Mining and characterization of ubiquitin E3 ligases expressed in the mouse testis

Background: Ubiquitin-mediated protein modification and degradation are believed to play important roles in mammalian spermatogenesis. The catalogues of ubiquitin activating enzymes, conjugating enzymes, and ligases (E3s) have been known for mammals such as mice and humans. However, a systematic characterization of E3s expressed during spermatogenesis has not been carried out. Results: In present study, we set out to mine E3s from the mouse genome and to characterize their expression pattern, subcellular localization, and enzymatic activities based on microarray data and biochemical assays. We identified 398 putative E3s belonging to the RING, U-box, and HECT subfamilies and found that most genes were conserved between mice and humans. We discovered that 73 of them were highly or specifically expressed in the testes based on the microarray expression data. We selected 10 putative E3 genes to examine their mRNA expression pattern, and several genes to study their subcellular localization and E3 ligase activity. RT-PCR results showed that all the selected genes were predominately expressed in the testis. Some putative E3s were localized in the cytoplasm while others were in both the cytoplasm and the nucleus. Moreover, all the selected proteins were enzymatically active as demonstrated by in vitro and in vivo assays. Conclusions: We have identified a large number of putative E3s that are expressed during mouse spermatogenesis. Among these, a significant portion is highly or specifically expressed in the testis. Subcellular localization and enzymatic activity assays suggested that these E3s might execute diverse functions in mammalian spermatogenesis. Our results may serve as an initial guide to the field for further functional analysis.