Evolutionary conservation of a testes-specific proteasome subunit gene in Drosophila

Proteasomes are large multisubunit particles that act as the proteolytic machinery for the ubiquitin-dependent proteolytic pathway. The core of this complex, the 20S proteasome, is made up of seven α-type and seven β-type subunits, arranged in an (α1–α7)(β1–β7)(β1–β7)(α1–α7) configuration. Previous work had shown that there exist alternative isoforms of the Drosophila melanogaster α4-type subunit, encoded by two distinct genes, α4t1_dm and α4t2_dm, and that these are expressed exclusively in the germline of the testes. We sought to investigate the evolutionary conservation of this phenomenon by screening for orthologs of the α4-type gene family in the distantly related Drosophila species, D. virilis. We isolated the D. virilis orthologs of the somatically expressed gene, α4_dm, and the testes-specific gene, α4t2_dm. We failed to find an ortholog of the other testes-specific gene, α4t1_dm. The α4_dv gene maps to the X chromosome at 12A-C, its product shares 90% amino acid identity with α4_dm, and it is expressed at high levels in both males and females. The other gene, α4t_dv, encodes a protein most similar to the testes-specific α4t2_dm proteasome subunit (59% a.a. identity), and it maps to position 27 on chomosome 2. The expression of the α4t_dv gene is testes-specific, like that of α4t2_dm. The existence of testes-specific α4-type subunits in two widely diverged subgenera of Drosophila suggests that these subunit isoforms have important functional roles in spermatogenesis.