The proteasome is a large, multisubunit complex that acts as the cell's "protein-degrading machine" in the ubiquitin-mediated proteolytic pathway for regulated protein turnover. Although proteasomes are usually thought of as being homogeneous structures, recent studies have revealed their more dynamic and heterogeneous nature. For example, in a number of plant and animal species, multiple isoforms of several proteasome subunits, encoded by paralogous genes, have been discovered, and in some cases, these alternative isoforms have been shown to be functionally distinct from their conventional counterparts. A particularly striking example of this phenomenon is seen in Drosophila melanogaster, where 12 of the 33 subunits that make up the 26S proteasome holoenzyme are represented in the genome by multiple paralogous genes. Remarkably, in every case, the "extra" genes are expressed in a testis-specific manner. Here, we describe the extent and nature of these testis-specific gene duplications and discuss their functional significance, and speculate on why this situation might have evolved.
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