We previously reported that the ascidian sperm proteasome degrades the egg-coat protein extracellularly during fertilization. In order to explore an extracellular transport signal, we purified the proteasome from ascidian sperm and compared its subunit structure with egg and muscle proteasomes. The results showed that PSMA1/alpha 6 subunit of the sperm proteasome is distinct from egg and muscle proteasomes. LC/MS/MS analysis revealed that the C-terminal 16 residues of sperm alpha 6 subunit are processed. Whereas sperm-specific paralogous genes of alpha subunits are reported, its sperm-specific C-terminal processing is a newly discovered novel post-translational modification of the proteasome. (C) 2011 Elsevier Inc. All rights reserved.