Spermatogenesis, the process by which sperms are generated within the male gonads, involves a number of events that occur only in the testis. Examples include the nucleus condensation as a result of histone sequential replacement by transition proteins 1, 2 and protamin, the formation of sperm-specific organelles such as acrosomes and sperm tails, and the shedding of majority of the cytoplasm as residual bodies. To support the accomplishment of these events, it is hypothesized that a large number of testis-specific genes need to be expressed, which is consistent with the estimation that about 4% of the genes in the mouse genome are specifically expressed in the testis 1. Identification of these genes and their biological functions will not only help us to elucidate the mechanisms of spermatogenesis but also provide potential targets for novel non-hormonal contraceptive drugs 2.

In this study, we identified a potential testis-specific gene named Rnf133 (NCBI: Mm.436547). It belongs to the RING finger protein (RNF) family, whose members are very divergent in terms of their sequences and functions except for sharing a RING domain (CX2CX(9-39)CX(1-3)HX(2-3)C/HX2CX(4-48)CX2C) 3. A number of RNFs have been reported to be expressed in the testis, but their functions are poorly understood 4. Multiple sequence alignment of Rnf133 proteins from five mammalian species (Mus musculus, Homo sapiens, Pan troglodytes, Rattus norvegicus, Macaca mulatta) indicated that they are highly conserved (∼70% identity among all species, Supplementary information, Figure S1). All these proteins possess a C3H2C3 RING finger domain, a protease-associated (PA) domain, a signal peptide (SP) and a transmembrane (TM) domain. These features are typical of the GREUL (for Goliath Related E3 Ubiquitin Ligase) proteins, a group of RNFs with some of its members being identified as E3 ubiquitin ligases 5.

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