models addressing these predictions. These data shed light on the mechanisms controlling sperm interactions with VC and on the evolutionary conservation of the proteasome-assisted fertilization mechanisms.