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 a-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, α4 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.