Characterization of the Testis-specific Proteasome Subunit α4s in Mammals

Background: Two subtypes of proteasome core particles (CPs) with tissue-specific subunits have been identified in mammals. Results: Mammals have an additional proteasome subunit, 4s, which forms the male germ-specific CP. Conclusion: The 4s-containing CP is a new subtype of CP with unique properties distinct from the constitutive CP. Significance: Our results provide a clue for understanding the role of the proteasome in mammalian testes. The 26 S proteasome is responsible for regulated proteolysis in eukaryotic cells. It is composed of one 20 S core particle (CP) flanked by one or two 19 S regulatory particles. The CP is composed of seven different -type subunits (1-7) and seven different -type subunits, three of which are catalytic. Vertebrates encode four additional catalytic subunits that are expressed predominantly in immune tissues and produce distinct subtypes of CPs particularly well suited for the acquired immune system. In contrast, the diversity of subunits remains poorly understood. Recently, another subunit, referred to as 4s, was reported. However, little is known about 4s. Here we provide a detailed characterization of 4s and the 4s-containing CP. 4s is exclusively expressed in germ cells that enter the meiotic prophase and is incorporated into the CP in place of 4. A comparison of structural models revealed that the differences in the primary sequences between 4 and 4s are located on the outer surface of the CP, suggesting that 4s interacts with specific molecules via these unique regions. 4s-containing CPs account for the majority of the CPs in mouse sperm. The catalytic subunits in the 4s-containing CP are 1, 2, and 5, and immunosubunits are not included in the 4s-containing CP. 4s-containing CPs have a set of peptidase activities almost identical to those of 4-containing CPs. Our results provide a basis for understanding the role of 4s and male germ cell-specific proteasomes in mammals.